Prescribed practical 6 - Section 1

Purpose

How to safely plan and carry out an investigation into Ohm's law

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; and
  • 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.

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.

Variables

In this experiment the:

  • Independent Variable is the electric current I
  • Dependent Variable is the voltage V
  • 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.

Prediction

As the current increases, the voltage will also increase.

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.

HazardConsequenceControl measures
WaterElectric shockDo not set up the experiment near taps, sinks etc.
Wire gets hotMinor burnsDo not handle the wire. Switch off between readings.