Describe an experiment to demonstrate electromagnetic induction

Cambridge IGCSE Physics 0625 – Topic 4.5.1 Electromagnetic Induction

Learning Objective

Describe an experiment that clearly demonstrates electromagnetic induction.

Key Concepts

• Faraday’s law of electromagnetic induction: the induced emf (\$\mathcal{E}\$) in a coil is proportional to the rate of change of magnetic flux linkage.
• Lenz’s law: the direction of the induced current opposes the change that produced it.
• Factors affecting the magnitude of induced emf:

  • Number of turns in the coil (\$N\$)
  • Speed of relative motion between magnet and coil (\$v\$)
  • Strength of the magnetic field (\$B\$)
  • Area of the coil (\$A\$)

Apparatus

Experimental Procedure

1. Connect the ends of the coil to the galvanometer using the connecting wires. Ensure the connections are secure.
2. Set the galvanometer to zero (null) before starting the experiment.
3. Hold the bar magnet vertically with its north pole facing the coil.
4. Rapidly insert the magnet into the centre of the coil and observe the galvanometer deflection.
5. Record the direction (deflection) and magnitude of the galvanometer reading.
6. Repeat the insertion, this time withdrawing the magnet from the coil, and again note the galvanometer response.
7. Perform the experiment several times, varying:

   • The speed of insertion/withdrawal (slow vs. fast).
   • The number of turns in the coil (if different coils are available).
   • The orientation of the magnet (north pole first vs. south pole first).

8. For each variation, record the galvanometer reading and the speed (if measured).

Observations

Typical observations include:

• A sudden deflection of the galvanometer needle when the magnet is moved.
• The direction of deflection reverses when the magnet is withdrawn instead of inserted.
• Faster motion of the magnet produces a larger deflection (greater emf).
• Increasing the number of turns in the coil results in a larger deflection.

Explanation Using Faraday’s Law

When the magnet moves relative to the coil, the magnetic flux (\$\Phi = B \cdot A \cdot \cos\theta\$) through the coil changes. According to Faraday’s law, the induced emf is

\$\mathcal{E} = -N\frac{d\Phi}{dt}\$

The negative sign represents Lenz’s law, indicating that the induced current creates a magnetic field opposing the change in flux.

Key points:

• Direction of induced current: Determined by Lenz’s law; the coil produces a magnetic field that tries to keep the flux constant, leading to opposite deflection when the magnet is withdrawn.
• Magnitude of emf: Proportional to the rate of change of flux. Faster motion (\$\frac{d\Phi}{dt}\$ larger) or more turns (\$N\$ larger) increase \$\mathcal{E}\$.
• Polarity of magnet: Reversing the magnet’s polarity reverses the sign of \$d\Phi\$, thus reversing the direction of the induced current.

Safety and Precautions

• Handle the magnet carefully; strong magnets can pinch fingers.
• Do not connect the galvanometer to a voltage source while the coil is open; this could damage the instrument.
• Ensure all connections are tight to avoid spurious readings.
• Keep the experiment away from electronic devices that could be affected by stray magnetic fields.

Extension Questions

1. Predict how the induced emf would change if the coil were rotated instead of the magnet being moved. Explain your reasoning.
2. If the coil is connected to a resistor, how would you calculate the induced current using Ohm’s law? Write the expression.
3. Describe how this experiment relates to the operation of an electric generator.