Question

A generator is constructed by rotating a coil of N turns in a magnetic field B at a frequency f. The internal resistance of the coil is R and the cross sectional area of the coil is A.

True or False

1. The average induced EMF doubles if the resistance R is doubled

2. The average induced EMF doubles if the magnetic field B is doubled

3. The maximum induced EMF occurs when the coil is rotated about an axis perpendicular to area A.

4. The average induced EMF doubles if the frequency f is doubled.

5. The average induced EMF doubles if the area A is doubled.

Answer 1

1) no
2) true .. E(rms) proportional to E(peak)
3) no .. not if field is parallel to rotation axis
4) true
5) uncertain .. if doubling R is due to doubling N, then true .. otherwise , no (E not dependent on R)

Answer 2

Firstly, the quantity which is measured in Teslas, and symbolised by B is magnetic flux density. Magnetic field strength is a completely different quantity and concept. It is symbolised by H and measured in units of amps/metre. If you are going to get anywhere in this topic it is essential that you distinguish between these two quantities and use the appropriate language. So you are talking about flux density (B).

A) False - the average emf is always zero when averaged over an integral number of whole revolutions
B) as (A)
C) False - the axis of rotation should be normal to the direction of B for maximum effect
D) False -the coil should be rotated about an axis which is normal to the axis of the coil (presumed to be a solenoid) for maximum effect
E) as (A)
F) as (A)

Answer 3

1. False. The induced EMF will be cut in half, not doubled.

2. True. You're increasing the area subjected to the magnetic field, so the induced EMF will increase.

3. True. You're increasing the intensity of the magnetic field, thereby increasing the intensity of the induced EMF.

4. True. The additional motion should translate into increased electric energy.

5. False. The frequency is independent of the induced EMF (unless you're approaching zero).

Answer 4

1. False. The induced EMF will be cut in half, not doubled.

2. True. You're increasing the area subjected to the magnetic field, so the induced EMF will increase.

3. True. You're increasing the intensity of the magnetic field, thereby increasing the intensity of the induced EMF.

4. True. The additional motion should translate into increased electric energy.

5. False. The frequency is independent of the induced EMF (unless you're approaching zero).

Answer 5

1. False. The induced EMF will be cut in half, not doubled.

2. True. You're increasing the intensity of the magnetic field, thereby increasing the intensity of the induced EMF.

3. True. You're increasing the area subjected to the magnetic field, so the induced EMF will increase.

4. False. The frequency is independent of the induced EMF (unless you're approaching zero).

5. True. The additional motion should translate into increased electric energy.