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Physics - Lizzie > Current Electricity > Flashcards

Flashcards in Current Electricity Deck (37):

What is the symbol for time, the unit and the symbol for the unit?

Symbol - t
Unit - second
Symbol for seconds - s


What is the symbol for current, the unit and the symbol for the unit?

Symbol - I
Unit - Amp
Symbol for Amps - A


What is the symbol for energy, the unit and the symbol for the unit?

Symbol - E
Unit - joule
Symbol for joules - J


What is the symbol for voltage, the unit and the symbol for the unit?

Symbol - V
Unit - volt
Symbol for volts - v


What is the symbol for resistance, the unit and the symbol for the unit?

Symbol - R
Unit - omh
Symbol for ohms - Ω


What is the symbol for power, the unit and the symbol for the unit?

Symbol - P
Unit - joules per second/watt
Symbol for units - J/s / W


What equation links current, charge and time?

I = Q/t


What equation links energy, voltage and charge?

V = E/Q


What equation links voltage, current and resistance?

R = V/I


What equation links power, energy transferred and time?

t = e.t./P


What equation links power, voltage and current?

V = P/I


Voltage is low ->

current is low -> temperature is low -> resistance of the filament is low
• Gradient is high


Voltage is high ->

current it high -> temperature is high -> resistance of the filament is high
• Gradient is low


What is the symbol for charge, the unit and the symbol for the unit?

Symbol - Q
Unit - coulombs
Symbol for coulombs - C


How to find out resistance when voltage is proportional to current and the gradient is known?

Resistance = 1/gradient


Rules for current and voltage in a series circuit

• Current doesn't change in a series circuit.
• In series, voltage is shared between all components.
• In parallel, voltage is the same with each branch of the circuit.


What is charge measured in?

Charge = Q
Measured in Coloumbs (C)


What is a thermistor and how does its resistance change?

• Thermistors can detect and sense temperatures.
• As the temperature increases, the resistance decreases.


What is an LDR and how does its resistance change?

• Light dependent resistors can detect light levels.
• As light intensity increases, their resistance decreases.


Where do you put an ammeter in a circuit?

• Anywhere in the main circuit in series, but not in parallel.


Where do you put the voltmeter?

• In parallel, across the component.
• Not around the variable resistor or battery!!!


What are alternating (ac) and direct (dc) currents?

• Alternating currents are constantly changing direction - mains electricity is an ac supply.
• Direct current are always the same direction - cells and batteries supply direct current.


Recipe for a safe plug:

• Firmly screw in the rightly coloured wire to its corresponding pin.
• No bare wires inside the plug.
• Fasten the cable grip tightly over the cable outer layer.
• Metal parts are made of copper or brass (good conductors).
• Case, cable grip and cable insulation are made of flexible rubber or plastic (good insulators).


How does the earth wire work?

• Live wire manages to touch the metal case.
• The case is earthed, so a large current flows through the live.
• The current then flows through the case and out down the earth wire.
• The surge of current causes the fuse to melt (or the circuit breaker to be tripped) and cuts off the live supply.
• The appliance cannot get electricity - no chance of an electric shock or fire from overheating.


What does it mean to double insulate an appliance?

• If an appliance has a casing made out of plastic (instead of metal - which conducts electricity making electric shocks possibles) and has no metal parts showing, it is double insulated.
• If an appliance is double insulated, it doesn't require an earth wire.


How do fuses work?

• Fuses contain a thin piece of wire which has a low melting point.
• If there is a current which is too large flowing in the circuit, the fuse wire will become very hot and melt.
• If the fuse has melted ('blown'), the circuit is shut off and you cannot get an electric shock or have an electrical fire.


How do circuit breakers work?

• Very similar to fuses.
• If a circuit breaker detects a surge in the current, it will break the circuit by opening a switch.
• To reset them, one flicks a switch on the device (more convenient than fuses, as once a fuse melts it has to be replaced).


Why is it bad to have water near electrical objects?

Water is a very good conductor, so the current would pass through it and cause an electric shock.


What are the different types of fuses?

• 3A, 5A and 13A.


Why do some appliances have thick cables?

• Thick cables have a lower resistance.
• Thick cables are easy to insulate.
• Thick cables produce less heat.


What is voltage?

The amount of energy transferred per unit of charge (Coulomb). 1 Volt = 1 Joule/Coulomb.


What is current?

The rate of flow of charge.


Why might a series circuit be more appropriate for a particular application?

• In a series circuit the voltage is shared out between all the components and therefore might be useful to supply low power things, such as fairy lights.
• In a series there are fewer wires used.
• In a series the resistance values are lower.


Why might a parallel circuit be more appropriate for a particular application?

• In a parallel circuit, each component receives full voltage, so it is useful for supplying power to high energy things.
• It might be more practical because if component in the circuit breaks, the rest of the circuit will still work because the different components are connected separately to the supply.


A kitchen has a water supply, an electricity supply and electric lighting. There is a toaster, a kettle, a clothes iron, a microwave oven and a dishwasher.
What are the hazards of electricity in this kitchen?

• Electrical devices with heating elements (e.g. the kettle) can reach high temperatures, which can cause (skin) burns and fires (if the insulation melts).
• Electricity coming into contact with water is hazardous (e.g. dishwasher) because water conducts electricity and can therefore cause an electric shock.
• Damaged equipment can cause many hazards, such as a fault microwave leaking cancer-causing microwaves. Live parts must not be exposed which could lead to electric shocks.
• With many electrical appliances, there is a danger of the sockets being overloaded (which can cause a fire). Circuit must have correct fuses and there must be sufficient sockets.
• Having many electrical appliances means there is a trip hazard of all the cables. Short leads could be used to reduce the hazard.
• There is a danger of sticking metal objects into sockets/heating elements which can cause an electric shock. Sockets could be covered to stop people being able to put things in them and tools must be insulated properly.


What does a variable resistor do?

It enables someone to control the current in a circuit.


Why would you want to keep the temperature of a wire constant when working out the difference in resistance by changing voltage?

• Resistance changes with temperature.
• A hot wire would be dangerous as it might melt or catch fire.
• Voltage is only proportional to current at a constant temperature.
• Ohm's law is only true if the temperature is constant.