electricity (p2)

• Charge and current

  • For charge to flow in a closed circuit there must be a source of potential difference
  • Current is a flow of electrical charge
  • Current flows from positive to negative terminals
• Charge

  Measured in Coulombs
• Current

  Measured in Amps

  flow of electrical charge
• Potential difference (voltage)

  • Measure of the energy given to the charge carriers in a circuit
  • Potential difference = current & resistance
• Investigating length of wire affecting resistance

  1. Set up simple circuit

  2. Attach wire along metre ruler

  3. Attach crocodile clips at different points

  4. Record current and voltage

  5. Calculate resistance at each point

  6. Plot graph
• Zero errors and systematic errors can affect measurements
• Heating effect


  As temperature of wire increases, resistance increases
• Investigating resistance within a circuit

  1. Construct series circuit, record voltage and current

  2. Construct parallel circuit, record voltage and current

  3. Calculate total resistance for each circuit
• Resistance

  • Opposition to the flow of charge
  • Measured in Ohms
• Series circuits

  • Same current through each component
  • Total potential difference is shared
  • Resistance of components = sum of resistance of each component
• Parallel circuits

  • Potential difference is same across components
  • Current is shared across components
  • More components = lower total resistance
• Mains electricity

  • AC supply
  • Frequency - 60Hz
  • Voltage - 230V
• Wires

  • Live wire - brown
  • Neutral wire - blue
  • Earth wire - green/yellow
• Fuses

  • Thin piece of wire connected to live wire
  • Heats up and melts when current increases, breaking the circuit
• Circuit breakers

  Turn off current when a surge happens, can be reset
• Earthing

  Earth wire provides alternative pathway for electricity, rather than through a person
• Double insulation

  Appliance covered in plastic coating that cannot conduct electricity, no electric shocks
• Energy transfer

  Energy transferred = power x time

  Energy transferred = charge x potential difference
• National grid

  • System of cables and transformers linking power stations to consumers

  Step up transformers increase voltage, decrease current to reduce energy loss

  Step down transformers lower voltage back down, increase current for domestic use
• Static electricity

  Build up of charge on insulating materials

  Friction causes electrons to be rubbed off one object and onto another

  Conducting materials allow electrons to move back, no charge builds up

  Large potential difference can cause a spark
• Electric fields

  • Strongest closest to charged particles

  Oppositely charged particles' fields interact attractively
• Strongly charged electric fields can cause air particles to lose electrons and become positive ions (ionisation)
• IV characteristics - required characteristics

  1.build the circuit

  2. set the variable power supply or variable resistor to the lowest setting for PD

  3. record the current and voltage over the resistor

  4. increase current from the power supply by 2V and repeat your readings

  5. change resistor to a filament lamp and repeat the experiment

  6. change the lamp to diode and protective resistor

  7. change the ammeter to a milliammeter

  8. plot a graph of current against PD for each component
• alternating current 


  -direction of current changes

  -use an alternative potential difference
• direct current

  -either positive or negative

  -charge is always flowing in the same direction
• live wires

  carries the alternating potential difference from mains
• neutral wire

  completes circuit by carrying away current
• earth wire 


  safety wire to stop appliance from becoming live
• electric shocks

  • Difference in electricity voltage that causes electricity to pass from one to another
  • Humans have a potential difference of 0v, if we touch a live wire it would cause a large current to flow through the body
• fixed resistor 


  limits the amount of electrical current

  
• variable resistor 


  adjusts the resistance in a circuit and decreases current

  
• LED - light emitting diode 


  have a low resistance in one direction and emits light when a current passes through it

  
• diode 


  only allows current to flow in one direction

  
• thermistor 


  resistance decreases and temperature increases

  
• LDR - light dependent resistor 


  the greater the intensity of light, the lower the resistance

  
• fixed resistor 


  temp must be constant

  assumed to stay constant

  
• filament lamp 


  resistance increases as temperature increases

  
• diode 


  current only flows through in one direction

  very high temperature in the reverse direction

  
• live wire


  brown

  230v

  carries alternating pd from the supply
• neutral wire 


  blue

  0v

  completes the circuit