Physics

Problem

In an experiment to verify Ohm’s law, a student is provided with a dry cell, a rheostat, a known resistor

R

, a key, connecting wires, an ammeter, and a voltmeter. a. (i) Draw a clearly labelled circuit diagram for the experiment showing the correct placement of the ammeter and the voltmeter. b. (ii) Explain why the ammeter used in the circuit must have a very low resistance. How does this property help in obtaining an accurate

I

–

V

graph? c. (iii) Let the resistance of the ammeter be

r_A

. Show that the effective resistance of the circuit becomes

R + r_A

when the ammeter is connected, and hence explain how the slope of the

V

–

I

graph is affected. d. (iv) If

R = 5\,\Omega

and

r_A = 0.5\,\Omega

, calculate the percentage error in the slope of the

V

–

I

graph that will arise if

r_A

is ignored.

Intuitive explanation

The core concept here is Ohm's law ( $V = IR$ ), and the key insight is that you need to measure both the voltage across and current through a resistor to verify the relationship holds. The ammeter must be in series with the circuit to measure current, while the voltmeter connects in parallel across the resistor to measure the potential difference. By varying resistance with the rheostat and recording multiple voltage–current pairs, you can show that $V/I$ remains constant (equal to $R$ ), confirming the law experimentally.

Intuitive explanation

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