Using Electricity

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Using Electricity

• Revision

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Electric current is a flow of
Charge
Electric current is measured in
Amperes Amps A
Charge is measured in
Coulombs c
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Current and Charge

• QIt

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QIt

• Q Charge in coulombs
• I Current in Amps
• t Time in seconds

Q
I
t
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Questions

• A charge of 30 coulombs flows into a heater in 10
  seconds, how much electric current is this?

3Amps
An electric lamp uses 0.25 Amps. How much charge
flows through it in 10 seconds?
2.5 coulombs
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Uses of Electric Current.

• Making things hot, lamps and cookers.
• Making magnetism, motors and meters.
• Changing chemicals, electrolysis.
• Producing light, fluorescent lights.

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Electrical Power
Watts

• What is electrical power measured in?

Joules
Energy is measured in
Each coulomb of charge transfers some energy. One
Joule of energy transferred per coulomb is called
one volt.
Voltage is the number of joules of energy
transferred per second.
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Electrical Power

• PIV

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P I V
P Power in Watts I Current in Amps V
Voltage in Volts potential difference
P
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V
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Questions

• An electric heater runs from the mains (230V) it
  draws a current of 4 Amps. Calculate the power of
  the heater.

4A X 230V 920W

• How much current flows when a 2kW electric heater
  is connected to the mains?

2000W / 230V 8.7A
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Resistance

• Some conductors are better than others.
• A good conductor has low resistance.
• Copper and gold are good conductors.
• Aluminium is not a particularly good conductor,
  it has a high resistance.

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Variable resistor

• A dimmer switch uses a variable resistor.
• With increased resistance you get less current
  flowing.
• With less current flowing you get a dimmer light.

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Voltage, resistance and current.

• Current Voltage
• Resistance

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V I R

• V Voltage (Volts) potential difference
• I Current (Amps)
• R Resistance
• (Ohms) ?

V
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R
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Questions

• What is the resistance of a washing machine which
  draws a 5A current from the mains?

230V / 5A 46?

• What current will a motor with a resistance of 25
  Ohms draw from the mains?

230V / 25 ? 9.2 A
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Mains and batteries

• Mains electricity is alternating current. a.c.

Batteries (cells) give out direct current d.c.
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Static electricity

• Plastic is an insulator, electric charge cannot
  flow along it.
• If any charge is given to a plastic rod, the
  charge will stay there.

Lightning is caused by static electricity.
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Problems of static

• Minor electric shocks from getting charged up on
  synthetic carpets.

Major sparks from aeroplanes during refueling!
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Uses of static electricity

• Spraying paint on new cars. The car is charged up
  and the paint sprayer has the opposite charge, so
  more paint reaches the car.

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Uses of static electricity

• Removing dust from the air. Dust is attracted to
  charged plates over which air is blown.
• Inkjet printers spray ink and the direction of
  the spray is controlled by charged plates through
  which the ink passes.
• Photocopiers use charge to attract toner onto the
  paper.

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Q
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V
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P
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Now go and learn it!