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43 Cards in this Set
- Front
- Back
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What is Direct Current (d.c.)
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Current that passes round the circuit in one direction only
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What is alternating current (a.c.)
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Current that repeatedly reverses its direction
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What type of current is the mains supply?
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Alternating current
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Describe the mains circuit and what type of wires it consists of.
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It has a live wire that is alternately positive and negative every cycle and a neutral wire at zero volts
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What is the frequency and voltage of the UK mains supply?
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50 hertz frequency and 230V
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What can a diode be used for?
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For half-wave rectification of ac. This is used to convert alternating current t direct current. An oscilloscope may be used to show how the potential different across the resistor varies with time.
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How can the frequency of an ac supply be determined?
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By connecting the supply to an oscilloscope. The time taken for one cycle, T, can be found from the oscilloscope trace by measuring the time taken for as many complete cycles as possible.
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How do you measure the frequency of the ac supply?
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Measure the time period of the waves then use the formula frequency = 1/ time taken for 1 cycle.
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What are sockets and plugs made of?
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Stiff plastic materials, which enclose the electrical connections
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What does the mains cable consist of?
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Two of three insulated copper wires surrounded by an outer layer of flexible plastic material.
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What colours are the wires in a three-pin plug or a three-core cable?
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The live wire is brown
The neutral wire is blue The earth wire is green and yellow |
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What is the earth wire used for?
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To earth the metal case of a mains appliance.
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What are the pins of the plug made of?
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Brass. its a good electrical conductor and is hard, won't rust or oxidise.
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Why do appliances with plastic cases not need to be earthed?
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They are said to be double insulated and are connected to the supply with two-core cable containing just a live and a neutral wire.
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What is a fuse?
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Contains a thin wire that heats up and melts if too much current passes through it, cutting the current off.
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What is a circuit breaker?
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An electromagnetic switch that opens ('trips') and cuts the current off if too much current passes through it.
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Why do appliances with metal cases need to be earthed?
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Otherwise if a fault develops, and the live wire touches the metal case, the case becomes live and could give an electric shock to anyone who touches it.
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What does a residual current circuit breaker (RCCB) do?
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It cuts off the current in the live wire if it is different to the current in the neutral wire.
It works faster than a fuse or an ordinary circuit breaker and is more sensitive than an ordinary circuit breaker |
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What is the power supplied to a device?
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The energy transferred to it each second.
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What is the equation for electric power supplied (watts)?
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current (amperes) x potential difference (volts)
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What is the equation for correct rating (amperes)?
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electrical power (watts) / potential difference (volts)
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What is the equation relating charge, current and time?
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Q (Charger in coulombs, C) = I (current in amperes) x t (time in seconds)
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What happens when an electrical charge flows through a resistor?
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Energy transferred to the resistor makes it hot
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What is the equation to calculate energy transferred?
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Energy (Joules) = Voltage (volts) x Q charge (coulombs, C)
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What happens when charge flows in a circuit?
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the components will heat up because of their resistance. This means that most electrical appliances have vents to keep them cool.
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For any appliance, the energy supplied to it depends n:
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How long it is switched on for,
The power supplied to it. |
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What is the unit of energy supplied by mains electricity?
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Kilowatt-hou (kWh)
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What is the equation for energy transferred to a mains appliance?
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E (kWh)= Power (kW) x time (hours)
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What is the equation for the total cost of energy used?
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Total cost = number of kWh used x cost per kWh
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what are electrical faults dangerous?
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they can cause electric shocks and fires
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What should you do when choosing an electrical appliance?
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Consider the power and efficiency rating of the appliance
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What is the efficiency of filament bulbs and halogen lamps compared to low energy lamps?
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Filament bulbs and halogen lamps are much less efficient than low enery lamps
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Why should you never touch a mains appliance with wet hands?
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Tap water conducts electricity. If your hands were wet, the electrical resistance would be lowered so a larger current would pass through your body if the appliance case was not at zero
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Why is a filament bulb very inefficient?
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It becomes hot and heats the bulb an its holder. It transfers more energy by heating than by emitting light.
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What is the National Grid?
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A network of pylons and cables that connects power stations to homes, schools, factories and other buildings.
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Give two advantages of everhead cables compared with underground cables
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They are more easily accessed. They are unaffected when the land floods.
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What is the national grid's voltage?
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132 000 V or more.
Power stations produce electricity at a voltage of 25 000 V |
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A high grid voltage reduces current needed, therefore
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reducing energy wastage and making the system more efficient
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What happens to the voltage before the electricity is transmitted across the national grid and why?
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The voltage is increased by Step-up transformers because transmission at high voltage reduces the current in the cables and therefore reduces the energy wasted in the cables, making the system more efficient.
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What happens at the local sub-stations and why?
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Step-down transformers are used to reduce the voltage to 230 volts for use in homes and offices and it would be too dangerous to supply electricity to consumers at the high voltages of the cables.
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what is the peak voltage of the mains supply?
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325 V
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State the possible faults in the cable and the plug of a mains appliance.
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Cable : Insulation worn away, Bare wire exposed
Plug: chipped or cracked, cable stay lose, wire broken from a terminal inside the plug |
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State one advantage and disadvantage of a circuit breaker compared with a fuse.
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The circuit breaker can be reused.
It cannot be replaced as readily by a circuit breaker with a different current rating. |
