Electricity

• Current


  A flow of electric charge. Can only flow if there's a potential difference
• Potential difference


  The driving force that pushes the charge around
• Resistance


  Anything that slows the flow down
• What happens when there's a high resistance across a component?

  The greater the resistance across a component the smaller the current that flows (for a given potential difference across the component)
• Charge equation


  charge = current x time
• Potential difference equation


  Potential difference = current x resistance
• Investigating Factors Affecting Resistance

  1) Attach a crocodile clip to the wire level with 0cm on the ruler

  2) Attach a second crocodile clip to the wire 10 cm away from the first clip

  3) Close the switch and record the current and pd flowing through it

  4) Open the switch, removing the second crocodile clip and moving it further 10 cm from the first.

  5) Repeat this for a number of lengths whilst recording current and pd

  6) Using measurements and calculations, you can work out the resistance.

  Your graph should should a directional proportional relationship between resistance and length of wire
• Ammeter


  A device used to measure current in a circuit
• Voltmeter


  A device used to measure voltage, or electrical potential energy difference
• Ohmic conductors


  Have a constant resistance

  The resistance of ohmic conductors don't change with current

  At a constant temperature the current flowing through an ohmic conductor is directly proportional to the potential difference across it
• 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
• Diodes


  Current will only flow through a diode in one direction and has a high resistance in the opposite direction
• Investigating I-V characteristics of components

  You can do this experiment to find a component's I-V characteristic:

  1) Set up the test circuit

  2) Begin to vary the variable resistor. This alters the current flowing through the circuit and the potential difference across the component.

  3) Take several pairs of readings from the ammeter and voltmeter to see how the potential difference across the component varies as the current changes. Repeat each reading twice more to get an average pd at each current.

  4) Swap over the wires connected to the battery, so the direction of the current is reversed.

  5)Plot a graph of current against voltage for the component.
• LDR


  Light dependent resistor. in bright light, the resistance falls and in darkness, the resistance is the highest.

  Eg: Automatic night lights

  Outdoor lighting

  Burglar detectors
• Thermistor


  Resistor which varies with temperature

  In hot conditions the resistance drops

  In cool conditions the resistance goes up

  
  

  -Used in electrical thermometers

  -Car engine temperature sensors
• Sensing circuits


  Can be used to turn on or increase the power to components depending on the conditions that they are in
• Series circuit


  A circuit in which all parts are connected end to end to provide a single path of current.
• Potential difference in series circuit

  Shared between components so that their sum is the same as the potential difference of the cell
• Current in a series circuit

  Is the same everywhere
• Resistance in a series circuit

  total resistance = sum of individual resistances

  This is because by adding a resistor in a series the pd has to be shared

  The potential difference across each resistor is lower so the current through it is also lower
• Parallel circuits


  Two or more paths that electricity can flow through, and there is a load connected to each path.
• Potential difference in parallel circuits

  Same across all components
• Current in parallel circuits

  Shared between branches
• Resistance in a parallel circuit

  Adding a resistor reduces the total resistance

  • In parallel, both resistors have the same potential difference across them as the source.

  • This means the 'pushing force' making the current flow is the same as the source pd for each resistor that you add.

  • But by adding another loop, the current has more than one direction to go in.

  • This increases the total current that can flow around the circuit. Using V = IR. an increase in current means a decrease in the total resistance of the circuit.
• Investigating resistance in a series circuit and parallel circuit

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

  2) Then build the circuit using one of the resistor.

  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
• How resistors affects a series circuit

  Adding resistors in series increases the total resistance of the circuit

  The more resistors added the more overall resistance of circuit
• How resistors affect parallel circuits

  The total current increases so total resistance decreases

  More resistors added means smaller overall resistance
• Mains supply


  AC (alternating current) created by alternating voltages
• Battery


  Direct current created by direct voltage
• Cables in electrical appliances

  Live wire

  Neutral wire

  Earth wire
• Live wire


  Brown colour

  Bottom right of the three pin plug

  Provides the alternating pd from the mains supply - 230V

  Current flows in through this wire
• Neutral wire


  Blue colour

  Bottom left of the three pin plug

  Completes the circuit and carries away current

  Electricity flows out through this wire

  Around 0V
• Earth wire


  Green and yellow colour

  Middle of the three pin plug

  Protects the wiring

  Stops the casing from becoming live

  Doesn't usually carry a current - only when there's a fault

  Around 0V
• Equation for power


  Power = potential difference x current
• Energy transferred equation


  Charge flow x potential difference
• Potential difference is energy transferred per charge passed


  1)When an electrical charge goes through a change in potential difference, then energy is transferred.

  2) Energy is supplied to the charge at the power source to 'raise' it through a potential.

  battery

  3) The charge gives up this energy when it 'falls' through any potential drop in components elsewhere in the circuit.

  This means that a battery with a bigger pd will supply more energy to the circuit for every coulomb of charge which flows around it
• The national grid


  A system of cables and transformers linking power stations to consumers
• How do transformers work?


  They step the potential difference up at one end for efficient transmission and then bring it back down again to safe, usable levels at the other end
• Static electricity


  A buildup of charges on an object.
• Build up of static

  Caused by friction

  When certain insulating materials are rubbed together negatively charged electrons will be scraped off one and dumped onto another

  This will leave the materials electrically charged with a positive static charge on one and and equal negative static charge on the other

  
  

  Eg polythene and acetate rods with cloth dusters