topic 2

• current is the flow of electric charge around a circuit
• in a single closed loop circuit current is the same everywhere
• potential difference is the driving force that pushes charge around the circuit and is measured in volts
• the greater resistance across a component the smaller the current flows through it
• size of current is the rate of flow of charge
• more charge passes around the circuit when a bigger current flows
• resistance doesnt change with current
• at a constant temperature current and potential difference are directly proportional
• ammeter measures the current in a circuit and is connected in series with the component being measured
• voltmeter measures the pd across a component and is connected in parallel to the component being measured
• a voltmeter has high internal resistance so that no current flows through it
• an ammeter has low internal resistance so that most of the current flows through it
• investigating resistance

  1. attach a crocodile clip to wire parallel to 0cm on the ruler
  2. attach a second clip to the wire 10cm away
  3. close the switch and record current and potential difference
  4. open the switch and move the clip another 10cm away
  5. repeat
  6. use R =V/I
  7. Draw a graph the line should go straight through 0 if it doesn't the clip hasn't being at 0cm
• practical

  1. set up a test circuit
  2. begin to vary variable resistors
  3. take several readings from ammeter and voltmeter to see how potential difference across components varies with current change
  4. repeat and find average
  5. swap over wires connected to cell and repeat
• in an ohmic conductor potential difference and current are directly proportional
• in a filament lamp as current increases, resistance decreases, so the temperature increases
• in a diode current only flows through in 1 direction as it has high resistance in other direction
• an LDR is a resistor dependant on light intensity
• a thermistor is a temperature dependent resistor
• bigger potential difference when there are more cells
• potential difference is added up across all components in a circuit, and is measured in volts
• total potential difference is shared between components in a series circuit, so the total potential difference is the same as the potential difference across each component
• the same current flows through all the components
• the size of the current is determined by the pd and the resistance
• total resistance is the sum of all the resistors in a circuit
• voltage same across all component
• increase in current = decrease in total resistance
• resistor in series

  1. build circuit with 1 resistor
  2. measure current and calculate resistance using R = V / I
  3. add another resistor
  4. measure current and calculate overall resistance
  5. repeat
• resistors in parallel

  1. use same circuit as for series circuit and calculate resistance
  2. add a resistor in parallel
  3. measure the total current through the circuit
  4. calculate overall resistance using current and pd
• adding a resistor in series increases the total resistance
• adding a resistor in parallel total current is increased so total resistance decreases
• alternating current supply is current that changes direction at regular intervals, it is used in domestic appliances
• ac is produced by altering pd
• in a direct current current is always flowing in one direction, the electrons flow in the same direction as the electric field
• a neutral wire is blue
• an earth wire is green and yellow
• a live wire is brown
• a neutral wire is a wire that has no charge and is used to carry electricity safely
• a live wire is a wire that carries a potential difference from the mains supply to the appliance
• an earth wire is a wire that is connected to the earth terminal of a plug