SAFETY FEATURES USED IN THE CIRCUITS OF THE MAINS SUPPLY Additional Science Chapter 14 Physics 2

Electrical Power Additional Science Chapter 14 Remember: Electrical Power (W) = Current (A) × Voltage (V) or P = IV Power = Current × Voltage = 0.25 × 240 = 60W Current = 0.25A Voltage = 240V Additional Science Chapter 14 Power = Current × Voltage = 12.5 × 240 = 3000W = 3kW Current = 12.5A Voltage = 240V

Electrical Power Additional Science Chapter 14 Current = 0.5A Remember: Electrical Power (W) = Current (A) × Voltage (V) or P = IV Current = 0.5A Voltage = 240V Power =? Current = 2A Voltage = 240V Power =? Answer: 480W Answer: 120W Additional Science Current = 0.4A Voltage = 240V Power =? Current = 3A Voltage = 240V Power =? Chapter 14 Answer: 720W Answer: 96W

Electrical Power Appliance Power(W) Current(A) Voltage(V) You must remember: Electrical Power (W) = Current (A) × Voltage (V) or P = IV Appliance Power(W) Current(A) Voltage(V) Hair dryer 3 240 heater 1200 iron 960 personal stereo 2.4 microwave 800 Additional Science Chapter 14

Electrical Power Appliance Power(W) Current(A) Voltage(V) You must remember: Electrical Power (W) = Current (A) × Voltage (V) or P = IV Appliance Power(W) Current(A) Voltage(V) Hair dryer 720 3 240 heater 1200 5 iron 960 4 personal stereo 2.4 0.01 microwave 800 3.33 Additional Science Chapter 14

Electrical plug Additional Science Chapter 14 Remember: Te live wire carries the electricity The fuse protects the appliance from damage The earth wire protects us from danger from the circuit The plastic and the rubber is the material for the plug and wire cover Earth Live Additional Science Fuse Neutral Chapter 14 Cord grip Cable

Electrical plug Which plug has been correctly wired? A B C Additional Science D E F Chapter 14

Electrical Power and the Fuse You must remember: Electrical Power (W) = Current (A) × Voltage (V) or P = IV Current = ? Voltage = 240V Power = 720W Current = ? Voltage = 240V Power = 120W Current = ? Voltage = 240V Power = 3000W Current = ? Voltage = 240V Power = 480W Additional Science Chapter 14 What fuse must be used? 3A 5A 13A

The work of the fuse Current travels along the live wire, through the appliance (bulb in this case) and back along the neutral wire. ` Additional Science Chapter 14

The work of the fuse This shows what happens when there is a fault in the circuit. The current is too strong for the fuse. It blows causing a break in the circuit. Additional Science Chapter 14 Part of the live wire touches the side of the appliance. A lot of current now flows along the earth wire.

Circuit Breaker Additional Science Chapter 14 Miniature circuit beakers (MCB) are used in modern consumer units. They replace fuses. When the current exceeds the rated value, the strength of the electromagnet is enough to separate the contacts. The circuit is switched off. Circuit breakers, like fuses, do not provide protection should you touch a live component. Additional Science Chapter 14 Advantages of the fuse:- It works a lot faster It is possible to reset it. Click here for further information from the web

(Residual Current Devices RCDs) These devices are designed to protect against electrical shock. This type of circuit breaker measures the current in the live and neutral wires. If everything is working correctly, then these two are equal. If there’s a fault, you get a small electrical shock but not strong enough to kill you. You should always use a RCD every time you work with outdoor equipment. If there’s a difference between the live wire and the neutral, the electromagnet in the live side will pull the iron bar downwards, pushing the button up to create a break in the circuit. The effect of this is to prevent electricity from reaching the appliance. Switch on the drill to see the RCD at work. You must reset the switch before using the RCD again. Additional Science Break in the circuit B Chapter 14 N pivot Iron bar Wire coils (electromagnet) Switch on the drill Current out Current in Power supply neutral live

DC and AC Additional Science Chapter 14 Oscilloscope DC and AC Voltage (V) Time (T) DC means “Direct Current” – the current travels in one direction only e.g. in a battery 1.5V 1.5V Oscilloscope Voltage (V) Time (T) 1 th/ s 50 Additional Science AC means “Alternating Current” – the current changes 50 times per second (frequency = 50Hz). This is the frequency that comes from the mains supply. 230V Chapter 14

Why 230V? Additional Science Chapter 14 The UK and European countries have used 230V since the start. It would be very costly to change this to a lower voltage – appliances use 230V. Many transformers would be required for our appliances to work at a lower voltage. Thicker and more flexible cables would be required (lower voltage higher current to ensure the same power. Many builders use tools that use 110V. This voltage reduces the risk of fatal electrical shocks. Additional Science Chapter 14 The USA has chosen a voltage of 120V for its mains supply. Because of this, some electrical appliance manufacturers include a switch so that the correct voltage can be chosen for the appliance – using the appliance with a voltage of 120V would be damaging in this country.