EMF, internal resistance and potential dividers

Potential along a uniform wire, potential divider circuits with thermistors and LDRs, and the e.m.f. and internal resistance of cells.

Understand how the potential along a uniform current-carrying wire varies with the distance along it

Understand the principles of a potential divider circuit and understand how to calculate potential differences and resistances in such a circuit

Be able to analyse potential divider circuits where one resistance is variable, including thermistors and light dependent resistors (LDRs)

Know the definition of electromotive force (e.m.f.) and understand what is meant by internal resistance, and know how to distinguish between e.m.f. and terminal potential difference

CORE PRACTICAL 8: Determine the e.m.f. and internal resistance of an electrical cell

A cell of e.m.f. $1.5\,\text{V}$ and internal resistance $0.50\,\Omega$ is connected to a $2.5\,\Omega$ resistor.

Calculate:

(a) the current in the circuit

(b) the terminal potential difference of the cell

$I = \varepsilon/(R + r) = 1.5/3.0$ (1)
$I = 0.50\,\text{A}$ (1)
$V = IR = 0.50 \times 2.5 = 1.25\,\text{V}$ (1)
$P = I^2 r = 0.50^2 \times 0.50 = 0.13\,\text{W}$ (0.125 W) (1)

A potential divider consists of a fixed $2.2\,\text{k}\Omega$ resistor in series with a light-dependent resistor (LDR), connected to a $9.0\,\text{V}$ supply of negligible internal resistance. The output voltage is taken across the LDR. In the dark, the LDR has resistance $6.8\,\text{k}\Omega$ .

(a) Calculate the output voltage in the dark.

(b) Explain what happens to the output voltage when light shines on the LDR.

Describe how a student could determine the e.m.f. and internal resistance of a cell using a variable resistor, an ammeter and a voltmeter. Include how the results are analysed.

Connect the cell in series with the ammeter and variable resistor, with the voltmeter across the cell terminals (1)
Vary the resistance and record pairs of terminal pd $V$ and current $I$ (1)
Plot a graph of $V$ against $I$ ; since this is a straight line (1)

Specification points

Stuck on something?

Sample questions