Flux today, emf tomorrow

Science · Energy · ages 17-18

Name ______________________ Date ____________

B equals 0.5 T and area is 0.2 square metres, coil face on to the field. What is the flux in webers?
Answer: ______________
What is magnetic flux through a loop?
- B A cos theta
- B over A times theta
- B plus A plus N
A coil edge on to a strong field still threads zero flux.
Circle one: True False
Linkage falls from 10 to 2 in 4 seconds. What emf in volts is induced?
Answer: ______________
A flux of 0.1 webers threads a 100 turn coil. What is the flux linkage?
Answer: ______________
A coil spins in a perfectly steady field. Why is there still an emf?
- Electrons fall out of the wire
- The wire heats as it turns
- Its projected area swings, so linkage changes
Rosa reads emf off the linkage value instead of its slope. What is her error?
- Linkage values are always zero
- Emf equals the turn count alone
- Emf follows the rate of change, so take the gradient
A graph shows linkage falling by 8 in 2 seconds. What emf does the gradient give?
Answer: ______________

Answer key

For grown-ups. Fold this page away before handing over the rest.

0.1 · Face on means cos theta is 1: 0.5 times 0.2 is 0.1 webers.
B A cos theta · Flux counts field times area, trimmed by the tilt from the normal.
True · Edge on, the tilt kills the threading part even when the field is strong.
2 · Divide the change by the time: 8 over 4 is 2 volts.
10 · Linkage counts flux once per turn: 100 times 0.1 is 10.
Its projected area swings, so linkage changes · Turning swings the projected area, so linkage keeps changing and emf flows.
Emf follows the rate of change, so take the gradient · Faraday prices change, not level, so only the gradient gives the emf.
4 · The gradient is change over time: 8 over 2 is 4 volts.