Electricity Flashcards
(112 cards)
1
Q
What is the voltage
A
Energy supplied to electrons
2
Q
Potential difference official definition
A
Energy transferred/work done per unit charge V = W / Q
3
Q
How many electrons in a coloumb
A
6.25 x 10^18
4
Q
What is a coloumb a unit of
A
Charge
5
Q
What is the charge of an electron
A
1.6 x 10^-19
6
Q
How much energy is supplied to each coloumb
A
Voltage of the circuit
7
Q
What does the voltmeter measure
A
Energy lost by each coloumb of charge
8
Q
What direction does convential current go
A
Positive to negative
9
Q
What direction do electrons flow
A
Negative to positive
10
Q
Q, E and V in a formula
A
V = E/Q
11
Q
What is the energy/work done per unit charge
A
Voltage
12
Q
Relationship between emf and current
A
Higher emf, electrons flow faster, higher current
13
Q
I, Q and T
A
I = Q/T or Q = IT
14
Q
What is the current
A
Rate of flow of charge in coloumb per second
15
Q
V / I =
A
Constant, resistance
16
Q
What is ohms law
A
For an ohmic conductor, current is directly proportional to the potential difference across it, given that physical conditions (temperature) is kept constant
17
Q
Graph of ohmic conductor
A
V-I graph and I-V graph are the same, goes through origin, straight line, positive gradient
18
Q
What is ohms law formula
A
V = IR
19
Q
What is resistance
A
How hard is it for the current to flow, ohms
20
Q
What energy changes take place in the resistor
A
Elecrical energy to heat
21
Q
What is power
A
How much energy is consumed (converted) or delivered every second
22
Q
Units for power
A
JS^-1 or Watts (W)
23
Q
P, I, R, and V
A
P = VI, V = IR, P = I^2R
24
Q
Formula for power lost in resistor
A
I^2R
25
Resistor symbol
Rectangle with wire coming out of each side (wire doesn't go through box)
26
Gradient of a V-I graph
Resistance
27
V-I graph, straight line, positive constant gradient, goes through origin, positive and negative points, what component
Resistors, thermistors, LDR
28
Features of resistors
Ohmic, not metal, semiconductors
29
V-I graph, sideways s, more vertical at ends, more horizontal at origin, goes through origin, what component is it
Lamp, filament bulb
30
Resistance of a filament bulb
Increases with I
31
Why does R increase with I for bulbs
Filament heats up, ions from metal move more, electrons collide with ions more, R increases
32
Thermistors symbol
Resistor symbol with positive, straight line going through with horizontal cap on the end
33
LDR symbol
Resistor symbol, circle around it, two arrows pointing from top left
34
Application of thermistors
Temperature sensor, can trigger an event to occur once the temperature drops or reaches a certain value - automatically turn on heating
35
Features of thermistors and LDR's
Contant R, ohmic at constant temp/light levels, higher temp/more light = lower R
36
Diode symbol
Sideways isoceles going into vertical line with horizontal lines coming out of it
37
Diode V-I graph
Negative or positive voltage, positive I, backwards curved l shape, levels out before I axis
38
Features of a diodes resistance
Very high in one direction, very low in other direction
39
Factors of resistance
Length (R = kL), Cross section (R = k/A), Resistivity (R = kP)
40
What is the weird slanted P
Resistivity
41
Formulas involving resistivity, resistance, length and cross-sectional area
R = (PL) / A, P = (RA) / L
42
Resistivity at a constant temp is…
Constant
43
Resistivity of a good conductor
Small
44
Resistivity of a good insulator
Large
45
How does resistivity change as the temp increases
Increases
46
Why does resistivity increase as the temp increases
Atoms vibrate more, more collision between electons and atoms, harder for electrons to pass through, harder for current to pass
47
Example of a semi-conductor
Thermistor
48
Semi-conductor temp and resistance relationship and explanation
Higher temp = more charge carriers available = lower resistance
49
Semiconductor resistance-temperature graph
Decreasing exponential
50
What are superconductors
Materials with potential for very small (0) resistance
51
What is the critical temperature of a superconductor
Temperature when the resistance drops to 0
52
A-C graph, B is a point on C, vertical line at B, then increases exponentially, what is A
Resistance
53
A-C graph, B is a point on C, vertical line at B, then increases exponentially, what is B
Critical temperature
54
A-C graph, B is a point on C, vertical line at B, then increases exponentially, what is C
Temperature
55
A-C graph, B is a point on C, vertical line at B, then increases exponentially, what is the graph of
Superconductors
56
Application of thermistors
Triggers event when temperature drops below a certain value (automatically triggers heating)
57
Where are superconductors used
Strong electromagnetics such as the large hadron collider or MRI scanners, magnetic fields, transmission of electrical power, reduces energy lost
58
Using superconductors in strong magnetic fields
Does not require a constant power source, could be used in maglev trains (no friction) or in certain medical applications
59
Why are superconductors used in electromagnets
large currents are needed
60
How superconductors are used in electromagnets
Coils are cooled below the critical temperature using liquid helium
61
When can a current flow without a power source
In superconductors, a power supply is needed to start it off then the current flows by itself
62
What benefits would cheap, room temperature superconductors have
Would save energy and money
63
Time, power, energy formula
Power = Energy / time
64
What is the terminal p.d
Voltage through the circuit
65
What is the EMF
Total voltage that can be supplied when the current is 0
66
Equation for total p.d and emf
EMF = terminal p.d + the lost volts
67
What are the lost volts
Voltage lost in the internal resistor - pd across r
68
Actual definition of EMF
Energy that is transferred from chemical to electrical energy in the cell per coloumb
69
Actual definition of terminal pd
Energy transferred from electrical energy to any other type in the external circuit
70
Actual definition of lost volts
Energy wasted by cell per coulomb of charge
71
EMF formula involving current and resistance
E = IR + Ir, E = I(R + r)
72
How to extrapolate the data to find the EMF in the required practical
Plot a V-I graph, Y-intercept is EMF, gradient is r, V = -Ir + E, y = mx + c
73
Correlation between current and the lost volts
Higher current means more volts lost
74
What does emf stand for
Electromotive force
75
What is the base unit for energy
Joules
76
What is 1 Joule equal to
Work done by a force of 1 Newton acting on a distance of 1 metre
77
What is a small unit of energy
eV
78
What is a large unit of energy
kWh
79
Can energy be created or destroyed
No
80
What is energy transferrerd equal to
Work done
81
What is a potential divider
A circuit with several resistors in series used to produce a specific fraction of the source pd - either a constant or variable pd supplied
82
In a potential divider ciruit, what share of the pd does the smaller resistor get
The smaller share
83
What is a potentiometer
A type of varaible resistor
84
What is the forward bias of a semiconductor diode
Direction in which it allows current to flow
85
Threshold voltage of a semiconductor diode
Minimum voltage needed to allow the current to flow
86
Assumption made about ammeters
Have 0 resistance so will not affect the measurement of current in a circuit
87
Assumption made about voltmeters
Infinite resistance so no current can flow through them
88
What is the value of the resistivity
Resistance through a material of length 1m and cross sectional area 1m^2
89
Is resistivity dependent on environmental factors such as temperature
Yes
90
Most known superconductors have a critical temp of close to…
0 K (-273 celcius)
91
How to add resistances in series
Rt = R1 + R2 + R3 …
92
How to add resistances in parallel
1 / Rt = (1 / R1) + (1 / R2) + (1 / R3) …
93
What does VIt equal
Energy
94
Current through a series circuit
Constant throughout
95
pd through a series circuit
Total sum of voltages across all elements is equal to the supply p.d.
96
Current through a prallel circuit
Sum of current in each parallel set of branches is equal to the total current
97
pd through a parallel circuit
Potential difference across each branch is the same
98
What happens to voltage when identical cells are joined in a parallel circuit
Total voltage = voltage of one cell
99
Total voltage = voltage of one cell (in a parallel circuit) explanation
Current is split equally between branches so overall pd is the same as if the total current was flowing through a single cell
100
What is always conserved in DC circuits
Charge and energy
101
Kirchoff's first law
Total current flowing into a junction is equal to the current flowing out of that junction (so no charge is lost at any point in the circuit)
102
Kirchoff's second law
Sum of all voltages in a series circuit is equal to the battery voltage
103
How can you make a potential divider supply a variable potential difference
Using a variable resistor
104
How does an LDR's resistance change with light intensity
Decreases as light intensity increases
105
What is a battery's internal resistance caused by
Electrons colliding inside battery
106
What does the emf formula suggest the definition is
Energy transferred by a cell per coulomb of charge
107
What is the emf a sum of
Terminal pd and lost volts; E = V + v
108
How to measure the emf of a battery
Measuring the voltage across a cell using a voltmeter when there is no current running through cell (open circuit)
109
What is an NTC
Negative temperature coefficient thermistor
110
What is a negative temperature coefficient thermistor
Thermistor who's resistance decreases as temperature increases
111
Application of potential divider
Triggering certain events; if light intensity falls, resistance across LDR increases so current decreases and resistance across a second resistor in series decreases, so pd would increase as light intensity decreased and circuit would go on to switch on light bulb because threshold voltage has been met
112
E / Q =
e
