Electricity

Current, potential difference and resistance
1. Circuit symbols
2. Current and charge
  1. cell/battery provides a potential difference
    1. which gives a current
    2. p.d. = voltage(Volts)
  2. current is carried by electrons which are free to move
    1. direction in which positive charge would flow
    2. measured in ampere(A)
  3. charge(Q) is measured in coulombs(C)
    1. 1C = 6 billion billion electrons
  4. Q = It
3. Controlling the current
  1. change in size of p.d. will change the size of the current (increase=increase)
  2. increasing resistance will decrease the current
4. Ammeters and voltmeters
  1. ammeter is set up in series with the resistor
  2. voltmeter is placed in parallel with the resistor
  3. the voltmeter only allows a very small current to flow through it, so it doesn't affect the current
5. Resistance
  1. depends on two things
    1. 1. resistance of component
    2. 2. p.d. across the component
  2. V = IR
  3. measured in ohms
6. Current-potential difference characteristics graph
  1. An ohmic conductor
    1. at constant temperature; current and p.d. is proportional
    2. straight line through the oigin
    3. resistor = ohmic
  2. A filament lamp
    1. line curves away from y-axis
    2. resistor = non-ohmic
    3. increase in current = increase in resistance
      1. current inc. = temperature inc.
  3. A diode
    1. current only allowed to go in one direction
    2. current starts when p.d. = 0.7V
    3. example: LED
    4. reversed p.d. = no current
  4. Changing resistance
    1. by reacting to surroundings
    2. i.e. thermistor (temperature)
      1. temp. inc. = resistance. dec.
      2. fire alarm
    3. LDR
      1. bright light = low resistance
      2. dark light = high resistance
7. Series and parallel circuits
  1. Series circuits
    1. same current through each component
    2. total p.d. of the power supply is shared between the components
      1. V(supply) = V1 + V2
    3. total resistance of two components is the sum of each coponent
      1. R(total) = R1 + R2
  2. Parallel circuits
    1. p.d. across each component is the same
    2. total current = sum of currents through separate components
    3. total resistance of two resistors in parallel is less than the resistance of the smaller individual resistor
  3. Cells and batteries
    1. battery = two or more electrical cells
    2. total p.d. is calculated by adding separate p.d.s together(only works if the cells are joined facing the same way(+ to -))
8. Circuit calculations
  1. use circuit rules to solve circuit problems
9. Domestic use and safety
  1. Direct and alternating potential difference
    1. direct p.d. = direct current(d.c.)
      1. current and p.d. flows in one direction
    2. alternating p.d. = alternating current(a.c.)
      1. current and p.d. switches direction many times er second
  2. Mains supply
    1. supplied by a.c.
    2. UK: p.d. of 230V and frequency of 50Hz
  3. Cables
    1. connect to a wall-socket using a three-core cable plug
    2. cross-sectional area of 2.5mm^2
      1. no more than 13A
    3. larger appliances = thicker cables
  4. Live, neutral and earth wires
    1. live
      1. alternating p.d. from the mains supply
      2. 230V
    2. neutral
      1. completes the circuit
        carry current to and from the electrical appliance
      2. very close to 0V
    3. earth
      1. stops appliance from becoming live
      2. 0V
  5. Earthing
    1. joining earth wire from three-core cable plug to metal casing of appliance
Power
1. Energy and charge
  1. charge passing through a resistor: resistor gets hot
    1. electrons passing through collide with atoms of the resistor
  2. amount of energy depends on two things
    1. p.d. across the resistor
    2. how much charge flowing through
  3. E = VQ
2. Power and energy
  1. energy transferred depends on two things
    1. power of appliance
    2. how long it's switched on
  2. E = Pt
  3. since I = Q/t
    1. P = VI
  4. since V = IR
    1. P = I^2 R
3. National grid
  1. 1. power station sends 1000A and 25000V
  2. 2. transformer transforms to 400 000V and 62.5A
  3. 3. transformer transforms to 230V and larger current (for us to use in our homes)
Static electricity
1. Making static
  1. Sparks
    1. the more teo insulating objects are rubbed against each other, the more charge is built up - can lead to a spark
    2. lightning
    3. walking on a carpet
  2. Energy transfers in lightning
    1. 1. thunder cloud is charged: energy is stored in an electrostatic store
    2. 2. cloud discharges: work is done when charge flows to the earth
    3. 3. energy is transferred to the surroundings by heating, radiation and sound
    4. 4. energy carried by sound and light is transferred to the thermal stores of the surroundings
  3. Forces on charges
    1. pos and pos // neg and neg repells
    2. pos and neg attracts
  4. rubbing insulating materials together
    1. neg. charged particles are rubbed off one and onto the other
  5. when combing hair with a nylon comb: comb becomes neg. charged
  6. when rubbing a perspex ruler with a duster: duster becomes neg. charged
2. Electric fields
  1. gravitational
  2. magnetic
  3. electric
    1. charged object creates this around itself
    2. strongest closest to the object
    3. direction of field is that of the force of a positive charge
    4. strength is increased by adding more charge to object

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	Criado por h g mais de 6 anos atrás