Ohm's Law Experiment (SQA National 5 Physics): Revision Note

Ohm's law experiment

• Ohm's law can be verified by measuring a range of values of current and voltage across a fixed resistor

• Apparatus for investigating Ohm's law:

  • Resistor

  • Ammeter

  • Voltmeter

  • Variable resistor

  • Power supply

  • Connecting wires

• The circuit diagram shows how these components should be connected

Circuit diagram for Ohm's law investigation

• The voltage is the independent variable

  • The variable resistor is used to change the voltage across the resistor

  • By adjusting the variable resistor, a range of voltages can be applied across the resistor and measured using a voltmeter

• The current is the dependent variable

  • For each voltage, the current in the resistor can be measured using an ammeter

• Temperature is an important control variable in this investigation

  • To ensure that the observed change in the dependent variable is brought about by the change in the independent variable, all other relevant factors must be kept constant

  • Temperature affects resistance; therefore, temperature must be controlled

  • Control variables ensure the investigation is a fair test

• Method:

  • Set up the circuit, making sure to connect the ammeter in series and the voltmeter in parallel with the resistor

  • Record the values of voltage and current from the readings on the voltmeter and ammeter, respectively

  • Adjust the resistance of the variable resistor multiple times to record a range of voltage and current values

  • Plot the results on a graph of voltage against current

• The resistor obeys Ohm's law if the V–I graph is a straight line which passes through the origin

Evaluating the experiment

• The experimental procedure could be improved to give more reliable results by

  • taking repeat readings of the current for each voltage and taking an average

  • checking for zero errors in the voltmeter and ammeter (i.e. if they give a reading of zero when voltage and current are zero)

  • switching off the power supply in between readings to prevent heating of the component and wires (which could affect the resistance)