1) A wire carrying a current is placed at right angles to a uniform magnetic field of magnetic flux density B. When the current in the wire is I, the magnetic force that acts on the wire is F.
What is the force on the wire, placed in the same orientation, when the magnetic field strength is $2B$ and the current is $\frac{I}{4}$? [1]
A: $\frac{F}{4}$
B: $\frac{F}{2}$
C: F
D: $2F$

2) There is an electric current in a wire of mass per unit length $40\,g\,{m^{ - 1}}$. The wire is placed in a magnetic field of strength $0.50 T$ and the current is
gradually increased until the wire just lifts off the ground.
What is the value of the current when this happens? [1]
A: $0.080 A$
B: $0.20 A$
C: $0.78 A$
D: $780 A$

3) A current-carrying wire is placed in a uniform magnetic field.
a: Describe how the wire should be placed to experience the maximum force due to the magnetic field. [1]
b: Describe how the wire should be placed to experience no force due to the magnetic field. [1]
[Total: 2]

4) A current-carrying conductor placed at right angles to a uniform magnetic field experiences a force of $4.70 \times {10^{ - 3}}\,N$. Determine the force on the wire when, separately:
a: the current in the wire is increased by a factor of 3.0 [2]
b: the magnetic flux density is halved [2]
c: the length of the wire in the magnetic field is reduced to $40\% $ of its original length. [2]
[Total: 6]

5) A copper wire carrying a current of $1.2 A$ has $3.0 cm$ of its length placed in a uniform magnetic field, as shown.

The force experienced by the wire is $3.8 \times {10^{ - 3}}\,N$ when the angle between the wire and the magnetic field is ${50^ \circ }$.
a: Calculate the magnetic flux density. [3]
b: State the direction of the force experienced by the wire. [1]
[Total: 4]

6) This diagram shows a view from above of two long, parallel strips of aluminium foil, A and B, carrying a current downwards into the paper.

a: On a copy of the diagram, draw the magnetic field around and between the two strips. [2]
b: State and explain the direction of the forces caused by the current in the strips. [4]
[Total: 6]

7) This diagram shows a wire XY that carries a constant direct current. Plotting compass R, placed alongside the wire, points due north. Compass P is placed below the wire and compass Q is placed above the wire.

a: State the direction of the current in the wire. [1]
b: State in which direction compass P points. [1]
c: State in which direction compass Q points if the current in the wire is reversed. [1]
[Total: 3]

8) This diagram shows a rectangular metal frame PQRS placed in a uniform magnetic field.

The magnetic flux density is $4.5 \times {10^{ - 3}}\,T$ and the current in the metal frame is $2.5 A$.
a: Calculate the force experienced by side PQ of the frame. [3]
b: Suggest why side QR does not experience a force. [1]
c: Describe the motion of the frame immediately after the current in the frame is switched on. [2]
d: Calculate the maximum torque (moment) exerted about an axis parallel to side PQ. [2]
[Total: 8]

9) This diagram shows a current-carrying wire frame placed between a pair of Magnadur magnets on a yoke. A pointer is attached to the wire.

A current of $8.5 A$ in the wire causes the pointer to move vertically upwards. A small paper tape is attached to the pointer and the current is adjusted until the
weight of the paper tape causes the pointer to return to its initial position (with no current and no paper tape). The mass of the paper tape is $60 mg$. The section of the wire between the poles of the magnetic has a length of $5.2 cm$.
a: State the direction of the magnetic field. [1]
b: Calculate the force on the wire due to the magnetic field when it carries a current of $8.5 A$. [2]
c: Calculate the magnetic flux density of the magnetic field between the poles of the magnet. [3]
d: Describe what happens to the frame if low-frequency alternating current passes through the wire. [1]
[Total: 7]

10) a: The size of the force acting on a wire carrying a current in a magnetic field is proportional to the size of the current in the wire. With the aid of a diagram, describe how this can be demonstrated in a school laboratory. [5]
b: At a certain point on the Earth’s surface, the horizontal component of the Earth’s magnetic field is $1.6 \times {10^{ - 5}}\,T$. A piece of wire $3.0 m$ long and weight $0.020 N$ lies in an east–west direction on a laboratory bench. When a large current flows in the wire, the wire just lifts off the surface of the bench.
i- State the direction of the current in the wire. [1]
ii- Calculate the minimum current needed to lift the wire from the bench. [3]
[Total: 9]

11) This diagram shows a fixed horizontal wire passing centrally between the poles of a permanent magnet that is placed on a top-pan balance.

With no current flowing, the balance records a mass of $102.45 g$. When a current of $4.0 A$ flows in the wire, the balance records a mass of $101.06 g$.
a: Explain why the reading on the top-pan balance decreases when the current is switched on. [2]
b: State and explain the direction of the current flow in the wire. [2]
c: The length of the wire in the magnetic field is $5.0 cm$. Calculate the average magnetic flux density between the poles of the magnet. [2]
d: Sketch a graph, with balance reading on the vertical axis and current on the horizontal axis, to show how the balance reading changes when the current is altered. [2]
[Total: 8]

12) a: Define magnetic flux density and explain the similarity with the definition of electric field strength. [3]
b: Two thin horizontal wires are placed in a north–south direction. One wire is placed on a bench and the other wire is held $3.0 cm$ directly above the first wire.
i- When equal currents flow in the two wires, the force exerted on the bench by the lower wire decreases. Explain why this is so. What can you say about the directions of the currents in the two wires? [4]
ii- The magnetic flux density B at a distance x from a long straight wire carrying a current I is given by the expression , where x is in metres and I is in amps. When the current in each wire is $4.0 A$, calculate the force per unit length on one wire due to the current in the other. [3]
[Total: 10]