Question 3A
A circuit is connected as shown in the diagram above.
The e.m.f. of battery is 1.5 V. with the resistance in the resistance box RB set at RI and the key K closed, the current II in the circuit is read with the ammeter A.
The procedure is repeated for five other values of R.
Fig. 3(a) represents the values of RI while fig. 3(b) shows the corresponding values of II where i = 1, 2, 3, 4, 5 and 6.
- Read and record the values of R.
- Read and record the corresponding values of I.
- Evaluate P = in each case.
- Tabulate your readings
- Plot a grace with P on the vertical axis and R on the horizontal axis starting both axes from the origin (0,0).
- Determine the slope, s, of the graph.
- Determine the intercept, c, on the vertical axis.
- State the physical quantity represented by the intercept on the vertical axis.
- State two precautions that are necessary to ensure accurate results when performing this experiment.
(b) (i) Define the internal resistance of a cell.
(ii) A cell of e. m. f. 4.5 v is connected across a resistor of resistance 4 Ω. If a current of 0.9 A passes through the circuit, calculate the internal resistance of the cell.
Observation
Part (a) This was popular question among the candidates. Many candidates could not read current to at least 1 d.p. in A and P = E/I to at least 2 d.p. Also, many candidates could not determine the intercept correctly since they did not start the graph from the origin. Candidates failed to realise the intercept on the y-axis is the internal resistance of the cell.
Part (b) Candidates performance was fair. Majority of the candidates could not solve correctly the problem.
The expected response is as stated below:
- Six values of R correctly determined
- Read correctly six values of current I
- Evaluate correctly six values of P = - - Composite table showing at least R, I and P
- Plot a graph using reasonable scales
- Draw line of best it
- Determine the slope of the graph
- Determine th3 intercept
- State any two of the following precautions.
e.g.
- Ensured clean terminals
- Ensured tight connections
- Removed key when not taking readings
- Avoided parallax error on ammeter
- Noted/corrected zero error on ammeter
- Repeated readings (stated)
b(i) Internal resistance of a cell is the opposition to the passage of current offered by the cell itself
OR
Internal resistance of a cell is the resistance within the cell that causes a drop in the source voltage when there is a current in the cell.
OR
Internal resistance (r) of a cell is the resistance obtained by dividing the difference between the emf (E) and the terminal p.d (V) of the cell by current (I) through the cell.
