Magnetic induction: Faraday's laws

This experiment investigates magnetic induction by dropping a magnet through a coil and observing the induced electromotive force (EMF) using a PC Oscilloscope (PicoScope 2205).

Faraday's laws of magnetic induction state:

• An electromotive force is induced in a conductor when the magnetic field surrounding it changes.
• The magnitude of the electromotive force is proportional to the rate of change of the field.
• The sense of the induced electromotive force depends on the direction of the rate of the change of the field. 

Equipment required

• The tube-coil arrangement, mounted vertically, as shown in Figure 1.
• PicoScope 2205 PC Oscilloscope, with the PicoScope software. 

Carrying out the experiment

Drop the magnet through the coil, making sure you catch it before it hits the floor! Measure the induced EMF with the PC Oscilloscope. Use the PicoScope rulers or automatic measurements to record the amplitude and save each of the waveforms.

• Investigate Faraday's first law by dropping the magnet through the tube with north pole facing down*.
• The north and south poles are not marked on the magnets.
• Investigate Faraday's second law by changing the position of the coil and therefore changing the speed the magnet will drop past the coil. Think about how this will affect the induced EMF. Keeping the north pole facing down, drop the magnet and see if you are are correct.
• Investigate Faraday's third law by dropping the magnet with the south pole facing down. Think about how the induced EMF will change - try it and see if you are correct. 

Figure 1: Diagram showing the experimental layout

Analyzing the results

A typical trace is shown below using PicoScope 6 as illustrated in Figure 2.

Figure 2

Answers to questions

• Q1. Why are there negative and positive voltages generated as the magnet passes through the coil?
• A1. Faraday's third law tells us that the opposing poles of a magnet will induce an EMF in opposite directions. As the first pole passes through the coil, a votage is induced. As the second pole passes through the coil, a voltage is induced in the opposite direction. 

Figure 3

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