Question

Harry sets up a circuit as shown in Figure 7 below. He measures potential difference ‘V’ across the variable resistor R with an instrument Y. He also measures current ‘I’ flowing through R with another instrument X.

1. Identify the instruments X and Y.
2. Using the graph of V against I shown below, calculate
   
   
   1. emf (E) of the cell.
   2. Internal resistance (r) of the cell.

Answer 1

a. X is an ammeter, and Y is a voltmeter.

b. We know that,

V = E − Ir

1. From the graph, where I = 0,

We have,

V = 2.5 V

∴ 2.5 = E − 0 × r

⇒ E = 2.5 V

2. When V = 0,

We have,

I = 0.5 A

∴ 0 = 2.5 − 0.5 r

⇒ r = `2.5/0.5`

= 5 Ω