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Physics AQA AS Level > Electricity Part 1 > Flashcards

Electricity Part 1 Flashcards

1
Q

To make an electric current pass round a circuit:

A

1) The circuit must be complete/closed.

2) A source of potential difference in the circuit

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2
Q

Define ‘Electrical current’

A

Electrical current is the rate of flow charge (in the wire or component)

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3
Q

Electrical current is due to…

A

….the flow of charged paticles, called charge carriers.

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4
Q

2 Examples of charge carriers?

A

1) In conducting metals, the charge carriers are delocalised electrons .
2) In a salt solution, the charge carriers would be the charged ions.

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5
Q

What is conventional current?

A

Conventional current describes the flow of current from the positive terminal to the negative terminal.

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6
Q

What is electron flow?

A

Electron flow is the opposite to the conventional current and flows from the negative terminal to the positive terminal (this is the direction of flow in reality)

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7
Q

What is the unit of charge?

A

Coulombs (C)

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8
Q

What is the unit of current?

A

Amperes (A)

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9
Q

What is 1 Coulomb defined as?

A

1 Coulomb is defined as the amount of charge that passes a fixed point in 1s if the current is 1A.

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10
Q

What is the equation to calculate charge flow in a circuit?

A

Q = It [ ΔQ = ΔI x Δt ]

Δ = Change in...
Q = Charge (in C - coulombs)
I = Current (in A - amperes)
t = Time (in s - seconds)
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11
Q

How to calculate number of electrons passing a fixed point along the wire each second?

A

Total Charge (passing a fixed point each second) / Charge of an electron = Number of electrons passing a fixed point each second)

e.g. If 1C of charge is flowing per second, the number of electrons flowing is equal to 1C / 1.6 x 10^-19 C = 6.25 x 10^18 electrons. Because 1C is carried each second (1s), 1A of current flows. So 1A is due to 6.25 x 10^18 electrons.

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12
Q

What device is used to measure the current?

A

An ammeter

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13
Q

What electrical terms can be used to describe materials?

A

1) Conductors
2) Insulators
3) Semiconductors

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14
Q

What is an electrical conductor?

A

An (electrical) conductor is a material that allows the flow of charge carriers through it towards the positive terminal, when a voltage is applied.

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15
Q

What is an electrical insulator?

A

An (electrical) insulator is a material that does not allow the flow of charge carriers through it, when a voltage is applied (the electrons remain bound to the atoms).

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16
Q

What is a semiconductor?

A

A semiconductor, is a material that conducts current but only partly. The conductivity of a semiconductor is somewhere between that of an insulator (no conductivity) and a conductor (full conductivity).

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17
Q

What is a pure semiconductor and what is it also referred to as?
*You do not need to know what a dopant specie is.

A

A pure semiconductor, AKA an ‘intrinsic semiconductor’ is a semiconductor, that does not include any dopant species (traces of impurity elements). The numer of charge carriers is therefore determined by the properties of the materials itslef than the amount of impurities.

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18
Q

For a pure semiconductor, what happens when you increase the temperature?

A

When you increase the temperature, the resistance across it decreases, as the number of charge carriers flowing through increases.

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19
Q

When a battery is not in a complete circuit…

A

…it has the potential to transfer energy from its chemical store to the electrical store of the charge carriers.

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20
Q

When a battery is in a complete circuit…

A

…each electron passing through a circuit component does work to pass through that component and therefore transfers some or all of its energy (that has been transferred from the chemical store of the battery).

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21
Q

The work done by an electron is therefore equal to…

A

The work done by an electron = the loss of energy by an electron

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22
Q

The potential difference/voltage across a component can be defined as?

A

The work done per unit charge across the component.

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23
Q

If work W is done, when charge Q flows through a component the pd across the component, V, is given by?

A

V = W/Q

V = Potential Difference (V)
W = Work done (J)
Q = Charge (C)
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24
Q

What is the electromotive force (emf) of a source of electricity?

A

The emf is defined as the electrical energy produced per unit charge passing the source.

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25
Q

What is the unit of emf?

A

The unit of emf is also the volt (V).

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26
Q

What is the equation to calculate electrical power, linking together current and voltage?

A

P = IV

P = Power (W)
I = Current (A)
V = Potential Differencce (V)
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27
Q

Define Electrical Resistance (of a component)

A

Electrical resistance of a component is a measure of how difficult it is to get a current to flow through it.

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28
Q

What is electrical resistance of a component caused by?

A

Electrical resistance of a component is caused by repeated collisions between the charge carriers and the fixed positive metal ions in the material of that component.

29
Q

For a component which passes current I, when the pd across it is V, the resistance R, is given by the equation?

A

V = IR

V = Potential Difference (V)
I = Current (A)
R = Resistance (Ω)
30
Q

1Ω is defined as?

A

1Ω = 1V per ampere

31
Q

How is an ammeter connected in a circuit?

A

An ammeter must be connected in series so that it can measure the current in the circuit (accurately).

32
Q

What is the resistance of an ammeter and why?

A
  • An ammeter has very little/zero resistance
  • so that current can flow through it
  • and it can measure the current flowing in the circuit accurately
  • (no additional resistance from ammeter).
33
Q

What device measures potential difference?

A

A voltmeter

34
Q

How is a voltmeter connected in a circuit?

A

A voltmeter is connected in parallel to the circuit (so that the pd across the component being measured, and the voltmeter, is the same)

35
Q

What is the resistance of a voltmeter and why?

A

Voltmeters have infintite resistance. This is to prevent current flowing through it, so that the ammeter in the circuit can measure the correct current.

36
Q

What is the role of a variable resistor?

A

A variable resistor is used to adjust current and pd as necessary.

37
Q

What does ohm’s law state?

A

Ohm’s law states that pd across an ohmic conductor is directly proportional to the current through it, provided that the physical conditions do not change (constant).

38
Q

What happens to the graph line (I/V characteristic), if the resistance of the resistor is decreased?

A

Steeper line (still directly proportional) with a greater gradient

39
Q

Describe the I/V characteristics of a filament lamp.

A

1) At low V, I increases proportionally.
2) As V increases, temperature of filament increases, and therefore resistance increases.
3) Rate of increase of I with V decreases (Gradient decreases)
4) Same form of the curve in negative quadrant

40
Q

Describe the I/V characteristics for a resistor.

A

1) A straight line through the origin.
2) Current is directly proportional to the potential difference, so as current doubles potential difference also doubles. Resistance remains constant (constant gradient)

41
Q

Describe the I/V characteristics of a diode

A

In a diode, current can only flow in one direction. This is because the resistance is very high in the negative direction/reverse bias, so the current flowing through it would be very tiny, but in the positive direction/forward bias, current increases with potential difference just after the origin, at a threshold voltage of 0.6V.

42
Q

Form the graphs, which of these devices are ohmic conductors?

A

Resistors are ohmic conductors, Filament Lamps and Diodes are not ohmic conductors

43
Q

The gradient of an I/V characteristic tells us?

A

Gradient tells us 1/R (if it was V/I, so Voltage on the y-axis, then gradient would simpley be R).

44
Q

Define Resistivity.

A

The resistivity of a material is defined as the resistance of a wire (made of that material) which is 1m in length and has a uniform cross-sectional area of 1m². Measured in ohm-metres (Ωm).

45
Q

For a conductor of length L and uniform cross-sectional area A, its resistance R is proportional to X and inversely proportional to Y.

A 
X = Length (the longer the wire = the higher the resistance because more collisions)
Y = Uniform cross-sectional area (the wider the wire, the lower the resistance, because the easier it is for electrons to pass through).
46
Q

What is the equation to calculate resistivity?

*Resistivity is the constant linking together resistance, cross-sectional area and length.

A

ρ = RA/L

ρ = Resistivity (Ωm - Ohm meters)
R = Resistance (Ω - Ohms)
A = Uniform cross-sectional area (m²)
L = Length (m - meters)
47
Q

How to calculate circular cross-sectioanl uniform area?

A

πr²

πd²/4

48
Q

So… what are the factors that determine resistance?

A

On top of temeprature,

1) Length,
2) Cross-sectional area
3) Resistivity of a material

49
Q

What is a superconductor?

A

A material that has zero resistivity at and below a critical temperature

50
Q

What happens when a current passes through a superconductor?

A

No pd across it, as resistance is zero. Current has no heating effect.

51
Q

What happens to the superconductor above the critical temperature?

A

Loses its superconductivity (zero resistance at critical temperature).

52
Q

Application of superconductors?

A
  • High-power electromagnets that generate very strong magnetic fields (e.g. in MRI scanners + particle accelerators)
  • Lightweight electrical motors + power cables that transfer electrical energy without energy dissipation.
53
Q

To measure the variation of current with pd for a component, you can use a:

A

1) Variable Resistor
2) Potential divider

*LOOK AT THE CIRCUIT DESIGNS

54
Q

What kind of IV characteristic graph is produced by a thermistor and LDR, at a CONSTANT TEMPERATURE and why?

CHECK IMAGE

A
  • A thermistor, at constant temperature, produces a straight line (through the origin). The current is directly proportional to the potential difference = constant gradient = value of resistance remains constant.
  • The higher the temperature, the greater the gradient of the line, as resistance falls with increase of temperature.
  • The same is obtained for an LDR if the light intensity remains constant.

(After-all, if the physical conditions remain the same, then they are basically just fixed resistors)

55
Q

What other device produces the same I/V characteristic graph?

A

An LDR

56
Q

How does a potential divider (in this case it means those with a sliding contact e.g. potentiometers) have an advantage over a variable resistor?

A

Using a potential divider (with a sliding contact) means that the potential difference across the component being measured can be reduced to zero (up to Vᵢₙₚᵤₜ) and thus the current can also be reduced to zero. This is not possible with a variable resistor.
*SMALL DIAGRAM TO SHOW THIS IN NOTES!

57
Q

.

A

.

58
Q

If the temperature of a conducting metal increases, what happens to the resistance?

A

Resistance increases with the increase of temperature.

59
Q

Why does resistance increase with temperature?

A
  • When the temperature increases, the resistance increases.
  • This is because the positive ions in the conductor have more internal energy and vibrate with greater amplitude about their fixed positions.
  • The frequency of the collisions between the charge carriers (conduction electrons) and the positive ions increases so the charge carriers do more work.
60
Q

Why does a metal have a positive temperature coefficient?

A

This is because its resistance increases with increasing temperature.

61
Q

What temperature coefficient would a thermistor made from an intrinsic semiconductor have?

A

A negative temperature coefficient this is because as the temperature increases, its resistance decreases (this is because, as we discussed, increase in temperautre results in an increased number of charge carriers).

62
Q

Why are thermistors used in temperature-sensitive components (e.g. thermometers) over metal wires.

A

The percentage change of resistance per kelvin change in temperature is much greater than a metal (effect is much more more observable, even for small changes)

63
Q

Describe the Resistance-Temperature graph for a metal wire and a thermistor.

A

At 0°C, both components have the same resistance.
As the temperature increases, the resistance of the thermistor decreases non-linearly whereas for the metal wire, the resistance increases much less but linearly over the same temperature range.

64
Q

What does the resistivity of a material depend on?

A

1) It is a constant that depends on the chemical composition/structure of the material (but not the length + cross-sectional area).
2) It is also dependant on environmental factors, such as temperature and light intensity.

65
Q

What is the effect on resistivity of a material, if the temperature is increase?

A

(Unless a semiconductor in which resistivity decreases with temperature)

  • When the temperature increases, the resistance increases.
  • This is because the positive ions in the conductor have more internal energy and vibrate with greater amplitude about their fixed positions.
  • The frequency of the collisions between the charge carriers (conduction electrons) and the positive ions increases so the charge carriers do more work.
66
Q

What is the difference between resistance and resistivity?

A

Resistivity is an electrical property of a material that is constant for that material which does not change with length or cross-sectional area, whereas, the resistance refers to only a specifc component of that material which changes with length and cross-sectional area.

67
Q

Outline an experiment to calculate the resistivity of a wire.

A

To determine resistivity, we can investigate how the resistance of a wire varies with length.

1) First calcualte a mean value for the diameter of the wire by measuring the diameter at several different points along the wire using a micrometer. This value can then be used to calculate the cross-sectional area of the wire using A = πd²/4
2) Using the circuit illustrated in notes, we can determine the pd and current across different lengths of the wire (at eqaual intervals) to calculate resistance using R = V/I
3) Next plot a graph of R against L which should produce a straight line through the origin.
4) gradient = ρ/A → gradient x A = ρ

68
Q

In a circuit, what direction of current flow should you always use?

A

Conventional current. For purpose of calculations, current flows from the positive terminal (longer line of cell symbol)

Physics AQA AS Level (16 decks)

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