Question

(a) What do you understand by the term "potential difference"?
(b) What is meant by saying that the potential difference between two points is 1 volt?
(c) What is the potential difference between the terminals of a battery if 250 joules of work is required to transfer 20 coulombs of charge from one terminal of battery to the other?
(d) What is a voltmeter? How is a voltmeter connected in the circuit to measure the potential different between two points. Explain with the help of a diagram.
(e) State whether a voltmeter has a high resistance of a low resistance. Give reason for your answer.

Answer 1

(a) The potential difference between two points in an electric circuit is equal to the amount of work done in moving a unit charge from one point to another point.
$\mathrm{Potential}\mathrm{difference}\left(V\right)=\frac{\mathrm{Work}\mathrm{done}\left(W\right)}{\mathrm{Quantity}\mathrm{of}\mathrm{charge}\mathrm{moved}\mathit{\left(}Q\right)}$
The SI unit of potential difference is volt.

(b) The potential difference between two points is said to be one volt if one joule of work is done in moving one coulomb of electric charge from one point to the other.
Thus:
1 volt = 1 joule/1 coulomb

(c) Here:
Work, W = 250 J
Charge, Q = 20 C
Using the relation, V = W/Q:
Potential difference, V = 250/20 = 12.5 V

(d) A voltmeter is an instrument that is used to measure the potential difference across a conductor.
A voltmeter is always connected in parallel across the points where the potential difference is to be measured.


Suppose we have a conductor AB, such as a resistance wire, and we want to measure the potential difference across its ends.
So one end of the voltmeter is connected to point A and the other end to the point B. We can read the value of potential difference in volts on the dial of the voltmeter.

(e) A voltmeter has a high resistance so that it takes a negligible current from the circuit.
In a circuit, a voltmeter is connected in parallel across the element of whose potential difference it has to measure. Since we don't want the current diverted through the voltmeter, the voltmeter has a high internal resistance so that the circuit maintains the same current as if the voltmeter was not present.