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P5 Electric Circuits (Electric Motors (An electric motor is a coil of wire…
P5 Electric Circuits
Electric Motors
An electric motor is a coil of wire that rotates between the opposite pole sof a permanent magnet when a current flows through the coil.
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Resistance
Components (resistors, lamps, motors etc) resist the flow of charge through them. The greater the resistance, the smaller the current.
When resistors are added in series, the total resistance increases. When you add resistors in parallel the total resistances reduces and the total current increases.
Resistance (ohms) = Voltage (volts, V) % Current (amps, A)
Electric Circuits
The work done on each unit of charge by the battery must equal the work done by it on the circuit components. A change in the resistance of one component will result in a change in p.d. across all the components.
When two components are connected in series to a battery, the current flowing through each component is the same, the p.d. across the components adds up to the p.d. across the battery, and the p.d. is largest across components with the greatest resistance. This also applies when more than two components are connected in series to a battery.
In a parallel circuit with one component per parallel path the current flowing through each component depends on the resistance of each component, the total current from the battery equals the sum of the current through each component, and the current is greatest through the component with the smallest resistance.
The current through each component is the same as if it were the only component present. If a second identical component is added in parallel, the same current flow should through each component and the total current increases. The same voltage causes more current to flow through a smaller resistance than a bigger one. The p.d. across each component is equal to the p.d. of the battery.
Transformers
Transformers are used to change the voltage of an alternating current. They consist of a primary and secondary coil wrapped around a soft iron core. Alternating current flowing through the primary coil creates an alternating magnetic field, and this changing field then induces an alternating current in the second coil.
The amount by which a transformer changes the voltage depends on the number of turns on the primary and secondary coils. The changing current in the primary coil will cause a changing p.d. across the secondary coil.
(Voltage on primary coil % voltage on secondary coil) = (number of turns on primary coil % number of turns on the secondary coil)
Static Electricity
When you rub two objects together they become electrically charged as electrons are transferred from one object to the other. The object receiving the electrons becomes negatively charged. The object giving up electrons becomes positively charged. This electrical charge is called static electricity.
When two charged objects are brought together they exert a force on each other: two materials within the same type of charge repeal each other, two materials with different charges attract each other.
Electric Currents
An electric current is a flow of charge measures in amperes (amps, A). A direct current always flows in the same direction. An alternating current changes the direction of flow back and forth continuously and is used for mains electricity.
Alternating current is used for mains because it is easier to generate, can be distributed more efficiently, and can be used in a transformer.
Potential difference (voltage) is measured in Volts (V) using a voltmeter connected across the circuit in parallel. The p.d. between two points on a circuit is the work done on a given amount of charge as it moves between these points.
When you add more batteries in parallel the total p.d. and current stay the same, but each battery supplies less current.
When you add more batteries in series, the p.d. and current increase. The greater the p.d. across a component the greater the current will be.
Electrical Components
The resistance of an LDR (light dependent resistor) depends on light intensity. As the amount of light falling on it decreases, its resistance decreases and more current flows.
The resistance of a thermistor depends on its temperature. As the temperature increases, resistance decreases and more current flows.
Power
Power is a measure of the rate of an energy transfer to an appliance/device and/or its surroundings and is measured in watts (W).
Power (watts, W) = Voltage (volts, V) x Current (amps, A)
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