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Electricity Flashcards

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1
Q

What is electric current measured in

A

Amperes (A)

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2
Q

Define electric current

A

Rate of flow of charge

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3
Q

Electric Current equation

A

I = Change in Q / Change in t

I = Current (A)
Q = Charge Transferred (C)
t = Time (s)

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4
Q

Describe current

A

Amount of charge passing a given point in a given circuit per unit time

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5
Q

What is 1 A the same as

A

One coulomb of charge passing a given point per second (1 C s^-1)

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6
Q

What is electrical charge

A

A physical property measured in coulombs (C)

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7
Q

Define Coulomb

A

The electric charge flowing past a point in one second when there is an electric current of one ampere

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8
Q

What is one coulomb equivalent to

A

One ampere second (A s)

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9
Q

What is e

A

Elementary charge

1.6 x 10^-19 C

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10
Q

Equation for net change on an object

A

Q = +-ne

Q = net charge on the object in coulombs
n = number of electrons
e = elementary charge

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11
Q

Describe charge on an object

A

Quantised - can only have certain values (integer multiples of e)

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12
Q

When is an ammeter used and how

A

Measure the electric current at any point in a circuit

Placed directly in series and at the point where you want to measure the current

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13
Q

Why are ammeters placed in series

A

They should have the lowest possible resistance - reduce the effect resistance has on current

Ideal ammeter has zero resistance- no effect on current it measures

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14
Q

Structure of a metal

A

Lattice of positive ions surrounded by delocalised electrons

Positive ions are not free to move - vibrate around fixed points - vibrate more vigorously when temperature increases

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15
Q

Why may current get larger

A

Greater number of electrons moving past a given point each second

Same number of electrons moving faster through metal

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16
Q

What does the conservation of charge state

A

Electric charge can neither be created nor destroyed

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17
Q

Kirchhoffs first law

A

For any point in a circuit - the sum of current into that point is equal to the sum of current out of that point

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18
Q

What is number density

A

Number of free electrons per cubic metre of material

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19
Q

Conductors number density

A

Order of 10^28 m^-3

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20
Q

Insulators number density

A

Much lower than conductors

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21
Q

Semiconductors number density

A

In between insulators and semiconductors

10^17 m^-3

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22
Q

What do semiconductors need to do carry the same amount of electrons as conductors and why

A

They need to move much faster

Lower number density

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23
Q

Equation for current with v

A

I = Anev

I = electric current (A)
A = cross sectional area (m^2)
e = elementary charge (C)
v = mean drift velocity (ms^-1]

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24
Q

Derive I = Anev

A

I = Change in Q / Change in T

I = neV / Change in T

V/ Change in T = Av

I = neV / Change in T = neAv

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25
Q

What is Potential Difference

A

Measure of the transfer of energy by charge carriers

Measured in Volts

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26
Q

What is one Volt

A

P.D across a component when 1 J of energy is transferred per unit charge passing through the component

1 V = J C^-1

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27
Q

Define Potential Difference

A

Energy transferred from electrical energy to other forms per unit charge

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28
Q

Potential Difference equation

A

V = W/Q

V = P.D (Volts)
W = Energy transferred by Q
Q = Charge (C)

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29
Q

What is a Voltmeter

A

Component used to measure P.D which is always connected in parallel

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30
Q

Conditions for ideal voltmeter

A

In parallel and infinite resistance - no current will pass through the voltmeter

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31
Q

Define EMF

A

Energy transferred from chemical energy to electrical energy per unit charge

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32
Q

EMF equation

A

W / Q = Weird E

Weird E (V)
Q = Charge (C)
W = Energy transferred by Q

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33
Q

Energy transfer equation (charges)

A

W = VQ

W = Weird E x Q

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34
Q

What is an electron gun

A

An electrical device used to produce a narrow beam of electrons

35
Q

What is thermionic emission

A

Emission of electrons through heat

36
Q

How does an electron gun work in most cases

A

A small metal filament is heated by an electric current - electrons in the wire gain kinetic energy - some of them gain enough KE to escape from the surface of the metal

37
Q

Heated filament in a vacuum - high p.d applied between filament and an anode - explain what happens

A

Filament acts as a cathode - free electrons accelerate towards the anode - gaining KE

If anode has a small hole - electrons in line with the hole can pass through it - gives rise to a beam of electrons with a specific KE

38
Q

Energy transfers in Electron Gun

A

Electrons accelerate towards anode - gain KE - Work done on single electron from cathode to anode is eV

Work done on electron = 1/2mv^2 (assumes electrons have negligible kinetic energy at cathode)

39
Q

What is resistance

A

The opposition to a flow of electric current

40
Q

How to determine resistance in a circuit

A

Measuring the current in a component and the p.d in a component

Using R = V/I

41
Q

What is the unit of resistance

A

Ohm

42
Q

Define the ohm

A

Resistance of a component when a P.d of 1V is produced per ampere of current

1 Ohm = 1 V A^-1

43
Q

Deduction made from IV graph for resistors

A

Straight line through O

P.D across resistor is directly propertional to the current in the resistor

Resistor obeys Ohms Law

Resistance of resistor is constant

Resistor behaves same way regardless of polarity

44
Q

Deductions made from IV graph of a filament lamp

A

Passes through O - straight through the middle - curves off

P.d is not directly proportional to current

Does not obey V = IR - non ohmic component

Resistance is not constant

45
Q

What is a diode

A

A component which allows a current in one particular direction

46
Q

What do LEDs emit

A

Light of a certain wavelength

47
Q

IV characteristic for a diode

A

P.D across a diode is not directly proportional to the current

Non-ohmic component

Resistance is not constant

Behaviour depends on polarity

Gradient stays flat until threshold p.d and slowly increases

48
Q

What factors affect resistance

A

Temperature

Material

Length

Cross sectional area

49
Q

Define resistivity

A

Electrical property of a material

50
Q

Relationship between resistance of a wire and it’s length

A

Directly proportional

51
Q

Relationship between resistance of a wire and cross sectional area

A

Cross sectional area increases - resistance decreases

R is directly proportional to 1 / A

52
Q

Calculate resistivity from resistance

A

R is directly proportional to L / A
Resistivity is the constant

R = (Resistivity x length) / Area

53
Q

Resistivity unit

A

Ohm meter

54
Q

Define Resistivity

A

Resistivity of a material at a given temperature is the product of the (resistance of the material and its cross sectional area) divided by length

55
Q

Resistivity equation

A

Weird p = RA/L

56
Q

Resistivity relationship with temperature

A

As temperature increases - resistivity increases

57
Q

Deduce resistivity from a graph

A

Multiplying gradient by cross sectional area

Gradient = resistivity / A

58
Q

What does negative temperature coefficient do to a component

A

Resistance drops as the temperature increases

59
Q

What is a thermistor

A

A semiconductor with a negative temperature coefficient

Change in resistance is often dramatic

60
Q

Thermistor experiment

A

Use a water bath to control the temperature of a thermistor

An ohmmeter for a quick and simple recording of the resistance

Ammeters and voltmeter CAN be used to measure current and P.D

Resistance calculated through R = V/I

61
Q

IV Characteristics of a thermistor

A

Temperature increases as current increases

Temperature increase leads to a drop in resistance - number density of charge carriers increase

Straight gradient curves vertically (graph)

62
Q

What is a LDR

A

A Semiconductor in which the number density of charge carriers changes depending on light intensity of the incident light

63
Q

When do LDRs have high and low resistance and why

A

Dark conditions - high resistance
Number density of free electrons is low

Bright conditions - low resistance
Number density of free electrons is high

64
Q

Investigate LDR

A

Resistance of LDR varies with distance form a constant light source

Narrow tube made of black cardboard placed around LDR greatly reduces the effect of other background sources of light

Will see a curved L graph

65
Q

Equations for electrical power

A

P = VI
Leads to (P = IR x I) = (P = I^2R)

P = VI and I = V/R
Leads to (P = V x V/R) = (P = V^2 / R)

66
Q

Derive P = IV

A

P = W/t
V = W/Q W = VQ
P = VQ/t Q/t = I
P = VI

67
Q

Calculate energy transferred in a circuit

A

P = W/t W = Pt
P= VI

W = VIt

68
Q

What is a kilowatt hour

A

Energy transferred by a device with power 1kW for 1h
1kWh = 3.6 MJ

J = Ws OR kWh = kW x h

69
Q

Kirchhoff’s Second Law

A

Sum of EMF = Sum of P.Ds

Total energy transferred to the charges = total energy transferred from the charges

70
Q

Current in a series circuit

A

Same in every position

Charge isn’t used up - just flows around

71
Q

EMF in a series circuit

A

EMF is shared between components

Components with greater resistances take up a greater proportion of EMF

Same rule applies for circuits with more than one source of EMF

If sources of EMF are connected with opposing polarities - sum EMF is the difference

72
Q

Resistance in a parallel circuit

A

Greater resistance of the branch - lower the current that passes through it

Changes made to one branch does not affect the other branches

73
Q

EMF and P.D in a parallel circuit

A

Total p.d in each branch = total emf from the power supply

74
Q

Resistors in series

A

Total resistance = sum of individual resistances

75
Q

Resistors in parallel

A

1 / Total Resistance = 1 / R1 + 1 / R2…

76
Q

Four key electrical relationships

A

I = change in Q / change in t

V = W / Q

P = VI

V = IR

77
Q

Size of internal resistance required for large current

A

Small internal resistance

78
Q

What is lost volts

A

When Terminal P.D is less than actual EMF

79
Q

Relationship between EMF, Terminal P.D and lost volts

A

EMF = Terminal P.D + Lost volts

80
Q

How does changing current affect terminal p.d and lost volts

A

More charges travel though the cell per second - more work done is done by charges - increasing lost volts - lowers terminal p.d

81
Q

Equation for lost volts applying V=IR to the internal resi

A

I x r (TIR)

If r is fixed - current is directly proportional to the lost volts

82
Q

EMF compared to terminal p.d

A

EMF always more than terminal p.d unless there is no current

When current is very small, EMF = P.d

83
Q

Potential Divider Equation

A

V out = (R2 / R1 + R2) x V in

84
Q

Simplest way to vary V out

A

Replace one of the fixed resistors with a variable resistor

Low voltage electric circuits use a potentiometer

Physics (19 decks)

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