electricity

• Electric current


  The rate of flow of electrical charge (which requires a potential difference)
• Charge flow, current and time

  1. Charge flow = current × time

  2. Q = I t
• Charge flow, Q

  In coulombs, C
• Current, I


  In amperes, A (amp is acceptable for ampere)
• Time, t


  In seconds, s
• Potential difference (Voltage)

  The energy per unit charge
• Voltage


  1. Voltage = Energy / Charge

  2. V = E/Q
• Energy, E


  In Joules, J
• Voltage, V

  In Volts, V
• Resistance


  The opposition to the flow of charge
• Potential difference, current or resistance

  • Potential difference = current × resistance
  • V = I R
• Potential difference, V


  In volts, V
• Resistance, R


  In ohms, Ω
• Measuring resistance


  1. Use either a voltmeter and ammeter and use the equation V = I R

  2. Use a multimeter
• Ohmic Conductor (metal or resistor at constant temperature)


  • The current through an ohmic conductor (at a constant temperature) is directly proportional to the potential difference across the resistor
  • The resistance remains constant as the current changes
  • A conductor with a lower resistance lets more current through for the same voltage so has a steeper gradient (linear graph)
• Filament Lamp


  • As the current increases, so does the temperature
  • This causes ions in the filament to vibrate faster and hence there are more frequent collisions with the electrons so a higher resistance
  • The gradient becomes lower as the same increase in voltage does not increase the current as much (non linear graph)
• Diode


  • The current through a diode flows in one direction only (forward bias)
  • The diode has a very high resistance in the reverse direction (reverse bias)
• LDR and Thermistor


  • The resistance of a thermistor decreases as the temperature increases
  • The resistance of an LDR decreases as light intensity increases
  • Both graphs are non linear
• LDR circuit


  • As the light intensity increases, the resistance of the LDR decreases
  • This causes the current flowing to increase as the cell's voltage is fixed
  • The LDR now has a lower resistance than before, it takes a smaller share of the voltage which is shared between the resistor and LDR as they are in a series
• Series circuits

  • There is the same current through each component
  • The total potential difference of the power supply is shared between the components
  • The total resistance of two components is the sum of the resistance of each component
• Parallel circuits

  • The potential difference across each component is the same
  • The total current through the whole circuit is the sum of the currents through the separate components
  • The total resistance of two resistors is less than the resistance of the smallest individual resistor
• Mains electricity is an ac supply and in the United Kingdom it has a frequency of 50 Hz and is about 230 V
• Most electrical appliances are connected to the mains using three-core cable
• Live wire


  Carries the alternating potential difference from the supply and is at a pd of 230V compared to the Earth (0V)
• Neutral wire


  At or close to 0V, completes the circuit
• Earth wire


  A safety wire to stop the appliance becoming live and only carries a current if there is a fault
• If someone was to touch the live wire at 230V then since their potential difference is 0V, there is a large potential difference between the two so a large current may flow which could be dangerous
• For objects with a metal case, this can become live and therefore if you touch it you could get an electric shock
• Electrical Power


  1. Power = potential difference × current

  2. P = V I

  3. Power = current2 × resistance

  4. P = I2R
• Energy transferred by electrical work

  1. Energy transferred = power × time

  2. E = P t

  3. Energy transferred = charge flow × potential difference

  4. E = Q V
• Energy transferred, E


  In joules, J
• Power, P


  In watts, W
• The National Grid


  • Step-up transformers are used to increase the potential difference from the power station to the transmission cables
  • Step-down transformers are used to decrease, to a much lower value (230V), the potential difference for domestic use
• Ratio of the potential differences across the primary and secondary coils of a transformer


  Vp/Vs = np/ns
• Static electricity


  • When certain insulating materials are rubbed against each other they become electrically charged
  • Negatively charged electrons are rubbed off one material and on to the other
  • The material that gains electrons becomes negatively charged
  • The material that loses electrons is left with an equal positive charge
  • Rubbing conductors does not allow a charge to build up
• Two electrically charged objects are brought close together

  • They exert a force on each other
  • Two objects that carry the same type of charge repel
  • Two objects that carry different types of charge attract
• Electric Fields


  • A charged object creates an electric field around itself
  • The electric field is strongest close to the charged object
  • The further away from the charged object, the weaker the field
  • A second charged object placed in the field experiences a force
  • The force gets stronger as the distance between the objects decreases
  • Field lines show the way a positive charge would move if placed in the field at that point