Case Study:

During a school visit near a railway station, students observed several birds comfortably sitting on high-voltage electric wires. These wires carried currents at thousands of volts, yet the birds appeared unaffected. However, warning signs nearby clearly stated that humans should not touch these wires, as they could lead to fatal electric shocks.

Curiously, the students discussed this with their physics teacher. The teacher explained that electric current flows only when there is a potential difference across two points of a conductor. In the case of birds, both their feet are on the same wire, so there is no significant potential difference across their bodies. As a result, no current flows through them.

To really grasp how these invisible forces work before moving on, you might find it helpful to look at these Visual Concepts of Electric Charges and Fields. If you’re preparing for your finals, don't miss these Critical Electric Field Questions that often catch students off guard.

In contrast, if a human touches a high-voltage wire while standing on the ground, their body provides a path between the wire and the earth. This creates a potential difference, allowing current to pass through the body, which can be dangerous. The concept of earthing plays a crucial role here, as it provides a low-resistance path for current to flow into the ground.

This observation highlights how understanding electric charges and conductors helps explain real-life electrical safety.

CASE-BASED QUESTIONS

Before you test yourself with the questions below, you can sharpen your skills using this Class 12 Physics Worksheet. Once you feel confident, try timing yourself with an Unsolved Practice Paper or review the step-by-step logic in this Solved Practice Paper.

MCQ

Q1. Why do birds not get electrocuted while sitting on a single high-voltage wire?
A. Their bodies have high resistance
B. There is no potential difference across their bodies
C. They are insulated from the wire
D. The voltage is too low

Q2.  A person touching a live wire while standing on the ground receives an electric shock because:
A. The body stores electric charge
B. The body acts as an insulator
C. A potential difference is created across the body
D. The current avoids the ground

Assertion - Reason

Q3. Assertion (A): Birds are safe on high-voltage wires because no current flows through their bodies.
Reason (R): Current flows only when there is a potential difference between two points.

A. Both A and R are true, and R is the correct explanation of A
B. Both A and R are true, but R is not the correct explanation
C. A is true, but R is false
D. A is false, but R is true

Application-Based

Q4. What would happen if a bird touches two wires of different potentials simultaneously? Explain.

Application-Based

Q5. Why is earthing essential in household electrical systems in the context of human safety?

Data/Logic-Based

Q6. A wire carries a high voltage of 11,000 V. A bird sits on it with both feet 5 cm apart. Explain why no current flows through the bird despite the high voltage.

ANSWER KEY WITH EXPLANATION

A1. B - Explanation: Both feet of the bird are at the same potential, so no potential difference exists; hence no current flows.

A2. C - Explanation: The human body connects the wire and ground, creating a potential difference, allowing current to flow.

A3. A - Explanation: Both statements are true, and the reason correctly explains that current requires potential difference.

A4. If the bird touches two wires at different potentials, a potential difference develops across its body, causing current to flow and possibly leading to electrocution.

A5. Earthing provides a low-resistance path for excess current to flow into the ground, preventing electric shocks and protecting humans.

A6. Despite high voltage, both feet are at nearly the same potential due to the negligible distance, so no effective potential difference exists to drive current through the bird.

HOTS EXTENSION QUESTIONS

1. If a person is insulated from the ground using rubber shoes, would touching a live wire still be dangerous? Justify with reasoning.

2. a safety mechanism for birds on power lines that could prevent accidental electrocution when they spread their wings across wires of different potentials.

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