Magnetic Effect of Electric Current questions

A rectangular coil is placed perpendicular to a magnetic flux whose density changes uniformly. Which of the following graphs represents the relation between the total flux $(\phi_m)$ through the coil and the magnetic flux density (B) in which the coil is placed?

1. A A
2. B B
3. C C
4. D D

The opposite figure shows a circular coil placed perpendicular to a uniform magnetic field. If the coil rotates clockwise through 90° about an axis perpendicular to its plane, the flux passing through the coil ..........

1. A increases
2. B equals zero
3. C decreases
4. D does not change

A ring of cross-sectional area $0.2$ m² is placed perpendicular to the lines of a uniform magnetic flux of density $0.04$ Wb/m². The magnetic flux passing through the ring equals ..........

1. A 0.002 Wb
2. B 0.004 Wb
3. C 0.006 Wb
4. D 0.008 Wb

A square frame of side length 20 cm is placed in a uniform magnetic field of flux density $3 \times 10^{-2}$ Tesla. If the flux passing through the frame is $6 \times 10^{-4}$ Weber, the angle between the plane of the frame and the flux lines equals ..........

1. A 20°
2. B 30°
3. C 45°
4. D 90°

A coil of area 2 m² is placed in a magnetic field of flux density 0.05 Wb/m² so that the flux through it is maximum. The magnetic flux through the coil when the coil rotates from this position through an angle of:
30° equals ..........

1. A 0.01 Wb
2. B -0.1 Wb
3. C 0.7 Wb
4. D 0.087 Wb

A coil of area 2 m² is placed in a magnetic field of flux density 0.05 Wb/m² so that the flux through it is maximum. The magnetic flux through the coil when the coil rotates from this position through an angle of:
45° equals ..........

1. A -0.01 Wb
2. B 0.03 Wb
3. C 0.07 Wb
4. D 0.09 Wb

A coil of area 2 m² is placed in a magnetic field of flux density 0.05 Wb/m² so that the flux through it is maximum. The magnetic flux through the coil when the coil rotates from this position through an angle of:
135° equals ..........

1. A 0.1 Wb
2. B 0.005 Wb
3. C -0.07 Wb
4. D -0.09 Wb

A coil of area 2 m² is placed in a magnetic field of flux density 0.05 Wb/m² so that the flux through it is maximum. The magnetic flux through the coil when the coil rotates from this position through an angle of:
180° equals ..........

1. A -0.1 Wb
2. B 0.2 Wb
3. C -0.3 Wb
4. D 0.4 Wb

Several rectangular coils of different areas are placed in a uniform magnetic field, inclined to it at 30°. The opposite graph shows the relation between the total flux through the coil $(\phi_m)$ and the area of the coil (A). The magnetic flux density acting on all the coils is ..........

1. A 1 T
2. B 4 T
3. C 0.5 T
4. D 0.8 T

The opposite figure shows two different positions (x) and (y) of a coil of area 0.2 m² placed in a uniform magnetic field of flux density 0.8 T. The change in magnetic flux $\Delta\phi_m$ through the coil between the two positions equals ..........

1. A 0
2. B 0.08 Wb
3. C 0.4 Wb
4. D 0.16 Wb

Two rectangular coils (x) and (y) of the same area are fixed together so that one is inclined to the other at 30°. If both coils are exposed to a magnetic flux of density B in the direction shown in the opposite figure, the ratio between the magnetic fluxes through the two coils $\left(\frac{(\phi_m)_x}{(\phi_m)_y}\right)$ equals ..........

1. A $\dfrac{1}{2}$
2. B $\dfrac{2}{1}$
3. C $\dfrac{\sqrt{3}}{2}$
4. D $\dfrac{2}{\sqrt{3}}$

A coil of face area A is placed perpendicular to a uniform magnetic flux of density B. The change in the magnetic flux through the coil when it rotates from the perpendicular position about an axis in its own plane through:
a quarter of a turn equals ..........

1. A zero
2. B BA
3. C $\dfrac{3}{2}$ BA
4. D 2 BA

A coil of face area A is placed perpendicular to a uniform magnetic flux of density B. The change in the magnetic flux through the coil when it rotates from the perpendicular position about an axis in its own plane through:
half a turn equals ..........

1. A zero
2. B BA
3. C $\dfrac{3}{2}$ BA
4. D 2 BA

A coil of face area A is placed perpendicular to a uniform magnetic flux of density B. The change in the magnetic flux through the coil when it rotates from the perpendicular position about an axis in its own plane through:
a third of a turn equals ..........

1. A zero
2. B BA
3. C $\dfrac{3}{2}$ BA
4. D 2 BA

The opposite figure shows a square coil moving at constant speed to the right of the page through a uniform magnetic field perpendicular to the page and into it. The relation between the magnetic flux $(\phi_m)$ through the coil while it moves from position A to B and the time (t) is ..........

1. A A
2. B B
3. C C
4. D D

Two cubes x and y of side lengths ℓ and 2ℓ respectively each have a uniform magnetic field acting perpendicular to its upper face, as in the figure. Which of the following relations represents the relation between the magnetic fluxes through their upper faces?

1. A $(\phi_m)_y = \dfrac{1}{4}(\phi_m)_x$
2. B $(\phi_m)_y = (\phi_m)_x$
3. C $(\phi_m)_y = 2(\phi_m)_x$
4. D $(\phi_m)_y = 4(\phi_m)_x$

A coil of area A placed completely in a uniform magnetic field perpendicular to its plane has a magnetic flux of $10^{-3}$ Wb through its face. The coil was moved so that the area through which the flux passes decreased by 10 cm², as in the opposite figure, and the magnetic flux through the coil became $7.5 \times 10^{-4}$ Wb. The area of the coil (A) equals ..........

1. A 10 cm²
2. B 20 cm²
3. C 40 cm²
4. D 80 cm²

A coil is placed in a uniform magnetic field so that its plane is inclined to the field lines at 60°, as in the figure, and the magnetic flux through it is $2 \times 10^{-6}$ Wb. The flux through the coil when it rotates:
clockwise through 30° equals ..........

1. A $3.854 \times 10^{-8}$ Wb
2. B $1.155 \times 10^{-6}$ Wb
3. C $9.731 \times 10^{-5}$ Wb
4. D $8.514 \times 10^{-7}$ Wb

A coil is placed in a uniform magnetic field so that its plane is inclined to the field lines at 60°, as in the figure, and the magnetic flux through it is $2 \times 10^{-6}$ Wb. The flux through the coil when it rotates:
clockwise through a quarter of a turn equals ..........

1. A $6.25 \times 10^{-6}$ Wb
2. B $7.93 \times 10^{-4}$ Wb
3. C $1.155 \times 10^{-6}$ Wb
4. D $9.11 \times 10^{-4}$ Wb

A coil is placed in a uniform magnetic field so that its plane is inclined to the field lines at 60°, as in the figure, and the magnetic flux through it is $2 \times 10^{-6}$ Wb. The flux through the coil when it rotates:
anticlockwise through 30° equals ..........

1. A $1.155 \times 10^{-6}$ Wb
2. B $2.31 \times 10^{-6}$ Wb
3. C $4.692 \times 10^{-7}$ Wb
4. D $3.854 \times 10^{-8}$ Wb

A coil is placed in a uniform magnetic field so that its plane is inclined to the field lines at 60°, as in the figure, and the magnetic flux through it is $2 \times 10^{-6}$ Wb. The flux through the coil when it rotates:
anticlockwise through a quarter of a turn equals ..........

1. A $1.155 \times 10^{-6}$ Wb
2. B $2.231 \times 10^{-5}$ Wb
3. C $4.692 \times 10^{-7}$ Wb
4. D $3.854 \times 10^{-8}$ Wb

In the opposite figure, a coil of area A is placed in a magnetic field of density B so that it is inclined to the field at 30°, and the total flux through the coil is $\phi_m$. The smallest angle the coil must rotate through so that the flux through it becomes:
$2\phi_m$ is ..........

1. A 30°
2. B 45°
3. C 60°
4. D 90°

In the opposite figure, a coil of area A is placed in a magnetic field of density B so that it is inclined to the field at 30°, and the total flux through the coil is $\phi_m$. The smallest angle the coil must rotate through so that the flux through it becomes:
$\frac{2}{3}\phi_m$ is ..........

1. A 20.31°
2. B 10.53°
3. C 13.9°
4. D 15.52°

The opposite figure shows a rectangular coil rotating in a uniform magnetic field about an axis parallel to its long sides. The graph that represents the relation between the magnetic flux $(\phi_m)$ through the coil and the angle (θ) the coil rotates through during half a turn, starting from the position perpendicular to the magnetic field, is ..........

1. A A
2. B B
3. C C
4. D D

The unit $Wb.A^{-1}.m^{-1}$ is the unit of ..........

1. A magnetic flux
2. B magnetic flux density
3. C magnetic permeability
4. D electric power

In the figure shown, a long straight wire has a resistance of 0.2 Ω and the potential difference across it is 1 V. The magnetic flux density at point x is ..........

1. A $5 \times 10^{-5}$ T, into the page
2. B $5 \times 10^{-5}$ T, out of the page
3. C $2 \times 10^{-5}$ T, into the page
4. D $2 \times 10^{-5}$ T, out of the page

Which of the following graphs represents the relation between the magnetic flux density (B) at point x and the distance (d) of point x from the axis of the wire shown in the figure?

1. A A
2. B B
3. C C
4. D D

The opposite figure shows a vertical straight wire (P) surrounded by four compasses, the needle of each in the magnetic meridian plane. When an electric current passes through the wire downward (into the page), it produces a magnetic field stronger than the Earth's magnetic field. The compass whose needle's north pole does not change the direction it points to is ..........

1. A x
2. B y
3. C z
4. D k

A wire ab carries a direct current, with the free electrons moving through it from end a to end b. Which of the following figures correctly represents the pattern of magnetic flux lines produced around the wire?

1. A A
2. B B
3. C C
4. D D

The opposite figure shows a straight wire perpendicular to the plane of the page carrying a current into the page. Points a, b, c and d lie in the plane of the page at equal distances from the wire. The point at which the arrow correctly shows the direction of the magnetic field of the wire's current is ..........

1. A a
2. B b
3. C c
4. D d

In the opposite figure, a straight wire perpendicular to the plane of the page carries a current out of the page. The magnetic field points north-west at point ..........

1. A a
2. B b
3. C c
4. D d

A straight wire carries a current of 4 A. If the magnetic flux density at a point at a certain distance from its axis is $2 \times 10^{-5}$ T, the distance of the point from the axis of the wire equals ..........

1. A 0.01 m
2. B 0.02 m
3. C 0.03 m
4. D 0.04 m

A current I passes through a straight wire, and the magnetic flux density it produces at a point 5 cm perpendicular distance from the wire is $5 \times 10^{-5}$ T. The rate of flow of electrons through a cross-section of the wire equals ..........

1. A $5.61 \times 10^{21}$ electron/s
2. B $7.81 \times 10^{21}$ electron/s
3. C $5.61 \times 10^{19}$ electron/s
4. D $7.81 \times 10^{19}$ electron/s

A battery of emf 8 V and internal resistance 2 Ω is connected to a straight wire of length 20 cm, cross-sectional area $3 \times 10^{-8}$ m² and resistivity $4.5 \times 10^{-6}$ Ω.m. The magnetic flux density at a point 10 cm perpendicular distance from the axis of the wire equals ..........

1. A $4 \times 10^{-6}$ T
2. B $5 \times 10^{-7}$ T
3. C $6 \times 10^{-8}$ T
4. D $7 \times 10^{-9}$ T

In the figure shown, the magnetic flux density produced by the current in the wire at point x ..........

1. A equals $4 \times 10^{-6}$ T
2. B is greater than $4 \times 10^{-6}$ T
3. C is less than $4 \times 10^{-6}$ T
4. D cannot be determined

If the ratio between the magnetic flux densities at two points (X, Y) near a straight wire carrying a current $\left(\frac{B_X}{B_Y}\right)$ is $\frac{2}{3}$, the ratio between the perpendicular distances of the two points from the wire $\frac{d_X}{d_Y}$ is ..........

1. A $\dfrac{2}{3}$
2. B $\dfrac{1}{3}$
3. C $\dfrac{1}{6}$
4. D $\dfrac{3}{2}$

The opposite figure shows a straight wire carrying a direct current. The ratio between the magnetic flux densities produced by that current at points x and y $\left(\frac{B_x}{B_y}\right)$ equals ..........

1. A $\dfrac{1}{2}$
2. B $\dfrac{3}{1}$
3. C $\dfrac{2}{1}$
4. D $\dfrac{1}{3}$

The opposite figure represents a long straight wire carrying a direct current. If the magnetic flux density at point (z) equals (B), the magnetic flux density:
at point (x) is .......... the magnetic flux density at point (z).

1. A greater by $\dfrac{2}{3}$ B than
2. B less by $\dfrac{2}{3}$ B than
3. C greater by 2 B than
4. D less by 2 B than

The opposite figure represents a long straight wire carrying a direct current. If the magnetic flux density at point (z) equals (B), the magnetic flux density:
at point (y) is .......... the magnetic flux density at point (z).

1. A greater by $\dfrac{1}{3}$ B than
2. B less by $\dfrac{1}{3}$ B than
3. C greater by $\dfrac{1}{2}$ B than
4. D less by $\dfrac{1}{2}$ B than

A straight wire is placed perpendicular to the page and carries a direct current out of the page, as in the opposite figure. The correct order of the magnetic flux density at the points shown is ..........

1. A $B_z > B_x = B_y > B_k$
2. B $B_y = B_z = B_k > B_x$
3. C $B_z > B_y = B_k > B_x$
4. D $B_z = B_x > B_y = B_k$

The opposite graph represents the change of the magnetic flux density (B) produced by a current in a straight wire with the current intensity (I) at two points x and y. So ..........

1. A the distance of point x from the axis of the wire is greater than that of point y
2. B the distance of point x from the axis of the wire is less than that of point y
3. C the distance of point x from the axis of the wire equals that of point y
4. D cannot be determined

The opposite graph represents the relation between the magnetic flux density (B) produced by a current in a straight wire at a certain point and the current intensity (I). The distance of that point from the axis of the wire is ..........

1. A 10 cm
2. B 20 cm
3. C 40 cm
4. D 100 cm

A straight wire AB of resistance R is connected in an electric circuit as in the opposite figure. The magnetic flux density at point P produced by the current through wire AB is greatest when ..........

1. A switches $K_1$ and $K_2$ are both open
2. B switches $K_1$ and $K_2$ are both closed
3. C switch $K_1$ is closed and switch $K_2$ is open
4. D switch $K_1$ is open and switch $K_2$ is closed

Two identical metal rings x and y are placed in the same vertical plane, with a straight wire ab next to them in their plane, as in the opposite figure. If the free electrons move through the wire from end a to end b:
the direction of the magnetic flux lines through the two rings is ..........

1. A perpendicular to the plane of the rings and inward
2. B perpendicular to the plane of the rings and outward
3. C in the plane of the rings toward the wire
4. D in the plane of the rings away from the wire

Two identical metal rings x and y are placed in the same vertical plane, with a straight wire ab next to them in their plane, as in the opposite figure. If the free electrons move through the wire from end a to end b:
the ratio between the magnetic fluxes through the two rings $\left(\frac{(\phi_m)_x}{(\phi_m)_y}\right)$ is ..........

1. A greater than one
2. B less than one
3. C equal to one
4. D cannot be determined

In the figure shown, a long straight wire carries a current of 2 A and is placed perpendicular to a uniform magnetic field of flux density $4 \times 10^{-6}$ T. The resultant magnetic flux density at:
point A equals ..........

1. A $3.5 \times 10^{-6}$ T
2. B $6 \times 10^{-6}$ T
3. C $8 \times 10^{-6}$ T
4. D $9 \times 10^{-6}$ T

In the figure shown, a long straight wire carries a current of 2 A and is placed perpendicular to a uniform magnetic field of flux density $4 \times 10^{-6}$ T. The resultant magnetic flux density at:
point B equals ..........

1. A $2.5 \times 10^{-6}$ T
2. B $5 \times 10^{-6}$ T
3. C $2.67 \times 10^{-6}$ T
4. D $9 \times 10^{-6}$ T

In the opposite figure, a long straight wire perpendicular to the plane of the page carries a current of 40 A into the page. The wire is placed in a uniform magnetic field of flux density $6 \times 10^{-5}$ T directed to the left of the page. The resultant magnetic flux density at point P, 10 cm from the axis of the wire, is ..........

1. A $1.4 \times 10^{-4}$ T
2. B $1 \times 10^{-4}$ T
3. C $8 \times 10^{-5}$ T
4. D $2 \times 10^{-5}$ T

In the opposite figure, a straight wire is placed perpendicular to a uniform magnetic field of density B directed into the page. If a current I passes through the wire, the total magnetic flux density at point P becomes 3B. The resultant magnetic flux density at point Q is ..........

1. A zero
2. B B
3. C 2B
4. D 3B

The opposite figure shows a wire A placed perpendicular to the plane of the page carrying a current out of the page, producing a magnetic flux of density H tesla at the points shown. If the flux density of the horizontal component of the Earth's field is H tesla in the direction shown, the resultant flux density of these two fields at:
point (1) equals .......... tesla.

1. A $\sqrt{2}$ H
2. B $\sqrt{3}$ H
3. C H
4. D 2 H

The opposite figure shows a wire A placed perpendicular to the plane of the page carrying a current out of the page, producing a magnetic flux of density H tesla at the points shown. If the flux density of the horizontal component of the Earth's field is H tesla in the direction shown, the resultant flux density of these two fields at:
point (2) equals .......... tesla.

1. A 0
2. B H
3. C 3 H
4. D $\sqrt{5}$ H

The opposite figure shows a wire A placed perpendicular to the plane of the page carrying a current out of the page, producing a magnetic flux of density H tesla at the points shown. If the flux density of the horizontal component of the Earth's field is H tesla in the direction shown, the resultant flux density of these two fields at:
point (3) equals .......... tesla.

1. A 4 H
2. B 2.5 H
3. C $\sqrt{2}$ H
4. D $\sqrt{7}$ H

The opposite figure shows a wire A placed perpendicular to the plane of the page carrying a current out of the page, producing a magnetic flux of density H tesla at the points shown. If the flux density of the horizontal component of the Earth's field is H tesla in the direction shown, the resultant flux density of these two fields at:
point (4) equals .......... tesla.

1. A H
2. B 2 H
3. C 3 H
4. D 4 H

Which of the following figures correctly represents the pattern of magnetic flux lines produced by two equal currents flowing in opposite directions in two parallel wires?

1. A A
2. B B
3. C C
4. D D

The opposite figure shows two long parallel wires y and z, each carrying a current of the same intensity (I). The direction of the resultant magnetic flux density at point x is direction ..........

1. A (1)
2. B (2)
3. C (3)
4. D (4)

In the opposite figure, two parallel straight wires a and b perpendicular to the plane of the page carry currents I and 2I respectively. At point y, the resultant magnetic flux density (B) is calculated from the relation ..........

1. A $B = B_a + B_b$
2. B $B = B_a - B_b$
3. C $B = B_b - 2B_a$
4. D $B = \sqrt{B_a^2 + B_b^2}$

In the opposite figure, two long parallel wires each carry a current. The magnetic flux density is greatest at point ..........

1. A x
2. B y
3. C z
4. D k

Two long parallel straight wires carry currents (I, 2I) in opposite directions as in the opposite figure. The correct order of the magnetic flux density at points (x, y, z) is ..........

1. A $B_x > B_y > B_z$
2. B $B_z > B_y > B_x$
3. C $B_y > B_x > B_z$
4. D $B_y > B_z > B_x$

The opposite figure shows two very long parallel straight wires perpendicular to the plane of the page, each carrying a current. If the current in the first wire is greater than that in the second wire, the direction of the resultant magnetic flux density at point P is in the plane of the page and toward the ..........

1. A right
2. B left
3. C top
4. D bottom

In the opposite figure, two long parallel straight wires (1) and (2) carry currents I and 3I respectively. Comparing the resultant magnetic flux densities at points x and y, we find that the two resultants ..........

1. A have the same magnitude and direction
2. B differ in magnitude and direction
3. C have the same magnitude but different directions
4. D differ in magnitude but have the same direction

In the opposite figure, two parallel wires: wire (1) carries a current of 2 A and wire (2) a current of 4 A. The total magnetic flux density at point P if the two currents are:
in the same direction equals ..........

1. A $1.33 \times 10^{-5}$ T
2. B $2.66 \times 10^{-5}$ T
3. C $4.43 \times 10^{-6}$ T
4. D $6.65 \times 10^{-6}$ T

In the opposite figure, two parallel wires: wire (1) carries a current of 2 A and wire (2) a current of 4 A. The total magnetic flux density at point P if the two currents are:
in opposite directions equals ..........

1. A $4.14 \times 10^{-5}$ T
2. B $3.06 \times 10^{-7}$ T
3. C $2.9 \times 10^{-8}$ T
4. D $2.67 \times 10^{-6}$ T

Two parallel straight wires x and y are 5 cm apart. Free electrons pass through a cross-section of wire x at a constant rate of $2.5 \times 10^{20}$ electron/s, and wire y carries a current of 40 A. The flux density at a point midway between the two wires, if the direction of motion of the free electrons in wire x and the conventional current in wire y are:
in the same direction, equals ..........

1. A $1.6 \times 10^{-4}$ T
2. B 0
3. C $2.56 \times 10^{-4}$ T
4. D $6.4 \times 10^{-4}$ T

Two parallel straight wires x and y are 5 cm apart. Free electrons pass through a cross-section of wire x at a constant rate of $2.5 \times 10^{20}$ electron/s, and wire y carries a current of 40 A. The flux density at a point midway between the two wires, if the direction of motion of the free electrons in wire x and the conventional current in wire y are:
in opposite directions, equals ..........

1. A 0
2. B $6.4 \times 10^{-4}$ T
3. C $3.2 \times 10^{-4}$ T
4. D $1.92 \times 10^{-4}$ T

In the opposite figure, two long parallel wires (1) and (2) in the plane of the page are 20 cm apart. Wire (1) carries a current of 9 A. The magnitude and direction of the current in wire (2) that makes the resultant magnetic field of the two wires at point X zero are ..........

1. A 4.5 A, from a to b
2. B 4.5 A, from b to a
3. C 3 A, from a to b
4. D 3 A, from b to a

Two parallel straight wires each carry a current I in the same direction. While the two wires are moved a distance d in opposite directions at the same instant and with the same uniform speed (v), as in the opposite figure, the resultant magnetic flux density at points P and C respectively ..........

1. A at P: stays zero, at C: increases
2. B at P: stays zero, at C: decreases
3. C at P: increases, at C: increases
4. D at P: increases, at C: decreases

In the opposite figure, two long straight wires are perpendicular to each other with a shortest distance of 20 cm between them. The first wire is perpendicular to the page and carries a current of 20 A, and the second wire is in the plane of the page and carries a current of 15 A. The resultant magnetic flux density at point X equals ..........

1. A $3 \times 10^{-5}$ T
2. B $4 \times 10^{-5}$ T
3. C $5 \times 10^{-5}$ T
4. D $6 \times 10^{-5}$ T

The opposite figure shows two perpendicular insulated straight wires, each carrying a current I. The ratio between the flux densities at points x and y respectively is ..........

1. A $1 : 1$
2. B $2 : 1$
3. C $1 : 2$
4. D $3 : 2$

In the opposite figure, two parallel straight wires (1) and (2). If the total magnetic flux density $B_t$ at point P (midway between the two wires) equals $6 \times 10^{-5}$ T, the total magnetic flux density at point Q equals ..........

1. A $3.35 \times 10^{-5}$ T
2. B $2.68 \times 10^{-6}$ T
3. C $2.01 \times 10^{-5}$ T
4. D $6.7 \times 10^{-6}$ T

In the opposite figure, two long parallel straight wires (1) and (2) each carry a current. If the resultant magnetic flux densities at points x and y are equal and the resultant magnetic flux density at point z is (B), the resultant magnetic flux density at point x is ..........

1. A 0.75 B
2. B B
3. C 1.5 B
4. D 2 B

If the magnetic flux density at point x is B, the flux density at point y is ..........

1. A $\dfrac{B}{12}$
2. B 12 B
3. C $\dfrac{B}{2}$
4. D $\dfrac{B}{8}$

In the opposite figure, two very long parallel straight wires each carry a current. If the magnetic flux density produced by the current of wire (1) at point P equals B, the magnitude and direction of the resultant magnetic flux density at point P are ..........

1. A B, perpendicular to the page and inward
2. B 3 B, perpendicular to the page and inward
3. C B, perpendicular to the page and outward
4. D 3 B, perpendicular to the page and outward

In the opposite figure, when the current in wire (1) is increased to 6 I, the resultant magnetic flux density at point x ..........

1. A increases to three times
2. B increases to six times
3. C becomes zero
4. D decreases to one third

Two long parallel straight wires (1) and (2) carry currents in the same direction of I and 2I respectively, as in the opposite figure. Which of the following actions increases the magnetic flux density at both points (x) and (y)?

1. A Increasing the current in wire (1) to 2 I
2. B Increasing the current in wire (1) to 3 I
3. C Increasing the current in wire (2) to 4 I
4. D Passing no current in wire (2)

In the opposite figure, two long parallel straight wires (1) and (2) carry currents in opposite directions with $(I_1 > I_2)$. Which of the following actions increases the magnitude of the resultant magnetic flux density at point (m)?

1. A Increasing the current $(I_2)$ until it equals $(I_1)$
2. B Decreasing the current $(I_1)$ until it equals $(I_2)$
3. C Reversing the direction of the current in one of the wires
4. D Bringing one wire closer to the other

The opposite figure shows two very long parallel straight wires in the plane of the page, acted on by a uniform external magnetic field of flux density $2 \times 10^{-5}$ T perpendicular to the plane of the page and inward. If the resultant magnetic flux density at point P equals $2 \times 10^{-5}$ T into the page, the current in the second wire equals ..........

1. A 24 A
2. B 18 A
3. C 12 A
4. D 6 A

In the opposite figure, two parallel straight wires a and b carry currents I and 2I respectively as shown. If the flux density produced by wire a at point y is B, the resultant flux density at point y equals ..........

1. A B
2. B 2 B
3. C $\sqrt{3}$ B
4. D $\sqrt{5}$ B

Two long parallel straight wires (a) and (b) are placed perpendicular to the plane of the page and carry currents of the same intensity in opposite directions. The two wires are placed in different positions relative to each other as in the following figures.
At point (m), the magnitude of the resultant magnetic flux density is ..........

1. A greatest in figure (1)
2. B greatest in figure (2)
3. C greatest in figure (3)
4. D equal in the three figures

Two long parallel straight wires (a) and (b) are placed perpendicular to the plane of the page and carry currents of the same intensity in opposite directions. The two wires are placed in different positions relative to each other as in the following figures.
The resultant magnetic flux densities at point (m) in figures (2) and (3) ..........

1. A have the same magnitude and direction
2. B have the same magnitude but different directions
3. C differ in magnitude but have the same direction
4. D differ in magnitude and direction

In the opposite figure, if the distance between the two wires is 4d, the neutral point is point ..........

1. A x
2. B y
3. C z
4. D k

The opposite figure shows two long parallel wires M and N perpendicular to the plane of the page, carrying currents into the page. The neutral point is ..........

1. A between them, 8 cm from wire M
2. B outside them, 8 cm from wire M
3. C between them, 24 cm from wire M
4. D outside them, 24 cm from wire M

In the opposite figure, two parallel straight wires in the same plane each carry a current. If $I_1 > I_2$, in which of the regions X, Y, Z, K is the neutral point located if the currents in the two wires are:
in the same direction?

1. A X
2. B Y
3. C Z
4. D K

In the opposite figure, two parallel straight wires in the same plane each carry a current. If $I_1 > I_2$, in which of the regions X, Y, Z, K is the neutral point located if the currents in the two wires are:
in opposite directions?

1. A X
2. B Y
3. C Z
4. D K

In the opposite figure, two parallel straight wires (1) and (2) each carry a current I in the same direction. Which of the following actions makes the neutral point disappear?

1. A Moving one wire away from the other
2. B Doubling the current in one of the wires
3. C Reversing the direction of the current in one of the wires
4. D Reversing the direction of the current in both wires

In the figure, two perpendicular insulated wires each carry a current. Their resultant flux density is zero at point ..........

1. A A
2. B B
3. C C
4. D D

Two long insulated straight wires (1) and (2) carry currents $I_1$ and $I_2$ in the directions shown in the opposite figure. If point (c) is a neutral point, which of the following relations is correct for the currents of the two wires?

1. A $I_1 > I_2$
2. B $I_1 < I_2$
3. C $I_1 = I_2$
4. D cannot be determined

In the opposite figure, a long straight wire in the plane of the page carries a current of 10 A and is placed in a uniform magnetic field of flux density $2 \times 10^{-5}$ T perpendicular to the page and inward. The point at which the resultant flux density is zero is ..........

1. A A
2. B B
3. C C
4. D D

In the opposite figure, two long straight wires xy and ab, insulated from each other, are perpendicular and in the same plane. Wire (xy) carries a current of 4 A. The magnitude and direction of the current in wire (ab) so that the resultant flux density at point P is zero are ..........

1. A 8 A, from a to b
2. B 8 A, from b to a
3. C 16 A, from a to b
4. D 16 A, from b to a

A small compass is placed at a point between two parallel straight wires, 20 cm from the first wire, which carries a current of 2 A from south to north, and 40 cm from the second wire. The magnitude and direction of the current that, when passed through the second wire, causes no deflection of the compass needle are ..........

1. A 4 A, from north to south
2. B 2 A, from north to south
3. C 4 A, from south to north
4. D 2 A, from south to north

Two parallel straight wires (1) and (2) each carry a current as in the figure, so that point X is the neutral point, at a distance (a) from wire (2). If the current in wire (2) is increased to 4 A, the neutral point shifts by 10 cm. The distance d between the axes of the two wires equals ..........

1. A 18.51 cm
2. B 20.83 cm
3. C 24.75 cm
4. D 33.33 cm

In the figure shown, three long parallel straight wires each carry a current. The direction of the magnetic field at points P and Q is ..........

1. A at P: out of the page, at Q: out of the page
2. B at P: into the page, at Q: into the page
3. C at P: out of the page, at Q: into the page
4. D at P: into the page, at Q: out of the page

The opposite figure shows three wires perpendicular to the plane of the page, each carrying a current. The resultant magnetic flux density at point x equals ..........

1. A $2 \times 10^{-5}$ T
2. B $8 \times 10^{-5}$ T
3. C $9 \times 10^{-5}$ T
4. D $4 \times 10^{-4}$ T

Three vertical straight wires x, y and z in the same plane each carry a current I. A compass was placed between wires y and z, and its needle took the position shown in the opposite figure. Neglecting the Earth's magnetic field, the directions of the currents in wires x and y respectively are ..........

1. A out of the page, out of the page
2. B out of the page, into the page
3. C into the page, out of the page
4. D into the page, into the page

In the opposite figure, three long straight wires (a), (b) and (c) lie in the same plane and each carries a current. If point (m) is a neutral point, then ..........

1. A the current in wire (b) is upward, of intensity I
2. B the current in wire (b) is upward, of intensity 2I
3. C the current in wire (b) is downward, of intensity I
4. D the current in wire (b) is downward, of intensity 2I

In the opposite figure, three wires x, y and z in the same plane each carry a current. If the magnetic flux density at point c produced by the current in wire z equals $B_o$, the magnitude of the resultant flux density at point c produced by the currents in the wires equals ..........

1. A zero
2. B $2B_o$
3. C $3B_o$
4. D $4B_o$

In the opposite figure, four very long parallel wires perpendicular to the plane of the page are placed at the corners of a square, each carrying a current of the same intensity in the direction shown. The direction of the resultant flux density at point (m) is ..........

1. A perpendicular to the page and inward
2. B perpendicular to the page and outward
3. C in the plane of the page, to the right of the page
4. D in the plane of the page, to the left of the page

The opposite figure shows four long straight wires (1), (2), (3) and (4) in the same plane, each carrying a current I. The correct order of points x, y and z according to the resultant magnetic flux density at each is ..........

1. A z < y < x
2. B z > y > x
3. C z < y = x
4. D z > y = x

Two wires abc and adc of the same metal have the same length but different cross-sectional areas. They are joined together in the shape of a square as in the opposite figure. The two points which, when connected to an electric source, make the resultant magnetic flux density at point x zero are ..........

1. A a, d
2. B a, c
3. C a, b
4. D b, d

A wire of uniform cross-section is bent into a square abcd and a current is passed through it as in the opposite figure. If the magnetic flux density at point (m) produced by the current in part (ab) of the wire alone equals $(B_o)$, the magnitude of the resultant magnetic flux density at point (m) equals ..........

1. A zero
2. B $B_o$
3. C $2B_o$
4. D $4B_o$

In figure (1), a long straight wire in the plane of the page carries a current (I), and point (y) is at a distance (d) from it. The wire is rotated in the plane of the page about an axis through point (x) by 30° twice: first clockwise as in figure (2), and second anticlockwise as in figure (3).
The correct order of the magnetic flux density at point (y) in the three figures is ..........

1. A $B_1 = B_2 = B_3$
2. B $B_1 > B_2 = B_3$
3. C $B_2 > B_3 > B_1$
4. D $B_2 = B_3 > B_1$

A straight wire carrying a direct current is placed perpendicular to the plane of the page. It can be moved from position a to position b along one of two paths (1) or (2) (path (2) is a circle centred at point x), as in the opposite figure. What happens to the magnetic flux density at point x while the wire is moved along:
path (1)?

1. A it increases
2. B it does not change
3. C it increases then decreases
4. D it decreases then increases

A straight wire carrying a direct current is placed perpendicular to the plane of the page. It can be moved from position a to position b along one of two paths (1) or (2) (path (2) is a circle centred at point x), as in the opposite figure. What happens to the magnetic flux density at point x while the wire is moved along:
path (2)?

1. A it increases
2. B it does not change
3. C it increases then decreases
4. D it decreases then increases

A straight wire of length $1\ m$ is wound into a coil of one turn and placed perpendicular to a magnetic field of density $1\ T$. Find the magnetic flux through the turn if it is:
(1) circular.
(2) square.

The opposite figure shows a side view of a coil placed in a uniform magnetic field, and the flux through the coil is $4\times10^{-2}\ Wb$. What is the smallest angle the coil must rotate clockwise from the position shown so that the flux through the coil becomes:
(1) $2\times10^{-2}\ Wb$ ?
(2) $-2\times10^{-2}\ Wb$ ?

The opposite graph represents the relation between the magnetic flux $(\phi_m)$ through a coil rotating at a uniform rate in a magnetic field and the angle $(\theta)$ between the plane of the coil and the direction of the field. Find the value of z.

What is the direction of the magnetic field at each of points a and b produced by a current (I) in the straight wire shown in each of figures (1) and (2)?

A vertical straight wire xy carries a current and passes through a horizontal sheet of cardboard as shown in the figure. Draw on the sheet ABCD four of the magnetic field lines produced by the current in wire xy.

In the opposite figure, a very long straight wire carries a current, and three points x, y and z lie next to it on a line parallel to it. Compare the values of the magnetic flux density at these points, explaining your answer.

Write the mathematical relation represented by each of the following graphs and what the slope of the straight line represents:
(where: (B) is the magnetic flux density, (I) the current in the wire, (d) the distance of the point from the axis of the wire)

Explain: the magnetic flux density at point x produced by the current in wire ab increases when switch K in the opposite circuit is closed.

In the opposite figure, if the magnetic flux density produced by the current (I) in the wire at point x equals B in a certain direction, determine, with explanation, the point at which the flux density becomes $\frac{B}{2}$ in the opposite direction.

Figure (1) shows a compass placed in the magnetic meridian plane below a straight wire. When a current passes through the wire, the compass needle is deflected and settles as in figure (2). Determine the direction of the current in the wire.

Two parallel straight wires (x, y) are $0.3\ m$ apart. Wire x carries a current of $2\ A$ and wire y a current of $3\ A$. Calculate the distance of the neutral point from wire y if the two currents flow:
(1) in the same direction.
(2) in opposite directions.

Two parallel straight wires (1) and (2) each carry a current as in the figure. Find the resultant magnetic flux density at point C in terms of I, d and μ.

Two parallel straight wires x and y each carry a current as in the opposite figure. Explain what happens to the magnetic flux density at point a if wire x is moved a distance $\frac{1}{2}d$ toward wire y.

In the opposite figure, when currents $I_1$ and $I_2$ flow in the two parallel straight wires, point x is the neutral point. What change must be made to the current $I_2$ so that the neutral point becomes:
(1) at point y?
(2) at point z?

In the opposite figure, three long parallel straight wires carry currents. If the resultant magnetic flux density is zero at point Q, prove that: $I_1<(I_2+I_3)$

In the figure shown, three parallel straight wires x, y and z each carry a current I. If the magnetic flux density produced by wire x at point a equals B, calculate the resultant magnetic flux density at point a in terms of B when there is no current in:
(1) wire y
(2) wire z

In the opposite figure, two rectangular coils (x) and (y) of areas A and 3A respectively are placed in a uniform magnetic field. The ratio between the magnetic fluxes cutting them $\frac{(\phi_m)_x}{(\phi_m)_y}$ is .......... (Sample exam 2025)

1. A $\frac{\sqrt{3}}{2}$
2. B $\frac{2\sqrt{3}}{9}$
3. C $\frac{1}{\sqrt{3}}$
4. D $\frac{2\sqrt{3}}{5}$

A long straight wire carries a current I as shown in the figure. Which of the following relations correctly expresses the magnetic flux density (B) produced by the wire's current at points x, y and z, which lie in the same plane as the wire? (Trial exam / June 2021)

1. A $B_z>B_y$
2. B $B_y<B_x$
3. C $B_x<B_z$
4. D $B_y=B_z$

In the opposite figure, the magnetic flux density produced by the currents in wires (x) and (y) at point P equals $B_t$. If the direction of the current in wire (x) is reversed while the current in wire (y) stays as it is, the magnetic flux density at point P becomes .......... (Trial exam / June 2021)

1. A $\frac{3}{7}B_t$
2. B $\frac{2}{3}B_t$
3. C $\frac{3}{5}B_t$
4. D $\frac{3}{8}B_t$

The following figure represents four wires carrying different currents $I_1$, $I_2$, $I_3$ and $I_4$, and the flux density at points X, Y, Z and D is the same. The greatest current is .......... (1st session 2021)

1. A $I_1$
2. B $I_2$
3. C $I_3$
4. D $I_4$

The opposite figure represents two straight wires 1 and 2 in a plane perpendicular to the page, with a magnetic needle placed midway between them. If each carries a current I out of the page, the north pole of the needle .......... (2nd session 2021)

1. A is deflected toward point X
2. B is deflected toward point Y
3. C is deflected toward point Z
4. D stays in its position without deflection

The opposite graph represents the relation between the magnetic flux density (B) produced by a current at a point and the current (I) in three wires x, y and z, each separately. This point is .......... (2nd session 2021)

1. A closer to wire (z) than to wire (y)
2. B at equal distances from wires (x), (y) and (z)
3. C closer to wire (x) than to wire (y)
4. D closer to wire (y) than to wire (x)

In the figure shown, a straight wire carrying a current (I) is placed in a uniform magnetic field. The order of the resultant flux density (B) at points A, E, C and D is .......... (1st session 2022)

1. A $B_C>B_D>B_A>B_E$
2. B $B_D>B_C>B_E>B_A$
3. C $B_A>B_C>B_D>B_E$
4. D $B_E>B_C>B_D>B_A$

The opposite figure represents a straight wire carrying a current (I), with points A and C on either side of the wire. If the flux density at point A is $B_A$ and the magnetic flux density at point C is $B_C$, the ratio $\frac{B_A}{B_C}$ equals .......... (2nd session 2022)

1. A $\frac{1}{\pi}$
2. B $\frac{1}{2\pi}$
3. C $2\pi$
4. D $\pi$

Two long parallel wires (X) and (Y) are separated by a perpendicular distance of $0.5\ m$ and carry currents in the same direction of I and 3I respectively. The neutral point is at a distance of .......... (Trial exam 2023)

1. A $0.125\ m$ from wire Y
2. B $0.25\ m$ from wire Y
3. C $0.125\ m$ from wire X
4. D $0.625\ m$ from wire X

The opposite figure shows two long parallel wires (A) and (C), each carrying a current. To get a neutral point at point (Z), which of the following choices is correct for the magnitude and direction of the current in wire (C)? (1st session 2023)

1. A $2\ A$, in the same direction as the current in wire (A)
2. B $0.5\ A$, in the same direction as the current in wire (A)
3. C $0.5\ A$, opposite to the current in wire (A)
4. D $2\ A$, opposite to the current in wire (A)

The figure represents a straight wire carrying a current (I). Which of the following choices correctly expresses the relation between the magnetic flux densities (B) produced by the wire's current at points (Z) and (Y), which lie in the same vertical plane as the wire? (2nd session 2023)

1. A $B_Y=B_Z$, in opposite directions
2. B $B_Y=B_Z$, in the same direction
3. C $B_Y<B_Z$, in opposite directions
4. D $B_Y>B_Z$, in the same direction

The figure shows two parallel wires (1) and (2) carrying currents $I_1$ and $I_2$ respectively. For point (y) to be a neutral point of the two magnetic fields, the magnitude and direction of the current $I_2$ must be .......... (2nd session 2023)

1. A $\frac{I_1}{2}$, upward
2. B $I_1$, upward
3. C $2I_1$, downward
4. D $\frac{I_1}{3}$, downward

When a current passes through a straight wire of uniform cross-section and resistance (R), whose ends are connected to a DC source and which is placed in air, a magnetic field of flux density (B) is produced at a point near the wire. Which of the following changes decreases the flux density at the same point, other factors being constant? (1st session 2024)

1. A Replacing the wire with a shorter one
2. B Replacing the wire with a longer one
3. C Replacing the wire with one of larger cross-sectional area
4. D Replacing the source with one of greater emf

In the opposite figure, a straight wire carries a current. The ratio between the magnetic flux densities produced by the wire's current at the three points shown $B_z:B_y:B_x$ respectively equals .......... (2nd session 2024)

1. A $2:3:6$
2. B $1:2:3$
3. C $3:2:1$
4. D $2:6:4$

The opposite graph represents the relation between the magnetic flux density (B) produced by a current in a straight wire at a certain point and the current (I). The distance of that point from the axis of the wire is .......... (Sample exam 2025)

1. A $10\ cm$
2. B $8\ cm$
3. C $6\ cm$
4. D $4\ cm$

The opposite figure shows two long parallel straight wires, each carrying a current in the same direction. Which of the following graphs expresses the relation between the resultant magnetic flux density $(B_c)$ of the two wires at point (c) and the current $(I_2)$? (Sample exam 2025)

1. A A
2. B B
3. C C
4. D D

In the opposite figure, two very long parallel straight wires are in the plane of the page. If the resultant magnetic flux density at point (x) produced by the currents of the two wires is $2\times10^{-5}\ T$, the current in the second wire $(I_2)$ equals .......... (Sample exam 2025)

1. A $10\ A$
2. B $20\ A$
3. C $30\ A$
4. D $40\ A$

In the opposite figure, two very long parallel straight wires carrying the same current are placed in the plane of the page. The resultant magnetic flux density at point (x) equals .......... (Sample exam 2025)

1. A $10^{-5}\ T$
2. B $2\times10^{-5}\ T$
3. C $3\times10^{-5}\ T$
4. D $5\times10^{-5}\ T$

Two long parallel straight wires each carry a current as in the opposite figure. If the direction of the current in wire (X) is reversed, the magnitudes of the resultant magnetic flux densities at points K and L respectively .......... (Sample exam 2025)

1. A decreases, increases
2. B increases, decreases
3. C does not change, does not change
4. D decreases, decreases

Three long parallel straight wires (X, Y, Z) each carry a current as shown in the opposite figure. If the magnetic flux density produced by the current in wire Y at point P equals B, the magnitude of the resultant magnetic flux density at point P equals .......... (Sample exam 2025)

1. A B
2. B 2B
3. C 4B
4. D 8B

The figure shows two insulated perpendicular straight wires lying in the same plane, each carrying a current. If the four points (x, y, z, n) shown in the figure all lie in the plane of the wires, at which of these points is the resultant magnetic flux density zero? (2nd session 2025)

1. A Point (y)
2. B Point (x)
3. C Point (n)
4. D Point (z)

The figure shows a long straight wire of uniform cross-section and radius (R) carrying a direct current (I) in the direction shown. The relation between the magnetic flux density at point P and the magnetic flux density at point Q is .......... (Sample exam 2026)

1. A $B_P=2B_Q$
2. B $B_P=3B_Q$
3. C $B_P=4B_Q$
4. D $B_P=2.5B_Q$

The figure represents a long straight wire carrying a current (I), placed in position (1), and the magnetic flux density produced at point (P) is $(B_1)$. When the wire is rotated in the directions marked on the figure to position (2) and then position (3), the magnetic flux density produced at point (P) is $(B_2)$ and $(B_3)$ respectively. The correct order of the magnetic flux density at point (P) produced by the same current in the wire in the three positions is .......... (Sample exam 2026)

1. A $B_1>B_2>B_3$
2. B $B_3>B_2>B_1$
3. C $B_1>B_3>B_2$
4. D $B_2>B_3>B_1$