10 minutes maximum! Can you do it in 5?

1. Magnetic Flux Φ through a surface is defined as Φ = BA cos θ.

A is the area of the surface. Which of these descriptions correctly identifies B and θ?

 B θ A Magnetic field strength angle between surface and magnetic field lines B Magnetic flux linkage angle between surface and magnetic field lines C Magnetic field strength angle between line perpendicular to surface and magnetic field lines D Magnetic flux linkage angle between line perpendicular to surface and magnetic field lines

2. The units for B and Φ are:

 B Φ A weber tesla m-2 B tesla m-2 tesla C tesla weber D tesla m-2 weber

3. A coil is placed perpendicular to a uniform magnetic field.

Which of the methods listed on the right will induce an emf in the coil?

• A. I only
• B. I and II
• C. II and III
• D. I, II and III

I. Rotate the coil.

II. Produce a varying magnetic field strength.

III. Move the coil backwards and forwards parallel to the field lines.

4+5. The diagrams here show a wire XY being moved through two magnetic fields, P and Q.

The wire is connected to an external circuit (not shown) so a current can flow. 4. Which of the answers below show the direction of the induced current in the wire in diagrams P and Q?

 field P field Q A X to Y X to Y B X to Y Y to X C Y to X X to Y D Y to X Y to X

5. In the diagrams above, The magnitude of the induced emf can be varied in several ways. Which of these changes will have no effect on the magnitude of the induced emf?

• A. Increasing the length of the wire XY without increasing the dimensions of the magnetic field.
• B. Increasing the length and width of the magnetic field, without changing the field strength.
• C. Increasing the velocity of the wire through the field.
• D. Increasing the field strength without increasing the area of the field.

6. The north pole of a magnet is moved downwards at 1 ms-1 past a stationary wire XY as shown in the diagram: The magnetic field is uniform and perpendicular to the wire, of magnetic flux density 0.1 mT. The wire is completely within the field, and is 2 cm long.

The induced emf magnitude and direction (high to low potential) in the wire is:
• A. 0.2 mV from X to Y
• B. 0.2 mV from Y to X
• C. 2 µV from X to Y
• D. 2 µV from Y to X

7+8. "The direction of the current induced in a conductor by a changing magnetic field is such that the magnetic field created by the induced current opposes the initial changing magnetic field." *

*(Wikipedia)

7. This law is:

• A. Lenz's Law
• B. Malus' Law
• D. Tesla's Law

8. A magnet is dropped towards a coil as shown here: Due to the law stated above, the current flowing in the front section of the coil PQ will be:
• A. From P to Q so the magnet is repelled from the coil.
• B. From Q to P so the magnet is repelled from the coil.
• C. From P to Q so the magnet is attracted towards the coil.
• D. From Q to P so the magnet is attracted towards the coil.

9 & 10. A rectangular coil of 1000 turns and area 0.01 m2 is within a magnetic field of strength 0.1 T.

It is rotating with a frequency of 0.25 Hz.

9. If the coil is initially perpendicular to the field, what is the minimum elapsed time before the flux linkage is zero?

• A. 4 seconds
• B. 2 seconds
• C. 1 second
• D. 0.5 seconds

10. What is the average emf induced in the coil during this time?

• A. 1 volt
• B. 0.1 volts
• C. 0.01 volts
• D. 0.001 volts