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Electrical Components (Edexcel GCSE Physics)
Revision Note
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I-V Graphs
Fixed Resistors
- The current through a fixed resistor increases as the potential difference across it increases
- In other words, current is directly proportional to the potential difference for a fixed resistor
- An I-V graph shows that the line is straight and goes through the origin, as shown in the I-V graph below:
I-V graph for a fixed resistor. The current is directly proportional to the potential difference as the graph is a straight line through the origin
- This relationship is true because the resistance of a fixed resistor is constant
Filament Lamps
- For a filament lamp, current and potential difference are not directly proportional
- This is because the resistance of the filament lamp increases as the temperature of the filament increases
- The I–V graph for a filament lamp shows the current increasing at a proportionally slower rate than the potential difference
I-V graph for a filament lamp
- This is because:
- As the current increases, the temperature of the filament in the lamp increases
- The higher temperature causes the atoms in the metal lattice of the filament to vibrate more
- This causes an increase in resistance as it becomes more difficult for free electrons (the current) to pass through
- Resistance opposes the current, causing the current to increase at a slower rate
- Where the graph is a straight line, the resistance is constant
- The resistance increases as the graph curves
- Reversing the potential difference reverses the current and makes no difference to the shape of the curve
Diodes
- A diode allows current to flow in one direction only
- This is called forward bias
- In the reverse direction, the diode has very high resistance, and therefore no current flows
- This is called reverse bias
I-V graph for a semiconductor diode
- The I–V graph for a diode is slightly different:
- When the current is in the direction of the arrowhead symbol, this is forward bias
- This is shown by the sharp increase in potential difference and current on the right side of the graph
- When the diode is switched around, this is reverse bias
- This is shown by a zero reading of current or potential difference on the left side of the graph
- When the current is in the direction of the arrowhead symbol, this is forward bias
Examiner Tip
Make sure to practice drawing which current-voltage graph is for which component, as this is a common exam question!
LDRs
- A light-dependent resistor (LDR) is a type of sensory resistor
- This means it is a resistor which has a resistor that changes with its surroundings
- The circuit symbol for an LDR is:
LDR circuit symbol
- The resistance of an LDR changes depending on the light intensity on it
- As the light intensity increases the resistance of an LDR decreases and vice versa
The resistance of an LDR is dependent on the amount of light intensity on it
- LDRs can be used as light sensors, so, they are useful in circuits which automatically switch on lights when it gets dark, for example, street lighting and garden lights
Thermistors
- A thermistor is also a type of sensory resistor
- It is represented by the following circuit symbol:
Thermistor circuit symbol
- The resistance of a thermistor changes depending on its temperature
- As the temperature increases the resistance of a thermistor decreases and vice versa
The resistance through a thermistor is dependent on temperature
- Thermistors are temperature sensors and are used in circuits in ovens, fire alarms and digital thermometers
- As the thermistor gets hotter, its resistance decreases
- As the thermistor gets cooler, its resistance increases
A digital thermometer uses a thermistor
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