Electricity Flashcards
(114 cards)
1
Q
current
A
rate of flow of charge
2
Q
potential difference
A
work done per unit charge
3
Q
resistance
A
voltage / current
4
Q
equation linking current, charge and time
A
Current = change in charge / change in time
5
Q
Equation in terms of voltage, work done and charge
A
Voltage = work done / charge
6
Q
conventional current
A
flow of positive charge from the positive terminal of a cell to the negative terminal
7
Q
why is conventional current the opposite to electron flow
A
conventional current was described before electric current was understood
8
Q
what is current measured in
A
Amps
9
Q
how do you measure current
A
using an ammeter, connected in series
10
Q
how do you measure PD
A
using a voltmeter, connected in parallel
11
Q
what is PD measured in
A
volts
12
Q
what is resistance
A
the opposition to current
13
Q
the higher the resistance ….
A
the lower the current
14
Q
why are wires commonly made of copper
A
it has a low electrical resistance ( it is a good conductor)
15
Q
what is resistance measured in
A
Ohms
16
Q
what is Ohms law
A
voltage is proportional to current under constant physical conditions (temperature)
17
Q
constant temperature =
A
constant resistance
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
19
Q
what does the IV graph of a fixed resistor look like
A
a straight line through the origin
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
21
Q
what are some ohmic electrical components
A
fixed resistor and wire
22
Q
what are some non-ohmic electrical components
A
diode, filament lamp bulb, thermistor
23
Q
what does a filament lamp do
A
it transfers electrical energy into light and heat as the current flows through it
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.
25
higher current =
higher temp = higher resistance
26
what does the IV graph for a filament lamp look like
a curve throughout
27
what do semiconductor diodes do
they act as one way gates, preventing the current from flowing back through the circuit
28
what are semiconductor diodes useful in
converting ac to dc in circuits
29
forward bias
in the direction of the arrow on the symbol
30
in what direction do semiconductor diodes work
in forward bias
31
how can you tell which end is the forward bias end
the component has different coloured ring at the forward bias end
32
what is the resistance of the diode in reverse bias
infinite
33
what is the resistance of diodes in forward bias
very low resistance
34
what does the IV graph of a semiconductor diode look like
straight line through the negative end and then steep uphill slightly curved
35
total resistance in series =
R1 + R2 + R3
36
total resistance in parallel
1/R = 1/R1 + 1/R2 + 1/R3
37
What happens to the total resistance if more resistors are added in parallel
Resistance decreases
38
Current in series
The same for all components
39
Current in parallel
Split between the different branches
40
Total current into a junction =
Total current out of a junction
41
What does the amount of current in each branch depend on
The resistance of components in the branch. More resistance = less current
42
Voltage in series
Shared between components but depends on their resistance. More resistance = more voltage (USE RATIOS)
43
Voltage in parallel
Equal
44
Voltage in cells connected in series
total voltage between end of the chain of cells is the sum of PD across each cell
45
Voltage in cells connected in parallel
Total voltage across arrangement is the same as for 1 cell
46
Conservation of Charge
Charge is never used up or lost at a circuit
47
Kirchoff's first law
Sum of current entering junction = Sum of current leaving the junction
48
Conservation of energy
Energy is never used up or lost in a circuit
49
Kirchoff's Second law
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
how does voltage/ current flow in a circuit
From the long end of a cell to short end
51
PD
Work done per unit charge
52
Equation linking power, energy and time
Power = Energy / Time
53
Equation linking power, work done and time
Power = Work done / Time
54
Equation linking power, current and voltage
Power = Current x Voltage
55
Equation linking power, current and resistance
Power = Current^2 x Resistance
56
Equation linking power, voltage and resistance
Power = Voltage ^2 / Resistance
57
Equation linking energy, current, voltage and time
Energy = current x voltage x time
58
Purpose of potential dividers
Get a variable PD / a constant PD
Choose a specific PD
Split PD between 2 components
59
Voltage out when using potential dividers =
R2 / R1 + R2 x Voltage in ( when the voltmeter is on R2)
60
Why are variable resistors used in potential dividers
to vary the voltage out and cause external components to switch on or off
61
What happens to the resistance of an LDR when the light intensity increases
Resistance decreases
62
What happens to the resistance of an LDR when the light intensity decreases
Resistance increases
63
What happens to the resistance of a thermistor when temperature increases
Resistance decreases
64
What happens to the resistance of a thermistor when temperature decreases
Resistance increases
65
What happens to voltage out in a potential divider circuit when resistance increases
Voltage out also increases as voltage out and resistance in this type of circuit are proportional
66
Why is their electrical heating in wires
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
What does resistance depend on
Length of the wire
Cross sectional area of the wire
Resistivity of the material
68
Resistance =
Resistivity x Length / Cross sectional area
69
What is the units of resistivity
ohmic metres
70
What is the units of the area
m^2
71
What happens to resistance if the length of the wire doubles
the resistance also doubles
72
What happens to the resistance if the thickness of the wire doubles
The resistance will half
73
What is resistivity
Property that describes how much a material opposes the flow of electric current through it
74
Increase in resistivity =
Increase in resistance
75
Why is copper used for wires
It has low resistivity so current flows through easily
76
If the cross sectional area is a circle, what is the relationship between area and diameter
Area is directly proportional to diameter^2
77
What happens to the area and resistance if the diameter doubles
area x 4
resistance x 1/4
78
What happens to the speed of atoms if temperature increases
Atoms move faster
79
What happens to resistance of a metallic conductor which obeys Ohm's law if temperature increases
resistance increases
80
What happens to resistance of a metallic conductor which obeys Ohm's law if temperature decreases
Resistance decreases
81
Thermistor
A non-ohmic conductor and sensory resistor whose resistance varies with temperature
82
What happens to the resistance of a thermistor as temperature increases
Resistance decreases
83
What are thermistors used in
Ovens, fire alarms and thermometers
84
What happens if a material is cooled below the critical temp
Its resistivity disappears completely and is now a superconductor
85
Superconductor
A material with no resistance below a critical temp
86
Critical temp
The temp at which a material becomes superconductive
87
What are superconductors useful for the productions of
Strong magnetic fields
Reduction of energy loss in the transmission of electrical power
88
Describe how to determine the resistivity of constant wire (from the gradient of a graph)
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
What are the control variables in the resistivity experiment
Voltage in through the wire
Cross sectional area of the wire
90
What is the independant variable in the resistivity experiment
Length of the wire
91
What is the dependant variable in the resistivity experiment
Current through the wire
92
Why should we only use small currents in the resistivity experiment
To keep the temperature constant since it affects the resistivity and resistance
93
Why should we use low voltages in the resistivity practical
If a high voltage was used, the wire would become very hot and would be dangerous to touch
94
What is the gradient of the graph in the resistivity practical
resistivity / area
95
What happens as charge passes through a power supply
it gains electrical energy
96
What is EMF
The amount of chemical energy converted to electrical energy per coloumb of charge when passing through a power supply
97
equation linking EMF, energy transferred to electrical energy and charge
EMF = E / Q
98
How is EMF measured
By connecting a high resistance voltmeter (so there is no/little current) around the terminals of a cell in an open circuit.
99
Terminal PD
PD across terminals of a cell
100
If there is no internal resistance, terminal PD =
EMF
101
Benefits of superconductors in electrical transmission over long distances
there is 0 resistance which means there is a reduced power loss
102
Difficulty of superconductors in electrical transmission over long distances
Very low temperature is needed to achieve the critical temperature.
It must be kept at the critical temperature
103
Why is the terminal PD always lower than the EMF
due to internal resistance
104
Lost volts
Work done per unit charge to overcome internal resistance.
OR
Voltage lost in the cell due to internal resistance
105
lost volts (v) =
EMF - V
106
EMF =
current x ( resistance of load x internal resistance)
107
IR =
terminal PD
108
Ir =
lost volts
109
Internal resistance
resistance of the materials within the battery
110
Describe the procedure for the EMF and internal resistance RP
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
What is the aim of the EMF and internal resistance RP
Investigate the relationship between emf and internal resistance, by measuring variation of I + V using a variable resistor
112
Independant variable of the EMF and internal resistance RP
Voltage and current
113
Dependant variable of the EMF and internal resistance RP
resistance
114
Control variable of the EMF and internal resistance RP
EMF of the cell
Internal resistance of the cell
