Resistors (AQA GCSE Physics Combined Science): Revision Notes
Resistors
What are I-V graphs?
An I-V graph shows how current changes when you alter the potential difference across a component. These graphs help us understand different types of resistors and how they behave in circuits.
I-V graphs are fundamental tools in electronics that allow us to visualise and analyse the electrical behaviour of components. They provide insights into how components respond to changing electrical conditions.
The current (I) goes on the vertical y-axis and potential difference (V) goes on the horizontal x-axis.
Types of resistors and their I-V graphs
Fixed resistor
A fixed resistor has a constant resistance that doesn't change. This creates a straight line through the origin on an I-V graph.
Key features:
- Temperature stays the same
- Resistance stays the same
- Straight line relationship
- Current increases at a steady rate as potential difference increases
This shows a linear relationship between current and potential difference. This linearity is what makes fixed resistors predictable and useful in circuit design.
Filament lamp
A filament lamp behaves differently because it gets hot when current flows through it.
Key features:
- As potential difference increases, current gets bigger
- The filament gets hotter and vibrates more
- This makes the resistance increase
- The line curves upward (non-linear relationship)
The relationship is non-linear because resistance changes with temperature. As the filament heats up, the atoms vibrate more, making it harder for electrons to flow through - this increases resistance.
Diode
A diode only lets current flow in one direction, making it very useful in electronics.
Key features:
- Current only flows in the forwards direction
- There's a threshold voltage needed before current starts to flow
- The graph is flat until this threshold is reached
- Very high resistance in the reverse direction
- Behaves like a fixed resistor once current starts flowing
The threshold voltage is typically around 0.7V for silicon diodes. Below this voltage, virtually no current flows, making diodes excellent for controlling current direction in circuits.
Drawing I-V graphs in practice
To collect data for an I-V graph, you need this circuit setup:
Practical Method: Creating an I-V Graph
Equipment needed:
- Ammeter (to measure current)
- Voltmeter (to measure potential difference)
- Variable resistor (to change the potential difference)
- The component you're testing
Method:
- Close the switch to complete the circuit
- Change the variable resistor to get different potential difference readings
- Record current and potential difference values
- You can reverse the cell to get negative values for a complete graph
Understanding gradients
The gradient (slope) of an I-V graph tells you about resistance:
- Steeper gradient = lower resistance
- Gentler gradient = higher resistance
Remember: gradient =
So if you want to find resistance from an I-V graph, you use:
Where you divide the potential difference value by the current value at that point.
Identifying components from graphs
You can tell what component you're looking at from its I-V graph:
- Straight line through origin = fixed resistor
- Curved line that gets less steep = filament lamp
- Flat then straight line = diode
The shape of the I-V curve is like a "fingerprint" for each component type. Once you understand these characteristic shapes, you can quickly identify unknown components just by looking at their I-V graphs.
Remember!
Key Points to Remember:
- I-V graphs plot current (y-axis) against potential difference (x-axis)
- Fixed resistors show straight lines because resistance stays constant
- Filament lamps curve because they get hot and resistance increases
- Diodes only allow current in one direction after a threshold voltage
- Use variable resistors in circuits to collect different readings for your graph