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Physics GCSE 2020 > Higher: P2 - Electricity > Flashcards

Higher: P2 - Electricity Flashcards

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1
Q

What is potential difference?

A

Energy transferred per unit charge.

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2
Q

What is charge flow?

A

The total charge which has passed through a circuit in a given time.

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3
Q

What is the unit for charge flow?

A

Coulombs (C)

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4
Q

What is resistance? What causes it?

A

Something that opposes flow of charge, transferring energy away as heat.

Caused by electrons colliding with each other and the wire.

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5
Q

What is the unit for resistance?

A

The ohm (Ω).

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6
Q

What is power and its unit and symbol?

A

Rate of energy transfer to charges. Unit = watts (W); symbol = P.

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7
Q

What is current?

A

Rate of flow of charge.

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8
Q

What equation links voltage, current and resistance?

A

V = IR

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9
Q

What equation links current, time and charge flow?

A

Q = It

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10
Q

What equation links energy, voltage and charge flow?

A

E = QV

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11
Q

The length of a wire is directly proportional to its what?

A

Resistance.

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12
Q

Describe how you would investigate how wire length affects resistance.

A
  • Connect an ammeter in series with a test wire, using crocodile clips to attach the wire to the test wire. Connect a voltmeter in parallel to the test wire.
  • Place the 0cm mark on a metre ruler level with one clip, and attach the other clip a set distance away, recording the length of the wire between the clips.
  • Record the current through the wire (ammeter) and pd across it (voltmeter).
  • Open the switch and move the 2nd clip further down the metre ruler, again measuring the length between clips. Repeat step 3 with several different wire lengths.
  • Turn off the circuit between readings to stop the wire heating.
  • Use R = V/I to calculate the resistance for each length of wire, then plot the wire length against resistance. The graph should show a directly proportional relationship.
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13
Q

A student used the following method to investigate how wire length affects resistance:

  • Connect an ammeter in series with a test wire, using crocodile clips to attach the wire to the test wire. Connect a voltmeter in parallel to the test wire.
  • Record the length of the wire between the clips.
  • Record the current through the wire (ammeter), and the pd across it (voltmeter).
  • Repeat step 3 with several increasing wire lengths.

State the independent, dependent and control variables for this investigation.

A

Vind = length of wire (use metre ruler w/ mm intervals)

Vdep = resistance (calculated w/ current, pd)

Vcon = temp. of wire (turn off circuit between readings), type and diameter of wire, pd of cell/battery, other components kept the same.

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14
Q

A student used the following method to investigate how wire length affects resistance:

  • Connect an ammeter in series with a test wire, using crocodile clips to attach the wire to the test wire. Connect a voltmeter in parallel to the test wire.
  • Record the length of the wire between the clips.
  • Record the current through the wire (ammeter), and the pd across it (voltmeter).
  • Repeat step 3 with several increasing wire lengths.

Suggest how they could improve the validity of their results.

A
  • Use a metre ruler with mm intervals to increase the accuracy of length readings.
  • Turn off the circuit between readings to prevent the wire from heating, which would affect its resistance.
  • Take several readings for each wire length, omitting any anomalies and calculating an average.
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15
Q

How do circuits work?

A

1) Current flows clockwise from the negative side of the cell. 2) This side releases electrons, which push away electrons further down the wire. 3) At the other end of the circuit, the electrons are attracted to the positive end of the cell. 4) The space they leave causes electrons behind them to be attracted to the positive ions there.

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16
Q

How do cells work?

A

1) Chemicals in the negative end react to release electrons. 2) In the positive end, chemicals react to take in electrons.

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17
Q

How is pd shared in series circuits?

A

Shared between components, according to the ratio of resistance of these components.

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18
Q

How is current shared in series circuits?

A

Current is the same everywhere as there’s only one path for electrons to take.

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19
Q

How is resistance shared in series circuits?

A

The sum of all the resistors’ resistances amounts to the total resistance. Resistors share the potential difference. The bigger a component’s resistance, the bigger its share of the pd.

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20
Q

Describe how you could investigate the effect of adding identical resistors to a series circuit’s total resistance.

A
  • You’ll need mimimum 4 identical resistors.
  • Assemble a series circuit, including one of the resistors and an ammeter.
  • Measure the current (ammeter) and calculate the resistance using Ohm’s law, R = V/I. (pd = pd of battery.)
  • Add another resistor in series, and repeat the same steps, measuring the current and calculating the resistance.
  • Repeat step 4 until you’ve added all the resistors.
  • Plot the number of identical resistors against the total resistance of the circuit (relationship will be proportional).
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21
Q

How is pd shared in parallel circuits?

A

Pd is the same across all branches because components are not having to share it, like in series.

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22
Q

Compare the brightness of identical bulbs connected in parallel.

A

They will be at the same brightness.

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23
Q

How is current shared in parallel circuits?

A

It diverges. The current of each branch adds to make the total current. If identical components are connected in parallel, the same current will flow through each branch.

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24
Q

How is resistance shared in parallel circuits?

A

Total resistance is always smaller than the resistance of the smallest resistor. Adding a resistor in parallel decreases the total resistance, because fewer electrons pass through each resistor, making it easier for each one to pass through.

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25
Q

What effect does adding a resistor in parallel have?

A

It decreases the total resistance, because fewer electrons pass through each resistor (an extra branch has been added). Since V = IR, and V is constant in parallel circuits, when the resistance decreases, it causes an increase in current on each branch.

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26
Q

In a parallel circuit, as you add identical resistors in parallel to one another, the resistance __.

A

Decreases.

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27
Q

Describe how you could investigate the effect of adding identical resistors to a parallel circuit’s total resistance.

A
  • You’ll need mimimum 4 identical resistors.
  • Assemble a series circuit, including one of the resistors and an ammeter.
  • Measure the current (ammeter) and calculate the resistance using Ohm’s law, R = V/I. (pd = pd of battery.)
  • Add another resistor in parallel to the first; repeat the same steps, measuring the total current and calculating the resistance.
  • Repeat step 4 until you’ve added all the resistors.
  • Plot the number of identical resistors against the total resistance of the circuit. The resistance will decrease with no. resistors (but not proportionally - see graph).
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28
Q

Describe how you could investigate how the pd through a filament lamp affects its current.

A
  • Attach in series a variable resistor, ammeter, and bulb, and a voltmeter in parallel with the bulb.
  • Take current readings with the ammeter as you alter the pd across the bulb (measured with the voltmeter) using the variable resistor.
  • Turn the circuit off between readings to stop the wire heating up.
  • Plot the pd (independent variable) against the current (dependent variable) on a graph.
  • Control variables: component under investigation, type of wire, cell/battery and other components.
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29
Q

What are ohmic conductors?

A

Components which follow Ohm’s law (V=IR). This includes wires and resistors. Their resistance is constant and doesn’t vary with current.

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30
Q

The resistance of Ohmic conductors is __ and changes/doesn’t vary with current.

A

1) Constant
2) Doesn’t vary

31
Q

Describe how the graph for an ohmic conductor would look. What would this show?

A

It would be a straight line which passes through the origin ([0,0]).

This shows that, at a constant temperature, the current flowing through an ohmic conductor is directly proportional to the pd across it.

32
Q

On the IV graph for an ohmic conductor, the __ the gradient, the __ the resistance.

A
  1. The steeper the gradient, the lower the resistance.
  2. The shallower the gradient, the higher the resistance.
33
Q

What are non-ohmic conductors?

A

Components which do not follow Ohm’s law.

34
Q

Give 4 examples of non-ohmic conductors.

A
  1. Diodes
  2. Filament lamps
  3. LDRs
  4. Thermistors
35
Q

Describe how the IV graph for a diode (non-ohmic) would look. Explain the shape.

A

Diodes only allow current to flow in one direction; they have an extremely high resistance if the current is reversed.

The line is an asymptote at the x axis- current cannot be negative.

36
Q

What makes filament blubs glow?

A

They have a high resistance. When charges flow through, they collide and transfer energy to the thermal energy store of the filament, which is designed to heat up.

37
Q

Describe how the IV graph for a filament lamp would look. Explain the shape.

A

S - shape graph.

When charges flow through, they collide and heat the filament. As the bulb heats, particles in the filament vibrate more, making it harder for electrons to move- i.e. more resistance.

Therefore, an increase in resistance leads to more resistance, explaining the exponential curve.

(Remember: lower gradient = higher resistance.)

38
Q

Why is no device 100% efficient?

A

The higher the current, the more energy transferred to the thermal energy stores of components (and then the surroundings).

39
Q

What is direct current (d.c.)?

A
  • Current flows in one direction around the circuit, due to a direct pd.
  • Supplied by cells and batteries.
40
Q

What is alternating current (a.c.)?

A
  • Current alternates direction.
  • Caused by a pd where the positive and negative ends alternate.
41
Q

UK mains electricity is an __ supply.

A

A.c.

42
Q

UK mains supply is at __V to -__V.

A
  1. 230
  2. -230
43
Q

What is the frequency of UK mains electricity?

A

50Hz.

44
Q

Why must voltmeters be placed in parallel to the component of which they are measuring the pd?

A
  • They cannot go in series since they have an extremely high resistance and would block the current.
  • They measure potential difference, which has to be measured across a component or battery.
45
Q

Why are ammeters placed in series with the component of which they are measuring the current?

A

They measure current, and putting them in parallel would affect it.

46
Q

As light increases, the resistance of LDRs __ - and vice versa.

A

As light increases, the resistance of LDRs decreases - and vice versa.

47
Q

Give three uses of LDRs.

A
  1. Automatic night lights
  2. Outdoor lighting
  3. Burglar detectors.
48
Q

As temperature increases, the resistance of thermistors __ - and vice versa.

A

As temperature increases, the resistance of thermistors decreases - and vice versa.

49
Q

Thermistors make useful temperature detectors - give 2 examples.

A
  1. Car engine temperature sensors
  2. Electronic thermostats
50
Q

Why do LDRs’/thermistors’ resistances decrease as light/temperature increases?

A

They are both semiconductors: components in which, when given energy via heat/light, fixed electrons can become delocalised, decreasing the resistance of the component.

51
Q

What are semiconductors?

A

Components in which, when given energy via heat/light, fixed electrons can become delocalised, decreasing the resistance of the component.

52
Q

What circuit could be used to increase the power received by a fan when temperature increases? How would it work?

A
  • Connect a thermistor in series with a fixed resistor, then connect the fan in parallel to the resistor.
  • As temperature rises, the thermistor’s resistance decreases, so it takes a smaller percentage of the total pd.
  • The fixed resistor and fan are connected in parallel, so will always have an equal pd across them.
  • The branch with the fan on therefore receives a larger share of the pd, so it goes faster.
53
Q

What circuit can be used to dim a filament lamp as a room gets brighter? How would it work?

A
  • Connect a fixed resistor in series with an LDR, then connect the lamp in parallel to the LDR.
  • The fixed resistor takes a fixed proportion of the total pd.
  • As light intensity increases, the LDR’s resistance decreases, so it takes a smaller percentage of the total pd.
  • The lamp and LDR are connected in parallel, so will always have an equal pd across them.
  • So, when the pd across the LDR decreases, the pd across the lamp also decreases, causing it to dim.
54
Q

What colour is the neutral wire?

A

Blue

55
Q

What is the blue wire?

A

Neutral

56
Q

What colour is the earth wire?

A

Green + yellow

57
Q

What is the green + yellow wire?

A

Earth

58
Q

What colour is the live wire?

A

Brown

59
Q

What is the brown wire?

A

Live

60
Q

What is the purpose of the neutral wire?

A

Completes the circuit; carries current away.

61
Q

What is the purpose of the live wire?

A

Provides the alternating potential difference flowing to the fuse.

62
Q

What is the purpose of the earth wire?

A

Protects the circuit and stops the appliance casing from becoming live. Only carries current if there is a fault.

63
Q

What is the pd of the neutral wire?

A

0V

64
Q

What is the pd of the live wire?

A

About 230V

65
Q

What is the pd of the earth wire?

A

0V (unless there’s a fault)

66
Q

What happens if you touch the live wire?

A
  • A large pd is produced across your body
  • A current flows through you
  • Causes an injuring/lethal electric shock

Even if the socket is turned off, although a current isn’t flowing, there is still a pd in the live wire. Your body would provide a link between the mains supply and the earth, so a current would flow through you and you would get a shock.

If the link creates a low resistance path to earth, a huge current would flow and could cause a fire.

67
Q

What is the national grid?

A

A system of cables and transformers, transferring electrical power from stations to consumers.

68
Q

How does the national grid respond to changing demand?

A
  1. Predicting when peaks in demand will be.
  2. Stations run at well below maximum output, so there is spare capacity.
  3. Smaller power stations are kept in standby.
69
Q

What voltage does current in the national grid travel at?

A

400,000V.

70
Q

Why does current travel at 400,000V through the national grid?

A

The higher the current, the more collisions of electrons with each other and the wires, meaning the more energy lost to the surroundings as heat. It is more efficient to keep the current relatively low and boost the pd (to maintain the same power).

71
Q

What happens at step-up transformers?

A

Pd increased, current lowered.

72
Q

What happens at step-down transformers?

A

Pd lowered, current increased.

73
Q

Why do we need step-down transformers?

A

Electricity has to be reduced to mains pd (230V) to be safe and usable.

Physics GCSE 2020 (9 decks)

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