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51 Cards in this Set
- Front
- Back
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Identify and explain the hazards of electricity |
Frayed wires and broken plugs - Exposes the metal wires that carry electricity and cause electric shock if touched. Poking metal in socket - Causes electric shock. Long cables - Should be kept short to prevent spills. Spilled water - Dangerous as conducts electricity at high voltages. |
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How does a fuse work? |
Contains a thin wire with a low melting point. If the current is too large the wire melts. The fuse blows shutting off the circuit. Fuse then must be replaced before appliance can be used. |
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What are the sizes of a fuse? |
3A, 5A and 13A. |
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How does a circuit breaker work? |
If too large a current flows through a circuit breaker, a switch opens making the circuit incomplete. The circuit breaker resets once the circuit has been corrected and never has to be replaced. |
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How do earth wires work? |
Connected to metal casing on appliances. If the live wire breaks and comes in contact with the casing, the earth wire provides a low resistance path for the current. The current would then blow the fuse. Anyone touching the casing would receive a severe electric shock.
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What is double insulation? |
It is used on appliances with insulated casing opposed to metal. All electrical parts are insulated this way so that they cannot be touched. Appliances insulated this way use a two wire flex and have no earth wire. |
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How does the heating effect of a current work> |
The current passes through the element. The energy transfers and the element heats up. |
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which appliances use the heating effect? |
Toaster, kettle, dishwasher, cooker, washing machine, electric fire, fan heater and hair dryers. |
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How do light bulbs work? |
They have a very thin filament which gets hot when the current passes through and they glow white. |
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What is the equation relating voltage, power and current? |
Power = Current x Voltage P = I x V |
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What is power measured in? |
Joules per second or watts |
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What is current measured in? |
Amps |
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What is voltage measured in? |
Volts |
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What is energy measured in? |
Joules |
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What are the two formulas for calculating energy from an appliance? |
Energy = Power x Time (E = P x t) Energy = Current x Voltage x T (E = I x V x t) As power = current x voltage |
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What is the difference between ac and dc? |
Ac flows in alternating directions where as dc only flows in one direction. |
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What is an electric current? |
A flow of charge |
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What is charge carried by? |
Electrons
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How can you calculate the charge passing along a wire? |
Charge = current x time Q = I x t |
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How can charge, current and time be rearranged? |
Q = I x t I = Q / t t = Q / I |
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What is charge measured in? |
Coulombs |
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What is current measured in? |
Amps |
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What does the voltage across a component tell us? |
How much energy it is converting |
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How can we measure voltage? |
Using a voltmeter |
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What is a series circuit? |
A circuit with no branches or junctions |
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What is a parallel circuit? |
A circuit with branches and junctions |
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Give 3 components of a series circuit |
One switch will turn off all the components together If one bulb blows, all bulbs will as there is a gap in the circuit The voltage is shared between all components |
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Give 3 components of a parallel circuit? |
Switches can be placed in different parts of the circuit, so bulbs can be turned off individually If one bulb blows only bulbs on the same branch will be affected Each branch of the circuit receives the same voltage |
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How are decorative lighting wired? |
In series |
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Do wires have resistance? |
Very low resistance |
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Do components have resistance? |
High resistance |
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Formula for voltage, current and resistance |
Voltage = current x resistance V = I x R |
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How can you rearrange voltage, current and resistance? |
V = I x R I = V / R R = V / I |
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What is resistance measured in? |
Ohms |
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What are fixed resistors used for? |
They control the sizes of currents and voltages |
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What are variable resistors used for? |
It is possible to alter their resistance |
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What are thermistors? |
A resistor whose resistance changes dramatically with temperature |
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What are thermistors used in? |
Fire alarms and thermostats |
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How do thermistors work? |
At room temp the number of free electrons is small and so the resistance is large. If warmed the number of free electrons increases and the resistance decreases. |
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What are light dependent resistors? |
The resistance of a thermistor is lower in light conditions |
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What are light dependant resistors used in? |
Photographic equipment, automatic lighting controls and burglar alarms |
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How do light dependant resistors work? |
In dark conditions they contain few free electrons and have high resistance. If light is shone on them more electrons are freed and the resistance decreases. |
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What are diodes? |
Resistors that behave like one way valves |
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How do diodes work? |
When current flows through them in one direction it can do so easily as the diode has low resistance. If the current tries to flow in the opposite way the diode has a very high resistance and very little current can now flow. |
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What are diodes used for? |
Circuits where the current needs to flow in one direction |
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What is ohms law? |
The current that flows through a conductor is directly proportional to the potential difference across its ends, provided its temperature remains constant |
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What is voltage? |
Energy transferred per unit charge passed |
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What is a volt? |
A joule per coulomb |
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How can you investigate how insulating materials can be charged by friction? |
Take a rod of plastic such as acetate or polythene and rub a cloth duster against them. This builds up static charge. Then take a positively charged material and tie it to a string so its suspended at the halfway line. Take the plastic rod and place it near to this material. If the rod is positive it will repel and if it's negative then it will attract. |
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Explain that positive and negative electrostatic charges are produced on materials by the loss and gain of electrons |
When two insulating materials are rubbed together, they become electrostatic ally charged. This is because the electrons are transferred from one material onto the other. For example, when polythene rod (this has a negative charge) is rubbed against a cloth, (this has a positive charge), all the electrons move from the cloth to the polythene rod. This leaves the polythene rod negatively charged (as it has gained electrons) and the cloth positively charges (as it has lost electrons). |
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The forces of attraction between unlike charges and forces of repulsion between like charges |
Two materials with opposite electrostatic charges are attracted to each other Two materials with the same electrostatic share will repel |
