Electricity

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  • Created by: holly6901
  • Created on: 22-05-19 11:34

Electric charge

Current

  • An electric current is the flow (movement) of electric charges.
  • Electric current is measured in amperes (A). Current has the same value at any point in a single closed loop.
  • The flow of charge (C) = current (A) x time (s).
  • A current has the same value at every point in a single closed loop.

Conductors and insulators

  • An electrical conductor is material that allows electric currents (flow of electric charges) to pass through it.
  • All metals are electrical conductors.
  • An electrical insulator is a material that doesn’t allow electric currents (flow of electric charges) to pass through it.
  • Plastic and glass are electrical insulators.
  • An electrical conductor has lots of charges that are free to move.
  • In a metal, the charges that are free to move are electrons.
  • The electric current through the metal is the flow of these free electrons.
  • An electrical insulator is a material that doesn’t allow electric currents (flow of electric charges) to pass through it.
  • Plastic and glass are electrical insulators.

Potential difference/voltage

  • Potential difference (p.d.) is measured in volts (V) and is also called voltage.
  • The energy is transferred to the electrical components in a circuit when the charge carriers pass through them.
  • We use a voltmeter to measure potential difference (or voltage).
  • A potential difference of one Volt is equal to one Joule of energy being used by one Coulomb of charge when it flows between two points in a circuit.
  • Voltmeters measure potential difference (voltage) between two points in a circuit. For example between two points either side of component X above.
  • Voltmeters must always be connected in parallel (on a separate branch of the circuit) with the two points being measured.

Ammeters

  • An ammeter measures the flow of current that passes through it.
  • Ammeters have to be connected in series (in the same loop of the circuit) with the electrical component whose current you are measuring. 
  • The current is a flow of charge. Current is measured through a component.
  • Potential difference is the energy used between two points in a circuit, therefore it is measured between two points either side of a component.
  • We describe this as the potential difference measured across a component.

Resistance

  • Resistance is measured in ohms, Ω.
  • When charge flows in an electric circuit, the size of the current is affected by two things:
    1. The resistance: If the resistance is increased, the current will decrease.
    2. The potential difference: If the potential difference is increased the current will increase.
  • To measure the resistance of a component (e.g. lamp or resistor), measure the potential difference across the component.
  • Use an ammeter to measure the current flowing through a component.
  • The longer the wire, the higher the resistance.
  • The current through an ohmic conductor (at a constant temperature) is directly proportional to the potential difference across the resistor.
    • This means that the resistance remains constant as the current changes.

Current-voltage graphs

  • We can plot a current-voltage graph by varying the potential difference across a circuit component.

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