Practical - 17 :- Demonstrating Self & Mutual Induction

 

This experiment uses a primary circuit to first show the principle of self-induction and then uses the same circuit to induce a current in a separate secondary circuit, demonstrating mutual induction.

Materials Needed:

• Two coils of insulated copper wire (a primary coil with fewer turns and a secondary coil with more turns is ideal).

• A soft iron core that fits inside both coils (e.g., a large iron bolt or rod).

• A low-voltage DC power supply (e.g., 6V or 12V).

• A switch.

• A low-voltage light bulb (e.g., 6V).

• A galvanometer or a sensitive multimeter set to measure DC current.

• Connecting wires.

---

Experimental Setup

Assemble the circuit as shown in the diagram below. The primary circuit consists of the power supply, switch, light bulb, and the primary coil. The secondary circuit consists only of the secondary coil and the galvanometer. Place both coils around the same iron core.

---

Part 1: Observing Self-Induction

This part of the experiment focuses only on the primary circuit.   

Procedure:

1. Pay close attention to the light bulb in the primary circuit.

2. Close the switch and observe how quickly the bulb lights up.

3. Now, remove the iron core from the center of the primary coil.

4. With the iron core removed, close the switch again and observe the bulb.

Observation and Explanation:

You will notice that with the iron core inside, the light bulb takes a noticeable moment to reach its full brightness. Without the iron core, it lights up almost instantly.

This delay is caused by self-induction. When the switch is closed, the growing current creates a changing magnetic field, which is intensified by the iron core. This change induces a "back EMF" (electromotive force) in the coil itself, which opposes the incoming current and slows down its rise. Without the core, the magnetic field is weaker, the back EMF is smaller, and the effect is negligible.

---

Part 2: Observing Mutual Induction  

This part focuses on the interaction between the primary circuit and the secondary circuit.

Procedure:

1. Ensure the iron core is placed through both coils.

2. Watch the galvanometer in the secondary circuit.

3. Close the switch in the primary circuit. Observe the galvanometer needle.

4. Keep the switch closed for a few seconds. Observe the galvanometer.

5. Open the switch. Observe the galvanometer needle again.

Observation and Explanation:

• When you close the switch, the galvanometer needle will briefly deflect in one direction and then return to zero.

• While the switch remains closed, the galvanometer will show no reading (zero).

• When you open the switch, the needle will briefly deflect in the opposite direction and then return to zero.

This happens due to mutual induction. The changing magnetic field created by the primary coil when the current starts or stops extends to the secondary coil. This changing magnetic flux through the secondary coil induces an EMF and a temporary current, which the galvanometer detects. When the primary current is steady, the magnetic field is constant, no EMF is induced, and thus no current flows in the secondary circuit.