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Physics > topic 2 - electricity > Flashcards

topic 2 - electricity Flashcards

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1
Q

What must be included for electrical charge to flow through a closed circuit?

A

a source of potential difference must be included

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2
Q

What is electric current?

A

the rate of flow of electrical charge

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3
Q

State the equation linking charge, current and time

A

Q = IT
charge = current x time

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4
Q

What is the value of current at any point in a single closed loop?

A

current is the same at all points in a closed loop

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5
Q

what two factors does the current in a circuit depend on?

A
  • potential difference
  • resistance
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6
Q

what occurs to the current when the resistance increases?

A
  • the greater the resistance the smaller the current for a given potential difference across the component
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7
Q

What equation should be used to calculate the potential difference if current and resistance are known?

A

potential difference = current x resistance
V = IR

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8
Q

What is a zero error?

A
  • reading on a measuring instrument when the value should be zero
  • this is a systematic error. We cannot reduce it by carrying repeats
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9
Q

What is an “Ohmic Conductor”? State the condition required

A
  • A conductor for which current and potential difference are directly proportional
  • Resistance remains constant as current changes
  • Temperature must be constant
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10
Q

How does the resistance change with the current?

A
  • As current increases, electrons have more energy
  • when electrons flow through a resistor, they collide with the atoms in the resistor
  • this transfers energy to the atoms, causing them to vibrate more
  • this makes it more difficult for electrons to flow through the resistor
    -> so as the current increases, the resistance decreases
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11
Q

How does the resistance change with temperature

A
  • in hotter temperatures, the thermistors resistance decreases
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12
Q

How does the resistance change with length?

A
  • the greater the length, the greater the resistance and the lower the current
  • electrons have to make their way through more resistor atoms, so it is harder than using a shorter wire
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13
Q

How does the resistance change with light?

A
  • the greater the intensity of light, the lower the LDR’s resistance
  • so the resistance is greatest when it is dark
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14
Q

How does the resistance of a diode change with voltage?

A
  • diode allows current to flow freely in one direction
  • in the opposite direction, it has a very high resistance so no current can flow
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15
Q

Give two examples of when a thermistor may be used

A
  • in a thermostat to turn a heater on below a certain temperature
  • in a freezer to turn on a cooler when the temperature becomes too high
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16
Q

What is an application for an LDR?

A
  • street lights often use LDRs
  • When light levels become too low, the light gains sufficient current to turn on
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17
Q

what are the two ways that a component can be connected in a circuit?

A
  • series (same loop)
  • parallel (adjacent loop)
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18
Q

state the rules for current, p.d., and resistance in a series circuit

A
  • the current is the same through each component
  • the total potential difference of the power supply is shared between the components
  • the total resistance of two components is the sum of the resistance of each component
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19
Q

When two resistors are connected in series, how can u calculate the total resistance?

A

Rtotal = R1 +R2

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20
Q

Explain why in a series circuit, if one bulb goes out all of them will get out.

A
  • An electron will pass through every component on its way around the circuit.
  • If one of the bulbs is broken then current will not be able to flow round the circuit
  • If one bulb goes out, they all go out
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21
Q

What happens to the resistance of a parallel circuit when another resistor is added in parallel?

A
  • the resistance decreases
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22
Q

State the rules of current, potential difference and resistance in parallel circuits

A
  • the p.d across each component is the same
  • the total current through the whole circuit is the sum of the currents through the separate components
  • the total resistance of two resistors is less than the resistance of the smallest individual resistor
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23
Q

Describe the current in a series circuit

A

In a series circuit, the current is the same at all positions since the charge only has one path to flow through

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24
Q

Describe the current in a parallel circuit

A
  • In a parallel circuit, the current is shared between the different branches.
  • When the charge reaches a junction it splits
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25
Q

How should you connect a voltmeter in a circuit to measure p.d?

A

Voltmeters should be connected in parallel to the component that they are measuring the p.d of

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26
Q

How should you connect an ammeter in a circuit to measure current?

A

Ammeters should be connected in series with the component that they are measuring the current through

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27
Q

Describe how to perform an experiment to compare the total resistance in series and parallel arrangements

A
  • set up the circuit, turn the power supply on and close the switch
  • Record the voltmeter and ammeter readings and calculate the resistance of the resistor using R = V/I, where R is resistance, v is the potential difference and I is current
  • change the resistor and repeat step two to find the resistance of a second resistor
  • arrange the two resistors in series and close the switch
  • record the voltmeter and ammeter readings once again and determine the total resistance of both resistors in series using R = V/I
  • arrange the two resistors in parallel and close the switch
  • Record the voltmeter and ammeter readings once again and calculate the total resistance of both resistors in parallel
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28
Q

What is the difference between the total resistance when resistors are added in series and parallel circuits?

A
  • In series, the resistance of the network is equal to the sum of the two individual resistances
  • In parallel, the resistance of the network is less than either of the two individual resistances
29
Q

What is the mains electricity?

A
  • Mains electricity refers to the power that is brought into people’s homes
30
Q

Is mains electricity an a.c supply or a d.c supply? What does each of these stand for?

A

Mains electricity is an a.c supply
a.c - alternating current
d.c - direct current

31
Q

What is alternating current?

A

a current that continuously changes direction at a specific frequency

32
Q

What direct current?

A

One directional current flow

33
Q

What is the frequency and voltage of the UK mains electricity supply?

A
  • frequency: 50 Hz
  • Voltage: 230 V
34
Q

What are the wires that are usually in the cables connecting electrical appliances to the mains?

A
  • live wire
  • neutral wire
  • earth wire
35
Q

what are the two colours of the earth wire

A

green and yellow stripes

36
Q

what is the colour used on the live wire?

A

brown

37
Q

what is the colour used on the neutral wire?

A

blue

38
Q

what is the purpose of the live wire?

A
  • the live wire carries the alternating potential difference from the supply
39
Q

what is the purpose of the neutral wire?

A

the neutral wire completes the circuit

40
Q

what is the purpose of the earth wire?

A

it is a safety wire to stop the appliance becoming live

41
Q

What is the potential difference of the earth and neutral wire

A

0V, it only carries a current if there is a fault

42
Q

Explain why a live wire may be dangerous even when a switch in the mains circuit is open

A

The first part of the live wire would still contain a voltage/p.d
- if someone touched the live wire, a current would flow through the person, into the earth and they’d be electrocuted

43
Q

Why are appliances with a metal case dangerous?

A
  • If the live wire comes loose and touches the metal case, the case can become live
  • this could give someone a fatal electric shock if they touch the live case
44
Q

For appliances with a metal case, where is the Earth wire connected? Why?

A
  • the earth wire is connected to the metal casing of the appliance
  • if the live wire becomes loose and touches the casing, the current will flow through the earth wire, preventing electrocution
45
Q

What are the two equations for the power of a circuit?

A

power = current x potential difference
power = (current)^2 x resistance

46
Q

State the equation linking energy transferred, charge flow and potential difference

A

energy transferred = charge flow x potential difference

E = Q X V

47
Q

What two main factors do the amount of energy transferred by an appliance depend on?

A
  • how long the appliance is switched on for
  • the power of the appliance
48
Q

Describe the energy transfers in a battery-powered torch

A
  • Battery converts chemical energy into electrical energy
  • Bulb convert electrical energy into light energy as well as waste energy in the form of heat
49
Q

Describe the energy transfers in a battery-powered motor

A
  • battery converts chemical energy into electrical energy
  • motor converts electrical energy into kinetic energy as well as waste energy in the form of heating due to friction
50
Q

What energy is transferred in a kettle?

A

electrical energy is transferred to thermal energy

51
Q

When is work done?

A

When charge flows in a circuit

52
Q

what three things determine the power of a circuit device?

A
  • the p.d across the circuit
  • the current through the circuit
  • the amount of energy transferred in a given time
53
Q

What is the National Grid?

A

a system of cables and transformers linking power stations to consumers
- electrical power is transferred from power stations to consumers using the National grid

54
Q

What are the two types of transformers used in the national grid?

A
  • step-up trnasformers
  • step-down transformers
55
Q

What is the purpose of step-up transformers?

A
  • step-up transformers are used to increase the potential difference from the power station to the transmission cables
56
Q

what is the purpose of step-down transformers?

A
  • step-down transformers are used to decrease, to a much lower value, the pd for domestic use
57
Q

Why is the National Grid system an efficient way to transfer energy?

A
  • when electricity is transmitted over large distances, the current in the wires heats them, resulting in energy loss
  • by increasing the potential difference at which the electricity is transmitted, the same amount of power can be transmitted using a much smaller current
  • this results in less heat being produced in the wire and hence less energy loss
58
Q

what is an electrical insulator?

A

an insulator does not allow electrical charges to flow through a material

59
Q

what is a static charge?

A

an electrical charge that accumulates on an insulated object

60
Q

Why can insulators become electrically charged when rubbed together?

A
  • electrons are rubbed from one material onto the other
  • the material gaining electrons becomes negatively charged
  • the material losing electrons becomes equally positively charged
61
Q

What happens when two electrically charged objects are brought close together?

A

they exert a force on each other

62
Q

What happens when two identically charged objects are brought close together?

A

They exert a repulsive force on each other and repel

63
Q

what happens when two oppositely charged objects are brought close together?

A

they exert an attractive force on each other an attract

64
Q

What is an electric field?

A

A region in which a charged object will experience a non-contact electrical force

65
Q

What do all electrically charged objects have around them?

A

An electric field

66
Q

Where is the electrical field stronger?

A

the closer to the charged object

67
Q

How do the electrical field lines show how strong the field is?

A

the closer together the field lines, the stronger the field is

68
Q

How can a build-up of static charge lead to a spark

A
  • an object with a large static charge has a large potential difference between it and the earth
  • a large potential difference means a strong electric field. this ionises the air and makes it conductive
  • this allows charge to flow in the form of a spark
69
Q

What precautions must be taken to prevent the build-up of static charge?

A

any surfaces that are rubbing against each other should be earthed to allow the charge to flow off the materials

Physics (8 decks)

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