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Paper 3 2022 AQA Physics > Current electricity (RP 5) > Flashcards

Current electricity (RP 5) Flashcards

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1
Q

What is the resistivity of a material?

A

The resistance of a 1m length of the material of 1m2 cross-sectional area.

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2
Q

What 3 factors affect the resistance of a piece of wire?

A

1) Length
2) Area (Cross-sectional)
3) Resistivity

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3
Q

How does length of a wire affect its resistance?

A

The longer the wire, the higher the resistance (More difficult to make current flow through)

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4
Q

How does the cross-sectional area of a wire affect its resistance?

A

The wider the wire, the lower the resistance. (Easier for electrons to pass through)

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5
Q

What factors affect the resistivity of a material (not including resistance, length, area)?

A

Property of a material:

Structure of material and environmental factors (e.g. temperature and light intensity)

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6
Q

What is the unit for resistivity?

A

Ohm-metre (Ωm)

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7
Q

What is the symbol for resistivity?

A

p (Greek letter “rho”)

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8
Q

What is the equation for resistivity?

A 
p = RA/L
Where:
p - Resistivity (Ωm)
R - Resistance (Ω)
A - Area (m2)
L - Length (m)
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9
Q

Is the resistivity of a material a set quantity?

A
  • No, it depends on temperature.

* Resistivity is usually quoted at a set temperature (e.g. 25*)

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10
Q

What is the size of a typical value for the resistivity of a conductor?
Is this good for conducting?

A

Very small - e.g. 1.72 x 10^-8 Ohm meters.

Lower resistivity = better at conducting electricity

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11
Q

Describe how you can calculate the resistivity of a piece of wire.

A

Risk assessment first.

Calculate area:
1) Measure the diameter at at least 3 points along the wire using a micrometre -> Find average -> Divide by two to get radius
2) Area = πr^2
Calculate R/l:
1) Set up a circuit with an ammeter, wire and voltmeter.
2) Attach a test wire along a ruler -> Attach one end where the ruler reads 0cm
3) Move the crocodile clip at the other end to adjust the length of the wire
4) Record the length of the wire and the resistance (R = V/I), keep V constant by adjusting the power supply.
5) Repeat this to find an average resistance for that length
6) Vary the length from 0.10 to 1.00m
7) Plot a graph of resistance (y) against length (x) + draw a line of best fit (should be straight line through origin)
Find the resistivity:
1) The gradient is R/l, so it can be subbed in to the equation p = RA/l by multiplying by the area.
2) Take note to maintain the temperature of the wire constant at all times (since resistivity depends on temperature) do this by turning off the power supply between readings to avoid the wire heating.

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12
Q

When calculating the resistivity of a piece of wire, what is it important to keep constant and how?

A
  • Temperature -> Resistivity depends on it

* Only have small currents flow through the wire

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13
Q

What is the problem with most materials having some resistivity?

A

When current is passed through them, they heat up and energy is wasted as thermal energy.

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14
Q

What is the easiest way to lower the resistivity of most materials?

A

Cool them down.

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15
Q

What is a superconductor?

A

A wire or device made of a material that has 0 resistivity below a critical temperature, which depends on the material.

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16
Q

Describe how the resistance of a superconductor changes with temperature.

A
  • Below critical temperature -> Zero resistance

* Above critical temperature -> Resistance increases

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17
Q

What is the temperature below which a superconductor has no resistance called?

A

Threshold temperature / Critical temperature

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18
Q

Describe how the resistance of these components varies with temperature:
• Metal
• Semiconductor
• Superconductor

A
  • Metal -> Resistance increases as temperature increases
  • Semiconductor -> Resistance decreases as temperature increases
  • Superconductor -> Non-zero resistance above critical temperature, zero resistance below it
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19
Q

What are the positives of a superconductor and why?

A
  • With no resistance, there is no heating effect -> No energy lost
  • You could start a current using a magnet and it would flow forever
20
Q

What is the problem that makes the use of superconductors difficult?

A
  • Most have a transition temperature below 10 kelvin (-263*C)
  • It is hard and expensive to cool things that much
21
Q

What is a high-temperature superconductor?

A

A superconductor with a critical temperature above 77K (the boiling point of nitrogen).

22
Q

What is 77K the boiling point of?

A

The boiling point of nitrogen.

23
Q

Describe the graph of resistivity (y) against temperature (x) for a superconductor.

A
  • Line along the x-axis up to critical temperature
  • Vertical spike at the critical temperature
  • Straight line with upwards gradient after this
24
Q

What is the next development in terms of superconductors?

A

Room-temperature superconductors

25
Q

Give some current uses of superconductors.

A

High-power electromagnets used in:
• MRI scanners
• Particle accelerators

26
Q

Give some potential uses of superconductors.

A
  • Power cables that transmit energy without energy loss
  • Strong electromagnets without a constant power source -used in medicine and Maglev trains
  • Electronic circuits that work really fast since there’s no resistance to slow them down (therefore minimal energy loss)
27
Q

What does resistivity measure?

A

How much a particular material resists current flow

28
Q

What is a semiconductor and why does it behave like this?

A
  • Number of charge carriers increases with temperature -> Resistance decreases and resistivity decreases.
  • This is because as temperature increases, electrons break free and act as charge carriers
29
Q

What is a pure semiconducting material called and why?

A

Intrinsic semiconductor (intrinsic means natural) -> Conduction is due to electrons that break free from atoms of the material

30
Q

What are the uses of semiconductors?

A

Sensors for detecting changes in the environment (e.g. thermistors and diodes).

31
Q

Give two examples of semiconductors.

A

Thermistors and diodes.

32
Q

Compare the resistance of semiconductors and metals..

How does it vary with temperature?

A
  • Metals are better conductors (i.e. lower resistance)

* Resistance of metals increases with temperature, while resistance of semiconductors decreases with temperature

33
Q

At low temperatures, why are metals better conductors than semiconductors?

A

There are more charge carriers available.

semiconductors have electrons stuck to atoms and aren’t free moving

34
Q

What is a thermistor?

A

A resistor with a resistance that changes with temperature.

35
Q

What type of thermistor do you need to know about?

A

Negative Temperature Coefficient (NTC) - Resistance decreases as temperature increases

36
Q

How does the resistance of a thermistor change with temperature?

A

As temperature increases, resistance decreases.

37
Q

Describe the graph of resistance (y-axis) against temperature (x-axis) for a thermistor.

A
  • Downwards curve

* Gradient becomes less steep with temperature

38
Q

Describe the I/V graph for a thermistor.

A
  • Upwards curve
  • Curved upwards away from x-axis
  • Rotated around origin
39
Q

Explain the I/V graph for a thermistor.

A
  • As p.d. increases, current increases
  • This causes temperature to increase
  • More electrons have enough energy to escape from their atoms
  • More charge carriers -> Resistance decreases
  • More current can flow, so graph curves upwards
40
Q

What is a diode?

A

A component that allows current to flow in one direction only
Made from semiconductors

41
Q

What is the forward bias of a diode?

A

The direction in which the current is allowed to flow.

42
Q

What does LED stand for?

A

Light emitting diode.

43
Q

Describe how the resistance of a diode changes with potential difference.

A
  • With a negative voltage (in reverse bias) the resistance is very high
  • Up to a threshold voltage (usually about 0.6V) the resistance remains high
  • After this voltage, the resistance falls rapidly
44
Q

What is the threshold voltage of most diodes?

A

About 0.6V in the forward direction before they conduct.

45
Q

What is an LED?

A

A diode that emits light when current flows through it.

46
Q

How can you investigate the resistance of a thermistor?

A

Use water bath:

Put the thermistor into a beaker and pour boiling water into it, covering the thermistor.

Measure the temperature of the water using a digital thermometer.
Record the current through the circuit with the ammeter.

The potential difference needs to be kept constant throughout the experiment.

Keep recording current and temperature for every 5 degree drop.

From the values for current and potential difference, work out resistance for each temperature.
For NTC resistors, as temperature decreases, resistance increases (current will decrease).

Paper 3 2022 AQA Physics (3 decks)

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