DEMONSTRATION 20.8 Demonstration of Electromagnetic Damping
(a) Freely spinning aluminium disc Fig 20.26 Apparatus Aluminium disc on an axis, horseshoe magnet
Procedure ■ Set disc spinning freely away from the magnet. ■ Bring the magnet close to the spinning disc. Position the disc within the magnetic field.
Observation/Conclusion The disc stops spinning when placed within the magnetic field, showing electromagnetic damping.
Explanation ■ There is an induced emf in the aluminium causing eddy currents to flow in the disc.These eddy currents generate their own magnetic fields. In accordance with Lenz’s law, the induced field opposes the rotation, thus slowing it down quickly.
Note The main energy conversions that take place as the disc slows down are kinetic to electrical to heat. By combining Lenz’s law with Faraday’s law we get:
E d dt
Often, when doing calculations involving electromagnetic induction, we deal with solenoids passing through magnetic fields. When this happens, the total induced emf is the sum of the emf induced in each loop. Therefore, in a solenoid:
The ‘–’ is not important for numeric calculations and may be overlooked. It is there to signify that the induced emf ‘opposes’ the change causing it.
INDUCED EMF IN A SOLENOID
E N d dt
E emf
N the number of turns in the solenoid
magnetic flux t time
SAMPLE PROBLEM 20E
A rectangular planar coil of wire with 200 turns and dimensions 3 cm by 4 cm is rotated at a constant speed 8 rad s1 within a uniform magnetic field of flux density 2.5 T. Given that the axis of rotation is perpendicular to the magnetic field, calculate the induced emf in the coil.
