Curriculum
SQA National 5 Physics
1. Dynamics
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2. Space
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3. Electricity
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3.1.1 Electric Current
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3.1.2 Alternating and Direct Current
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3.2.1 Electric Fields
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3.2.2 Potential Difference
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3.3.1 Voltage-Current (V-I) Graphs
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3.3.2 Calculating Potential Difference, Current and Resistance
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3.3.3 Resistance and Temperature
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3.3.4 Ohm's Law Experiment
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3.4.1 Measuring Current, Potential Difference and Resistance
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3.4.2 Circuit Components
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3.4.3 Transistors
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3.4.4 Series and Parallel Circuits
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3.4.5 Combined Resistance
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3.5.1 Electrical Energy and Electrical Power
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3.5.2 Electrical Power Relationships
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3.5.3 Fuses
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4. Properties of Matter
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5. Waves
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6. Radiation
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Text lesson
3.4.2 Circuit Components
3.4.2 Circuit Components: Recognise common circuit symbols, state the function of each component, and select a series resistor for an LED.
These components supply energy and decide when current flows.
Cell and battery
- A cell supplies electrical energy to the charges, and a battery is two or more cells joined together.
Switch
- Opens or closes the circuit, so it can stop or allow the current to flow.
Fuse
- Contains a thin wire that melts if the current becomes too large, breaking the circuit safely.
Variable resistor
- Has a resistance that can be changed by hand to alter the current in the circuit.
Battery symbol
- A battery is drawn as several cell symbols together, and the longer line of each cell is the positive side.
Output devices transform electrical energy into another form.
Lamp
- Transforms electrical energy into light and heat when a current flows through its filament.
LED
- A light-emitting diode gives out light and only conducts current in one direction.
Motor
- Transforms electrical energy into kinetic energy through rotation.
Loudspeaker and buzzer
- Transform electrical energy into sound energy.
Heater
- Transforms electrical energy into heat energy and has a resistance wire that warms when current flows.
| Component | Energy change |
|---|---|
| Lamp | Electrical to light (and heat) |
| Motor | Electrical to kinetic |
| Loudspeaker | Electrical to sound |
| Heater | Electrical to heat |
Some components respond to their surroundings.
Sensors and Protection
Thermistor
- Its resistance falls when the temperature rises, so it senses heat.
LDR
- Its resistance falls when the light intensity rises, so it senses light.
Diode
- Allows current in one direction only and blocks it in the opposite direction.
LED protection
- An LED needs a resistor in series to limit the current and prevent it burning out.
Why resistors matter
- Choosing the correct value of series resistor keeps the current within the rating of delicate components.
Circuit diagrams
- Standard symbols let anyone read a diagram, so draw them neatly with straight lines and complete loops.
Question: An LED needs 2.0 V across it and a current of 20 mA. It is connected to a 5.0 V supply. Calculate the resistance of the series resistor.
- Potential difference across the resistor = 5.0 − 2.0 = 3.0 V.
- Current I = 20 mA = 0.020 A.
- R = V ÷ I = 3.0 ÷ 0.020 = 150 Ω.
Answer: A 150 Ω resistor is needed.
3.4.2 – Key Points for the Exam
- Resistor: A component that limits the current in a circuit.
- Diode: A component that conducts in one direction only.
- LED: A diode that emits light when current flows through it.
- Series resistor: A resistor in line with an LED to limit its current.
Common Mistakes
- Connecting an LED the wrong way round so that it never lights.
- Leaving out the series resistor and destroying the LED.
- Confusing the symbols for a cell and a battery.
- Drawing an ammeter in parallel or a voltmeter in series in a circuit diagram.