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Physics The Good One > 03 electric circuits > Flashcards

03 electric circuits Flashcards

1
Q

Define current

A

A rate of flow of charge.

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2
Q

What is the gradient on a charge-time graph?

A

Current (I = Q/t).

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3
Q

How do you work out charge in a current-time graph?

A

Area under the graph (Q = It).

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4
Q

An Ammeter must…

A

Have a low resistance and must be placed in series.

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5
Q

A voltmeter must…

A

Have an infinitely high resistance and must be placed in parallel.

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6
Q

Another word for voltage is…

A

emf / potential difference.

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7
Q

Potential difference is…

A

A measure of the amount of energy or unit of charge transferred between 2 points in a circuit.

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8
Q

What is Kirchhoff’s first law?

A

The total current flowing into a point is equal to the current flowing out of that point. Conservation of Charge.

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9
Q

What is Kirchhoff’s second law?

A

Around any closed loop in a circuit, the sum of the pd across all the components is the pd of the supply. Conservation of energy.

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10
Q

Power (watts) is…

A

Rate of transferring energy/ rate of doing work.

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11
Q

1 watt of power means…

A

1 joule of energy is used every second.

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12
Q

Ohm’s law:

A

V = IR.

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13
Q

In a series circuit the current is…

A

The same across all the components.

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14
Q

In a parallel circuit the current…

A

Total current = sum of the current in all the branches.

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15
Q

Potential difference in a series circuit…

A

Total = sum of pd in all appliances.

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16
Q

Potential in parallel circuits is…

A

The same across each branch.

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17
Q

Resistance in a series circuit…

A

Total resistance = the sum of the resistance in all the components.

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18
Q

Resistance in a parallel circuit…

A

Reciprocal of Resistance = sum of all the reciprocals of the resistance of all the components.

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19
Q

Is a resistor a conductor?

A

Yes, it’s a conductor so that current can still flow around a circuit, however, resistors let less current flow than other conductors.

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20
Q

What affects the resistance of a resistor?

A

Cross-sectional area of the wire, length of the wire, material of the wire, temperature of the wire.

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21
Q

What is resistance proportional to?

A

Length.

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22
Q

What is resistance inversely proportional to?

A

Cross-Sectional Area.

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23
Q

What is the Resistivity?

A

The resistance of a 1m² 1m long sample (the resistance per unit cube). It’s a property of a material.

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24
Q

The resistivity is constant providing…

A

The temperature is constant.

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25
Q

What is the conductivity?

A

1/ resistivity.

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26
Q

On a resistance-Length graph, what does the line look like?

A

Positive straight gradient.

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27
Q

On a resistance-length graph, what does the gradient of the line tell you?

A

Resistivity/ Area.

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28
Q

What does a Resistance-Area graph look like?

A

A curve getting less steep as it goes down (negative gradient). It’s a 1/X curve.

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29
Q

What does a resistance 1/cross-sectional area graph look like?

A

Positive straight gradient.

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30
Q

What does the gradient of a resistance 1/cross-sectional area graph mean?

A

Resistivity multiplied by length.

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31
Q

Prove 1/R1 + 1/R2 + 1/R3 = 1/R:

A

Itotal = i1 + i2 + i3; i = v/r, hence v/Rtotal = v/R1 + v/R2 + v/R3; v is constant and is the same across all branches, therefore 1/R1 + 1/R2 + 1/R3 = 1 / R.

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32
Q

What is a potentiometer?

A

A device that provides a potential difference ranging from 0V to Vs (voltage of supply).

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33
Q

Length needed =

A

Length (of wire) * (voltage out / total voltage).

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34
Q

What do the letters stand for in I = nAvq?

A

I = Current (A)
, v = Drift velocity (m/s),
A = Cross-sectional area of the conductor (m²)
, n = Charge density (m⁻³),
q = Charge on each charge carrier (C).

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35
Q

What is the charge density?

A

The number of charge carriers that can move per m³.

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36
Q

A poor conductor has ___ charge carriers:

A

Few.

37
Q

A GOOD conductor has ___ charge carriers:

A

Many.

38
Q

4V means 4J per…

A

Coulomb of charge.

39
Q

5Ω resistance means 5V is needed for…

A

1A of current.

40
Q

Vout / Vsupply =

A

R2 / (R1 + R2).

41
Q

Prove Vout / Vsupply = R2 / (R1 + R2):

A

For the whole circuit
Vsupply = I(R1 + R2)
; Vout = IR2, I = Vout / R2;
Vsupply = Vout / R2 x (R1 x R2)
R2 / (R1 + R2).

42
Q

Define emf:

A

Energy supplied per unit of charge by the supply.

43
Q

Define terminal pd:

A

Energy transferred per unit of charge to the load.

44
Q

Define lost volts:

A

Energy transferred per unit of charge as the electrons flow through the cell.

45
Q

What is the difference between R and r?

A

R denotes the load resistance and r denotes the internal resistance.

46
Q

On a graph of current against voltage, what does the y-intercept and gradient show?

A

Y-intercept shows the emf, gradient shows the internal resistance.

47
Q

What happens to the graph of current against voltage if there are two cells?

A

The gradient is twice as steep and the intercept is twice as far up.

48
Q

Define thermistor:

A

Resistor whose resistance varies with temperature.

49
Q

Define LDR:

A

A light-dependent resistor is a resistor whose resistance varies with light intensity.

50
Q

LDR relationship with incident light:

A

As more light is incident, the resistance decreases.

51
Q

What are the two types of thermistors?

A

Negative temperature coefficient (NTC) and positive temperature coefficient (PTC).

52
Q

What is an NTC?

A

A thermistor where as the temperature increases, the resistance decreases.

53
Q

What is a PTC?

A

A thermistor where as the temperature increases, the resistance increases.

54
Q

What are the two factors which describe how thermistors work (PTC, NTC, and LDR, the first factor)?

A
  1. Increased temperature means increased lattice vibrations, which leads to more electron collisions. This reduces v in I = nAqv and reduces the current (i.e., increases resistance).
55
Q

What are the two factors which describe how thermistors work (NTC and LDR, the second factor)?

A
  1. Energy absorbed (light or heat) results in the release of conduction electrons (this increases n in I = nAqv so increased current, therefore reduced resistance).
56
Q

How does current flow?

A

Free electrons (already in the conductor) are repelled by the negative terminal and attracted by the positive terminal. The positive terminal is the long side of a cell. The arrows in a circuit are drawn the opposite way around.

57
Q

Insulators:

A

Have a low number of electrons per m³ and so do not conduct well.

58
Q

Number of electrons =

A

Total charge / charge of an electron.

59
Q

1 coulomb:

A

The amount of charge that passes a point where a current of 1 amp flows for 1 second.

60
Q

1 volt of p.d.:

A

A joule of electrical energy transferred for each coulomb of charge.

61
Q

Basic points of I = nAvq proof:

A

Wire volume = Ad (area * length); number of electrons in volume of wire = nAd;
total charge of all electrons = nAdq
I = Q/t, so I = nAdq/t;
V = d/t, so I = nAvq.

62
Q

define resistance

A

The ratio of the p.d. applied across it to the current passing through it.

63
Q

What affects the resistance of a resistor/wire?

A

• Length (longer means more resistance), • Cross-sectional area (smaller means more resistance), • Temperature (in metals, a hot wire means more resistance), • Type of material.

64
Q

define Resistivity ρ:

A

The resistance of a 1m² 1m long sample, measured in ohmmeters.

65
Q

Filament lamp current voltage graph:

A

The resistance increases as the current increases. As the wire gets hotter, the current doesn’t increase as much.

66
Q

LED/diode current voltage graph:

A

Semiconductor; therefore, the reverse diode has very high resistance, and the forward diode has very low resistance. They only conduct when they are forward biased.

67
Q

As temperature increases, resistance increases because (in metals):

A

The ions in the metal lattice vibrate faster, with greater amplitude. This means electrons collide more with ions, opposing the flow and increasing resistance.

68
Q

As temperature increases, resistance decreases because (in semiconductors):

A

At room temperature, there are few free electrons available for conduction. At high temperatures, some electrons have enough energy to ‘escape’ from their atoms and be able to conduct.

69
Q

what is a Superconductors

A

If a wire is cooled to a low enough temperature, it loses all its resistance. Electrons flow through them without any transfer of energy.

70
Q

what is a Fuse

A

A thin wire that melts if the current gets too high.

71
Q

Kilo-watt hour:

A

The electrical energy transferred by a 1kW device in 1 hour.

72
Q

Variable resistor uses:

A
  • Control current (rheostat)
    , * To control voltage (potentiometer).
73
Q

how to work out the Root mean square:

A

Square all the values to make them positive. Then find the mean, then square root this mean.

74
Q

Root mean square uses in electricity:

A

Finding an average voltage or current when it’s alternating. (A regular average would just be zero).

75
Q

what is the Potential:

A

The amount of energy available at a point in a circuit.

76
Q

How does the potential along a wire vary with distance?

A

As distance increases, potential decreases.

77
Q

What is current?

A

A flow of charged particles which are already in the conductors. Free electrons are repelled by the negative terminal and attracted by the positive terminal.

78
Q

Which way does current flow?

A

Short side of the cell to the long side of the cell (negative to positive, it’s drawn the other way around as conventional current).

79
Q

Precautions to obtain accurate values for temperature when making a thermistor:

A

• Stir the water / make sure the thermometer and coil are in the same part of the beaker, • Check for zero error, • Read temperature at eye level to avoid parallax, • Switch on/off between readings to avoid the wire heating up, • Use a small current to avoid the wire heating up.

80
Q

Explain why resistance increases using the structure of the metal:

A

• As temperature increases, the lattice ion vibrations increase. Therefore, electrons will collide more frequently with the vibrating ions. • More energy dissipated by collisions means greater V required. • Since V increases and R = V/I, R will increase with temperature.

81
Q

Explain, in terms of energy, the difference between potential difference (p.d.) and electromotive force (e.m.f.):

A

• p.d. is electrical energy transferred between two points in a circuit, • emf is the energy supplied to the circuit.

82
Q

What happens inside a cell as load resistance increases?

A

• Total resistance increases. • If emf remains constant, then the current inside the cell must increase.

83
Q

Why may large wires be used?

A

• Prevent energy dissipation, • Allows large currents, • Lower cable resistance.

84
Q

Vout/Vsupply = for a potentiometer:

A

Given length / full length.

85
Q

Vout/Vsupply = for a potential divider:

A

R2 / (R1 + R2); R2 is the resistor with the voltmeter in parallel over it.

86
Q

If there is a parallel loop over part of a series circuit and a component blows…

A

The resistance of the parallel increases, therefore it gets more voltage.

87
Q

What does increasing r do?

A

Reduces the terminal pd, V due to the lost volts, so less energy/power output to load.

88
Q

Frequency of mains supply:

A

50Hz.

Physics The Good One (16 decks)

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