# 2.34 Ohm's Law

## Defining Resistance

#### 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

#### Worked example

A charge of 5.0 C passes through a resistor of resistance R Ω at a constant rate in 30 s.

If the potential difference across the resistor is 2.0 V, calculate the value of R. ## Ohm's Law

• 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

#### Worked example

The current flowing through a component varies with the potential difference V across it as shown Which graph best represents how the resistance R varies with V?  #### Exam Tip

Resistance is used to control current. Increasing the resistance in a circuit will reduce the current. Don't get the cause and effect mixed up here. Reducing current does not increase resistance - it's the other way round!

Using graphs;

• In maths, the gradient is the slope of the graph
• The graphs below show a summary of how the slope of the graph represents the gradient  ### Get unlimited access

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