Required practical - Investigating I-V characteristics (AQA GCSE Physics): Revision Notes
Required practical - Investigating I-V characteristics
What are I-V characteristics?
I-V characteristics show how current (I) changes when you change the potential difference (V) across different electrical components. This practical helps you understand how resistors, filament lamps, and diodes behave differently in circuits.
I-V characteristics are fundamental to understanding how electrical components behave in circuits. The shape of these graphs tells us whether a component follows Ohm's law or has more complex electrical properties.
Aim of the practical
The main goal is to investigate I-V graphs for three different components:
- A resistor (at constant temperature)
- A filament lamp
- A diode
Each component produces a different shaped graph, which tells us about their electrical properties.
Why these three components? These components represent three different types of electrical behaviour:
- Linear relationship (resistor)
- Non-linear relationship (filament lamp)
- One-way conduction (diode)
Equipment needed
You will need these components to build your circuits:
- Ammeter - measures current
- Voltmeter - measures potential difference
- Variable resistor - changes the voltage in the circuit
- Component holders - to hold your test components safely
- Filament lamp, resistor, and diode - the components you're testing
- Protective resistor - keeps current safe
- Switch - to control the circuit
- Connecting leads - to join components together
Method
- Set up the circuit as shown in your textbook with the ammeter and voltmeter connected correctly
- Start with low voltage using the variable resistor
- Record the current that flows through each component
- Gradually increase the potential difference using the variable resistor
- Take several readings for different voltage values
- Reverse the battery or cell to get negative values for current and potential difference
- Repeat for all three components (resistor, filament lamp, diode)
Critical Setup Points:
- Always start with the lowest possible voltage setting
- Check your ammeter and voltmeter connections before switching on
- Make sure the variable resistor is set to maximum resistance initially
Safety considerations
Be very careful when working with electrical circuits:
- Electric current produces heat and can cause burns
- Use a switch in your circuit so current doesn't get too large
- Large currents increase resistance due to heating effects
- This heating can cause vibrations in the metal lattice which affects your results
Recording your results
Create a data table for each component with two columns:
- Potential difference (V) - in volts
- Current (I) - in amperes
Take readings for both positive and negative values by reversing your power supply.
Data Collection Tips:
- Take at least 6-8 readings for each component
- Include zero values in your measurements
- Record readings to appropriate precision (typically 2-3 significant figures)
- Note any unusual observations during the experiment
Drawing I-V graphs
Plot your results on a graph with:
- Current (I) on the y-axis
- Potential difference (V) on the x-axis
The shape of each graph depends on the component you're testing:
- Resistor - straight line through origin
- Filament lamp - curved S-shape
- Diode - current only flows in one direction
Graph Characteristics to Expect:
Resistor: A straight line passing through the origin with constant gradient
- Gradient = 1/resistance
- Same behaviour for positive and negative voltages
Filament Lamp: An S-shaped curve
- Steeper at low voltages, flattens at higher voltages
- Resistance increases as the filament gets hotter
Diode: A sharp corner shape
- No current for negative voltages (reverse bias)
- Rapid increase in current above ~0.7V (forwards bias)
Improving accuracy
To get the most accurate results:
- Take repeat readings and check they're similar
- Use precise measurements
- Keep the current low to avoid heating effects
- Close readings that match show your data is reliable
What the graphs tell us
Each component has a different I-V characteristic because:
- Resistors follow Ohm's law (at constant temperature)
- Filament lamps get hotter as current increases, changing their resistance
- Diodes only allow current to flow in one direction
The key insight is that electrical components don't all behave the same way - some follow simple mathematical relationships while others have more complex behaviours due to physical changes like heating.
Key Points to Remember:
- I-V graphs show current vs potential difference for different components
- Use a variable resistor to change the voltage in your circuit
- Safety first - electrical circuits can get hot and cause burns
- Repeat readings make your results more reliable
- Different components produce different shaped graphs due to their electrical properties