Electricity

• What is electric current?


  The flow of electrical charge
• State the equation linking charge, current and time


  Q = I T

  charge (coulombs) = current (amperes) x time (seconds)
• In a single closed loop at any point, the value of current is the...


  SAME

  current is the same at any point in a single closed loop
• What are the two factors that current depends on?


  Potential difference (voltage)

  Resistance

  -remeber PR, because what to celebrities rely on? PR (potential difference and resistance)
• If current and resistance are known, what is the formula for the potential difference?


  V = I R

  potential difference = current x resistance
• What is an ohmic conductor?


  A conductor in which current and potential difference are directly proportional, resistance remains constant as current changes, but the temperature must be constant.
• Which four components resistance is not constant as the current changes?

  Lamps

  Diodes

  Thermistors

  LDR's
• What happens to the resistance of a filament lamp as the temperature increases?


  Resistance increases, ions in the metal have a high amount of free ions, with high amounts of energy which vibrate more as a current is passed through them, this causes more collisions with the electrons as they flow through the metal, creating greater resistance to current flow
• What is different about current flow through a diode?


  It only flows through one direction, as resistance is very high in the other direction preventing current flow
• What happens to the resistance of the thermistor as the temperature increases?


  The thermistors resistance decreases
• Give two examples of thermistor use


  In a thermostat to turn on a heater below a certain temperature

  In a freezer to turn on a cooler when the temperature becomes too hight
• What happens to the resistance of the LDR as light intensity increases?


  The LDR's resistance increases
• What is one application of an LDR


  Street lights, when the light level is too low, the light gains sufficient current to turn it on!
• In what two ways that a component can be connected to a circuit?

  In series (same loop)

  In parallel (adjacent loop)
• How does the potential difference vary across components when connected in series and parallel?


  SERIES: total PD is shared between each component

  PARALLEL: PD is same across all components
• If two resistors are connected in parallel what can be said about their combined total resistance?


  The total resistance is LESS than the smallest of the individual resistences (SUBTRACT)
• If two resistors are connected in series, what can be said about their total resistance?


  The total combined resistance is the SUM of the two individual resistances (ADD)
• Describe current in a series circuit


  In a series circuit, the current is the same at all positions, since it has only one path to flow through
• Describe the current in a parallel circuit


  In a parallel circuit, the current is shared between the branches, when a charge reaches a junction it splits
• How do you connect an ammeter to the circuit to measure current?


  Ammeters should be in series to the component they are measuring current through
• How should you connect a voltmeter to the circuit to measure potential difference?


  Voltmeters should be connected in parallel to the component they are measuring the potential difference of
• Why is it advantageous to connect lamps in parallel?


  Because if one lamp blows, the rest will be unaffected and can still receive current as the circuit is still complete!
• Is the mains supply A.C or D.C?


  Mains electricity is A.C

  A.C - alternating current

  D.C - direct current
• Define alternating and direct current


  Alternating current - current that continuously changes direction at a specific frequency

  Direct current - one directional flow
• What is the frequency and voltage of the UK mains supply? 


  FREQ : 50Hz

  VOLTAGE : 230V
• Name the three wires typically found in a plug

  Live - brown

  Neutral - blue

  Earth - green and yellow stripes
• Why does the earth wire not carry a current?


  Usually the earth wire does not carry a current, it is sort of a safety measure, for example if a fault occurs in the applience (IE. surge or case becoming live) the current will flow to the earth/ground and not cause harm
• What potential is the neutral wire at?


  0V
• State the potential difference between live wires and earth wires


  230V
• What is the purpose of the neutral wire?


  To complete the circuit by connecting the appliance back to the mains supply
• State two equations for power in a circuit


  P = I V

  (power = current x potential difference)

  P = I^2 R

  (power = current ^2 x resistance)
• State an equation linking energy transfered, power and time


  E = P T

  (energy = power x time)
• State the equation linking energy transfered, charge flow and potential difference


  E = Q V

  (energy = charge x potential difference)
• What two main factors does the amount of energy transferred by an appliance depend on?


  How long the appliance is being used for

  The power of the applience
• Describe the energy transfered of a battery powered torch

  Battery converts chemical energy into electrical energy

  Bulb converts electrical energy into light as well as the waste energy in the form of heating
• Describe the energy transfers in a battery powered motor

  Battery converts chemical energy into electrical energy

  Motor converts electrical energy into kinetic energy as well as waste energy in the form of heating due to friction
• What three things determine the power of a circuit device?

  -potential difference

  -current

  -amount of energy transferred
• What is the purpose of the national grid?


  To link powerstations to consumers so they have access to a source of electricity
• What are the two types of transformers used by the national grid?


  Step-up transformers

  Step-down transformers
• Where are the step up transformers in the national grid and what is their function?

  Step up transformers are used when connecting power stations to transmission cables

  They increase the potential difference