5. Electricity Flashcards
1
Q
What is electric current?
A
Net flow of charged particles
2
Q
What is the metal wire made up of?
A
A lattice of positive ions, surrounded by ‘free electrons’
3
Q
How do ions move in a metal wire?
A
They vibrate about fixed positions
4
Q
How do electrons move in a wire?
A
They are free to move from one ion to another
5
Q
What happens to electrons when a battery is connected to a wire?
A
Free electrons are repelled by the negative terminal and attracted to the positive
6
Q
What is drift velocity?
A
When a wire is connected to a battery and electrons move randomly, but in the same direction
7
Q
At what point is there a current in a wire?
A
When electrons move in the same direction (electrons carry charge, current = flow of charge)
8
Q
How does electric current flow through liquids?
A
- electrolyte (e.g. salt solution)
* when power supply connected, +ve ions move to -ve terminal and vice versa
9
Q
In the equation ▲Q / ▲T, what is the quantity being calculated?
A
Rate of flow of charge
10
Q
How to calculate rate of flow of charge?
A
▲Q / ▲T
11
Q
How is the coulomb defined?
A
The quantity of electric charge that passes a point in a circuit in 1s when a current of 1A is present
12
Q
What direction does the current go in?
A
In the direction of positive charge
13
Q
How is the ohm defined in words?
A
A conductor has resistance of 1Ω if a current of 1A flows across when a p.d. of 1V is applied
14
Q
How is the ohm defined in an equation?
A
1Ω = 1V / 1A
15
Q
What is Ohm’s Law?
A
Current through a wire is proportional to the p.d. across it (if temperature is constant)
16
Q
What are Ohmic conductors?
A
Materials that obey Ohm’s law
17
Q
What are the charge carriers in metals?
A
Conduction electrons
18
Q
What are the charge carriers in a salt solution?
A
Ions
19
Q
What is the convention for the direction of a current in a circuit?
A
Positive to negative
20
Q
What can materials be classified as in electrical terms?
A
Conductors, insulators or semiconductors
21
Q
How is potential difference defined?
A
As the work done (or energy transfer) per unit charge
22
Q
What is resistance caused by?
A
The repeated collisions between the charge carriers in the material with each other and with the fixed ions of the material
23
Q
How is the resistance of any component defined?
A
The p.d. across the component / the current through it
24
Q
How is an Ohmic conductor represented on an I-V graph?
A
A straight line through the origin
25
How will an I-V graph show that a component is obeying Ohm's Law?
Straight line through the origin
26
Why does a straight line through the origin on an I-V show that Ohm's Law is being obeyed?
Gradient is constant so resistance is constant - for both directions of current flow
27
What is the I-V graph for a filament lamp?
A curve
28
What does the curved graph for a filament lamp mean?
Resistance increases as current increases
29
For a filament lamp, why does resistance increase as current increases?
The current has a heating effect
30
What is the I-V graph for a semiconductor diode?
Reverse bias for a negative p.d. and forward biased p.d. of 0.6V needed before diode conducts in forward direction
31
For which component's graph showing I-V characteristics can the gradient be used for resistance for all values?
For ohmic conductors - for filament lamps and diodes, read values off the graph for R at a certain point
32
What is resistance proportional to, in terms of length and area?
Resistance of a uniform conductor is ∞ to length and 1/∞ to its cross sectional area
33
Equation for resistance? (using resistivity)
R = ρl / A
34
Equation for resistivity?
ρ = RA / l
35
Unit for resistivity?
Ωm
36
Units for length, area and resistivity for the equation?
length = m
cross-sectional area = m²
resistivity = Ωm
37
How can resistivity be defined?
As numerically equal to the resistance of a sample of the material of unit length and unit cross-sectional area at a particular temperature
38
Typical value of resistivity of copper at 20°C?
Good conductor - 1.7x10^-8 Ωm
39
Typical value of resistivity of nichrome at 20°C?
Conductor - 1.1x10^-6 Ωm
40
Typical value of resistivity of silicon at 20°C?
Semiconductor - 2.3x10^3 Ωm
41
Typical value of resistivity of glass at 20°C?
Insulator - 1.0x10^12 Ωm
42
A wire of uniform cross-section has a resistance of RΩ. What would be the resistance of a similar wire, made of the same material, but twice as long and twice the diameter?
1/2 RΩ
43
A wire of uniform cross-section has a resistance of RΩ. It is drawn to three times the length, but the volume remains constant. What will be its resistance?
9 RΩ
44
In general, how are resistivity and temperature of metals linked?
As temperature ↑, resistivity ↑
45
What happens, in metals at high temperatures, to resistance as temperature increases?
Resistance increases linearly with temperature
46
What does increased temperature mean for particles within a wire?
Ions vibrate faster, with greater amplitude, so it's more difficult for the electrons to pass through the lattice (resistance ↑)
47
When can a metal become a superconductor?
Below its critical temperature
48
What does it mean if a metal becomes a superconductor?
It loses all of its resistance
49
Why do superconductor wires not become hot?
As electrons can flow through them without any transfer of energy
50
What are the particles like in insulators at room temperature?
There are few free electrons available for conduction
51
What are the particles like in insulators at high temperatures?
Some electrons have enough energy to 'escape' from their atoms and the insulator is able to conduct
52
What happens to resistance in insulators as temperature increases?
Resistance decreases
53
What is one of the best known semiconductor materials?
Silicon
54
How are semiconductors at low temperatures?
Poor conductors
55
What happens in semiconductors as temperature rises?
Electrons break free from their atoms and so it becomes a better conductor
56
What happens when silicon reaches 150°C?
Breakdown occurs and it becomes permanently damaged
57
When might a semiconductor material become permanently damaged?
When it reaches a certain temperature (e.g. when silicon reaches 150°C)
58
What is the relationship between temperature and resistance for semiconducting materials?
As temperature ↑, resistance ↓ (for many)
59
For materials where temperature ↑ as resistance ↓, what is a term that can be used to describe them?
They have a negative temperature coefficient of resistance (NTC)
60
What is an NTC?
A negative temperature coefficient of resistance
61
What are semiconductor materials used to make?
Thermistors
62
Is there a p.d. when a material is a superconductor?
No - as a current flows but resistance = 0
63
When will a superconductor lose its superconductivity?
When it raises above its critical temperature
64
What is electrical power?
The rate at which electrical energy is converted to other forms of energy
65
What is the unit of power?
The watt
66
What is 1J per second equivalent to?
1 W
67
Equation for power?
P = W/t (P=ItV/t)
68
Equation for energy in terms of charge?
W=QV (W=ItV)
69
What happens in terms of energy transfers when current flows through a resistor?
Electrical energy is transferred to heat
70
What equation is given when P=IV and V=IR are combined?
P=I²R
71
In which direction is the conventional current?
The flow of positive charge
72
In liquids, gases and semiconductors, which particles move in the direction of the conventional current?
Positive ions
73
In liquids, gases and semiconductors, which way do charge carriers flow?
In the opposite direction to conventional charge
74
In metals, what is the only type of charge that flows?
Negative - as the only charge carrier is the electrons
75
How does the current behave in a series circuit?
It is the same at all points
76
How does the current behave in a parallel circuit?
Current leaving and returning to the supply is the sum of the currents in the separate branches
77
What is Kirchoff's first law?
The sum of the currents flowing into any junction in a circuit is equal to the sum of currents flowing out of that junction i.e. charge is conserved
78
What is Kirchoff's second law?
Around any closed loop in a circuit, the sum of the emfs is equal to the sum of the p.d.s
79
Explanation of Kirchoff's second law?
* we know that a coulomb gains electrical energy as it moves through each emf and loses electrical energy as it moves through each p.d.
* after one loop of the circuit, the energy it has gained must be equal to the energy it has dissipated
80
What does emf stand for?
Electromotive force
81
If two identical lamps are connected in series, what is their brightness?
They are equally bright, but not as bright as if they were connected to the battery on its own
e.g. total p.d. across both = 6V, and shared between 2 = 3V each
82
In a series circuit, what is the total p.d. equal to?
total p.d. across all components = sum of p.d.s across the separate components
83
When two identical lamps are connected in parallel, what happens to their brightness?
Each lamp is as bright as if it were connected to the battery on its own
84
What happens to p.d. in a parallel circuit?
It is the same across each branch
85
What model of energy transfer is used to explain why p.d. is equal across all branches in parallel?
* each coulomb transports all 6J of energy to the lamp
* each lamp receives same energy as if it were connected to battery on its own
* twice as many coulombs pass per second through the battery - so battery runs out more quickly
86
How is electromotive force defined?
The energy (chemical, mechanical, thermal etc,) converted into electrical energy when unit charge (ie 1C) passes through it
87
What is emf and what is p.d. in any closed loop?
emf - supply of electrical energy
p.d. - conversion of electrical energy to other forms
88
For cells in series, how is total emf of their combination calculated?
By adding their individual emfs
89
For identical cells in parallel, how is total emf of their combination calculated?
Total emf is the same size as each of the cells individually
90
Why, for identical cells in parallel, is the total emf the same size as each of the cells individually?
Each charge only passes through one cell so gains energy from the single cell
91
Why, for a combination of cells in series, is the total emf calculated by adding the individual emfs?
Each charge passes through each cell so gains energy from all three
92
When resistors are in series, what is the current going through them and p.d. across them?
Current through each resistor is the same, and total p.d. across resistors is the sum of the p.ds across the separate resistors ie. V=V1+V2+V3
93
How is the combined resistance in a series circuit calculated?
R=R1+R2+R3
94
When resistors are in parallel, what is the current going through them and p.d. across them?
Current in main circuit is the sum of the currents in each parallel branch, and p.d. across each resistor is the same
95
How is the combined resistance in a parallel circuit calculated?
1/R= 1/R1 + 1/R2 + 1/R3
96
In a simple potential divider (two resistors in series), what happens to p.d. when resistors are identical?
Resistors share voltage equally
97
In a simple potential divider (two resistors in series), one resistor has a value of 200Ω and the other 100Ω, and the emf is 6V. What is the p.d. through each resistor?
200Ω = 4V
100Ω = 2V
98
Equation to calculate output and input voltage in a potential divider?
Vₒᵤₜ = Vᵢₙ (R₁ / R₁+R₂)
99
What components can be used as sensors in electronic circuits?
Thermistors and light dependant resistors
100
As a thermistor cools, what happens to its resistance?
Resistance rises
101
As an LDR receives less light, what happens to its resistance?
Resistance rises
102
In a circuit diagram, what is a loop around a cell and a resistor showing?
They are one component; the resistor is the cells internal resistance
103
What is emf equal to?
Terminal p.d. + lost volts
104
What two equations are used to form the equation emf = I (R +r)?
emf = IR and emf = Ir
105
What happens if a driver starts a car with the head-lamps on?
The current through the battery is so large that the 'lost volts' are high - even though the battery's internal resistance is low. Terminal p.d. drops and headlights dim.
106
What is emf equal to when there is no current in a cell?
The terminal p.d.
107
What is terminal p.d. ?
The p.d. across the terminals of the cell
108
What happens to p.d. as soon as current is drawn from the cell?
P.d. drops
109
Why does p.d. drop as soon as current is drawn from the cell?
The cell has internal resistance
110
What is an open circuit?
A circuit that is off, with no current
111
What is the p.d. defined as?
The amount of electrical energy changed to other forms of energy per coulomb of charge flowing between them
112
How is p.d. calculated with energy and charge?
V = W / Q
113
How can p.d. be defined using an equation?
p.d. = energy used by the component / charge passing
114
What does W equal in terms of I, t and V?
W = ItV
115
What is one volt?
The p.d. between 2 points in a circuit in which 1J of energy in converted to other forms when 1C of charge passes between them
116
How is the unit of 1V defined?
1J / 1C
117
Why is a loop drawn around E and r (battery and resistor)?
To show that they are one component
