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Sensing Flashcards

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1
Q

What are electronic sensors?

A

Things designed to sense things we either can’t or don’t want to and so any change in whatever the sensors detecting will change the current in the connected circuit and the current is processed to give a reading

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2
Q

What is current?

A

The rate of flow of charged particles
(no. coulombs transferred per second)

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3
Q

What is the defining equation for current?

A

I = Q/t

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4
Q

How do you define one coulomb?

A

The amount of charge that passes a point in one second when the current is ones amp

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5
Q

How do you measure current and how do you connect it in circuit?

A

Ammeter attached in series

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6
Q

What direction does conventional current flow?

A

From positive to negative
(Conventional current is in the opposite direction to electron flow)

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7
Q

How you make electric charge flow through a conductor?

A

Do work on it

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8
Q

What is potential difference?

A

Energy transferred per unit charge

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9
Q

How do you define one volt?

A

When you convert one joule of energy moving one coulomb of charge through the component

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10
Q

How do you measure voltage and how is it connected in a circuit?

A

Voltmeter connected in parallel

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11
Q

What is power?

A

The rate of transfer of energy

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12
Q

How do you calculate power in electrical circuits?

A

P=IV

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13
Q

What is the defining equation of power?

A

P=W/t

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14
Q

What does the amount of current you get for a particular p.d. depend on?

A

Resistance

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15
Q

What is a simplified definition of resistance?

A

How difficult it is for current to flow

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16
Q

What is the defining equation of resistance?

A

R=V/I

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17
Q

What is power dissipation?

A

The rate at which a component converts electrical energy into other types of energy

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18
Q

What does a shallow gradient of an I-V graph mean?

A

Component has a high resistance

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19
Q

What does a curved line on an I-V graph mean?

A

The resistance changes with the potential difference across it

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20
Q

How do you investigate the I-V characteristic of a graph?

A

Use a variable resistor to alter the potential difference across a component and therefore its current, and then record V and I.

Repeat measurements and take averages to reduce the effects of random errors

Plot a graph of the results and you can then use the I-V graph to see how resistance changes

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21
Q

What are ohmic conductors?

A

Conductors that obey ohms law, meaning that resistance is always constant

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22
Q

What is Ohm’s law?

A

Provided external factors such as temp. are constant, the current through an ohmic conductor is directly proportional to the potential difference across it

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23
Q

Describe the I-V graph of an ohmic conductor

A

Straight line through the origin

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24
Q

Describe the I-V graph of a filament lamp?

A

A curve which starts steep and gets shallower as potential difference increases

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25
Q

Explain the shape of the I-V graph of a filament lamp?

A

Current flowing through the lamp increases its temperature so its resistance increases as current increases.

Filament lamp is just a coiled up length of metal wire so temp. increases resistance as metal ions in the wire vibrate more making it more difficult for electrons to flow

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26
Q

What is a thermistor and how are they used?

A

A resistor whose resistance depends on its temperature so they can be used as temp. sensors

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27
Q

How does the resistance of a NTC thermistor vary with temperature?

A

Resistance decreases as temp. increases

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28
Q

What is the shape of the I-V graph of a thermistor?

A

A curve with increasing gradient (starts shallow and gets steeper)

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29
Q

What is an LDR?

A

Light dependant resistor so the more light falls on it the lower its resistance

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30
Q

What is a diode?

A

Only allows current to flow in one direction

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31
Q

What direction do diodes allow current to flow?

A

In the direction that the circuit symbol points to

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32
Q

What is the shape of the I-V graph of a diode?

A

Nothing to the left of axis and then at around 0.6V (threshold voltage) a straight line graph

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33
Q

What three things determine the resistance of a wire?

A

length
cross sectional area
resistivity of the material

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34
Q

What does a longer wire mean for resistance?

A

Greater resistance as more difficult to get current to flow

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35
Q

What does a greater cross sectional area mean for resistance?

A

The wider it is the easier it is for electrons to flow so the lower the resistance

36
Q

What is the resistivity of a material and its units?

A

The resistance of 1m length with a 1m2 cross sectional area (Ωm)

37
Q

What is the conductivity of a material and its units?

A

The conductance of a 1m length with 1m2 cross sectional area (Sm-1, siemens per metre)

38
Q

What is conductance?

A

The opposite of resistance so a measure of how easy it is for electrons to flow

39
Q

How do you find the resistivity of a wire?

A

Measure the cross sectional area of the wire by measuring the diameter in three different places with a micrometer and taking an average and using A=πr2

Then connect the wire in a circuit with a voltmeter and ammeter and vary the length of wire connected to the circuit by moving one of the crocodile clips

Record the length of the wire and the ammeter and voltmeter readings

Then plot a graph of R against L and draw a line of best fit. The gradient = resistivity/area

40
Q

What assumptions do you need to make about a wire when finding its resistivity?

A

The wire is cylindrical with uniform cross sectional area

41
Q

What must be kept constant when finding the resistivity of wire?

A

Temperature

42
Q

What does how conductive a material depend on?

A

The number density of mobile charge carriers

43
Q

What makes a good conductor?

A

High density of mobile charge carries

44
Q

Why are metals good conductors?

A

In a metal the charge carriers are delocalised electrons meaning they have a high density of mobile charge carriers

45
Q

How does temperature affect the conductivity of metal and why?

A

As temp. increases the lattice vibrates more which increases the electron scattering so they are slightly less free to move meaning conductivity decreases (resistivity increases)

46
Q

What are conductors?

A

Materials (such as metals) which have a high proportion of mobile charge carriers and so can conduct electricity well

47
Q

What are semiconductors?

A

Materials with a low proportion of mobile charge carriers but often the no. increases with a factor such as light or temp which enables them to conduct electricity. However under normal conditions they do not conduct very well

48
Q

What are insulators?

A

Materials with no or very few mobile charge carriers meaning they don’t conduct electricity

49
Q

What is a perfect insulator?

A

Has no mobile charge carriers

50
Q

What are some examples of semiconductors?

A

Thermistors - heat liberates electrons

LDRs - light liberates electrons

51
Q

Where does resistance come from?

A

Electrons colliding with atoms and losing energy to other forms

52
Q

What does a battery do?

A

Uses chemical energy to make electrons move
(converts chemical energy to electrical energy)

53
Q

What is internal resistance?

A

When electrons collide with electrons inside the battery creating resistance. This causes batteries and cells to warm up when they’re used

54
Q

What is load resistance?

A

The total resistance of all the components the external circuit

55
Q

What is the e.m.f?

A

Electromotive force is the amount of electrical energy the battery produces per coulomb of charge

56
Q

What is the terminal p.d?

A

The potential difference across the load resistance (the energy transferred per coulomb that flows through the load resistance)

57
Q

When is terminal pd the same as the emf?

A

When there is no internal resistance

58
Q

What are lost volts?

A

The energy wasted per coulomb in overcoming the internal resistance

59
Q

What is emf equal to?

A

ε = V + v
V = terminal pd
v = lost volts

60
Q

How do you work out the total emf of multiple cells in series?

A

Total emf is found by adding all the individual emus (given they’re all connected in the same direction)

61
Q

How do you find the total emf of multiple identical cells in parallel?

A

For identical cells in parallel the total emf is the same size of the emf of each individual cell

62
Q

What is an ideal ammeter and why?

A

Resistance is negligible so that the voltage across it is negligible so that the ammeter doenst affect the pd across other components

63
Q

What is an ideal voltmeter and why?

A

Very high internal resistance so that the current through them is so low you can assume its negligible, this is so the voltmeter doesn’t affect the current through the component its connected to

64
Q

Why do we include switches in circuits?

A

To turn off the current when circuit not in use to reduce the effect of heating in the wires on the resistance of the circuit

65
Q

How can you investigate internal resistance and emf?

A

Set up a circuit with a battery, variable resistor, ammeter and voltmeter.

Use the variable resistor to change the value of the load resistance and measure the different pds (this value is the terminal pd) and currents

Plot a graph of V against I and using the equation V = ε -Ir we know the yintercept is ε and the gradiet is r

66
Q

How do you decide the values for the load resistance when investigating emf and internal resistance?

A

A low load resistance will give a large current which will reduce percentage uncertainty in the ammeter reading but large currents causes significant heating the wires which will invalidate the results so you need a compromise

67
Q

What is an easy way to measure the emf of a power source?

A

Connect a voltmeter across its terminals (voltmeter has negligible current so any difference between actual value and measured value will be so small its insignificant)

68
Q

What is conserved in an electrical circuit?

A

Energy and charge

69
Q

How is charge conserved in a circuit?

A

Charge doesn’t get used up or lost

70
Q

What is Kirchhoff’s first law?

A

The total current entering a junction = total current leaving it

Charge is conserved

71
Q

What is the difference between emf and potential difference?

A

emf is the energy transferred to charge and pd is the energy transferred from a charge

72
Q

What is true about emf and pd in a closed loop?

A

They are the same (as energy is conserved)

73
Q

What s Kirchhoff’s second law?

A

total emf around a series = the sum of the pds across each component

ε = ΣIR

74
Q

In series circuit what is true of current, p.d and resistance?

A

current is the same at all points (as there are no junctions)

p.d. is split between components (Kirchhoff’s second law)

total resistance is the sum of all the resistance

75
Q

Why is current the same at all points in a series circuit?

A

There are no junctions in a series circuit

76
Q

What is true of current, pd and resistance in parallel circuits?

A

Current is split at each junction

Same pd across all components

1/Rtotal = 1/R1 + 1/R2 + …

77
Q

Why do we use potential dividers?

A

To get a fraction of an input voltage

78
Q

What is a potential divider put simply?

A

A circuit with a voltage source and a couple of resistors in series

79
Q

What is a use of potential dividers?

A

Calibrating voltmeters

80
Q

Why can’t resistors in a potential divider circuit have a low resistance?

A

Then you effectively have two resistors in parallel which will have a total resistance less than the resistance of the resistor which means Vout will be less than youve calculated

81
Q

How can potential dividers be made into sensors?

A

Include components whose resistance changes with external factors eg LDRs, thermistors

82
Q

What is a potentiometer?

A

Uses a variable resistor to give a variable voltage

83
Q

When you make a potential divider that works as a sensor what must you do and why?

A

Calibrate the circuit so you know how the voltage across the component and Vout varies as external factors change

84
Q

What goes on the axis of a thermistor calibration curve and how can it be used?

A

Plot temperature against voltage

You can then find the temperature if you know the voltage across it effectively making it a thermometer

85
Q

Why is there a potential difference across the plates of a battery?

A

The two terminals have different charge built up on each plate which is stored as electrical potential energy meaning there is a potential difference

86
Q

What is the mean drift velocity of electrons in a wire?

A

The average distance of an electron travelled along the wire per second. The electrons move slowly in one direction as they are constantly colliding

Physics (16 decks)

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