WASSCE SHS
Voltage, current and resistance are linked. In a series circuit the resistances add. In a parallel circuit the currents add and the combined resistance falls.
Potential difference, or voltage, is measured in volts. Current is measured in amperes. Resistance is measured in ohms. Ohm's law says that for a metal conductor at constant temperature, voltage equals current times resistance. Rearrange it to find whichever quantity the question leaves out. An ammeter is placed in series. A voltmeter is placed in parallel across the component. A fuse melts when current is too high and breaks the circuit.
Resistors in series share one path. The total resistance is the sum, and the current is the same in each. Resistors in parallel have the same voltage across them, and the total current is the sum of the branch currents. The combined resistance is less than the smaller branch, because the battery sees more than one path. For two resistors the reciprocal of the total is the sum of the reciprocals. Calculate the reciprocal sum first, then invert it. Leaving the answer as a reciprocal loses the final mark.
Units are part of the answer. 2.4 with no ohms can lose the unit. Show the substitution into voltage = current × resistance, or into the series or parallel formula. In parallel, invert at the end. A candidate who adds parallel resistors as if they were in series has used the wrong model and loses the method marks even if a later number is close.
These are practice questions in the style of the paper, with a model answer and a mark scheme. They are not a promise of what your paper will ask.
2 marks
A resistor of 12 ohms carries a current of 0.5 A. Calculate the potential difference across it. (2 marks)
Voltage = current × resistance = 0.5 × 12 = 6 V.
4 marks
A 4 ohm resistor and a 6 ohm resistor are connected in series to a battery.
(a) Calculate the total resistance. (1 mark)
(b) The same two resistors are then connected in parallel. Calculate the combined resistance. (3 marks)
(a) 4 + 6 = 10 ohms.
(b) 1/R = 1/4 + 1/6 = 3/12 + 2/12 = 5/12. R = 12/5 = 2.4 ohms.
2 marks
Explain why the parallel combination in the previous question has a smaller resistance than either resistor alone. (2 marks)
There are two paths for the current. The battery can push more current for the same voltage, which means the combined resistance is lower.
More questions on this topic are inside GLearn, marked as you answer, with the reason written out. The first ones need no account.
Practise Integrated Science freeIn series, add them. A 4 ohm resistor and a 6 ohm resistor in series make 10 ohms. In parallel, add the reciprocals and then invert. Those two give 2.4 ohms. The parallel value is smaller than either resistor.
For a metal conductor at constant temperature, potential difference equals current times resistance. If you know two of the three, you can calculate the third. Keep the units: volts, amperes and ohms. All WASSCE Integrated Science past questions