Resistors (AQA GCSE Physics): Revision Notes
Resistors
What are I-V graphs?
An I-V graph shows the relationship between current and potential difference in electrical components. These graphs help us understand how different components behave in circuits and are essential tools for analysing electrical behaviour.
Key points about I-V graphs:
- Current (I) goes on the vertical y-axis
- Potential difference/voltage (V) goes on the horizontal x-axis
- The shape of the line tells us about the component's properties
Three types of components
Understanding how different components appear on I-V graphs is crucial for circuit analysis. Each component type has a distinctive pattern that reveals its electrical characteristics.
Fixed resistor
A fixed resistor creates a straight line through the origin on an I-V graph. This linear relationship is one of the most fundamental concepts in electrical circuits.
Key features of fixed resistors:
- The resistance stays constant as temperature remains steady
- Shows a linear relationship between current and voltage
- These are called ohmic conductors
- The gradient (slope) stays the same throughout
Filament lamp
A filament lamp creates a curved line on an I-V graph. This non-linear behaviour occurs due to the temperature-dependent nature of the filament material.
Key features of filament lamps:
- As potential difference increases, the filament gets hotter
- Higher temperature means higher resistance
- The gradient gets steeper as voltage increases
- Shows a non-linear relationship
Diode
A diode creates a distinctive flat then steep line on an I-V graph. This unique characteristic makes diodes essential components for controlling current direction in circuits.
Key features of diodes:
- Current only flows in one direction
- Needs a threshold voltage before current flows
- Graph is flat initially, then rises steeply
- Acts like a fixed resistor once current starts flowing
- Has very high resistance in the reverse direction
Drawing an I-V graph
To collect experimental data for an I-V graph, you need specific equipment and must follow a systematic procedure. This experimental approach helps verify theoretical predictions about component behaviour.
Equipment needed:
- A circuit with the component being tested
- An ammeter to measure current
- A voltmeter to measure potential difference
- A variable resistor to change the voltage
Experimental steps:
- Close the switch to complete the circuit
- Read current from the ammeter and voltage from the voltmeter
- Change the variable resistor to get different voltage values
- Reverse the cell to get negative voltage values
- Plot your results on a graph
Finding resistance from I-V graphs
Calculating resistance from I-V graphs is a fundamental skill that requires careful attention to avoid common mistakes. The resistance can be determined at any point on the graph using Ohm's law.
Critical formula:
Resistance = Potential difference ÷ Current
Common mistake to avoid: The gradient of the line is NOT the resistance. You must divide the x-axis value (voltage) by the y-axis value (current) at any chosen point.
Example
Let's work through a practical example to demonstrate how to calculate resistance from I-V graph data.
Worked Example: Calculating Resistance
Given: A component has 12V across it and 0.6A flowing through it
Step 1: Apply the resistance formula
Step 2: Substitute the values
Step 3: Calculate the result
Conclusion: The straight line through the origin shows this is a fixed resistor with constant resistance.
Remember!
Key Points to Remember:
- I-V graphs show how current changes with voltage for different components
- Fixed resistors give straight lines - constant resistance
- Filament lamps give curved lines - resistance increases with temperature
- Diodes only conduct one way - flat then steep lines
- To find resistance: divide voltage by current, don't use the gradient