Mains Electricity GCSE Revision

Tried to cover the entire 'Mains Electricity' unit, so sorry its quite a lot, all important through :-) Let me know if I've missed anything, thanks!

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Mains Supply

UK mains supply is approximately 230 volts

It is AC (Alternating Current) - constantly changing direction. 

Frequency of AC mains supply - 50 cycles / second (50Hz)

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Cathode ray oscilloscope (CRO)- similar too a voltmeter. 

'Trace' on screen shows how the voltage of supply over time 

AC supply goes up and down in a regular pattern. Sometimes its positive, sometimes its negative. 

The vertical height of the AC trace shows the imput voltage at that point. The height = Potential difference of AC supply. 

DC supply is just a straight line- Voltage= distance from straight line trace too the center line. 

Gain Dial controls how many volts each cm division represents on the horizontal axis. 

The Timebase controls how many milliseconds (0.001s) each division represents on the horizontal axis.

Frequency (Hz) = 1 / Time Period (s) 

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Dangers of Electricity in the Home

  • Long cables
  • Frayed cables
  • Cables in contact with something hot or wet
  • Water near sockets
  • Shoving things into sockets
  • Damaged plugs
  • Too many plugs in one socket
  • Lighting sockets without bulbs in 
  • Appliances without covers on 
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Cables and Wires

Most electrical appliances connected to mains supply by three core cables. - Three wires inside of them- each with core of copper and coloured plastic coating. 

Live Wire- Brown live wire in mains supply alternates between a high +VE and -VE voltage. 

Neutral Wire- Blue neutral wire always at OV. Electricity normally flows in and out through neutral and live wire only. 

Earth Wire- Green and yellow earth wire for safety. Prevents fire and shocks (along with fuse). Attached to the metal casing of the plug and carry the electricity away to earth if theres a fault and the live or neutral wires touch the metal case. 

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Plug Safety

Correct Wiring

1- The right coloured wire must be connected to each pin and firmly screwed in. 

2- No bare wires showing in plug. 

3. Cable grip tight over the cable outer layer. 

4- Different appliances need different amounts of electrical energy. Thicker cables have less resistance, so they carry more current. 

Plug Features

Metal parts made of copper or brass- very good conductors. 

Case, cable grip and cable insulation made of rubber or plastic- very good insulators and flexible. 

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Fuses and Earthing

If a fault develops in which the live wire touches the metal case, if the case is earthed, too much current flows through the live wire and out down the earth wire. 

This surge in the current melts the fuse (or trips the circuit breaker in the live wire) when the amount of current is greater than the fuse rating. This cuts off the live supply and breaks the circuit. 

This isolates the whole appliance, so you can't get an electric shock. It also prevents the fire hazard due to the heating effect of a large current. 

Fuses and earthing also there to protect appliance- stop wiring breaking due to current surges. 

Fuses should be rated as close as possible to normal operating current- always higher. 

Larger the current = thicker the cable you need to carry it. This is why fuse rating needed for cables usually increases with cable thickness. 

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Circuit Breakers vs. Fuses

Circuit breakers are like fuses- protect circuit from damage if too much current flows. 

When circuit breaker detects a surge in current, break the circuit by opening a switch. 

Circuit breakers can be easily reset by flicking a switch on device. More convenient than fuses, as do not have to be replaced every time. However more expensive to buy than fuses. 

Residual Current Circuit Breaker (RCCB) used instead of fuse and earth wire:

Normally, same current flows through live and neutral wire. If somebody touches the live wire, small but deadly current flows through them to earth. This means neutral wire carries less current than live wire- RCCB detects this difference and cuts of power by opening switch. 

Faster than fuses- break circuit as soon as current surge- no waiting for fuse to melt, therefore safer.

More sensitive- work for small current charges. Small currents can be fatal, so RCCB more effecting in protecting against electrocution. 

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Energy and Power in Circuits

Electricity is a form of energy- always conserved. 

Anything that supplies electricity is also supplying energy, so cells, generators ect. all transfer energy to components in a circuit, e.g motion to motors and heat to kettles. 

All resistors produce heat when a current flows through them- electrical energy converted into heat energy when it passes through anything with electrical resistance.

More current flows, more heat produced. 

Bigger voltage - more heating, because it pushes more current through. 

Filament bulbs work by passing a current through a very thin wire, heating it up so much it glows- a lot of energy wasted as heat. 

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Efficient = Wastes less energy. 

Appliances that are energy efficient transfer more of their total electrical energy output to useful energy. 

e.g. less energy wasted as heat in power saving lamps - compact fluorescent lamps (CFL's) and light emitting diodes (LEDs)- than in filament bulbs. 

More energy efficient bulbs cost more to buy, but over time you save money on electrical bills, makes up for initial investment. The time it takes to make your money back - payback time. 

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Power Ratings

Power (W) = Energy transferred (J) per second (s). 

Total energy transferred by an appliance depends on how long the appliance is on and its power rating

Energy Transferred (J) = Power rating x Time (s)

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Power and Energy Change

Charge goes around the circuit- when they go through an electrical component, energy transferred to make component work. 

Electrical Power (W) = Current (A) x Potential Difference (V) 

Potential difference = Energy Transferred per Charge Passed

When an electrical charge (Q) goes through change in PD (V), energy (E) is transferred. 

Energy supplied to the charge at power source to 'raise' it through a potential. 

Charge gives up this energy when it 'falls' through any potential drop in components elsewhere in the circuit. 

Energy transformed (J) = Charge (C) x Potential Difference (V)

The bigger the change in PD (or voltage) the more energy transferred for a given amount of charge passing through the circuit. 

Therefore, battery with bigger voltage supplies more energy per coulomb of charge, because charge is raised 'higher' at start- more energy dissipated too. 

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Formulas in this Unit

Power (W) = Energy Transferred (J) / Time (s)

Power (W) = Current (A) x Voltage/Potential Difference (V) 

Energy Transferred (J) = Power rating x Time (s)

Energy Transferred (J) = Potential Difference (V) x Charge (C)

Current (A) = Rate of flow of charge / Time (s)

Charge (C) = Current (A) x Time (s)

Frequency (Hz) = 1 / Time Period (s) .

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