6: Ohm's law

remember to

• demonstrate understanding that the temperature of the wire is kept constant using a switch and small currents;

• recall that this shows that the current and voltage are proportional for a metal wire at constant temperature, and that this is known as Ohm’s law.

apparatus

1m length of constantan wire, a metre rule, a low voltage power pack, a variable resistor, a voltmeter, an ammeter, connecting leads, a switch, 2 crocodile clips, Sellotape.

Method

1. Set up the circuit,

2. Adjust the variable resistor until the current on the Ammeter is 0.1 A. Record the current in a suitable table.

3. Read the corresponding value of voltage across the wire on the voltmeter and record in the table.

4. Turn the switch off between all readings to prevent the temperature of the wire rising.

5. Turn on again. Ensure that the current is still 0.1 A and repeat the voltage reading. Calculate the average voltage.

6. Repeat the procedure for six values of current up to 0.7 A.

Avoiding errors

The temperature of the wire must be kept constant.

To ensure the temperature of the wire does not increase, switch off between readings and keep the current as small as possible.

Read the ammeter and voltmeter accurately by reading the scale from directly above the pointer or use digital instruments

independent variable

electric current

dependent variable

voltage

controlled variable

material , length, cross section area and temperature of the wire

risks

hazard	Consequence	Control measures
Water	Electric shock	Do not set up the experiment near taps, sinks etc.
Wire gets hot	Minor burns	Do not handle the wire. Switch off between readings.

conclusion

as the current increases the voltage also increases.

its directly proportional provided all physical conditions such as temperature remain cosntant

ohms law equation

voltage V = current I x resistance R

V= IR

V = voltage in V

I = current in A

R = resistance in Ω (ohms)