Static electricity
• When materials are rubbed against each other they can
become electrically charged. Negatively charged electrons
are rubbed off one material onto the other.
• The material that gains electrons becomes negatively charged.
The material that loses electrons has an equal positive charge.
• Two charged bodies will exert a force on each other.
• Like charges repel, unlike charges attract.
• Electric charges move easily through metals.
• The rate of flow of electric charge is called the current.
current I = charge / time
• A charged body can be discharged by connecting it to earth
with a conductor. Charge then flows through the conductor.
The greater the charge on an isolated body the greater the potential
difference between the body and earth. If the pd is high enough a
spark may jump to earth.
• Electrostatic charges can be useful, eg in photocopiers
and smoke precipitators
Electric Current
• Current-potential difference graphs are used to show how
current through a component varies with pd across it.
A resistor A filament lamp A diode
• The current through a resistor (at a constant temperature)
is proportional to the voltage across the resistor.
• Voltage = current × resistance
(volt, V) (ampere, A) (ohm, O)
• The resistance of a filament lamp increases as the
temperature of the filament increases.
• The current through a diode flows in one direction only.
The diode has a very high resistance in the reverse direction.
• The resistance of a light-dependent resistor (LDR)
decreases as light intensity increases.
• The resistance of a thermistor decreases as the temperature increases.
• The current through a component depends on its resistance,
the greater the resistance the smaller the current.
• The voltage from cells in series is the sum of the voltage of each cell.
• For components connected in series:
- total resistance = sum of the resistance of each component
- there is the same current through each component
- the total voltage of the supply is shared between the components.
• For components connected in parallel:
- voltage across each component is the same
- the total current through the whole circuit is the sum
of the currents through the separate components.
Mains electricity
• Cells and batteries supply current which always passes in
the same direction. This is called direct current (d.c.).
• An alternating current (a.c.) is one which is constantly
changing direction. Mains electricity is an a.c. supply.
In the UK it has a frequency of 50 cycles per second (50 Hz).
• UK mains supply is about 230 volts.
• Know the structure of electrical cable.
• Know the structure and wiring of a three-pin plug.
• If an electrical fault causes too great a current, the circuit
should be switched off by a fuse or a circuit breaker.
• When the current in a fuse wire exceeds its rating the
fuse will melt, breaking the circuit.
• Appliances with metal cases are usually earthed. The earth
wire and fuse together protect the appliance and the user
• The live terminal of the mains supply alternates between
positive and negative potential with respect to the neutral terminal.
• The neutral terminal stays at a potential close to zero
with respect to earth.
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