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Physics - Lizzie > Current Electricity > Flashcards

Flashcards in Current Electricity Deck (37):
1

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

Symbol - t
Unit - second
Symbol for seconds - s

2

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

Symbol - I
Unit - Amp
Symbol for Amps - A

3

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

Symbol - E
Unit - joule
Symbol for joules - J

4

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

Symbol - V
Unit - volt
Symbol for volts - v

5

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

Symbol - R
Unit - omh
Symbol for ohms - Ω

6

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

7

What equation links current, charge and time?

I = Q/t

8

What equation links energy, voltage and charge?

V = E/Q

9

What equation links voltage, current and resistance?

R = V/I

10

What equation links power, energy transferred and time?

t = e.t./P

11

What equation links power, voltage and current?

V = P/I

12

Voltage is low ->

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

13

Voltage is high ->

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

14

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

Symbol - Q
Unit - coulombs
Symbol for coulombs - C

15

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

Resistance = 1/gradient

16

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.

17

What is charge measured in?

Charge = Q
Measured in Coloumbs (C)

18

What is a thermistor and how does its resistance change?

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

19

What is an LDR and how does its resistance change?

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

20

Where do you put an ammeter in a circuit?

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

21

Where do you put the voltmeter?

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

22

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.

23

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).

24

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.

25

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.

26

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.

27

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).

28

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.

29

What are the different types of fuses?

• 3A, 5A and 13A.

30

Why do some appliances have thick cables?

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

31

What is voltage?

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

32

What is current?

The rate of flow of charge.

33

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.

34

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.

35

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.

36

What does a variable resistor do?

It enables someone to control the current in a circuit.

37

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.