Electricity

• Electric current


  The rate of flow of electrical charge (which requires a potential difference)
• Potential difference


  The energy per unit charge
• Resistance


  The opposition to the flow of charge. The greater the resistance of the component, the smaller the current for a given potential difference (pd) across the component. It arises from collisions between the charges (electrons) and the ions.
• Measuring resistance


  1. Use either a voltmeter and ammeter and use the above equation or a multimeter

  2. Set up the circuit to the right and adjust the variable resistor to get pairs of readings

  3. Turn the current off between readings as an increase in temperature can lead to a change in resistance
• Ohmic Conductor


  • 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
  • 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 – when current arrow matches diode arrow)
  • The diode has a very high resistance in the reverse direction (reverse bias – when current arrow and diode arrow are in opposite directions) (non linear graph)
• Thermistor


  • The resistance of a thermistor decreases as the temperature increases so they can be used in circuits which depend on temperature – e.g. thermostats
• LDR


  • The resistance of an LDR decreases as light intensity increases so could be used for example to switch lights on when it gets dark
• Example LDR circuit


  1. As the light intensity increases, the resistance of the LDR decreases

  2. Therefore the resistance of the whole circuit decreases

  3. This causes the current flowing to increase as the cell's voltage is fixed

  4. Since 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 series
• Series


  One single loop
• Parallel


  More than one loop – plural
• Components connected in series

  • 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
• Required practical – length affecting resistance


  1. Set up the circuit with the wire taped to a ruler

  2. Set the crocodile clips 10cm apart and measure the voltage and current

  3. Move them 20cm apart and repeat

  4. Continue until you have done 1m of wire

  5. For each length you could adjust a variable resistor in the circuit to obtain several results and find a mean

  6. This will reduce the effect of random errors and enable you to spot anomalies

  7. If you do get anomalous results, discard them before calculating a mean

  8. Calculate resistance using resistance = voltage/current

  9. Keep currents low as large currents lead to heat which could cause burns/wires melting

  10. Turn off the current in between readings to reduce wires heating which could cause inaccurate results
• Components connected in parallel

  • 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. This is because the charges now have a choice of paths to take (or consider two wires as being the same as one wider wire – which would be easier to pass through)
• Mains electricity


  An AC supply in the United Kingdom with a frequency of 50 Hz and about 230 V
• Mains electricity wires


  • Live wire – brown
  • Neutral wire – blue
  • Earth wire – green and yellow stripes
• 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 and 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
• Energy transfers in electrical appliances

  The amount of energy an appliance transfers depends on how long the appliance is switched on for and the power of the appliance
• Work


  Done when charge flows in a circuit
• National Grid


  A system of cables and transformers transferring electrical power which links power stations to consumers
• Transformers in the National Grid

  1. Step-up transformers are used to increase the potential difference from the power station to the transmission cables

  2. This decreases the current reducing the heat energy lost and increasing the efficiency

  3. Then step-down transformers are used to decrease, to a much lower value (230V), the potential difference for domestic use and in doing so increase the current
• Reason a step-up transformer increases the voltage


  The output coil has more turns on it. In fact, if the output has twice as many turns as the input, the voltage doubles.
• Ratio of potential differences across transformer


  Depends on the ratio of the number of turns on each coil, np and ns
• Charging by rubbing


  1. When certain insulating materials are rubbed against each other they become electrically charged

  2. Negatively charged electrons are rubbed off one material and on to the other

  3. The material that gains electrons becomes negatively charged

  4. The material that loses electrons is left with an equal positive charge

  5. If charge builds up, so does the potential difference between the object and an earthed conductor which can lead to a current flowing between the two to 'earth' the charge
• Attraction and repulsion between charged objects

  • Two objects that carry the same type of charge repel
  • Two objects that carry different types of charge attract
  • Attraction and repulsion between two charged objects are examples of non-contact force
• 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