p2 electricity

• Current


  The flow of electrical charge
• Potential difference (voltage)


  The driving force that causes the charge to flow around a circuit
• Resistance


  Anything that slows the flow of current down
• The current flowing through a component

  Depends on the potential difference across it and the resistance of the component
• The greater the resistance across a component


  The smaller the current that flows (for a given potential difference across the component)
• Investigating the factors affecting resistance

  1. Attach crocodile clip to wire at 0 cm

  2. Attach second crocodile clip, e.g. 10 cm away

  3. Close switch, record current and potential difference

  4. Open switch, move second clip, e.g. another 10 cm

  5. Repeat for different wire lengths

  6. Calculate resistance for each length using R=V/I

  7. Plot graph of resistance against wire length
• The graph should be a straight line through the origin, meaning resistance is directly proportional to length
• If the graph doesn't go through the origin, it could be due to a systematic error in the first clip not being at exactly 0 cm
• Q = IT

  charge flow (coulombs,C) = current (A) x time (s)
• the unit for resistance

  ohm
• voltage unit

  volt
• V = IR


  potential difference = current x resistance
• ammeter

  measure current, in amps

  must be placed in series
• voltmeter

  measures potential difference, volts

  must be placed in parallel
• ohmic conductors have a constant resistance
• at a constant temperature, the current flowing through an ohmic conductor is directly proportional to the potential difference across it
• ohmic conductors that don’t change with current

  wire or a resistor
• resistors and components that do change

  a diode or a filament lamp
• ohms law


  V = IR
• Linear components

  • Have an I-V characteristic that's a straight line (e.g. a fixed resistor)
• Non-linear components

  • Have a curved I-V characteristic (e.g. a filament lamp or a diode)
• Experiment to find a component's I-V characteristic

  1. Set up test circuit

  2. Begin to vary the variable resistor

  3. Take several pairs of readings from ammeter and voltmeter

  4. Swap over the wires connected to the battery

  5. Plot a graph of current against voltage for the component
• 1. V characteristics for an ohmic conductor, filament lamp and diode

  • Ohmic conductor: straight line
  • Filament lamp: curve gets shallower
  • Diode: current only flows in one direction
• LDR

  light dependent resistor
• LDR

  • dependent on the intensity of light
  • bright light, resistance falls
  • darkness, resistance is highest
  • uses as automatic night lights, outdoor lighting and burglar detectors
• thermistor

  • temperature dependant resistor
  • hot conditions, resistance drops
  • cool conditions, resistance goes up
  • make useful temperature detectors eg electronic thermostats
• series circuits

  if you remove one component, the circuit is broken
• (series) potential difference is shared

  V (total) = V1 + V2
• (series) current is the same everywhere

  I1 = I2 = I3
• (series) resistance adds up

  the bigger a components resistance , the bigger it’s share of the total potential difference

  R (total) = R1 + R2
• parallel circuits

  if you remove a component , the others a hardly effected
• (parallel) potential difference is the same across all components

  V1 = V2 = V3
• (parallel) current is shared between branches 


  I (total) = I1 + I2
• parallel circuits bulbs have the same brightness due to same potential difference 


  yes
• ac

  alternating current
• dc 


  direct current
• in ac supplies the current is constantly changing direction

  alternating currents are produced by alternating voltages in which the positive and negative ends keep alternating
• the UK mains supply is an ac supply at around 230 V
• the frequency of the ac mains supply:

  50 cycles per second or 50Hz (hertz!)
• cells and batteries supply direct current