Mains Electricity, AC vs DC, Three-Core Plugs & The National Grid

1

Alternating Current (AC) vs Direct Current (DC)

In direct current (DC, supplied by cells and batteries), electric charge/current always flows in the same direction. In alternating current (AC, UK mains supply), the potential difference and current constantly change direction back and forth. UK mains electricity is an AC supply with a frequency of 50 Hz (changes direction 50 cycles per second) and a potential difference of ~230 V.

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2

Three-Core Cable & UK Plug Wiring

1. Brown = Live wire (right pin, connected to fuse): carries the 230 V alternating potential difference from the supply. 2. Blue = Neutral wire (left pin): completes the circuit back to the substation, at or near 0 V. 3. Green and Yellow stripes = Earth wire (top longest pin): safety wire at 0 V connected to the metal casing of the appliance to prevent it becoming live. Double-insulated appliances have a plastic casing (an electrical insulator) so they do not need an earth wire.

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3

Fuses & Circuit Breakers (RCCBs)

A fuse contains a thin metal wire placed in series with the live wire; if the current exceeds the fuse rating (standard ratings: 3 A, 5 A, or 13 A), the wire heats up and melts, breaking the circuit. Always choose a fuse rating slightly above the normal operating current calculated from I = P / V. Residual Current Circuit Breakers (RCCBs) are electromagnetic switches that trip much faster than fuses and can be reset.

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4

Electrical Power & Energy Equations

1. Power (W) = Potential difference (V) x Current (A) -> P = V x I. 2. Power (W) = (Current)² (A²) x Resistance (Ω) -> P = I² x R. 3. Energy transferred (J) = Power (W) x Time (s) -> E = P x t. 4. Energy transferred (J) = Charge flow (C) x Potential difference (V) -> E = Q x V.

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5

The National Grid

A system of cables and transformers linking power stations to consumers (homes, factories, offices) across the country. Notice that power stations are not part of the National Grid definition; only the cables and transformers are.

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6

Step-Up & Step-Down Transformers in the Grid

Power stations generate electricity at ~25,000 V. 1. Step-up transformers increase the potential difference to ~400,000 V (400 kV) for transmission along overhead cables, which decreases the current (P = VI) and minimises wasted heating power (P = I²R) in the cables. 2. Step-down transformers decrease the potential difference back down to 230 V so it is safe for domestic consumers.

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1. Q1

   Explain why transmitting electricity through the National Grid at 400,000 V is much more efficient than transmitting it at 230 V, and state the role of both types of transformer in the grid. [4 marks]

   Check Mark Scheme

   Click “Check Mark Scheme” to compare your answer, 1. Step-up transformers at power stations increase the potential difference to 400,000 V [1 mark]. 2. Because electrical power is given by P = V x I, increasing the potential difference (V) for a given power reduces the current (I) flowing through the transmission cables [1 mark]. 3. Since the power wasted by heating the resistance (R) of the long cables is given by P = I² x R, a low current drastically reduces the thermal energy wasted to the surroundings, making transmission much more efficient [1 mark]. 4. Step-down transformers near towns and homes then decrease the potential difference down to 230 V so that it is safe for domestic appliances and consumers [1 mark].

2. Q2

   A washing machine has a metal outer casing. Explain how the earth wire and the fuse in the plug work together to protect the user if a fault develops and the live wire touches the metal casing. [4 marks]

   Check Mark Scheme

   Click “Check Mark Scheme” to compare your answer, 1. The earth wire is connected directly to the metal casing of the washing machine and provides a very low-resistance path to the ground (0 V) [1 mark]. 2. If the 230 V live wire comes loose and touches the metal casing, a very large current immediately flows through the live wire, the casing, and down the earth wire [1 mark]. 3. This sudden surge of current is much greater than the rating of the fuse fitted in the live wire, causing the fuse wire to overheat and melt (blow) [1 mark]. 4. Melting the fuse breaks the circuit and disconnects the washing machine from the 230 V live supply, preventing anyone who touches the metal casing from receiving an electric shock [1 mark].

3. Q3

   An electric immersion heater operates at a potential difference of 230 V and draws a current of 12 A for 15 minutes. Calculate: (a) the total charge that flows through the heater, (b) the electrical power of the heater, and (c) the total energy transferred by the heater in megajoules (MJ). [5 marks]

   Check Mark Scheme

   Click “Check Mark Scheme” to compare your answer, 1. Convert time to seconds: t = 15 x 60 = 900 s [1 mark]. 2. (a) Charge flow: Q = I x t = 12 A x 900 s = 10,800 C (coulombs) [1 mark]. 3. (b) Electrical power: P = V x I = 230 V x 12 A = 2,760 W (or 2.76 kW) [1 mark]. 4. (c) Total energy transferred in Joules: E = P x t = 2,760 W x 900 s (or E = Q x V = 10,800 C x 230 V) = 2,484,000 J [1 mark]. 5. Convert Joules to megajoules (divide by 1,000,000): E = 2.484 MJ (or 2.5 MJ) [1 mark].

4. Q4

   An overhead transmission cable has a total resistance of 0.50 Ω. Compare the power wasted as heat in the cable when 10 MW of electrical power is transmitted at (a) 25,000 V versus (b) 400,000 V. [4 marks]

   Check Mark Scheme

   Click “Check Mark Scheme” to compare your answer, 1. At 25,000 V: Current I₁ = P / V = 10,000,000 W / 25,000 V = 400 A [1 mark]. 2. Power wasted as heat at 25,000 V: P_waste = I₁² x R = (400)² x 0.50 = 160,000 x 0.50 = 80,000 W (80 kW) [1 mark]. 3. At 400,000 V: Current I₂ = P / V = 10,000,000 W / 400,000 V = 25 A [1 mark]. 4. Power wasted as heat at 400,000 V: P_waste = I₂² x R = (25)² x 0.50 = 625 x 0.50 = 312.5 W (stepping up voltage by a factor of 16 reduces wasted power by a factor of 16² = 256!) [1 mark].

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