Electricity

ELECTRICITY

• power = current x voltage
• energy = power x time
• energy = current x voltage x time
• Current in amps = charge in coulombs / time in seconds
• Voltage = current x resistance
• electric current is the rate of flow of electric charge
• 1 volt = 1 joule per coulomb

Slide 2

Basics and Plugs

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• Safety Features:
• Insulation
• Double insulation - wiring insulated and cased in plastic or other insulating material
• Earth Wire - provides a very low resistance path for current in the event the live wire touches the case.
• Fuses - ceramic heat proof tubes with a thin wire designed to melt when certain current exceeded breaking the circuit.
• Circuit breaker - do the same as fuses but don't need to be replaced and reset by pressing a button

Measuring Current and Voltage

• Current is measured using an ammeter placed in series with the component through which the current passes.
• Voltage is measured using a voltmeter placed in parallel with the component through which the current passes.

Series vs Parallel

• Same current flows through all the components
• Any break in the circuit stops current flowing
• If the lamps are identical they will get an equal share of the voltage and be the same brightness
• Current can split / take two routes
• I = I1 + I2
• If the lamps are identical the current will divide equally - same voltage + brightness
• If the lamps are not identical the current will not divide equally
• If one of the lamps blows the other will be unaffected but the current supplied by the battery will half

Resistance

• For metal wires, current varies in proportion to the voltage as long as the wire doesn't get hot.
• Different wires allow different sized currents to flow for a given voltage; some wires allowed current to flow easily while others didn't
• Resistance is measured in ohms
• A component with a resistance of 1 ohm will pass a current of 1 Amp when a voltage of 1 volt is applied across it
• There are fixed resistors and variable resistors. Resistors have many uses such as lamp dimmers, speed control in slot car games and volume control in audio equipment

Components with resistance

• Thermistors:
• Resistance changes with change in temperature
• Resistance decreases as temperature increases
• Used in temperature sensing circuits

Components with resistance

• Light-Dependent resistors (LDR)
• Resistance changes with the amount of light shining on the LDR
• More light means less resistance
• Used in light sensitive circuits like those to turn on street lights in the dark

Components with resistance

• Diodes and Light emitting diodes (LEDs)
• Only allow current to flow in the direction of the arrow (circuit symbol)
• That is because in one direction they have little resistance and in the other very high resistance.
• LED's light up when a current passes through them
• Diodes control the direction in which current can flow in a circuit and are used in power supplies for DC equipment

Current voltage graph - resistor

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• Resistors and wires obey Ohm's law - current is proportional to voltage and the graphs are straight lines which pass through the origin.

Current voltage graph - filament lamp

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• The filament lamp is a metal wire but it gets very hot. The resistance of a metal increases with temperature so the graph curves.

Current voltage graph - diode

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• Diodes have a very high resistance when the voltage is applied in the 'wrong' direction - shown by the horizontal line when the voltage is negative.
• When the voltage is applied in the right direction, when it reaches about 0.7 volts the resistance drops to a small value - the graph curves and becomes very steep