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P5 – Electric Circuits

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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

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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

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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

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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

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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

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Ohm’s 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.

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Voltage / Potential Difference The energy given or taken away from the charge as it moves between two points

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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: 1.Increasing the speed of rotation of the magnet (or coil) 2.Increasing the strength of the magnetic field 3.Increasing the number of turns on the coil 4.Placing an iron core inside the coil

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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.)

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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

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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

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