topic p2- electricity

• current

  flow of electrical charge (amps)
• potential difference

  driving forces that pushes charge around (volts)
• resistance

  anything that slows down the charge flow (ohm)
• current

  • the greater the resistance, the smaller the current
  • the greater the potential difference, the larger the current
• size of the current=

  rate of flow of charge
• current and circuit symbols

  
• unit of resistance

  ohm
• unit of current

  amps
• unit of P.D

  volts
• current

  is the flow of electrical charge
• electrical charge will only flow around a complete (closed) circuit if there is a potential difference
• in a single closed loop the current has the same value everywhere
• potential difference is the driving force that pushes the charge round
• resistance is anything that slows the flow down
• the greater the resistance across a component, the smaller the current that flows
• charge=

  current x time
• circuit symbols:

  
• resistance and V=IR

  
• resistance and V=IR

  1. attach a crocodile clip to the wire level with 0cm on the ruler
  2. attach the 2nd crocodile clip to the wire 10cm away from the first clip
  3. close the switch, then reread the current through the wire and the pd across it
  4. open the switch, then move the 2nd crocodile clip another 10cm along the wire- record the new length, current, and pd
  5. repeat this for a number of different lengths across the wire
  6. use your measurements to calculate the resistance R=V/I
  7. plot a graph of resistance against wire length and draw a line of best fit
• ammeter

  • measures current (in amps) the wire flowing through
• voltmeter

  • measures potential difference across the wire (in volts)
• diode:

  current will only flow through a diode in one direction as shown; the diode has very high resistance in the reverse direction

  
• ohmic conductor

  the current through an ohmic conductor (at constant temp) is directly proportional to potential difference so you get a straight line

  
• filament lamp

  as the current increases, the temperature of the filament increases, so the resistance increases; this means less current can flow per unit pd so the graph gets shallower- hence the curve

  
• LDR is short for light dependent resistor

  • a resistor dependent on light
  • in bright light, the resistance falls
  • in darkness, the resistance is highest
• the resistance of a thermistor depends on temp

  • a thermistor is a temperature dependent resistor
  • in hot conditions, the resistance drops
  • in cool conditions, the resistance goes up
• LDR- light dependent resistor

  
• The resistance of a thermistor depends on temp

  
• series circuit

  
• V=

  potential difference
• I=

  current
• R=

  resistance
• series circuit

  • the different components are connected in a line, end to end, between the +Ve and -Ve of the power supply
  • if you remove or disconnect one component, the circuit is broken and they all stop
• P.D round a series circuit always add up to equal the source of P.D

  V total= V1 + V2 + ...
• some current flows through all components

  I 1 = I 2 = ...
• the total resistance of 2 components is just the sum of their resistances

  R total = R1 + R2
• parallel circuit

  
• parallel circuits

  • each component is separately connected to the +Ve and -Ve of the supply
  • if you remove or disconnect one of them, it won't affect the others
• all components get the full source of P.D, so the potential difference is the same across all components

  V1 = V2 = ...
• the total current flowing around the circuit is equal to the total of all the currents through the separate components

  I total = I 1 + I 2 + ...