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Mains electricity

Mains electricity. Revise the following Direct and alternating current 3-pin plugs and electrical cable Safety – fuses and earth wire Period and frequency (Higher tier) Calculating Power. Direct current. Alternating current. What are the parts of a plug?. live terminal. earth terminal.

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Mains electricity

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  1. Mains electricity Revise the following • Direct and alternating current • 3-pin plugs and electrical cable • Safety – fuses and earth wire • Period and frequency (Higher tier) • Calculating Power

  2. Direct current

  3. Alternating current

  4. What are the parts of a plug? live terminal earth terminal fuse neutral terminal cable grip cable

  5. Label the plug parts

  6. The names and jobs of each plug part

  7. What’s inside an electrical cable? • An electrical cable contains three wires. • These wires are made of copper because it is a good conductor of electricity • Each wire is made of thin strands of copper to keep the cable flexible • Each wire is encased in plastic to stop the wires touching and causing a short circuit • Plastic is used as it is a goodinsulator, as well as being tough and flexible • The whole cable is encased in another layer of plastic insulating plastic copper

  8. The live wire(brown)carries a current that alternates between a negative and positive voltage. The earth wire (green and yellow) is a safety wire that is needed to earth appliances with a metal case. This makes it safer to touch the appliance if it develops a fault. The neutral wire(blue) completes the circuit. It is kept at a zero voltage by the electricity company. What does each wire do?

  9. Safety using Fuses and Earthing FUSES • A fuse contains a thin wire, which melts if the current is too high. • This breaks the circuit and so electricity is unable to flow through the appliance. • The appliance stops working and any danger has been averted. • Fuses act as an early warning system, preventing appliances from being damaged by surges in electricity and warning owners of faults. EARTHING • Cables contains an earth wire to • provide an alternativepath for • current if appliances develop • faults. • An earth wire is essential for an • appliance with a metal case. • If the live wire becomes loose • and touches the metal case, a • very large current flows to earth • and blows the fuse, breaking the • circuit • When an appliance is working • correctly, no electricity flows • through the earth wire.

  10. Frequency and period (Higher tier) • We can use an oscilloscope to work out the frequency of the ac supply by measuring the period (time taken for one complete cycle) • Frequency and period are connected by the equation: Frequency (Hertz) = 1 . period (seconds)

  11. Using oscilloscope traces to work out period and frequency Each division will have a potential difference (voltage) value The 0 voltage represents the Neutral terminal (stays at a potential close to zero – compared to the Earth) Zero voltage (0V) Time

  12. Work out the frequency of the supply shown below? Each time division =1 milliseconds Period = 4 divisions x1 = 4ms 4 ms = 0.004 seconds Frequency = 1/Period (seconds) Frequency = 1/0.004 = 250Hz

  13. Calculating Power • Electric current is the rate of flow of charge. • When an electrical charge flows through a resistor, electrical energy is transformed into heat energy. • The rate at which energy is transformed in a device iscalled the power. Power = current x potential difference (Watts/W)) (Amps) (Voltage/V)) energy = potential difference x charge transformed (Joules/J) (Voltage/V) (Coulombs/C) charge = current x time (Coulomb/C) (Amps) (secs)

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