3.3 Ohm's Law

Resistance

• Resistance is defined as the opposition to current
• It is further defined by Ohm's Law, which says that the resistance of a conductor is given by the ratio of potential difference across it to the current flowing in it

• For a given potential difference, then, the higher the resistance the lower the current

Resistance of a component is the ratio of the potential difference and current

• Resistance is measured in Ohms (Ω)
• An Ohm is defined as one volt per ampere
• The resistance controls the size of the current in a circuit
  • A higher resistance means a smaller current
  • A lower resistance means a larger current

• All electrical components, including wires, have some value of resistance

Measuring Resistance

• To find the resistance of a component, a simple circuit can be used, containing:
  • A power supply
  • A component (such as a lamp or resistor)
  • An ammeter in series with the component
  • A voltmeter in parallel with the component

A circuit to determine the resistance of a component

• The power supply should be set to a low voltage to avoid heating the component, typically 1-2 V
• Measurements of the potential difference and current should then be taken from the voltmeter and ammeter respectively
• Finally, these readings should be substituted into the resistance equation

• Ohm’s law is defined as:

 The current through a component is directly proportional to the potential difference across it, providing the temperature is constant

• Constant temperature implies constant resistance
• This is shown the equation below:

Ohm’s law

• By adjusting the resistance on the variable resistor, the current and potential difference will vary in the circuit.
• The variation of current with potential difference through the fixed resistor can be plotted on a pair of axes
  • This will produce a straight-line graph

Circuit for plotting graphs of current against voltage

• Since the gradient is constant, the resistance R of the resistor can be calculated by using 1 ÷ gradient of the graph
• An electrical component obeys Ohm’s law if its graph of current against potential difference is a straight line through the origin
  • A resistor does obey Ohm’s law
  • A filament lamp does not obey Ohm’s law
• Any metal wires will follow Ohm's Law, provided that the current isn’t large enough to increase their temperature