ELECTRICITY

MAINS ELECTRICITY

The electricity that we use for heating, lighting, and air conditioning in our homes is referred to as mains electricity and is supplied by power stations.

Introduction to Mains Electricity

  • Mains electricity is essential for powering our appliances in homes.

  • This chapter discusses how mains electricity is delivered to homes and the safety measures in appliances.

Ring Main Circuits

  • Definition: Ring main circuits consist of three wires:

    • Live wire: Carries electrical energy from the power station.

    • Neutral wire: Completes the circuit.

    • Earth wire: Usually has no current; provides safety by allowing current to escape safely if there is a fault.

Advantages of Ring Main Circuits

  • Allows multiple appliances in different parts of the same room to connect to mains with minimal wiring.

  • Prevents excessive wiring use if only one socket is available in a room.

Learning Objectives

  • Understand how insulation, earthing, fuses, and circuit breakers protect users and devices in various domestic appliances.

  • Recognize that the current in a resistor transfers energy and causes a temperature increase, applicable in many household contexts.

  • Comprehend the relationship between power, current, and voltage, expressed as:
    P = I imes V

  • Differentiate between alternating current (AC) mains electricity and direct current (DC) from cells or batteries.

  • Calculate energy transferred using current, voltage, and time with the formula:
    E = I imes V imes t

Electrical Supply to Homes

  • Mains electricity in the UK, China, India, and other countries typically ranges from 220V to 240V.

  • Higher voltage than batteries makes contact with mains electricity dangerous, potentially leading to severe, fatal electric shocks.

Construction of Plugs

  • Plastic casing is used for insulation to protect users from electric shocks.

  • Brass pins are used for connections due to brass's excellent conductivity.

Safety Devices

Fuses

  • Fuses are usually cylindrical cartridges containing a thin wire made of a low melting point metal.

  • If the current exceeds the rated amount, the fuse wire melts, breaking the circuit to prevent electric shock and electrical fires.

  • Fuses must be replaced with the correct size after blowing, typically in sizes of 3 A, 5 A, and 13 A in the UK.

    • Example: For a circuit with a nominal current of 2 A, a 3 A fuse should be used; for 4 A, a 5 A fuse is appropriate.

Circuit Breakers

  • Modern safety devices in consumer units act like fuses but can be reset after a fault correction.

  • Circuit breakers open when a current exceeds the safe limit, and they do not need replacement after being triggered.

Earth Wire Function

  • Appliances with metal casings must connect to the earth wire to prevent electric shock in case of a fault.

  • The earth wire directs current away safely, causing the fuse to blow if a fault occurs.

Double Insulation

  • Appliances with casings made from insulators (e.g., plastic) do not require an earth wire as they prevent user contact with electrical parts.

Switches

  • Switches in circuits should always be placed in the live wire to ensure no electricity flows to an appliance when the switch is off.

The Heating Effect of Current

  • Low Resistance Wiring: Wiring is designed to pass current easily without heating.

  • High Resistance Heating Elements: Specifically designed (e.g., in kettles or toasters) to become hot and transfer energy.

  • Common appliances leveraging heating effect: kettles, dishwashers, electric cookers, washing machines, electric fires, hairdryers.

Traditional Light Bulbs

  • The filament in light bulbs becomes hot and glows, converting electrical energy to heat and light.

  • Inefficient as over 90% of energy is lost as heat.

  • Modern bulbs (e.g., LEDs) are more efficient, reducing energy wastage.

Electrical Power

  • Power (measured in watts) indicates energy transfer rates.

  • Higher power-rated devices transfer energy quickly (e.g., kilowatts, where 1 kW = 1000 W).

  • Power of an appliance is given by:
    P = I imes V

Examples of Power Calculation

  1. Example 1: A 230 V television takes 3 A current.

    • P = I imes V = 3 imes 230 = 690 ext{ W}

  2. Example 2: A 1 kW electric hairdryer at 230 V:

    • P = rac{1000}{230}
      ightarrow 4.35 ext{ A}

    • Fuse: Should use a 5 A fuse.

Energy Transfer by an Appliance

  • The total energy (E) transferred by an appliance is calculated by:
    E = P imes t

Energy Transfer Examples

  1. Example 3a: Energy from a 60 W bulb for 20 seconds:

    • E = 60 imes 20 = 1200 ext{ J} ext{ or } 1.2 ext{ kJ}

  2. Example 3b: Energy from the same bulb for 5 minutes (300 seconds):

    • E = 60 imes 300 = 18000 ext{ J} ext{ or } 18 ext{ kJ}

Alternating Current vs. Direct Current

  • Alternating Current (AC): Fluctuates and can be visualized as a wave.

    • Generated at power stations, varies in direction and value.

  • Direct Current (DC): Constant direction and value, like that from batteries.

    • Graphically represented as a straight horizontal line.

Comparison

  • AC varies with time, while DC remains steady.

  • Voltage behavior contrasts in AC (wave-like) and DC (flat line).

Conclusion

  • Safety is paramount when dealing with mains electricity.

  • Understanding wiring, appliances, power and energy relationships is essential for safe usage in homes.