Electromotive Force
Electricity will only flow around a circuit if an electrical energy source is present to “push” the charges around the circuit. This “pushing” is referred to as an electromotive force (e.m.f.).
Examples of e.m.f. sources would be: Battery, Cell, Solar Panel, Generator, etc.
| Definition: Electromotive Force |
|---|
| The electromotive force of an electrical source is the work done by the source in driving a unit charge around a complete circuit. |
| OR |
| The electromotive force of a cell is defined as the energy converted from non-electrical forms to electrical form, when one coulomb of positive charge passes through the cell. |
| OR |
| The electromotive force of a cell is defined as the rate at which energy is drawn from this source when unit current flows through the cell. |
This can be expressed in an equation as:
| ε = W/Q |
Where:
ε – electromotive force of the cell/battery (V)
W – work done by the cell/battery (J)
Q – charges moved through the cell / around the circuit (C)
Measuring EMF
Electromotive force (e.m.f.) can be measured with a voltmeter.
Voltmeter measuring the e.m.f. of the battery.
Note how it is fixed across across the terminals of the battery.
Cell
A single cell has the following symbol:
The long side is the positive terminal, the short side the negative.
diagram of a typical 1.5 V cell
(although in normal conversation
this is usually referred to as a battery)
Battery
If several cells are connected together they form a battery.
The symbol for a battery is:
3 cells connected in series
If each cell is 1.5 V, the battery would be 4.5V
2 cells connected in parallel
If each cell is 1.5 V, the battery would be 1.5V
| Question 1 |
|---|
| What is meant by the phrase “a cell has an e.m.f. of 1.5 V? |
| Question 2 |
|---|
| Four 1.5 V cells are connected in series and used as the supply for a circuit. (a) State the p.d. at the terminals of the supply. (b) How many joules of electrical energy does 4.0 C of charge gain on passing through the four cells? |
| Question 3 |
|---|
| What is the e.m.f. of the following batteries if each cell is 1.5 V?
(a)
(b)
(c) |
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