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Physics A - Level > Electricity (1): Electric Current > Flashcards

Electricity (1): Electric Current Flashcards

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1
Q

What is the definition of electric current?

A

The rate of flow of charge around a circuit/ flow of charge per unit time

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2
Q

What is required for a current in a circuit to be produced?

A
  • The circuit needs to be complete
  • There must be a potential difference
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3
Q

What type of particles create a current?

A

Charged particles known as charge carriers

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4
Q

In metals what are the charge carriers?

A

The conducting electrons

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5
Q

In salt solutions what are the charge carriers?

A

The ions present in the solution

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6
Q

What is the direction of conventional current?

A

+ to - terminal of battery/cell

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7
Q

In what direction do electrons flow around a circuit?

A
  • to + terminal
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8
Q

What is the unit of charge?

A

Coulomb, C

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9
Q

What is the unit of current?

A

Amp, A

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10
Q

What is the equation that relates charge flow, current and time?

A

(delta) Q = I (delta)T

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11
Q

What are the three ways that materials can be classified?

A
  • Insulator
  • Conductor
  • Semiconductor
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12
Q

What happens when a voltage is applied to an insulator?

A

No current passes through as no electrons can move through the insulator as each electron is attached to an atom and cannot move away from it

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13
Q

What happens when a voltage is applied to a conductor?

A

Current flows as the delocalised electrons (the charge carriers) are attracted towards the positive terminal

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14
Q

What are the rules of current and voltage values in a series circuit?

A

Voltage - Is split between components depending on their relative resistances
Current - Is the same across all components

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15
Q

What are the rules of current and voltage values in a parallel circuit?

A

Voltage - Is the same across each loop in the parallel circuit
Current - Splits at junctions where: (sigma)I in = (sigma)I out

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16
Q

What happens when a voltage is applied to a semiconductor?

A

Semiconductors have a conductivity between that of an insulator and conductor (the amount of charge carriers is less). Therefore, a smaller current will flow comparative to a conductor

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17
Q

What is a pure semiconducting material also known as?

A

Intrinsic semiconductor

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18
Q

What is the defining characteristic difference between conductors and semiconductors?

A

The response to changes in temperature

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19
Q

What happens to the flow of current and the resistance of a semiconductor when the temperature is increased?

A
  • No. of charge carriers increase, therefore current increases
  • Resistance decreases
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20
Q

What happens to the flow of current and the resistance of a semiconductor when the temperature is decreased?

A
  • No. of charge carriers decrease, therefore current decreases
  • Resistance increases
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21
Q

What is the definition of potential difference?

A

The work done/energy transferred per unit of charge

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22
Q

What is the unit for potential difference/voltage?

A

Volts, V

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23
Q

What is the equation relating voltage, charge and energy transferred/work done?

A

V = E or W/ Q

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24
Q

What is the definition of the emf of a source?

A

The chemical energy transferred to electrical energy per unit charge that passes through a cell

NB - Emf = V term when no current flows through a circuit

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25
Q

What is the term used for the voltage across a battery/cell/source?

A

Terminal voltage

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26
Q

Why is the terminal voltage not the same as the emf when current flows?

A

Due to internal resistance of the source

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27
Q

It what case is the terminal voltage the same as the emf?

A

When no current flows (or if the question says that internal resistance is negligible)

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28
Q

What is the definition of resistance?

A

A measure of how difficult it is for charge carriers to pass through a component

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29
Q

What is the definition of 1 volt?

A

The potential difference when you convert 1 joule of moving 1 coulomb of charge through a component

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30
Q

Why aren’t devices 100% efficient at transferring chemical energy to kinetic energy?

A
  • Devices have resistance
  • Work done on the device is transferred to thermal energy as the charge carriers repeatedly collide with atoms in the device, transferring energy to them so the atoms vibrate more and more.
  • The resistor becomes hotter
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31
Q

What is the equation for power supplied/produced to/by a device?

A

P = VI

32
Q

What is the unit of power?

A

Watt, W

33
Q

What is the equation for energy transferred/work done to/by the charge carries relating voltage, current and time?

A

E or W = IV(delta)t

34
Q

What does the prefix Giga represent?

A

10^9

35
Q

Describe what a resistor is?

A

A component that is designed to have a certain resistance which is the same regardless of the current flowing through it

36
Q

When measuring the voltage across a component what type of voltmeter should be used? Why?

A

A high resistance voltmeter - so that no current can flow through

37
Q

Describe the experiment in order to produce an IV graph of a component (therefore determining its resistance)

A

Dependant Variable: Changing the resistance of variable resistor
Independent Variable: current and pd values

Equipment:
- Cell/Batter
- Variable Resistor -> varies values of current and voltage
- Ammeter in series
- High resistance Voltmeter across the component

Method
- Adjust the variable resistor by each step and record the the current and voltage, from the ammeter and voltmeter respectively, at each of these steps

Data Analysis
- Plot a graph of current against pd

38
Q

What does Ohm’s Law state

A

That the potential difference across a metallic conductor is proportional to the current through it, provided the physical conditions (e.g temperature) do not change

39
Q

What do IV graphs show?

A

The IV characteristics of a component, how the current across the component changes as the voltage is increased

40
Q

What does the gradient of a V-I graph represent?

A

resistance

41
Q

What does the gradient of an I-V graph represent?

A

1/resistance

42
Q

What is an Ohmic conductor?

A

One that follows Ohms Law and whose resistance remains constant no matter the current flowing through it

43
Q

For a conductor of length, L and uniform cross-sectional area, A, what is the equation for its resistance, R?

A

R = pL/A

p (slanted) = resistivity

44
Q

As length of a conductor increases resistance…?

A

Increases

45
Q

As uniform cross-sectional area increases resistance…?

A

Decreases

46
Q

What is the equation for the resistivity of a sample of material length L, cross sectional area A?

A

resistivity, p = RA/L

47
Q

What is the unit of resistivity?

A

ohm(metre)

omega(m)

48
Q

How do you determine A from the diameter/radius of a conductor?

A

A = (pi)d^2/4

49
Q

Do resistors follow ohms law?

A

Yes

50
Q

Do metal wires follow ohms law?

A

Yes

51
Q

Do components follow ohms law?

A

no

52
Q

Describe the experiment in order to determine the resistivity of a component

A

Dependant Variable: Resistance, R
Independent Variable: Length of wire

Equipment:
- 1m length conducting wire
- Voltmeter
- Ammeter
- Low Voltage, variable power supply
- Metre ruler

Method
- Measure the diameter of the conducting wire at multiple points along it using a micrometer and record the mean diameter. Use this value to calculate the uniform cross-sectional area of the wire.
- Set up the low voltage power supply, Ammeter and conducting wire in series (using crocodile clips in order to hold the wire)
- Set up a voltmeter in parallel to the wire
- Adjust the length of the wire to read 1m and record the values of current and voltage using V/I = R to determine the resistance
- Repeat twice more and find the mean R for the the length
- Repeat with other lengths

Analysis of Results:
- Plot a graph of mean R against L
- Gradient = R/L, therefore, resistivity = gradient x A

Safety:
- Disconnect the wire between readings/ use a low voltage supply

Improvements:
- Heating of the wire may cause the resistance of it to increase therefore to reduce the accuracy of your voltage and current readings -> RANDOM ERROR
- The wire should be free of kinks and held straight so that the measurement for length is as accurate as possible

53
Q

What is the definition of a superconductor?

A

A wire or device made from a material which below a certain temperature, its critical temperature, has 0 resistivity

54
Q

What is the temperature below which a superconductor has zero resistivity called?

A

Critical temperature

55
Q

Is resistivity different to resistance? How?

A

Resistivity - Intrinsic property of all materials
Resistance - changes depending on current flowing through it/ relevant to a particular circuit

56
Q

When a current passes through a superconductor and the temperature is below its critical value (in Kelvin), describe what happens?

A

There is no resistance in the device, therefore, the current has no heating effect -> makes superconductors very efficient

57
Q

What is the characteristic of superconductors called?

A

superconductivity

58
Q

As the temperature is increased the resistivity of a superconductor…?

A

increases

59
Q

Describe what a resistivity against temperature graph looks like for a superconductor

A
  • At 0K resistivity is 0
  • As the temperature is increased resistivity will remain at 0 until the critical temperature is reached
  • At the materials’ critical temperature resistivity will spike straight up to a value
  • As temperature is increased the resistivity will increase proportionally
60
Q

A superconductor with a critical temperature above 77K is known as?

A

A high-temperature superconductor

61
Q

What is a use of a superconductor?

A

TI make high power electromagnets that generate very strong magnetic fields in devices such as MRI scanners and particle accelerators

62
Q

What is the rule for resistances in series?

A

Total resistance = sum of resistances

63
Q

What is the rule for resistances in parallel?

A

1/Total Resistance = sum of :1/resistance 1 + 1/resistance 2 etc..

64
Q

Describe the IV graph for an ohmic conductor

A
  • Current and voltage are directly proportional therefore line drawn is a straight line through the origin
  • The graph shows that the resistance is constant
65
Q

Describe the IV graph for a bulb

A
  • Initially as voltage is increased current increases proportionally (either negative or positive) and ohms law is followed
  • However, as more voltage is applied the current begins to plateau
  • This is because the resistance in the filament of the bulb increases due to the heating effect -> more charge carriers collide with atoms transferring kinetic energy causing them to vibrate more
66
Q

Describe the IV graph for a diode

A
  • Diodes only allow current to flow in one direction
  • In the forward bias current increases exponentially when the threshold voltage is reached - up until then the increase in current that can flow is small
  • In the reverse bias the resistance of the diode is very high so very little current can flow
67
Q

Describe the IV graph for a thermistor

A
  • A thermistor at a constant temperature gives a straight line
  • In a higher temperature environment the resistance will be lower, therefore, the gradient will be steeper
  • In a lower temperature environment the resistance will be higher, therefore, the gradient shallower
68
Q

Describe the graph for resistance against temperature for a metal wire versus a thermistor

A
  • At 0 degrees C both the metal wire and thermistor have the same resistance
  • As temperature increases the resistance of the metal wire will increase proportionally to the temperature (line is straight)
  • As temperature increases the resistance of the thermistor will decrease significantly in a non-linear way until it plateaus its lowest resistance
69
Q

What is the definition of the phrase ‘positive temperature coefficient’

A

When the resistance of a component increases with temperature

70
Q

What is the definition of the phrase ‘negative temperature coefficient’

A

When the resistance of a component decreases with temperature

71
Q

What is something you need to check when looking at an IV graph?

A

Which way round the axis are:
- If IV then gradient = 1/R
- If VI then gradient = R

NB: Its how current changes as voltage is increased

72
Q

What is the definition of breakdown voltage and when does it occur?

A
  • Diodes
  • In the reverse bias the breakdown voltage is roughly at about -55V which is when the resistance is no longer infinitely high and then the current flows exponentially (the same as the forward bias)
73
Q

What is the definition of a potential divider?

A

A circuit with several (more than one) resistors in series connected across a voltage source

74
Q

What piece of apparatus should you use in an experiment in order to get the full range of voltages?

A

Use a rheostat - using a variable resistor will only be able to get you a minimum voltage (as the component in series has resistance) not 0V

75
Q

What is a rheostat?

A

A potential divider where you can change the connection to it using the slider which alters the potential difference across the parallel circuit

76
Q

When the slider is moved to the beginning of the rheostat what is the potential difference across the external circuit?

A

0V as there is no pd between the legs

77
Q

As the slider is moved across the rheostat what happens to the potential difference across the external circuit?

A

It increases from 0 -> Maximum

Physics A - Level (23 decks)

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