GCSE P2 Chapter 4

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  • Created by: emma998
  • Created on: 09-03-14 14:00

Electrical charges

  • Charging by friction = When two electrically insultaing materials are rubbed together, electrons are rubbed off one of the materials and deposited on the other. The material which loses electrons becomes positively charged and the other gains electrons and becomes negatively charged.
  • Materials that have opposite charges attract
  • Materials that have like charges repel
  • The larger the distance between the two charged objects the weaker the force of attraction
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Electric circuits

  • Every component in an electric circuit has its own symbol
  • Switch = Can be opened or closed to enable the current to be switched on or off
  • Cell = Necessary to push electrons round the circiut, a battery is made up of 2+ cells
  • Ammeter = Measures the current through a component, is always placed in series
  • Voltmeter = Measures potential difference across component, placed in parallel
  • Diode = Allows current through in 1 direction only, high resitstance in the other direction
  • Indicator = Emits light when current passes through as signal or used as a light source
  • Light emitting diode = Emits light as a current passes through it
  • Heater = Transfers electrical enrrgy into heat energy as current passes through it
  • Fixed Resistor = Limits the current to a certain value
  • Variable resistor = Allows the current to be varied
  • Fuse = Melts and breaks the circuit if the current exceeds a specific value
  • Current(Amps(A)) = charge(Coulombs(C)) / time(Seconds(S))
  • I = Q / T
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Resistance and potential difference

  • Voltmeters measure the potential difference across a component and are always placed in parallel with the component
  • Potential difference(V) = work done(J) / charge(C)
  • Potential difference(V) = energy transfered(J) / charge(C)
  • Potential difference(V) = resistance(O) X current(A)
  • Resistance is measured in ohms and is a measure of how difficult it is for the electrons to pass through a component. It is the opposition to current flow
  • Resistance(O) = Potential difference(V) / Current(A)
  • Current-potential difference graphs show the relaionship between the current and potential difference through a compnent
  • Ohm's law states that the current through resistor at a constant temperature is diectly proportional to the potential difference across it.
  • Components which obey this law are said to be Ohmic Conductors 
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Current-potential difference graphs

Filament bulb - current is not directly proportional to potential differnce across it. Because:

  • As the current through it increases
  • The temperature increases
  • So resistance increases
  • So potential difference increases at a greater rate than the current ( V = I X R )

Diode - Current through it is not proportional to potential difference bcause:

  • The current through a diode can flow in one direction only
  • At a negative potential difference current is always zero because resistance in opposite direction is extremley high 
  • As soon as potential difference is positive current increases much more than potential difference

Light dependant resistor - As light falling on it increases, the resistance decreases so in parallel circuits, current increases at faster rate than potential difference across it

Thermistor - As temperature increases the resistance of component decreases

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Series circuits

  • In a series cricuits all components are connected one after the other
  • If there is a break anywhere in the circuit, charge stops flowing
  • In a series circuit current is the same through each component (only one path for charge to flow) (Current depends on pd of power supply and total resistance of circuit - I = V/R) 
  • The larger the resistance the smaller the current
  • Resistance of the circuit is found by adding up the resistance of each component
  • Potential difference of each compnent is shared by each component (the components with larger resistances get a larger potential difference)       
  • (V = I X R) - Larger resistance, larger potential difference
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Parallel circuits

  • In parallel circuits each component is connected across from the supply
  • There are several different routes which the charge can flow in
  • If there is a break in one area of the circuit then charge will continue to flow just to other areas of the circuit

The current through each component varies:

  • Charge has a choice of routes
  • The routes with a high resistance will have a low current through the components
  • Components which have a low resistance will have a much higher curent flowing through them (I = V/R) - The smaller the resistance the larger the current
  • The total current of circuit can be found by adding up current through each component

The potential differnce across each component is the same:

  • All components placed in parallel to the power supply
  • So potential difference is the same across each component

Total resistance of the circuit is found by adding up the resistance of each component

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