electricty and circuits basics

• current is

  A flow of electric charge around a closed circuit
• potential difference or voltage

  a difference in the electric potentials between 2 points. Charge flows only when it is "pushed" by a potential difference. Symbol is V. Volts are the units
• resistance

  anything that slows current down. (pd cant be slowed)
• V=IR


  Ohm's Law

  pd = current x resistance
• total charge through a circuit depends on

  current and time
• charge measured in

  Coulombs
• Q=It


  charge = current x time
• more charge passes around circuit when

  a bigger current flows
• ohmic conductors have a


  constant resistance at a constant temperature
• ohmic conductor examples

  wire or resistor
• resistance increases with

  temperature
• the resistance of some resistors and components does change:

  eg. filament lamp or diode
• when an electrical charge flows through a filament lamp-

  transfers some energy to thermal energy store of filament, which is designed to heat up, and therefore resists more (resistance increases with temp)
• For diodes the resistance depends on

  direction of current (high resistance if current reversed)
• ammeter

  measures current in wire

  
  

  must always be placed in series with whatever is being investigated
• voltmeter

  measures potential difference (voltage) across wire

  
  

  must be placed in parallel to what is being investigated
• method for investigating resistance

  1. attach crocodile clip to wire, level with 0cm on ruler

  2. attach second crocodile clip to wire, eg 10cm down ruler. record length between wires

  3. close switch, then record current and pd

  4.open switch, move second crocodile clip further down wire and repeat

  5.use measurements of current and pd to calculate resistance for each length of wire

  6. plot graph with resistance on y and length of wire on x

  7. line of best fit should be straight showing direct proportionality
• what can resistance in a circuit depend on?

  - whether components are in series or parallel

  - length of wire used in the circuit
• what is an I-V characteristic

  The term I-V characteristic refers to a graph which shows how the current flowing through a component changes as the potential difference across it is increased.
• method to find I-V characteristics

  1. set up circuit with battery, variable resistor, component and voltmeter in parallel, and then ammeter in series

  2. begin to vary variable resistor. this alters current and therefore p.d across component

  3.take readings from both ammeter and voltmeter as increase current in circuit

  4.swap over wires in battery so current reversed?

  5. plot graph of current on y and voltage on x
• ohmic conductor graph

  directly proportional (straight line)
• filament lamp graph

  S-shaped curve
• diode graph


  Current will only flow through a diode in one direction. goes from flat line to exponential increase
• light dependant resistor

  A resistor that changes its resistance with a change of light intensity. DARK=HIGH RESISTANCE

  BRIGHT=RESISTANCE FALLS
• thermistor

  resistance decreases as temp increases
• thermistor use


  temp detectors, thermostats
• ldr use

  in burglar detectors and digital cameras
• sesning circuits

  can be used to turn on or increase the power to components depending on the conditions that they are in
• series circuits


  A circuit that has only one path for the electric current to follow. If this path is

  broken then the current no longer will flow and all the devices in the circuit stop working.
• cell pd in series

  adds up
• total potential difference in series is

  shared, so all pd adds up
• current in series circuits is

  the same everywhere
• resistance in series

  adds up
• parallel circuits

  If you remove/disconnect a component, it will hardly affect the others - useful for most things e.g. cars, household electrics
• p.d in parallel circuits

  same across all components as all get full source p.d
• current is shared between

  branches in parallel.
• adding a resistor in parallel

  reduces the total resistance.
• if you have two resistors in parallel,

  their total resistance is less than the resistance of the smallest of the two resistors
• resistors in series practical

  1) First, you'll need to find at least four identical resistors

  .2) Then build the circuit shown on the right using one of the resistors.Make a note of the potential difference of the battery (V).

  3) Measure the current through the circuit using the ammeter.Use this to calculate the resistance of the circuit using R = V = 1.

  4) Add another resistor, in series with the first.

  5) Again, measure the current through the circuit and use this and the potential difference of the battery to calculate the overall resistance of the circuit.

  6) Repeat steps 4 and 5 until you've added all of your resistors.

  7) Plot a graph of the number of resistors against the total resistance of the circuit
• resistors in parallel practical

  1) Using the same equipment as before (so the experiment is a fair test), build the same initial circuit

  2) Measure the total current through the circuit and calculate the resistance of thecircuit using R = V= / (again, V is the potential difference of the battery).

  3) Next, add another resistor, in parallel with the first.

  4) Measure the total current through the circuit and use this and the potential difference of the battery to calculate the overall resistance of the circuit.

  5) Repeat steps 3 and 4 until you've added all of your resistors.

  6) Plot a graph of the number of resistors in the circuit against the total resistance.