Traditionally physics teachers have illustrated electromagnetic induction by moving a magnet inside an electric coil and connecting the coil to a sensitive ammeter, or galvanometer. The experiment that follows improves upon this demonstration by using a data logger to capture the induced EMF and determining the relationship between the rate of change of magnetic flux and the induced EMF.
This experiment was conducted using the Ezilog USB data logger.
Aim: To determine the relationship between the velocity of a moving magnet and the EMF it generates in a coil.
Theory:
Faraday's Law states the relationship between the EMF generated in a coil of wire (solenoid) and the rate of change of magnetic flux as
EMF = - N dφ/dt
Where φ= Magnetic Flux
N = Number of turns of the solenoid
As the EMF depends upon the rate at which the magnetic flux (φ) changes, we would expect the EMF generated by a magnet dropped into a wire coil to depend upon the velocity of the magnet as it enters the coil.
The velocity (v) of a dropped magnet depends upon the height from which it is dropped and is given by the equation v2 = u2 + 2as, where a is the acceleration, u = initial velocity and s is the height.
Equipment:
Solenoid (Coil), magnet, ruler, foam, data logger with voltage sensor, computer.
Method :
I. Connect a solenoid to a suitable voltage (+/- 2.5 V) sensor and the voltage sensor to a data logger, which in turn is connected to a computer.
II. Setup the solenoid with a foam (or other suitable material) inner core. Place a ruler into the core as indicated below
III. Set the sampling rate to a minimum of 5,000 samples s-1 and an appropriate sample number. Start the data logger
IV. Place the magnet at 0.04 cm above the solenoid and drop. Your data logger results should look similar to that shown below
V. Measure and record the value of the first peak.. Enter this value into the Max EMF column. Determine the velocity using the equation in the Theory section with u = 0 and enter this in the Velocity column of the results table.
VI. Repeat for the other values of height in the results table.
Finally when all values of EMF and velocity have been determined plot a graph of Max EMF vs velocity.
Results:
Height
|
Velocity
|
Max emf
|
(cm)
|
(m.s-1)
|
(volts)
|
0.04
|
||
0.06
|
||
0.08
|
||
0.1
|
||
0.12
|
Discussion:
1. What is the shape of the line in your graph?
2. What is the relationship between EMF generated in the coil and the velocity of the magnet?
3. Are your results consistent with Faraday's Law?
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