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Physics > Electricity > Flashcards

Electricity Flashcards

1
Q

current

A

rate of flow of charge

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2
Q

potential difference

A

work done per unit charge

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3
Q

resistance

A

voltage / current

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4
Q

equation linking current, charge and time

A

Current = change in charge / change in time

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5
Q

Equation in terms of voltage, work done and charge

A

Voltage = work done / charge

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6
Q

conventional current

A

flow of positive charge from the positive terminal of a cell to the negative terminal

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7
Q

why is conventional current the opposite to electron flow

A

conventional current was described before electric current was understood

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8
Q

what is current measured in

A

Amps

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9
Q

how do you measure current

A

using an ammeter, connected in series

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10
Q

how do you measure PD

A

using a voltmeter, connected in parallel

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11
Q

what is PD measured in

A

volts

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12
Q

what is resistance

A

the opposition to current

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13
Q

the higher the resistance ….

A

the lower the current

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14
Q

why are wires commonly made of copper

A

it has a low electrical resistance ( it is a good conductor)

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15
Q

what is resistance measured in

A

Ohms

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16
Q

what is Ohms law

A

voltage is proportional to current under constant physical conditions (temperature)

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17
Q

constant temperature =

A

constant resistance

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18
Q

how do you set up a circuit to investigate the relationship between PD across an electrical component and the current

A

set up a circuit with a cell, and a variable resistor and a voltmeter and another electrical component

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19
Q

what does the IV graph of a fixed resistor look like

A

a straight line through the origin

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20
Q

how can you tell if an electric component obeys Ohms law

A

if its IV graph is a straight line through the origin

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21
Q

what are some ohmic electrical components

A

fixed resistor and wire

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22
Q

what are some non-ohmic electrical components

A

diode, filament lamp bulb, thermistor

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23
Q

what does a filament lamp do

A

it transfers electrical energy into light and heat as the current flows through it

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24
Q

what happens as the current flowing in a filament lamp increases

A

As the current flowing increases, the temperature also increases. This causes an increase in the movement of the lattice/ions. Therefore there are more frequent collisions between the electrons and the positive metal ions so the resistance increases.

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25
Q

higher current =

A

higher temp = higher resistance

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26
Q

what does the IV graph for a filament lamp look like

A

a curve throughout

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27
Q

what do semiconductor diodes do

A

they act as one way gates, preventing the current from flowing back through the circuit

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28
Q

what are semiconductor diodes useful in

A

converting ac to dc in circuits

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29
Q

forward bias

A

in the direction of the arrow on the symbol

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30
Q

in what direction do semiconductor diodes work

A

in forward bias

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31
Q

how can you tell which end is the forward bias end

A

the component has different coloured ring at the forward bias end

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32
Q

what is the resistance of the diode in reverse bias

A

infinite

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33
Q

what is the resistance of diodes in forward bias

A

very low resistance

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34
Q

what does the IV graph of a semiconductor diode look like

A

straight line through the negative end and then steep uphill slightly curved

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35
Q

total resistance in series =

A

R1 + R2 + R3

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36
Q

total resistance in parallel

A

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

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37
Q

What happens to the total resistance if more resistors are added in parallel

A

Resistance decreases

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38
Q

Current in series

A

The same for all components

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39
Q

Current in parallel

A

Split between the different branches

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40
Q

Total current into a junction =

A

Total current out of a junction

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41
Q

What does the amount of current in each branch depend on

A

The resistance of components in the branch. More resistance = less current

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42
Q

Voltage in series

A

Shared between components but depends on their resistance. More resistance = more voltage (USE RATIOS)

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43
Q

Voltage in parallel

A

Equal

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44
Q

Voltage in cells connected in series

A

total voltage between end of the chain of cells is the sum of PD across each cell

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45
Q

Voltage in cells connected in parallel

A

Total voltage across arrangement is the same as for 1 cell

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46
Q

Conservation of Charge

A

Charge is never used up or lost at a circuit

47
Q

Kirchoff’s first law

A

Sum of current entering junction = Sum of current leaving the junction

48
Q

Conservation of energy

A

Energy is never used up or lost in a circuit

49
Q

Kirchoff’s Second law

A

The total emf in a closed circuit = sum of PD’s across each component

in parallel circuits :

The sum of the voltages in each closed circuit loop is equal to the total e.m.f of the power supply

50
Q

how does voltage/ current flow in a circuit

A

From the long end of a cell to short end

51
Q

PD

A

Work done per unit charge

52
Q

Equation linking power, energy and time

A

Power = Energy / Time

53
Q

Equation linking power, work done and time

A

Power = Work done / Time

54
Q

Equation linking power, current and voltage

A

Power = Current x Voltage

55
Q

Equation linking power, current and resistance

A

Power = Current^2 x Resistance

56
Q

Equation linking power, voltage and resistance

A

Power = Voltage ^2 / Resistance

57
Q

Equation linking energy, current, voltage and time

A

Energy = current x voltage x time

58
Q

Purpose of potential dividers

A

Get a variable PD / a constant PD
Choose a specific PD
Split PD between 2 components

59
Q

Voltage out when using potential dividers =

A

R2 / R1 + R2 x Voltage in ( when the voltmeter is on R2)

60
Q

Why are variable resistors used in potential dividers

A

to vary the voltage out and cause external components to switch on or off

61
Q

What happens to the resistance of an LDR when the light intensity increases

A

Resistance decreases

62
Q

What happens to the resistance of an LDR when the light intensity decreases

A

Resistance increases

63
Q

What happens to the resistance of a thermistor when temperature increases

A

Resistance decreases

64
Q

What happens to the resistance of a thermistor when temperature decreases

A

Resistance increases

65
Q

What happens to voltage out in a potential divider circuit when resistance increases

A

Voltage out also increases as voltage out and resistance in this type of circuit are proportional

66
Q

Why is their electrical heating in wires

A

As the free electrons move through a metal wire, they collide with ions which get in their way.

This means they will transfer some/al their kinetic energy on collision.

This causes heating

67
Q

What does resistance depend on

A

Length of the wire
Cross sectional area of the wire
Resistivity of the material

68
Q

Resistance =

A

Resistivity x Length / Cross sectional area

69
Q

What is the units of resistivity

A

ohmic metres

70
Q

What is the units of the area

A

m^2

71
Q

What happens to resistance if the length of the wire doubles

A

the resistance also doubles

72
Q

What happens to the resistance if the thickness of the wire doubles

A

The resistance will half

73
Q

What is resistivity

A

Property that describes how much a material opposes the flow of electric current through it

74
Q

Increase in resistivity =

A

Increase in resistance

75
Q

Why is copper used for wires

A

It has low resistivity so current flows through easily

76
Q

If the cross sectional area is a circle, what is the relationship between area and diameter

A

Area is directly proportional to diameter^2

77
Q

What happens to the area and resistance if the diameter doubles

A

area x 4
resistance x 1/4

78
Q

What happens to the speed of atoms if temperature increases

A

Atoms move faster

79
Q

What happens to resistance of a metallic conductor which obeys Ohm’s law if temperature increases

A

resistance increases

80
Q

What happens to resistance of a metallic conductor which obeys Ohm’s law if temperature decreases

A

Resistance decreases

81
Q

Thermistor

A

A non-ohmic conductor and sensory resistor whose resistance varies with temperature

82
Q

What happens to the resistance of a thermistor as temperature increases

A

Resistance decreases

83
Q

What are thermistors used in

A

Ovens, fire alarms and thermometers

84
Q

What happens if a material is cooled below the critical temp

A

Its resistivity disappears completely and is now a superconductor

85
Q

Superconductor

A

A material with no resistance below a critical temp

86
Q

Critical temp

A

The temp at which a material becomes superconductive

87
Q

What are superconductors useful for the productions of

A

Strong magnetic fields
Reduction of energy loss in the transmission of electrical power

88
Q

Describe how to determine the resistivity of constant wire (from the gradient of a graph)

A

Measure the thickness of the constantan wire using the micrometer in at least 3 places and find the mean diameter ‘d’.

Set up the apparatus with an ammeter and a voltmeter parallel to the wire

Attach the crocodile clips so that the length of wire between the crocodile clips, L = 1.000m measured on the metre rule.

Set the voltage, V, to a suitable value.

Record the current and record values for L, I, and V in the table.

Repeat the procedure for L = 0.900, 0.800, 0.700, 0.600, 0.500, 0.400 and 0.300m.

Repeat experiment for each length so you have a total of 3 values for I for each length L.

Calculate average current for each length.

Calculate the resistance R = V/I in Ω for each length and record values in your table.

Plot a graph of the mean R against L.

Draw the best straight line of fit through the points and find the gradient (the graph should be a straight line through the origin).

Calculate the cross-sectional area of the wire A = πd2/4 in m2.

From the gradient of your graph calculate the resistivity of constantant. The accepted value is 4.9 x 10-7 Ωm

89
Q

What are the control variables in the resistivity experiment

A

Voltage in through the wire
Cross sectional area of the wire

90
Q

What is the independant variable in the resistivity experiment

A

Length of the wire

91
Q

What is the dependant variable in the resistivity experiment

A

Current through the wire

92
Q

Why should we only use small currents in the resistivity experiment

A

To keep the temperature constant since it affects the resistivity and resistance

93
Q

Why should we use low voltages in the resistivity practical

A

If a high voltage was used, the wire would become very hot and would be dangerous to touch

94
Q

What is the gradient of the graph in the resistivity practical

A

resistivity / area

95
Q

What happens as charge passes through a power supply

A

it gains electrical energy

96
Q

What is EMF

A

The amount of chemical energy converted to electrical energy per coloumb of charge when passing through a power supply

97
Q

equation linking EMF, energy transferred to electrical energy and charge

A

EMF = E / Q

98
Q

How is EMF measured

A

By connecting a high resistance voltmeter (so there is no/little current) around the terminals of a cell in an open circuit.

99
Q

Terminal PD

A

PD across terminals of a cell

100
Q

If there is no internal resistance, terminal PD =

A

EMF

101
Q

Benefits of superconductors in electrical transmission over long distances

A

there is 0 resistance which means there is a reduced power loss

102
Q

Difficulty of superconductors in electrical transmission over long distances

A

Very low temperature is needed to achieve the critical temperature.

It must be kept at the critical temperature

103
Q

Why is the terminal PD always lower than the EMF

A

due to internal resistance

104
Q

Lost volts

A

Work done per unit charge to overcome internal resistance.

OR

Voltage lost in the cell due to internal resistance

105
Q

lost volts (v) =

A

EMF - V

106
Q

EMF =

A

current x ( resistance of load x internal resistance)

107
Q

IR =

A

terminal PD

108
Q

Ir =

A

lost volts

109
Q

Internal resistance

A

resistance of the materials within the battery

110
Q

Describe the procedure for the EMF and internal resistance RP

A

Set up the circuit as shown in the diagram.

Set the variable resistor at its maximum value.

With the switch open, record the reading, V, on the voltmeter.

Close the switch and take the readings of the terminal p.d., V, on the voltmeter and current, I, on the ammeter.

Adjust the variable resistor to obtain pairs of readings of V and I, over the widest possible range. Do this in eight approximately equal increments of the current.

Open the switch after each pair of readings. Only close it for sufficient time to take each pair of readings.

Rearrange the equation: ε = V+I r into the form y=mx+c and plot a suitable graph to determine the internal resistance being careful of the ‘sign’ of the gradient.

111
Q

What is the aim of the EMF and internal resistance RP

A

Investigate the relationship between emf and internal resistance, by measuring variation of I + V using a variable resistor

112
Q

Independant variable of the EMF and internal resistance RP

A

Voltage and current

113
Q

Dependant variable of the EMF and internal resistance RP

A

resistance

114
Q

Control variable of the EMF and internal resistance RP

A

EMF of the cell
Internal resistance of the cell

Physics (10 decks)

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