P7: Ohm's law - CCEA

To use a voltmeter to measure the voltage across a metal wire and an ammeter to measure the current passing through the wire, and:

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

• demonstrate understanding of the need to obtain sufficient values of voltage and current so that a voltage–current characteristic graph (V-I graph) can be plotted, with voltage on the y-axis and current on the x-axis.

• recall that the V-I graph is a straight line that passes through the origin.

• 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.

What are the main variables?

The main variables in a science experiment are the independent variable, the dependent variable and the control variables.

The independent variable is what we change or control in the experiment.

The dependent variable is what we are testing and will be measured in the experiment.

The control variables are what we keep the same during the experiment to make sure it’s a fair test.

In this experiment:

• The independent variable is the electric current I.

• The dependent variable is the voltage V.

• The control variables are the material, length, cross section area and temperature of the wire.

These are kept the same by not changing the wire during the experiment, by keeping the current small and opening the switching between readings.

Remember - these variables are controlled (or kept the same) because to make it a fair test, only 1 variable can be changed, which in this case is the current.

What is the prediction for this experiment?

As the current increases, the voltage will also increase.

What is the justification for the prediction?

Greater current will mean that more charge flows.

This means that more energy can be converted from electrical energy to other forms of energy and so voltage increases.

What apparatus is used in prescribed practical P7?

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.

Circuit diagram

1. Set up the circuit, as shown above.

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 more values of current up to 0.7 A.

How to avoid errors when carrying out prescribed practical P7

• The temperature of the wire must be kept constant.

• Whenever a current flows through a conductor there is a heating effect.

• Electrical energy is converted to heat energy.

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

Results

Graph

We can see from the graph that as the current increases the voltage also increases.

This agrees with our prediction.

In fact, since the line of best fit is a straight line through the origin, we can be even more precise.

We can say that the voltage across the wire is directly proportional to the current flowing through it.

As the voltage increases the current increases in direct proportion.

This is known as Ohm’s Law.

Key facts

• Ohm’s law states that the voltage across a conductor is directly proportional to the current flowing through it, provided all physical conditions, such as temperature, remain constant.

• A conductor that obeys Ohm’s law is called an ohmic conductor.

• Copper or constantan wire are examples of ohmic conductors.