Electricity

• Charged particles


  Have an electric field, positively charged particles have a field going away from themselves, negatively charged particles have a field going into themselves
• Interaction of charged particles

  • Like charges repel, opposite charges attract
• Charged particles


  Relate to electricity, can be observed through static electricity
• Atoms


  Have positively charged protons and negatively charged electrons in shells
• Transferring electrons


  Rubbing balloons on jumper, cloth on rod, can lead to electric shocks
• Current


  The flow of charge
• Voltage


  The push that drives the flow of charge around a circuit, like the steepness of a river pushing water
• Potential difference


  The difference in charge between two points that causes current to flow
• Resistance


  The opposition to the flow of charge, like obstacles in a river slowing the flow of water
• Current is often measured in milliamps or microamps
• Circuit symbols


  • Lamp
  • Cell
  • Resistor
  • Thermistor
  • Variable resistor
  • Light dependent resistor
  • Diode
  • Light emitting diode
  • Ammeter
  • Voltmeter
  • Alternating current supply
  • Open switch
  • Closed switch
• Factors affecting resistance


  • Width of wire
  • Type of wire
  • Temperature
  • Length of wire
• Experiment to investigate effect of wire length on resistance

  1. Set up crocodile clips on wire at 10cm intervals

  2. Measure voltage and current

  3. Calculate resistance using Ohm's law

  4. Repeat for different wire lengths
• Experiment to investigate component characteristics

  1. Set up circuit with voltmeter in parallel and ammeter

  2. Vary resistance using variable resistor

  3. Record voltage and current readings

  4. Flip battery to reverse current

  5. Repeat for different components (resistor, bulb, diode)
• Thermistor


  • Resistance decreases as temperature increases
• Light dependent resistor


  • Resistance decreases as light intensity increases
• Series circuit


  • Only one path for current, if one component breaks the whole circuit is affected
• Thermistors


  Devices used in appliances where the amount of current going through the appliance is really important and dependent on the temperature
• Light dependent resistor


  A device where the higher the light intensity, the lower the resistance
• Appliances that use light dependent resistors

  • Security lights
  • Street lamps
  • Alarm clocks
• Series circuit


  • Only one way for current to flow
  • Only one loop in the circuit
• If one light bulb breaks in a series circuit

  The other light bulb will also go out
• Current in a series circuit

  Always the same throughout the circuit
• Adding more components to a series circuit

  Reduces the current readings on the ammeters
• Increasing components or cells in a series circuit

  Increases the current readings on the ammeters
• Voltage in a series circuit

  Splits up between the components
• Resistance in a series circuit

  Adds up
• Parallel circuit

  • Current splits between the branches
  • Voltage is the same down each branch
• Calculating current in a parallel circuit with unequal resistors

  1. Use Ohm's law to calculate current in each branch

  2. Add up the branch currents to get the total current
• Resistance in a parallel circuit

  1/Rtotal = 1/R1 + 1/R2 + ...
• Alternating current (AC)


  • Continuously changing direction
  • Produces a sine wave on an oscilloscope
  • Mains electricity has a frequency of 50Hz and a maximum potential difference of 230V
• Direct current (DC)


  • Only flows in one direction
  • Produces a straight line on an oscilloscope
  • Supplied by batteries
• Plug


  • Contains a live wire (230V), neutral wire (0V), and earth wire (safety feature)
  • Cable clamp to keep wires in place
  • Insulators around copper wires
• Earth wire


  Provides a low resistance path for current to flow if a live wire touches the metal casing of an appliance
• Fuse

  High resistance wire that breaks the circuit if current gets too high, preventing damage
• Electricity generation and transmission

  1. Generated at power station

  2. Step-up transformer increases voltage for long distance transmission

  3. Pylons carry high voltage electricity

  4. Step-down transformer decreases voltage for household use
• Power rating

  Measures the amount of energy transferred per second by an appliance
• Power equation


  Power = Voltage x Current

  Power = Current^2 x Resistance
• Energy equation


  Energy = Charge x Voltage
• Efficiency

  Useful power / Total power