CP9 Electricity and circuits

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  • Created by: Sia11
  • Created on: 17-11-20 18:12

CP9a Electric circuits

  • All metals have electrons that are weakly attracted to the nucleus, these electrons can easily be removed, so a metal wire has many delocalised electrons
  • When a battery is attached to the wire the voltage pushes the delocalised electrons around the circuit, they are negatively charged so they move towards the positive terminal
  • Conventional current is when the current goes in the direction from the positive terminal to the negative terminal, this is used when working with circuits
  • Series Circuits:
    • has only 1 path for the current to follow
    • lamps cannot be switched on and off individually
    • if 1 lamp fails they will all switch off
  • Parallel Circuits:
    • have multiple paths for the current to follow
    • lamps can be switched separately
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CP9b Current and potential difference

  • Current is the rate of flow of charge, measured in amps using an ammeter
  • Potential difference is the amount of energy transferred by each coulomb of charge, it is measured in volts using a voltmeter
  • In a series circuit, current is the same everywhere and the potential difference is shared between components
  • In a parallel circuit the current splits at a junction but the total stays the same and the total potential difference goes to each branch
  • The total amount of current stays the same all the way around the circuit because current is conserved
  • The bigger the current the bigger the potential difference
  • A voltmeter is always connected in a parallel circuit to measure the potential difference across a component or circuit
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CP9c Current, charge and energy

  • Moving charged particles form an electric current, electric charge is measured in coulombs
    • one coulomb is the charge that passes a point in a circuit when there is a current of 1 amp for 1 second
  • In metals the current is the flow of electrons, each electron has a charge of -1.6 x 10^-19
  • The size of a current at any point in a circuit tells you how much charge flows past that point each second, electric current is the rate of flow of charge
  • Current (A) = charge (C) / time (s)                                                                                                  I = Q / t
  • The potential difference of a cell is the amount of potential energy the cell transfers to each coulomb of charge flowing through it
  • There is a potential difference of 1 volt when there is a transfer of 1 joule to each coulomb
  • potential difference (V) = energy (J) / charge (C)                                                                            V = E / Q
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CP9d Resistance

  • Resistance is an opposition to an electric current, the higher the resistance, the lower the current, it is measured in ohms
  • Resistance (Omega) = potential difference (V) / current (A)                                                          R = V / I
  • If there is more than one resistor in series, you add them together to get the total resistance
    • The resistance increases because the pathway becomes harder for current to flow through
    • The potential difference from a cell is shared between the resistors, but it may not be shared equally, there will be greater potential difference across resistors with higher resistances
  • If there is more than one resistor in parallel, the total is less than the smallest one
    • This is because there are now more paths for the current
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CP9e More resistance

  • A potential difference across a filament lamp causes a current to flow through it; the current causes the filament to heat up and glow
  • The greater the potential difference, the more current flows and the hotter and whiter the filament gets
  • As it heats up the filament's resistance increases meaning that when the potential difference changes, the current does not change by the same percentage
  • A diode has a low resistance if the potential difference is in one direction but a very high resistance if in the opposite direction meaning current can only flow in one direction
  • A light-dependent resistor (LDR) has a high resistance in the dark but it gets smaller when the light intensity changes
  • Thermistors have high resistances at low temperatures but as the temperature increases the resistance decreases
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CP9f Transferring energy

  • All circuits have some resistance, so they warm when there is a current, when a current passes through a resistor, energy is transferred because electrical work is done against the resistance, the energy is transferred by heating and the resistor becomes warm
  • Energy in a circuit is transferred to heat as electrons collide with ions in the metal lattice, this is useful if you want the device to get hot, but if you don’t, the energy is dissipated to the surroundings and wasted
  • Power is the amount of energy transferred each second
  • Inside a resistor, as electrons flow through the lattice of vibrating ions, they collide with ions, the more collisions they make with the ions, the harder it is for them to pass through, so the higher the electrical resistance; when the electrons collide with the ions, they transfer energy
  • Resistance in circuits can be reduced by using wires made from metals with low resistance (copper), thicker wires also have lower resistance; resistance can also be decreased by cooling metals so that the lattice ions are not vibrating as much
  • Energy transferred (J) = current (A) x potential difference (V) x time (s)                                        E = I x  V x t
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CP9g Power

  • The energy transferred by an electric current depends on the time taken, so it is often more useful to compare the power of the appliances
  • Power is the energy transferred per second, it is often shown on appliances as the power rating
  • Power is measured in watts (W), 1 W is a transfer of 1 J per second
  • Power (W) = energy transferred (J) / time taken (s)                                                                        P = E / t
  • The power transfer in a component or appliance is proportional to the potential difference across it and the current through it
    • Electrical power (W) = current (C) x potential difference (V)                                      P = A x V
    • Electrical power (W) = current^2 (I) x resistance (Omega)                                        P = A x R
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CP9h Transferring energy by electricity

  • Appliances that need a large amount power use mains electricity:
    • in a power station energy is transferred from a store of kinetic energy (e.g. a turbine), the electricity is carried to our homes through a network of wires and cables known as the national grid
  • In direct current, electrons all flow in one direction, as they move through a component, they deliver the energy they received from the power supply
  • In alternating current, electrons vibrate, changing direction many times per second, electrons pass on their energy to the next electron in a chain until it is delivered to the component
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CP9i Electrical safety

  • A 3-pin plug has designs to safely connect appliances to ains electricity:
    • Earth Wire (green and yellow) connects the case of the device to the Earth for safety
    • Neutral Wire (blue, 0V) connects the device to the power station
    • Fuse – melts if the current is too high
    • Live Wire (brown, 230V) connects the device to the power station
  • Switches - connected in live wire of a circuit; when off, no current goes through
  • A fuse is a tube with a thin wire inside; the current passes through the wire and the wire gets hotter, if it gets too hot, the wire melts which breaks the circuit and stops the current
  • If a faulty appliance draws too much current, it can cause overheating of the wiring in either the walls or in the appliance, this can cause fires; a fuse can stop this from happening
  • A fault causing live wire to touch a metal part makes a low resistance circuit; causing a very large current to flow to earth heating wire causing the current to blow fuse and cut off mains
  • Circuit breakers - alternative to fuses; detects changes in current, turns off supply
    • Advantage: once a fault is fixed they can be switched back on again whereas a fuse has to be replaced
    • Advantage: some work very quickly so saves lives while fuse takes a while to melt and will not prevent a shock
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