P5

1
P5 Electric Circuits
2
Static Electricity

• When two objects are rubbed together and become
  charged, electrons are transferred from one
  object to the other
• Repulsive forces between similar charges
• Attractive forces between opposite charges
• Explain simple electrostatic effects in terms of
  attraction and repulsion between charges

3
Electric Current

• Current is a flow of charge
• Measured in amperes (Amps)
• Wires, bulbs, etc are full of charges that are
  free to move
• The battery causes these free charges to move
• These charges are not used up but flow in a
  continuous loop

4
Conductors and Insulators

• Conductors
• Electrons free to move
• EG Metals
• Insulators
• Electrons are not free to move
• EG Plastic

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Resistance

• Components (for example, resistors, lamps,
  motors) resist the flow of charge through them
• The larger the resistance in a given circuit, the
  smaller the current will be
• The resistance of connecting wires is so small
  that it can usually be ignored

6
Resistors Get Hot

• Resistors get hotter when electric current passes
  through them
• This heating effect is caused by collisions
  between the moving charges and stationary atoms
  in the wire
• This heating effect makes a lamp filament hot
  enough to glow

7
Thermistors

• Low Temperatures High Resistance
• High Temperatures Low Resistance

Light Dependent Resistors

• Low Light High Resistance
• Bright Light Low Resistance

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Circuit Symbols

1. Open Switch
2. Closed Switch
3. Lamp
4. Cell
5. Battery
6. Voltmeter
7. Resistor
8. Ammeter
9. Variable resistor
10. Thermistor
11. Light dependent resistor (LDR)

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Series and Parallel

• Two (or more) resistors in series have more
  resistance than one on its own, because the
  battery has to push charges through both of them
• explain that two (or more) resistors in parallel
  provide more paths for charges to flow along than
  one resistor on its own, so the total resistance
  is less and the current is bigger

10
Ohms Law
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When the Resistance is Constant

• The current is directly proportional to the
  voltage.
• IE if you double the voltage, the current will
  also double.

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Voltage / Potential Difference

• The larger the voltage of the battery the bigger
  the current
• The voltage of a battery (measured in V) provides
  a measure of the push of the battery on the
  charges in the circuit

• Voltmeters are connected either side of the
  component you are measuring.

13
Voltage / Potential Difference

• The energy given or taken away from the charge as
  it moves between two points

14
Adding Batteries

• When you add batteries in Parallel, the Voltage
  and the Current stay the same
• When you add batteries in Series the Voltage and
  Current increase

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Series Circuits

1. Current through each component is the same
2. The voltage across the components add to the
   voltage across the battery (The total energy
   transferred to each unit of charge by the battery
   must equal the amount transferred from it to
   other components)
3. The voltage is largest across the component with
   the greatest resistance (more energy is
   transferred by the charge passing through a large
   resistance)

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Parallel Circuits

1. Voltage across each component is equal to the
   voltage of the battery
2. Current through each component is the same as if
   it were the only component present
3. Total current from (and back to) the battery is
   the sum of the currents through each of the
   parallel components
4. Current is largest through the component with the
   smallest resistance, because the same battery
   voltage causes more current to flow through a
   smaller resistance than a bigger one.

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Transformers

• Changing Current in a coil Changing Magnetic
  Field
• This changing magnetic field can induce a voltage
  in a neighbouring coil

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Transformers

• Transformer two coils of wire wound on an iron
  core
• A transformer can change the size of an
  Alternating Voltage

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Transformer Equation

• Secondary Voltage Number of Coils Secondary
• Primary Voltage Number of Coils Primary

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Generator

• A magnet or electromagnet is rotated within a
  coil of wire (or Rotating coil inside a magnet)
  to induce a voltage across the ends of the coil
• The size of the induced voltage can be increased
  by
• Increasing the speed of rotation of the magnet
  (or coil)
• Increasing the strength of the magnetic field
• Increasing the number of turns on the coil
• Placing an iron core inside the coil

21
Alternating Current

• The induced voltage across the coil of a
  generator changes during each revolution of the
  magnet or electromagnet and explain that the
  current produced in an external circuit is an
  Alternating Current (a.c.)

22
Alternating v Direct Current

• When the current is always in the same direction,
  it is a direct current (d.c.), e.g. the current
  from a battery
• Mains electricity is an a.c. Supply
• a.c. is used because it is easier to generate
  than d.c., and can be distributed more
  efficiently

23
Mains Electricity

• Mains electricity is produced by generators
• Mains supply voltage to our homes is 230 volts.

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Energy Transfer

• When electric charge flows through a component
  (or device), energy is transferred to the
  component
• Energy (J, kWh)
• Power (W, kW)
• Time (s, hr)
• A joule is a very small amount of energy, so a
  domestic electricity meter measures the energy
  transfer in kilowatt hours

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Power

• Power is a measure of the rate at which an
  appliance or device transfers energy

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Cost of Electricity

• Calculate the cost of electrical energy given the
  power, the time and the cost per kilowatt hour
• Multiply the Energy by the by the cost per kWh.

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Efficiency of Electrical Appliances

• Efficiency () (Useful Energy / Total Energy) x
  100