14.2.2 – Potential Difference | Physics with Mr Shone

Potential Difference

Definition: Potential Difference (p.d.)
The potential difference (p.d.) across a circuit component is the work done to drive a unit charge through the component.

This can be expressed in an equation as:

V = W/Q

Where:

V – potential difference across the component (V)
W – energy converted from electrical energy into non electrical energy in the component (J)
Q – charges moved through the component (C)

Both e.m.f and p.d. are measured in volts (V).

Caution
In everyday talk we often refer to p.d. and e.m.f. simply as “voltage”. Try to avoid doing this in Physics lessons.

Caution

In everyday talk we often refer to p.d. and e.m.f. simply as “voltage”.

Try to avoid doing this in Physics lessons.

Example 4

The cells and the lamps in the following diagrams are identical.

If lamp X is at normal brightness, compare the brightness of each lamp in the following circuit with that of lamp X.

(a)

Brighter than normal.

Although the emf (and hence current) is twice that of X, we cannot assume the bulb is “twice as bright”. We can only know that it will be brighter.

(b)

Normal brightness.

(c)

Normal brightness

(d)

Not lit.

Example 5A
In the diagram shown, the three identical bulbs connected to a 24 V supply are working at normal brightness. What is the voltage rating of each bulb? The voltage is 24 V Each bulb is connected across the cell in parallel.

Example 5A

In the diagram shown, the three identical bulbs connected to a 24 V supply are working at normal brightness.

What is the voltage rating of each bulb?

The voltage is 24 V

Each bulb is connected across the cell in parallel.

Example 5B
Twenty identical Christmas tree light bulbs work normally from a 240 V mains when they are connected as shown in the diagram. What is the voltage rating of each bulb? 240 ÷ 20 = 12 V The 20 bulbs are connected in series across the mains supply. The 240 V is therefore shared between them.

Example 5B

Twenty identical Christmas tree light bulbs work normally from a 240 V mains when they are connected as shown in the diagram.

What is the voltage rating of each bulb?

240 ÷ 20 = 12 V

The 20 bulbs are connected in series across the mains supply. The 240 V is therefore shared between them.

Measuring Potential Difference

Potential difference (p.d.) is measured by a voltmeter connected in parallel to the component being measured.

Voltmeter is measuring the p.d. across the resistor

Electromotive Force vs Potential Difference

Electromotive Force	Potential Difference
Associated with an electrical energy source (e.g. a dry cell)	Associated with two points in an electric circuit
It is work done to move each unit charge through the complete circuit	It is the work done by each unit charge passing through the components between two points
Non-electrical energy is converted to electrical energy	Electrical energy is converted to other forms of energy
E.m.f. is present even when no current is drawn from the source	P.d. across any electrical component is zero in absence of current

Potential Difference

Measuring Potential Difference

Electromotive Force vs Potential Difference

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