P2

• Current:

  • Flow of electrical charge
  • measured in amperes (A)
  • in a single Circuit, current is the same all the way around
  • In a parallel circuit, current varies in the different branches
• Potential difference:

  • Driving force that pushes the charge around
  • unit is volts (V)
  • Always the same throughout both Circuits
• Resistance

  • Any things that sIow the flow down
  • unit: Ohm
  • increases in a series circuit when more things are added
  • decreases in a parallel circuit when more things are added
• total Charge through a circuit depends on current and time

  Q=IXT
• Power = Current x Potential Difference
• Ammeter:

  • measures the current (in amps) flowing through the test wire
  • must always be placed in series to what you are investigating
• Voltmeter

  • measures the potential difference across the test wire
  • must always be placed parallel around what you are investigating
• The greater the resistance, the less electricity flows through it.
• The current through an ohmic conductor is directly proportional to potential difference so its graph is a straight line
• Filament lamp

  As the current increases, the temperature Of the filament increases, so the resistance increases. this means less current can fIow per unit pd, so the graph is a curve
• Diode:

  Current will only flow though the diode in one direction, as shown. The diode has very high resistance in the reverse direction
• LDR:

  1. An LDR is a resistor that is dependent on the intensity of light.
  2. In bright light, the resistance falls
  3. In darkness, the resistance is highest
  4. They can be used in automatic night lights, outdoor lighting and burglar alarms
• Thermistor

  1. temperature dependant resistor
  2. in hot conditions, the resistance drops
  3. In cool conditions the resistance goes up
  4. They make useful temperature detectors, e.g. car engine sensors and electronic thermostats
• LDRs and thermistors

  • Can be used in sensing circuits, e.g. a fan
• Sensing circuit with fixed resistor and fan

  Fixed resistor and fan always have the same potential difference across them because they are connected in parallel
• Potential difference sharing in sensing circuit

  1. The pd of the power supply is shared out between the thermistor and the loop made up of the fixed resistor and fan according to their resistance

  2. The bigger a component's resistance, the more potential difference it takes
• As the room gets hotter

  • The resistance of the thermistor decreases
  • The thermistor takes a smaller share of pd from the power supply
  • The pd across the fixed resistor and fan rises
  • The fan goes faster
• Series circuits:

  • components are connected in a line, end to end (exept for voltmeters which are always connected in parallel but they don't count as part of the circuit )
  • Potential difference is Shared

  total V= VI+ V2 + ...

  • current is the same everywhere

  The size of current is determined by the total pd of the cells and total resistance of the Circuit

  | = v÷ R

  • Resistance adds up

  the total resistance of two components is Just the sum of their resistances
• Parallel Circuits:

  • Each component is seperately connected
  • If you remove or disconnect something, it won't affect the others
  • Potential difference is the same
  • current is shared between branches
  • adding a resistor in parallel reduces the total resistance

  By adding another loop the current has more than one direction to go in, which increases the total current that can flow around the circuit
• power ratings:

  • labelled with the maximum safe power
  • tells you the maximum amount of energy transferred between stores per second
  • lower the power rating, the less electricity an appliance uses and the cheaper it is to run