Electricity

• • Current (I) through a component depends on:

  o potential difference (V) across the component – driving force that pushes charge round a circuit

  o resistance (R) of the component – resistance is the opposition to the flow of current (as resistance

  increases less current can flow per unit potential difference)

  • The greater the resistance of the component, the smaller the current for a given potential difference across the

  component (in the exam you can say potential difference or voltage)
• • Ohmic conductor (at a constant temperature)

  o E.g. wires or a resistor

  o The resistance remains constant as current changes

  o Current is directly proportional to the potential difference across the resistor
• Filament lamp

  ▪ When an electrical charge flows through a filament lamp, some energy

  is transferred to the thermal energy store of the filament (which is

  designed to heat up)

  ▪ As current increases the filament’s temperature increases, increasing

  the resistance (resistance increases as temperature increases)
• o Diode

  ▪ The current through a diode flows only in one direction

  ▪ The diode has very high resistance if the current is reversed

  o Thermistor

  ▪ Resistance decreases as temperature increases

  ▪ They are useful temperature detectors e.g. in car engines & electronic

  thermostats

  o Light dependent resistor (LDR)

  ▪ Resistance decreases as light intensity increases (so high resistance at night)

  ▪ Applications include automatic night lights (switch on when its dark) & burglar detectors
• o Alternating current (AC) – current is constantly changing

  direction, produced by alternating voltages in which the

  positive & negatives ends keep alternating

  o Direct current (DC) – current is always flowing in the

  same direction, created by direct voltage
• o live wire – brown

  ▪ carries alternating pd from the supply, 230V

  o neutral wire – blue

  ▪ completes the circuit with the supply & carries away current, at or close to 0V

  o earth wire - green/yellow stripes

  ▪ safety wire to stop the appliance becoming live, at 0V – only carries current if there is a fault
• The National Grid - A system of cables & transformers linking power stations to consumers within the UK
• e.g. a dry cloth rubbed on a polythene rod – both initially uncharged

  ▪ cloth transfers electrons to the rod

  ▪ cloth becomes positively charged; rod becomes negatively charged

  o Only electrons move, positive charges do not move

  ▪ Positive charge is caused by electrons moving away from the area
• Electric fields are represented around an object using field lines

  ▪ The field lines point in the direction a positive charge would go

  ➢ Away from a positive charge, towards a negative charge

  ▪ Always at a right angle to the surface

  ▪ The closer together the lines are, the stronger the field is

  ➢ As you get further from the charge, the further apart the

  lines are (so the field is weaker)
• o Sparking

  ▪ Sparks are caused when there is a high enough potential difference between charge objects or a

  charged object & earth

  ▪ A high pd causes a strong electric field between the objects

  ▪ The strong electric field causes electrons in the air particles to be removed (ionisation)

  ▪ Air is an insulator, but when it is ionised it is more conductive

  ➢ So current can flow through it – the spark