Ohm's Law
- Ohm's law states that:
For a conductor at a constant temperature, the current through it is proportional to the potential difference across it
- An electrical component obeys Ohm’s law if its graph of current against potential difference is a straight line through the origin
- A fixed resistor obeys Ohm’s law - i.e. it is an ohmic component
- A filament lamp does not obey Ohm’s law - it is a non-ohmic component
The current-voltage graph for a fixed resistor is a straight line through the origin. The fixed resistor is an ohmic component
- The resistance of an ohmic component can be calculated from the gradient of its current-voltage graph
- Since resistance is R = V/I
- R = 1/gradient
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?
ANSWER: D
Step 1: Write down the equation for the resistance R
Step 2: Link the resistance to the gradient of the graph
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- Gradient = I/V
R = 1/gradient
Step 3: Identify the gradient of different sections of the graph and use it to deduce what happens to the resistance
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- The first straight section of the graph has a constant gradient
- So the resistance remains constant
- The second section is curved and the steepness of the line increases, so the gradient increases
- So the resistance decreases
- The first straight section of the graph has a constant gradient
Step 4: Identify the correct graph out of the four proposed
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- Constant resistance is indicated by a straight horizontal line
- So either C or D are correct
- Decreasing resistance is indicated by a line curving downwards
- So only D is correct
- Constant resistance is indicated by a straight horizontal line
Examiner Tip
When solving problems about Ohm's law you will often deal with graphs. You need to be confident identifying and calculating their gradients.
- In maths, the gradient is the slope of the graph (i.e. rise/run)
- The graphs below show a summary of how the slope of the graph represents the gradient