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A Level OCR Physics > Electricity > Flashcards

Electricity Flashcards

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1
Q

The equation for current in terms of charge including units

A

I = Q(coulombs) / T(seconds)

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2
Q

What is the value of the elementary charge e?

A

e = 1.6x10^-19 C

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3
Q

What two things could cause an increase in rate of charge flow?

A

More electrons passing through a given point (greater cross sectional area)
The same number of electrons electrons moving faster through the wire

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4
Q

What is Kirchhoff’s first law?

A

The sum of the currents into a junction is equal to the sum of the currents leaving the junction

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5
Q

How are materials classified by conductivity?

A

Their number density (number of free electrons per cubic meter)

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6
Q

What are the three categories of conductivity of material?

A

Insulators, semiconductors, conductors

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7
Q

What is the equation for current in terms of mean drift velocity?

A

I = Anev where A is cross sectional area in meters squared and v is velocity in ms

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8
Q

How does reducing the cross sectional area of a wire affect the drift velocity if flow of charge remains constant?

A

The drift velocity increases in the smaller area to compensate

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9
Q

What is Kirchhoff’s second law?

A

The sum of the e.m.f.s is equal to the sum of the p.d.s in a closed loop

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10
Q

What can Kirchhoff’s second law be broken down to? (Interpreted as)

A

The total energy transferred to the charges in a circuit is always equal to the total energy transferred from the charges as they move around the circuit.

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11
Q

How is current affected by components in a series circuit?

A

It isn’t, it is the same all the way round

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12
Q

What is the sum of the p.d. Across components in a circuit equal to?

A

The e.m.f. (Components with more resistance get a higher share of the voltage)

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13
Q

In a parallel circuit, how much of the current will a branch with twice the resistance of another branch receive?

A

Half of the current

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14
Q

How does Kirchhoff’s second law work in parallel circuits?

A

The sum of the e.m.f. Is equal to the sum of the p.d. of all the components in that closed loop (branch of the parallel circuit)

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15
Q

What does a power source need in order to output a high current?

A

A low internal resistance, such as a car battery

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16
Q

How is energy ‘lost’ in the cell of an electrical circuit?

A

Energy is ‘lost’ to heat as work has to be done by the charge carriers in the power source, for a chemical source, this is due to reactions between chemicals.

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17
Q

What is terminal p.d.?

A

The p.d. measured at the terminals of a power source

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18
Q

How does terminal p.d. differ from e.m.f.?

A

The terminal p.d. Is lower than the actual e.m.f. due to energy lost from internal resistance, these are called lost volts.

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19
Q

What is the equation for e.m.f. in terms of terminal p.d.?

A

Electromotive force = terminal p.d. + lost volts

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20
Q

How does an increase in current affect terminal p.d. and lost volts when the emf remains constant?

A

And increase in current means more charge carriers doing work in the cell which increases lost volts and decreasing terminal p.d.

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21
Q

What is the equation for lost volts?

A

V (Lost volts) = I (current) x r (internal resistance)

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22
Q

What’s the equation for emf from a power source (derived from e = v + lost volts)

A

emf = V (terminal p.d.) + I (current) x r (internal resistance) or emf = I (current) x (R (resistance) + r (internal resistance))

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23
Q

What is Kirchhoff’s second law?

A

The sum of the e.m.f.s is equal to the sum of the p.d.s in a closed loop

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24
Q

What is the equation for potential difference in terms of charge?

A

V = W(energy transferred) / Q(charge)

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25
Q

What property would an ideal voltmeter have in order to function most efficiently?

A

It would have infinite resistance so when connected, no current would actually pass through

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26
Q

What is voltage in terms of work done by charge carriers?

A

Voltage is work done by the charge carriers, thus, the charge carriers are losing energy as they pass through components

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27
Q

Def electromotive force? (e.m.f.)

A

e.m.f. Is when work is done on the charge carriers, essentially the charge carriers gain energy as they travel through a component such as a cell, battery or power pack

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28
Q

What is the equation for the electromotive force?

A

E (e.m.f.) = W (energy transferred) / Q (charge)

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29
Q

What is the name of the process when electrons gain enough energy to escape the surface of a metal?

A

Thermionic emission - the emission of electrons through the action of heat

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30
Q

How does an electron gun work?

A

A heated filament is placed in a vacuum and a high p.d. Is applied between the filament and an anode, the filament then acts as a cathode and the freed electrons accelerate towards the anode gaining kinetic energy as they go. The electrons then go through a small hole in the anode creating a beam of electrons with a specific kinetic energy

31
Q

What is the equation for calculating the work done on a single electron travelling from the cathode to the anode in the electron gun experiment? (Hint, not kinetic energy)

A

W (work done) = e x v (accelerating p.d.)

32
Q

What is the equation for the work done on an electron to increase its kinetic energy?

A

eV (work done on an electron) = 1/2 x m (mass) x v^2 (velocity)

33
Q

What assumption is being made when calculating the work done on an electron to increase it’s kinetic energy (in the electron gun experiment)

A

The electrons have negligible kinetic energy at the cathode (initial energy is almost 0)

34
Q

How does increasing the accelerating p.d. In the electron gun experiment affect the velocity of the electrons?

A

It increases the kinetic energy (more energy being transferred to the electrons) so they have a greater velocity.

35
Q

What is the equation for resistance?

A

R (resistance) = p.d. (Voltage) / Current
V = IR

36
Q

What is the definition of an ohm?

A

The resistance of a component when a p.d. of 1 is produced per amp of current

37
Q

What is Ohm’s law?

A

For a metallic conductor kept at a constant temperature, the current in the wire is directly proportional to the p.d. across its ends.

38
Q

Why does resistance increase when heat increases?

A

When heat increases, the positive ions in the metal gain more energy thus vibrate more about their mean positions so the frequency of collisions between the positive ions and charge carriers increases so the charge carriers do more work (transfer more energy) as they travel through the wire.

39
Q

What does the I-V graph for a fixed resistor look like?

A

Straight line through the origin

40
Q

What is a resistor called if it obeys ohms law?

A

It is an ohmic conductor.

41
Q

How does a resistor behave under reversed polarity?

A

The same

42
Q

What is the relationship between the voltage and current in a filament lamp I-V graph and what does this say about the lamp?

A

Voltage is not directly proportional to current so is a non ohmic conductor and its resistance is not constant.

43
Q

How does the behaviour of a filament lamp change with polarity?

A

It behaves the same

44
Q

What is a main reason that LEDs are more effective than other bulbs?

A

They do not get hot so do not have much resistance and draw much less power

45
Q

What does the I-V graph of a diode look like?

A

Flat up until the origin where it starts to curve upwards before increasing at a linear rate

46
Q

What is the relationship between the p.d. and current in an LED and what does this say about and LED?

A

P.d. Is not directly proportional to current in an LED and so can be described as a non ohmic conductor

47
Q

How does the behaviour of an LED change with polarity?

A

A negative voltage will cause an infinite resistance in the LED so it will not work

48
Q

What is the name of the point on an I-V graph for an LED where the resistance starts to decrease?

A

The threshold p.d.

49
Q

What three factors (aside from temperature) affect resistance?

A

The material
The length of the wire L
The cross-sectional area of the wire A

50
Q

What is the relationship between Resistance and the length of a wire?

A

Resistance is directly proportional to the length of a wire (double R = double L)

51
Q

How is resistance related to cross-sectional area?

A

Resistance is inversely proportional to cross sectional area (2R = 1/2A)

52
Q

What is the equation for resistivity?

A

R (resistance) = (P (resistivity) x Length) / A (cross-sectional area)

53
Q

What does it mean if a material has a negative temperature coefficient?

A

It’s resistance drops as the temperature increases

54
Q

What is a thermistor made of?

A

A material with a negative temperature coefficient

55
Q

Where are thermistors used?

A

Thermometers
Thermostats
Engine temperature monitors

56
Q

What does the I-V graph for a thermistor look like?

A

It is an upwards curve (downwards cure in the negative V)

57
Q

How does the resistance of an LDR change with surrounding conditions?

A

As the surroundings get brighter, resistance decreases

58
Q

How does an LDR work?

A

When the light intensity increases, the resistance decreases because the number density of charge carriers increases.

59
Q

Define potential difference

A

The amount of work done per unit charge

60
Q

Define the volt

A

The potential difference across a component is 1 volt when you do 1 joule of work moving 1 coulomb of charge through a component

61
Q

Define mean drift velocity

A

The average velocity of all the charge carriers

62
Q

What are the charge carriers in liquids and gasses?

A

Ions

63
Q

What does the IV graph for a thermistor look like?

A

It is an upward curve in the top right and a downward curve in the bottom left

64
Q

Define Power in terms of an electrical circuit

A

Power is the rate of energy transfer

65
Q

What are 2 equations for electrical power?

A

P = W (work done in watts) / T (time)
P = VI

66
Q

What is the equation for work done in terms of power?

A

W = Power x Time
Also, W = VIT (voltage x current x time)

67
Q

When working with power, what should you always look out for?

A

Of the units, most questions deal with kWh but some only Wh

68
Q

What causes resistance?

A

Electrons collide with atoms and lose energy

69
Q

How do you calculate the total e.m.f. for cells in series?

A

Add together the individual e.m.f.s of all the cells in parallel

70
Q

How do you calculate the total e.m.f. for identical cells in parallel?

A

total e.m.f. will equal the e.m.f. of the individual cells (e = e1 = e2 etc)

71
Q

How can you measure terminal p.d?

A

Attach a voltmeter to the power supply terminals. This will be just slightly less than the e.m.f. due to lost volts.

72
Q

What is the equation to calculate output voltage in a potential divider circuit? And which resistor is the output attached to?

A

Vout = (R2 x Vin) / (R1 + R2) where R2 is the resistor connected in parallel with a voltmeter

73
Q

What is a potentiometer?

A

A potentiometer is a potential divider that uses a variable resistor instead of R1 and R2

74
Q

What ratio describes the distribution of p.d in a potential divider circuit? Explain what it means

A

V1 / V2 = R1 / R2. If a component has a higher resistance then it will receive a higher share of the voltage. Or, the greater the share of the total resistance, the greater the p.d supplied.

A Level OCR Physics (15 decks)

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