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
  • C. Faraday's 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