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Physics > Topic 7: Electricity and Circuits > Flashcards

Topic 7: Electricity and Circuits Flashcards

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1
Q

What’s current?

A

The flow of electrical charge (e.g. electrons)

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2
Q

What’s potential difference?

A

The energy transferred per unit of charge that passes between two points in a circuit (volts, V)

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3
Q

What’s resistance?

A

Anything that slows the charge down (Ohms)

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4
Q

How does the potential difference relate to the charge?

A

Generally speaking, the higher the potential difference, the higher the current will be
The higher the resistance, the smaller the current will be

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5
Q

How do you calculate charge?

A

Charge = current x time

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6
Q

How do you calculate energy transferred?

A

energy transferred = charge moved x potential difference

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7
Q

How do you calculate potential difference?

A

potential difference = current x resistance (V=IR)

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8
Q

What’s an Ammeter?

A

Measures the current flowing through the component (amps, A)
Must be placed in series, not in parallel

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9
Q

What’s a Voltmeter?

A

Measures the potential difference across a component
Must be placed in parallel with the component

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10
Q

How do you calculate the total resistance in a series circuit?

A

Sum of the resistance in each component (addition)

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11
Q

How is the potential difference and current in a series circuit?

A

Potential difference in series is shared across the whole circuit
The Current is the same across the whole circuit

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12
Q

How is the potential difference and current in a parallel circuit?

A

Potential difference in parallel is the same across all branches because charge can only pass through any one branch
The Current in parallel is shared between each of the branches

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13
Q

What do I-V graphs show?

A

How the current varies as you change the potential difference

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14
Q

How does the current vary as you change the potential difference in resistors and wires?

A

The current is directly proportional to the potential difference (if the temperature stays the same)

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15
Q

How would a filament lamp I-V graph look and why?

A

Increasing current increases the temperature of the filament lamp, which makes the resistance increase
So, the graph is curved

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16
Q

Explain the I-V graph for diodes.

A

Current will only flow through a diode in one direction (as shown in the graph)
The diode has very high resistance in the opposite direction

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17
Q

Which components of I-V graphs are linear/non-linear?

A

Linear - Fixed resistor
Non-linear - Filament lamp/diode

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18
Q

How does the gradient of the line on an I-V graph relate to the resistance?

A

The steeper the gradient, the lower the resistance

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19
Q

How does the number of components in parallel relate to the resistance/current and why?

A

The more components there are in parallel, the lower the resistance and higher the current
When there are more resistors each in parallel, there are more paths for the current to take
Using V=IR, an increase in the current means a decrease in the total resistance

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20
Q

What happens, in terms of energy transfers, when an appliance has a high current?

A

Energy is transferred to the thermal energy store, and then dissipates into the surroundings

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21
Q

What’s the power of an appliance?

A

The energy that it transfers per second

22
Q

How can you calculate the power of an appliance when given the time?

A

Power (W) = Energy transferred (J) / Time (s)

23
Q

What does the power rating on an appliance mean?

A

The maximum amount of energy that can be transferred between stores per second when the appliance is in use

24
Q

How can you calculate the power needed for an appliance when given the potential difference?

A

Electrical power (W) = Current (A) x P.D (V)

25
Q

How can you calculate the fuse needed for an appliance?

A

Fuse = current
Current (A) = Electrical power (W) / P.D (V)

26
Q

How can you find the power when you don’t know the potential difference or the energy transferred?

A

Power (watts) = Current (amps) squared x resistance (ohms)

27
Q

What are the two types of electricity supplies?

A

Alternating current (a.c)
Direct current (d.c)

28
Q

What’s an alternating current?

A
  • Comes from the mains supply
  • Current constantly changes direction (so, so does the charge)
  • Produced by alternating voltages (the positive and negative ends of the p.d keep alternating)
29
Q

What’s a direct current?

A
  • Produced by batteries
  • Current flows in one direction (so, so does the charge)
  • Created by direct voltage (a p.d that is only positive/negative, not both)
30
Q

What’s the frequency and voltage of the UK mains supply?

A

50Hz
230V

31
Q

What wires are found in a plug?

A

Live wire
Earth wire
Neutral wire

32
Q

What’s the Live wire?

A
  • Brown
  • Carries the voltage from the mains to the appliance (p.d)
  • Alternates between a positive and negative voltage of about 230V
33
Q

What’s the Neutral wire?

A
  • Blue
  • Completes the circuit
  • Around 0V
34
Q

What’s the Earth wire?

A
  • Green and yellow
  • The safety wire to prevent the appliance from becoming live
  • Carries current away
  • At 0V
35
Q

What is the potential difference between the live wire and the neutral wire?

A

It equals the supply’s potential difference (230V for the mains)

36
Q

What’s the potential difference between the live wire and the earth wire?

A

230V for an appliance connected to the mains

37
Q

What’s the potential difference between the neutral wire and the earth wire?

A

No potential difference, they’re both at 0V

38
Q

What feature do plug sockets have to reduce electric shocks?

A
  • Plug sockets have switches connected to the live wire
  • This is so the circuit can be broken
  • So, the appliance becomes isolated and the risk of an electric shock is reduced
39
Q

Why does touching the live wire give you an electric shock?

A
  • Your body is 0V
  • This means, if you touch a live wire (230V), a large potential difference is produced across your body and current flows through you (causing a large electric shock - could injure or even kill you)
  • Even if the plug socket is turned off, it could still chock you as there is still a potential difference in the live wire even though there is not a current flowing
  • When you make contact with the live wire, your body provides a link between the supply and the earth, so a current would flow through you
40
Q

Why can a connection between live and neutral be dangerous?

A
  • If the link creates a low resistant path to earth, a huge current will flow, which could result in a fire
41
Q

What is a surge?

A

A sudden increase in current that occurs due to changes in a circuit (e.g. appliance suddenly switching off, or a fault in an electrical appliance)
Surges can lead to the circuits and wiring in your appliances melting or causing a fire, faulty appliances can cause deadly electric shocks

42
Q

What are the safety measures taken to reduce risks associated with circuits and surges (e.g. electric shocks, fires)?

A

Fuses, earth wires

43
Q

How does a fuse work to prevent danger in a circuit/appliance?

A
  • The fuse contains a thin wire that will melt if the current gets greater than the fuse rating
  • The fuse is connected to the live wire, so breaking the fuse also breaks the circuit and cuts off the live supply
  • This isolates the whole appliance, making it impossible to get an electric shock from the case
  • It also prevents risk of fire caused by the heated effect of a large current
44
Q

Explain the thickness of the cable around fuses.

A

The fuse rating for cables usually increases with the cable thickness, because the cable needs to be thicker when the current is larger to stop the cable from getting too hot and melting

45
Q

What’s better and why: A circuit breaker, or a fuse?

A

Circuit breaker - Trips (turns off) instead of melts, so it can be reset rather than having to replace a fuse every time it melts
Turn off quicker than fuses
HOWEVER: Circuit breakers are more expensive than fuses

46
Q

How does a fault develop?

A

-The live wire touches the metal case
- Because the case is earthed, too great a current flows through the live wire, the case and the earth wire

47
Q

What’s Double Insulation?

A

An appliance has a plastic casing with no metal parts showing is double insulated, so it doesn’t need an earth wire

48
Q

What’s a two-core cable?

A

A cable that only carries the live and neutral wires

49
Q

What’s an LDR?

A
  • Light dependent resistor
  • A resistor that’s dependent on the intensity of light
  • In bright light, the resistance falls and in darkness, the resistance is highest
  • APPLICATIONS: Automatic night lights, outdoor lighting, burglar detectors
50
Q

What’s a thermistor?

A
  • A resistor that’s dependent on the temperature
  • In hot conditions, the resistance drops and in cool conditions, the resistance goes up
  • APPLICATIONS: Electronic thermostats, car engine temperature sensors

Physics (9 decks)

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