HS2 Physics Chapter 8 ELECTROMAGNETIC WAVES : Experiments

 

Experiment Title:

Demonstration of Displacement Current and Verification of Electromagnetic Wave Properties

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Aim:

1. To demonstrate the existence of displacement current using a charging capacitor.

2. To verify that the displacement current is equal to the conduction current.

3. To observe the nature of electromagnetic (EM) wave propagation (E ⊥ B ⊥ direction of propagation).

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Apparatus Required:

• Parallel Plate Capacitor

• DC Power Supply

• Switch

• Ammeter (for conduction current)

• Magnetic Field Sensor or small compass

• Connecting Wires

• CRO (optional for waveform observation)

• Function Generator (for AC EM wave demonstration)

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Theory:

• According to Maxwell, a time-varying electric field produces a displacement current which acts as a source of magnetic field similar to conduction current.

• Displacement current is given by:

  $$i_d = \varepsilon_0 \frac{d\Phi_E}{dt}$$

• In a charging capacitor, between the plates, there is no conduction current, yet magnetic field exists — evidence of displacement current.

• EM waves are transverse in nature, with electric field (E) and magnetic field (B) perpendicular to each other and the direction of wave propagation.

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Procedure:

Part A: Demonstration of Displacement Current

1. Connect the capacitor in series with a DC power supply, a switch, and an ammeter.

2. Close the switch to start charging the capacitor.

3. Observe the current in the ammeter — this is the conduction current in the wire.

4. Place a magnetic field sensor or a compass between the capacitor plates (ensure plates are partially open to access).

5. Detect magnetic field — this confirms the presence of a magnetic field inside the capacitor, despite no physical current flow.

6. Conclude that a displacement current exists inside the capacitor, equal in magnitude to the conduction current.

Part B: Verification of EM Wave Orientation (Optional with AC Source)

1. Connect a function generator to a dipole antenna (or capacitor).

2. Observe the oscillating electric field using an electric field probe.

3. Use a magnetic field sensor orthogonal to the electric field direction to detect B.

4. Verify that E ⊥ B ⊥ direction of propagation as expected from the EM wave model.

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Observations:

• Charging current observed on ammeter: ________ A

• Magnetic field detected between plates: Yes/No

• Orientation of E and B fields in AC demonstration: Confirmed / Not confirmed

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Calculations:

Calculate the displacement current:

$$i_d = \varepsilon_0 \frac{dE}{dt} \cdot A$$

Compare with the measured conduction current $i_c$.

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Result:

• The displacement current exists and equals the conduction current during capacitor charging.

• Magnetic field inside the capacitor confirms Maxwell’s prediction.

• Orientation of EM wave components follows the expected transverse nature.