SP10 Electricity and Circuits


SP10a Electric Circuits

  • Nucleus - includes protons, mass of 1 and charge of +1, and neutrons, mass of 1 and charge of 0. 
  • Shells - includes electrons, mass of 1/1835 and charge of -1.
  • Metals have one electron in the outer shell - weakly attracted to nucleus. Electrons can be easily removed, so many 'free' electrons. Voltage pushes the electrons arount the circuit
  • Convectional current - current flows +ve terminal to -ve terminal, opposite from electron flow.
  • Series circuit - one route current can take.
  • Parallel circuit - junctions allow current to take different routes. 
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SP10b Current and Potential Difference

  • Current - measured in units of amperes (amps, A), with an ammeter.
  • Current is conserved - total amount of current is the same around the circuit. The total amount entering junction is the same leaving.  
  • Potential difference/voltage - measured in volts, using a voltmeter and  is needed for electrons to flow. 
  • The bigger the voltage, bigger the current. 
  • Voltage across all components add up to give voltage produced by battery/cell. 
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SP10c Current, Charge and Energy

  • Electric charge - measured in Coulombs (C)
  • Electron charge - -1.6 x 10^-19
  • Size of current tells how much charge flows past each second.
  • Electic current - rate of flow of charge (electrons).
  • Charge (C) = current (A) x time (s)/ Q = I x t 
  • Energy transferred (J) = charge moved (C) x voltage (V)/ E = Q x V
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SP10d Resistance

  • Resistance - how hard it is for current to flow, measured in ohms (Ω).
  • Voltage (V) - current (A) x resistance (Ω)/ V = I x R
  • Resistors in series - resistance is increases, pathway harder for current to flow. Voltage shared betweeen resistors not always equal
  • Resistors in parallel - total resistance less than indivdual as there's more paths for current. 
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SP10e More about Resistance

  • Direct Proportion - two variables change by same percentage (straigh-line through origin).
  • (When voltage change:)Fixed resistor - current changes as voltage changes at direct proportion, resistance same.
  • Lamp - voltage causes current to flow and  it heats up lamp. Bigger the voltage, more current, hotter lamp. As it heats up - resistance increases (not direct proportion).
  • Diode - low resistance if voltage in 1 direction but high in opposite (current can only flow in 1 direction).
  • Light dependent resistor (LDR) - high resistance in dark, resistance decrease as light increases.
  • Thermistors - high resistance in low temps, temp increases as resistance decreases. 
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SP10f Transferring energy

  • All circuits have resistance - warm up when there's a current (heating effect).
  • Current passes through resistor - energy transferred by heating because electrical work done against resistance
  • Heating effect - useful in heaters/kettle. Not useful in a computer/plugs as useful energy is transferred from circuit by heating and dissipated into surroundings. 
  • Lattice in resistor - as electrons flow through lattice of vibrating ions, they collide with ions. More collisions, the harder it is to flow, more resistance. When they collide, they transfer energy to them. 
  • Reducing resistance - using wires of metals with low resistance. Cooling metals, so that lattice ions not vibrating as much. 
  • Energy transferred (J) = current (A) x voltage (V) x time (S)/ E = I x V x t
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SP10g Power

  • Power - energy transferred per second, Watts (W).
  • Power (W) = energy transferred (J) ÷ time (s)/ P = E ÷ t
  • Electrical power (W) = current (A) x voltage (V)/ P = I x V
  • Electrical power (W) = current² (A²) x resistance (Ω)/ P = I² x R
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SP10h Transferring Energy by Electricity

  • Energy from battery transfers by electricity to the motor, where it's transferred to store of kinetic energy in fan. Some energy transfers by heating wires, motor and surroundings. All energy will dissipate by heating, in the end.
  • Gloves contain wire with high resistance. Energy from battery transfers by electricity to wire, where it's transferred by heating to store of thermal energy. The energy transfers to gloves and hand, and dissipates to surroundings. 
  • National grid - network of wires that carries electricity to homes.
  • Direct current - a current flows in one direction only, the direction of movent of charge stays same. Voltage - 230V
  • Alternating current - current that changes direction many times each second. Voltage changes from peak voltage to zero (50 Hertz).  Average effect - 230V.
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SP10i Electrical Safety

  • Earth Wire - A safety wire to stop the appliance becoming live. 
  • Neutral Wire - carries current back to the source and completes circuit.
  • Live Wire - has a voltage of 230V and provides current.
  • Fuse - tube with thin wire that melts when current exceeds a certain value (3A, 5A and 13A), stops current/breaks circuit.
  • Fuses stops fire from overheating wires, and earth wire stops current from flowing in you if there's a fault
  • If live wire touches a metal part, there's large current flowing to earth, heats wire and causes fire. 
  • Circuit breakers - detect change in current and swithes off supply. Can be re-used, quick, saves lives. Fuse has to be replaced, fues takes time, doesn't save you from shock. 
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