The circuit diagram above illustrates a set-up to determine the internal resistance of a cell of e.m.f., E = 1.5 V. The cell is connected with ammeter, A, a resistance box Rband the key, K, in series.

With the resistance box sex at R and the key closed, the current I in the circuit is read and recorded.

The procedure is repeated for five other values of R and in each case, the corresponding values of I are recorded.

Fig. 3(a) and Fig. 3(b) on page 12, show Ri and Ii respectively where i=1,2,3,4, 5 and 6.

(i) Read and record Riand the corresponding Ii.

(ii) Evaluate P = E/I in each case.

(iii) Tabulate your readings.

(iv) Plot a graph of P on the vertical axis and R on the horizontal axisstarting both axes from the origin (0, 0).

(v) Determine the slope, s, of the graph and the intercept, c, on the vertical axis.

(vi) State two precautions you would have taken if you were to perform this experiment in the laboratory.

(b) (i) Explain the term internal resistance of a cell.

(ii) A cell has an e.m.f. of 3V. When it is connected across a resistor of resistance 4 Ω, a current of 0.5 A passes through the circuit. Calculate the internal resistance of the cell.

The expected response:



Six values of Rcorrectly read and recorded in ohms.

(Award ½ mark each)

Six values of Icorrectlyread and recorded to at least 1 d.p in amperes

(Award ½ mark each)

Six values of P = E/Icorrectly evaluated to at least 2 d.p

(Award ½ mark each)

Composite table showing at least R,I,and P.

Plot a graph of reasonable scales

Draw line of best fit

Determine the slope

Determine the intercept

State any two of the following precautions

e.g - Ensure clean terminals

- Ensure tight connection

- Remove key after each reading

- Note/correct zero error on ammeter

- Avoid parallax error on ammeter

- Repeated readings stated

(b) (i) The opposition to the flow of charges/electrons/current within a cell.

Its unit is ohm.

(ii) I = E/(R+r)

0.5 = 3/(4+r)

4 + r = 6

∴ = 2Ω