Physics P2B - Electricity

Physics P2B - Electricity

1. Mains Electricity

   • Mains Supply is AC, Battery Supply is DC
     • The UK mains supply is approximately 230 volts
       • It is an AC supply (alternating current) which means the current is constantly changing direction
     • The frequency of the AC mains supply is 50 cycles per second or 50 hertz
     • Cells and batteries supply direct current (DC)
       • This means that the current keeps flowing in the same direction
   • Electricity supplies can be shown on an oscilloscope screen
     • A cathode ray oscilloscope (CRO) is basically just a snazzy voltmeter
       • If you plug an AC supply into the oscilloscope, you get a 'trace' on the screen that shows how the voltage changes with time
         • This trace goes up and down in a regular pattern
           • Some of the time it's positive, some of the time it's negative
         • The vertical height of the AC at ay time shows the input voltage at that point
           • By measuring the height of the trace, you can find the potential difference of the AC supply
       • If you plug in a DC supply, the trace you get is just a straight line
         • For measuring the voltage of a DC supply, you just have to measure the distance between the straight line trace to the centre line

2. Electricity in the home

   • Hazards in the home
     • Long cables
     • Frayed cables
     • Damaged plugs
     • Cables in contact with something hot or wet
     • Water near sockets
     • Shoving things into sockets
     • Too many plugs in one socket
     • Lighting sockets with no bulbs in
     • Appliances without their covers on
   • Most cables have 3 separate wires
     • Most electrical appliances are connected to the mains supply by three core cables.
       • This means that they have three wires inside them, each with a core of copper and a coloured plastic coating
     • The brown wire is the LIVE WIRE and in a mains supply alternates between a HIGH POSITIVE VOLTAGE AND NEGATIVE VOLTAGE
     • The blue wire is the NEUTRAL WIRE and is always at 0 VOLTS
     • Electricity normally flows through the live and neutral wires only
     • The green and yellow earth wire is for protecting the wiring, and for safety
       • It works together with a fuse to prevent fire and shocks
         • It is attached to the metal casing of the appliance and carries the electricity to earth (and away from you) should something go wrong and the live or neutral wires touch the metal case
   • Plug features
     • The metal parts are made of copper or brass because these are very good conductors
     • The case, cable grip and cable insulators are made of rubber or plastic because they are very good insulators, and flexible too
     • This keeps all the electricity flowing where it should

3. Fuses and Earthing

   • Earthing and fuses prevent electrical overloads
     • If a fault develops in which the live wire somehow touches the metal case, then because the case is earthed, too great a current flows in through the live wire, through the case and out down through the earth wire
       • This surge in 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 isolates the whole appliance, making it impossible to get an electric shock from the case.
           • It also prevents the risk of fire caused by the heating effect of a large current
           • As well as people, fuses and earthing are there to protect the circuits and wiring in your appliances from getting fried if there is a current surge
             • Fuses should be rated as near as possible but just higher than the norma operating current
               • The larger the current, the thicker the cable you need to carry it. Thats why the fuse rating needed for cables usually increases with cable thickness
         • This cuts off the live supply and breaks the crcuit
   • Insulating materials make appliances 'double insulated'
     • All appliances with metal cases are usually 'earthed' to reduce the danger of electric shock
       • 'Earthing' just means the case must be attached to an earth wire
       • An earthed conductor can never become live. If the appliance has a plastic casing and no metal parts showing then it's said to be double insulated.
         • Anything with double insulating doesn't need an earth wire - just a live and a neutral
           • Cables that only carry the live and neutral wires are known as two-core cables
   • Circuit breakers have some advantages over fuses
     • Circuit breakers are an electrical safety device used in some circuits. Like fuses, they protect the circuit from damage if too much current flows through them
       • When circuit breakers detect a surge in current in a circuit, they break the circuit by opening a switch.
         • A circuit breaker (and the circuit they're in) can easily be reset by flicking a switch on the device
           • This makes them more convenient than fuses, which have to be replaced once they've melted
           • Circuit breakers are, however, more expensive than fuses
             • One type of circuit breaker used instead of a fuse and an earth wire is a Residential Current Circuit Breaker (RCCBs)
               • Normally exactly the same current flows through the live and neutral wires, If somebody touches the live wire, a small but deadly current will flow through them to the earth.
               • They also operate much faster then fuses - they break the circuit as soon as there is a current surge -no time is wasted waiting for the current to melt a fuse. This makes them safer.
               • RCCBs even work for small current charges that might not be large enough to melt a fuse. Since even small current charges can be fatal, this means RCCBs are more effective at protecting against electrocution

4. Energy and Power in Circuits

   • Energy is transferred from cells and other sources
     • Anything which supplies electricity is also supplying energy, so cells, batteries, generators etc. all transfer energy to components in the circuit
       • Motion: Motors
       • Light: Light bulbs
       • Heat: Hair dryers/kettles
       • Sound: Speakers
   • All resistors produce heat when a current flows through them
     • Whenever a current flows through anything with electrical resistance (which is pretty much everything) then electrical energy is converted into heat energy.
       • The more current that flows, the more heat is produced
         • A bigger voltage means 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 that it glows
             • This means they lose a lot of energy as heat
   • If an appliance is efficient, it wastes less energy
     • When you buy electrical appliances you can choose to buy ones that are more energy effiicient
       • These appliances transfer more of their total electrical energy output to useful energy
         • For example, less energy is wasted as heat in power-saving lamps such as compact fluorescent lamps (CFLs) and light emitting diodes (LEDs) than in ordinary filament bulbs
           • Unfortunately, they do cost more to buy, but over time the money you save on your electricity bills pays you back for the initial investment
   • Power of appliances
     • The total energy transferred by an appliance depends on how long the appliance is on and its power
       • The power of an appliance is the energy that it uses per second
         • ENERGY USED = POWER x TIME

5. Power and energy change

   • Electrical power and fuse ratings
     • Formula: POWER = CURRENT x POTENTIAL DIFFERENCE
       • P = I x V
     • You can use this equation to work out the fuse that should be used in an appliance. To work out the size of the fuse needed, you need to work out the current that the item will normally use
   • The potential difference is the energy transferred per charge passed
     • When an electrical charge (Q) goes through the change in potential difference (V), then energy (E) is transferred.
       • Energy is supplied to the charge at the power source to 'raise' it through a potential
         • The charge gives up this energy when it 'falls' through any potential drop in components elsewhere in the circuit
           • Formula: ENERGY TRANSFERRED = CHARGE x POTENTIAL DIFFERENCE
           • The bigger the change in PD (or voltage), the more energy is transferred for a given amount of charge passing through the circuit
             • This means that a battery with a bigger voltage will supply more energy to the circuit for every coulomb of charge which flows round it, because the charge is raised up 'higher' at the start, and more energy will be dissipated in the circuit too