2018 A/L Physics Past Paper · Question 09 (English Medium)
Source of electromotive force
Which of the following is not a source of e.m.f.?
Answer and explanation
Answer: (5) Charged capacitor
What the question asks
A source of e.m.f. changes some other energy into electrical energy.
Find the one that does not.
Relevant theory
The electromotive force of a cell is defined as the amount of form of energy existing in the cell which gets converted into electrical energy when a unit electric charge passes through a circuit which includes the cell.
The phenomenon of developing an e.m.f. across a junction of two different metals is called the thermoelectric effect or the Seeback effect.
The capacitance of a capacitor can be considered as a measure of its ability to store electric charges.
Reading the options
A charged capacitor does not change any other energy into electrical energy.
It only gives back the charge it stored. As it discharges, its voltage falls to zero.
So it is not a source of e.m.f.
Why the other options fail
Chemical energy becomes electrical energy.
So it is a source of e.m.f.
Light energy becomes electrical energy.
Mechanical energy becomes electrical energy.
Heat becomes electrical energy. An e.m.f. appears when the two junctions are at different temperatures.
Conclusion
A charged capacitor only gives back what it stored. It does not change energy.
It is not a source of e.m.f. The answer is (5).