CIE iGCSE Co-ordinated Sciences-P4.4 Electrical safety- Study Notes- New Syllabus
CIE iGCSE Co-ordinated Sciences-P4.4 Electrical safety – Study Notes
CIE iGCSE Co-ordinated Sciences-P4.4 Electrical safety – Study Notes -CIE iGCSE Co-ordinated Sciences – per latest Syllabus.
Key Concepts:
Core
- Describe the heating effect of current
- State the hazards of:
(a) damaged insulation
(b) overheating cables
(c) damp conditions
(d) excess current from overloading of plugs, extension leads, single and multiple sockets when using a mains supply - Explain the use and operation of trip switches and fuses and choose appropriate fuse ratings and trip switch settings (knowledge of RCDs (Residual Current Devices) is not required)
- Explain why the outer casing of an electrical appliance must be either non-conducting (double-insulated) or earthed
CIE iGCSE Co-Ordinated Sciences-Concise Summary Notes- All Topics
The Heating Effect of Current
When an electric current passes through a conductor (especially a resistor), electrical energy is converted into heat energy. This is known as the heating effect of current.
Explanation (Particle Model):
- In a conductor, current is due to the movement of free electrons.
- As electrons move, they collide with the ions of the metal lattice.
- These collisions transfer energy to the ions, causing them to vibrate more strongly.
- The increased vibrations raise the temperature of the conductor.
Formula (Joule’s Law of Heating):
The heat energy produced in a conductor is given by:
\(\mathrm{H = I^2 R t}\)
- \(\mathrm{H}\) = heat energy (J)
- \(\mathrm{I}\) = current (A)
- \(\mathrm{R}\) = resistance of the conductor (Ω)
- \(\mathrm{t}\) = time (s)
Key Point: The heating effect increases with higher current, larger resistance, and longer time.
Everyday Applications:
- Electric heaters and kettles: Heating coils (nichrome wires) convert electrical energy into heat.
- Electric bulbs: The filament gets hot and glows, producing both heat and light.
- Fuses: Thin wire melts when excessive current flows, protecting appliances.
- Electric irons and toasters: Use the heating effect to warm or cook food.
Example :
A current of 2 A flows through a 10 Ω resistor for 5 minutes. How much heat energy is produced?
▶️ Answer/Explanation
Step 1: Use Joule’s law: \(\mathrm{H = I^2 R t}\)
Step 2: Convert time: \(5 \, \text{min} = 300 \, \text{s}\)
Step 3: Substitute values: \(\mathrm{H = (2^2) \times 10 \times 300 = 4 \times 10 \times 300 = 12000 \, J}\)
Final Answer: \(\mathrm{12 \, kJ}\) of heat energy is produced.
Hazards of Using Mains Electricity
When using mains electrical appliances, certain conditions can be dangerous because they increase the risk of electric shock, fire, or equipment damage.
(a) Damaged Insulation
- If the plastic insulation around a wire is cracked or worn, the live wire may be exposed.
- Touching it can cause a severe electric shock.
- Exposed wires may also short circuit, leading to sparks or fire.
(b) Overheating Cables
- If too much current passes through a wire, it becomes hot due to the heating effect of current (\(\mathrm{H = I^2 R t}\)).
- Overheating may melt the insulation and cause fires.
(c) Damp Conditions
- Water is a conductor when it contains dissolved salts.
- Using electrical devices in damp or wet areas increases the risk of current flowing through your body → electric shock.
- This is why bathrooms and outdoor electrical use require special safety devices (e.g., RCDs).
(d) Excess Current from Overloading Plugs, Extension Leads, Sockets
- Plugging too many devices into one socket or extension can draw more current than the wires are designed to carry.
- This causes overheating of wires, damage to insulation, and possible fires.
- Also increases the chance of a short circuit if wires melt and touch.
Summary Table
| Hazard | Danger |
|---|---|
| Damaged Insulation | Exposed live wire → electric shock or fire |
| Overheating Cables | Excessive heating → melts insulation, causes fire |
| Damp Conditions | Water conducts electricity → increased risk of electric shock |
| Overloading Plugs / Extension Leads | Too much current → overheating, fire, short circuits |
Example :
Why is it unsafe to use a hairdryer in the bathroom?
▶️ Answer/Explanation
Step 1: Bathrooms are damp, and water is a conductor of electricity.
Step 2: If water touches the hairdryer or socket, current can flow through the water and your body.
Final Answer: Using electrical appliances in damp conditions greatly increases the risk of electric shock.
Fuses and Trip Switches
Purpose: Both fuses and trip switches are safety devices designed to protect electrical circuits and appliances from damage caused by excess current. They also reduce the risk of fire and electric shock.
1. Fuse
- A fuse is a thin wire enclosed in a cartridge or plug.
- When the current exceeds a safe value, the fuse wire heats up and melts.
- This breaks the circuit, stopping the current flow.
- Once a fuse blows, it must be replaced with a new one.
Choosing Fuse Ratings:
- The fuse rating should be slightly above the normal operating current of the appliance, but below the current that could cause damage.
- Common UK fuse ratings: 3 A, 5 A, and 13 A.
For example:
- A device rated at 700 W on a 230 V supply → \(\mathrm{I = P/V = 700/230 \approx 3 A}\). Use a 5 A fuse.
- A device rated at 2500 W on a 230 V supply → \(\mathrm{I = 2500/230 \approx 11 A}\). Use a 13 A fuse.
2. Trip Switch (Circuit Breaker)
- A trip switch (miniature circuit breaker, MCB) automatically switches off the current if it becomes too large.
- Unlike fuses, they do not melt — instead, a magnetic or thermal mechanism detects excess current and opens the circuit.
- After tripping, the switch can be reset manually without replacement.
- They are more convenient and faster than fuses.
Choosing Trip Switch Settings:
- Set slightly above the normal operating current of the circuit.
- For example: – A lighting circuit using 4 A → use a 6 A trip switch. – A socket ring circuit using 25 A → use a 30 A trip switch.
Comparison Between Fuses and Trip Switches
| Device | Operation | After Activation | Typical Rating |
|---|---|---|---|
| Fuse | Wire melts when current is too high | Must be replaced | 3 A, 5 A, 13 A (plugs) |
| Trip Switch | Switch trips off using magnetic/thermal action | Can be reset | 6 A, 30 A (circuits) |
Example :
An electric kettle is rated at 2000 W and connected to a 230 V supply. Which fuse should be used in its plug?
▶️ Answer/Explanation
Step 1: Calculate current: \(\mathrm{I = P/V = 2000/230 \approx 8.7 A}\).
Step 2: Choose the next higher standard fuse rating → 13 A.
Final Answer: A 13 A fuse should be used.
Safety of Appliance Casings
The outer casing of an electrical appliance must be safe to touch even if a fault occurs. This is achieved by either:
- Using a non-conducting casing (double insulation), or
- Connecting the casing to earth.
1. Non-Conducting / Double-Insulated Casing
- Some appliances (e.g., hairdryers, vacuum cleaners, electric drills) have casings made of plastic.
- Plastic is a good insulator, so it cannot conduct electricity.
- Even if a live wire inside the appliance touches the casing, the current cannot pass through to the user.
- These appliances are said to be double-insulated (insulation around the wires + plastic outer casing).
- Such appliances do not need an earth wire.
2. Earthed Metal Casing
- If the casing is made of metal (e.g., washing machines, ovens), it could conduct electricity if the live wire touches it.
- This would make the casing live, giving a dangerous electric shock if touched.
- To prevent this, the casing is connected to the earth wire in the plug.
- The earth wire provides a low-resistance path for current to flow safely to the ground.
- This causes a large current to flow, which blows the fuse or trips the circuit breaker, cutting off the supply quickly.
Example :
Why does a washing machine with a metal casing require an earth wire?
▶️ Answer/Explanation
Step 1: If the live wire touches the metal casing, the casing becomes live.
Step 2: Without earthing, a person touching the casing could receive an electric shock.
Step 3: With earthing, the current flows safely to ground, causing the fuse to blow or breaker to trip.
Final Answer: The earth wire prevents the casing from becoming dangerous by providing a safe path for current.