1
Q
What is the definition of electric current?
A
The rate of flow of charge (electrons)
2
Q
What is electric current measured in?
A
Amperes (A)
3
Q
What is the equation for electric current?
A
I = Q/t
Where:
* I = current (A)
* Q = charge (C)
* t = time (s)
4
Q
What is meant by the electric charge?
A
a fundamental property of matter that causes it to experience a force when placed within an electromagnetic field
5
Q
What is an equation for calculating charge?
A
Q = ne
Where:
* Q = charge (C)
* n = number of electrons
* e = electron charge (C)
OR
Q = It
Where:
* Q = charge (C)
* I = current (A)
* t = time (s)
6
Q
What happens when two oppositely charged conductors are connected together (by a length of wire)?
A
Charge will flow between the two conductors, creating a current
7
Q
What is the definition of conventional current?
A
The flow of positive charge from the positive terminal of a cell/battery to the negative terminal
8
Q
What is the difference between the flow of charge (electrons) and the conventional current?
A
Electrons flow away from the negative terminal of a cell towards the positive terminal.
Conventional current measures the positive charge flowing away from the positive terminal of a cell towards the negative terminal.
9
Q
How should ammeters be connected in a circuit?
A
In series within the part of the circuit current is to be measured through
10
Q
Why must ammeters be connected in series?
A
Current is the same in all components connected in series
11
Q
What is the definition of potential difference?
A
The work done per unit charge
12
Q
What is the unit of potential difference?
A
Volts (V)
OR
Joules per coulomb (J C-1)
13
Q
What is the equation to calculate potential difference?
A
V = W/Q
Where:
* V = potential difference (V)
* W = work done (J)
* Q = charge (C)
14
Q
How should voltmeters be connected in a circuit?
A
In parallel to (across) the component being measured
15
Q
Why must voltmeters be connected in parallel?
A
A potential difference across components in parallel is always the same due to the conservation of energy.
16
Q
What is the definition of resistance?
A
The opposition to current
17
Q
What does Ohm’s Law state?
A
The resistor of a conductor is given by the ratio of potential difference across it to the current flowing in it.
18
Q
What is the equation for Ohm’s Law?
A
R = V/I
Where:
* R = resistance (Ω)
* V = potential difference (V)
* I = current (A)
19
Q
What is the relationship between resistance and current?
A
Inversely proportional
(The higher the resistance, the lower the current and vice versa)
20
Q
What is the unit of resistance?
A
Ohms (Ω)
21
Q
How is the Ohm defined?
A
One volt per ampere
22
Q
In a circuit, how would a high resistance affect the current?
A
A higher resistance would result in a smaller current
23
Q
Which electrical components have some value of resistance?
A
All of them (including wires)
24
Q
Which 3 circuit components can be used to find the resistance of a component?
A
- A power supply
- An ammeter in series with the component
- A voltmeter in parallel with the component
25
What does Ohm's Law suggest about the relationship between the current and potential difference through a component?
Current ∝ Potential Difference
(as long as the temperature is constant)
26
If resistance on a variable resistor is increased, what would happen to the circuit's **potential difference**?
Potential difference would increase
(R ∝ V)
27
If resistance on a variable resistor is increased, what would happen to the circuit's **current**?
Current would decrease
(R ∝ 1/I)
28
How does a current-voltage graph show if an electrical component is obeying Ohm's law?
If it produces a straight line through the origin
29
Does a filament lamp obey Ohm's law?
No, as it's resistance increases as it heats up. This means that it is a **non-ohmic** conductor.
30
What will cause a metal wire to stop following Ohm's Law?
If the current going through it is too large, and increases its temperature
31
What is the definition of the electric current rule (Kirchhoff's First Law)?
The sum of the currents entering and leaving a junction is equal to zero, as due to the conservation of charge, current shouldn't decrease or increase in a circuit when it splits.
32
What is meant by a junction in a circuit?
A point where at least three circuit paths meet
33
What is meant by a branch in a circuit?
A path connecting two junctions
34
What should be observed about current if a circuit splits into two branches?
The current before the circuit splits should be equal to the current after it has split
(IIN = IOUT)
35
Alongside current, which other property is conserved on both sides of a junction in an electric circuit?
Charge
36
What is the rule of current in a series circuit?
Current is the same at all points in the circuit
37
What are the rules of current in a parallel circuit?
* The current divides at the junctions
* Each branch has a different value for current
* The sum of currents before the junction will equal the sum of the currents after the junction
38
What is the definition of the electrical voltages rules (Kirchhoff's Second Law)?
The sum of the e.m.f.s in a closed circuit loop is equal to the sum of the potential differences around that loop
39
What is the basic principle of the conservation of energy in terms of electric circuits?
Energy is never used up or lost in a circuit
40
What are the rules of voltage in a series circuit?
* The voltage is split across all components depending on their resistance
* The sum of the voltages is equal to the total e.m.f of the power supply
41
What are the rules of voltage in a parallel circuit?
* The voltage is the same across each closed loop
* The sum of the voltages in each closed circuit loop is equal to the total e.m.f of the power supply: (E1 + E2 = V1 + V2 = V3 + V4)
Where:
E = power supply
V1 & 2 = voltmeters around resistors
V3 & 4 = voltmeters around bulbs
## Footnote
https://cdn.savemyexams.com/cdn-cgi/image/f=auto,width=1920/https://cdn.savemyexams.com/uploads/2022/06/3-5-parallel-circuit-emf-and-pd_edexcel-al-physics-rn.png
42
What type of circuit is made up of two or more closed-circuit loops?
A parallel circuit
43
Why are parallel circuits better for temperamental components (e.g. faulty bulbs)?
If one component breaks, voltage and current can still flow through for the rest of the lights and appliances.
44
What is the rule for resistors in a series circuit?
When two or more components are connected in series:
* The combined resistance of the components is equal to the sum of individual resistances (RTOTAL = R1 + R2 + R3 ...)
45
What is the rule for resistors in a parallel circuit?
In a parallel circuit:
* The reciprocal of the combined resistance of two or more resistors is the sum of the reciprocals of the individual resistances (1/RTOTAL = 1/R1 + 1/R2 + 1/R3 ...)
46
In parallel circuits, why is the total resistance in the circuit less than the resistance of any of the individual components?
As the sum of the reciprocals of the individual resistances = the total resistance, meaning that if two resistors of equal resistance are connected in parallel, the combined resistance will halve
47
What is meant by the work done (to do work)?
A transfer of energy
48
Why must work be done when components transfer electrical energy to other stores?
Because energy is transferred
49
How can potential difference be defined in terms of work done?
Potential difference is the work done per unit charge
(V = W/Q)
50
What is the equation for work done in an electric circuit?
W = VIt
Where:
* W = work done / energy transferred (J)
* V = potential difference (V)
* I = current (A)
* t = time (s)
51
What is the definition of power?
The rate of doing work
52
What is the equation for power in terms of work?
P = W/t
Where:
* P = power (W)
* W = work done (J)
* t = time (s)
53
How can power be calculated in terms of electricity?
P = VI
P = I2R
P = V2/R
| P = W/t, W = VIt, P = VIt/t, P = VI
54
Due to the equations: P = I2R and P = V2/R, what would happen to power if the current or voltage **doubles**?
The power would be **4x** larger
55
Due to the equations: P = I2R and P = V2/R, for a given power, what would happen to current and voltage if the resistance **doubles**?
* The current would be 4x smaller
* The voltage would be 4x larger
56
How does the relationship between voltage and current change look for an **ohmic conductor** on an I-V graph?
A straight diagonal line through the origin (as potential difference increases, current increases)
57
How does the relationship between voltage and current change look for a **semi-conductor diode** on an I-V graph?
A horizontal line that goes sharply upwards when in the top right quarter (positive-positive)
58
How does the relationship between voltage and current change look for a **filament lamp** on an I-V graph?
An 'S' shaped curve
59
How and why does a **diode** in a circuit affect the relationship between current and potential difference?
A diode is used in a circuit to allow current to only flow in a specific direction:
* The sharp increase in potential difference and current on the right side of the graph shows the **forward bias** of the current (the current moves in the direction of the arrowhead symbol)
* If the diode is switched around, it doesn't conduct and is called **reverse bias**, which is shown by a zero reading of current or potential difference on the left side of the graph
The **threshold** voltage at which a diode starts to conduct is typically around **0.6V**
60
How and why does a **filament lamp** in a circuit affect the relationship between current and potential difference?
In a filament lamp, as current increases, the temperature of the filament in the lamp increases:
* Since the filament is a metal, the higher temperature causes an increase in resistance
* Resistance opposes the current, causing the current to increase at a slower rate
Where the graph is a straight line, the resistance is constant. The resistance increases as the graph curves, meaning that the filament lamp only obeys Ohm's Law for small voltages.
61
How does the relationship between voltage and current change look for a **thermistor** on an I-V graph?
A shallow curve upwards
62
How and why does a **thermistor** in a circuit affect the relationship between current and potential difference?
The increase in the potential difference results in an increase in current which causes the temperature of the thermistor to rise:
* As its temperature rises, its resistance decreases (meaning even more current is able to flow through)
* Since the current is not directly proportional to the potential difference (the graph is still curved), the thermistor does **not** obey Ohm's Law
The I-V graph for a thermistor shows the current increasing at a proportionally slower rate than the potential difference. This is because:
* As current increases, the temperature of the thermistor increases (which causes an increase in resistance)
* Resistance opposes the current, causing current to increase at a slower rate
63
How does electrical heating occur?
As free electrons (charge) move through a metal wire, they collide with ions which get in their way. As a result, they transfer some, or all, of their kinetic energy on collision, which causes electrical heating.
64
# Think resistivity equation
What are the 3 properties that **resistance** depends on?
* length of the wire
* cross-sectional area (through which the current is passing)
* the resistivity of the material
65
What is the resistivity equation?
R = ρL/A
Where:
* R = resistance (Ω)
* ρ = resistivity (Ω m)
* L = length (m)
* A = cross-sectional area (m2)
66
What does the resistivity equation show about the relationship between a **wires length** and its **resistance**?
The **longer** the wire, the **greater** its resistance
67
What does the resistivity equation show about the relationship between a **wires thickness** and its **resistance**?
The **thicker** the wire, the **smaller** its resistance
68
What would happen to resistance if the length of a wire was doubled?
Resistance would be doubled
69
What would happen to resistance if the cross-sectional area of the wire was doubled?
The resistance would be halved
70
What is meant by resistivity?
A property that describes the extent to which a specific material opposes the flow of electric current through it
71
Why is resistivity dependent on temperature?
At higher temperatures, conductors increase ionic vibrations, causing more collisions and therefore more resistance
72
What is the unit of resistivity?
Ohm-metres (Ω m)
73
Why is copper a good material for electrical wiring, in terms of its resistivity?
Copper has a relatively low resistivity at room temperature, meaning it has a low resistance. Therefore, current can flow through it very easily.
74
What equation should be used to calculate the cross-sectional area (A) of a wire?
A = πd2/4
Where:
* A = cross-sectional area (m2)
* d = diameter (m)
**OR**
A = πr2
Where:
* r = radius of the wire
75
What physics instrument should be used to measure the diameter of a wire?
A micrometer (with a mean diameter calculated through taking multiple readings on different points along the wire)
76
What are the safety risks with performing a practical to calculate resistivity?
* When there is a high current going through a thin wire, the wire will become very hot, so the wire shouldn't be touched directly whilst the circuit is switched on
* The wire may begin to produce a burning smell, in which case the power supply should be switched off immediately, but to prevent this, the power supply should be switched off between readings.
* Liquids must be kept away from the equipment
77
What is meant by a charge carrier in a conductor?
A particle that has a charge, which causes a flow of current due to its movement
78
Whether charge carriers are negative or positive, what property must always be taken to be in the same direction?
Current
79
What is meant by drift velocity?
The average velocity of the charge carriers travelling through a conductor
80
What is the primary example of charge carriers in conductors?
Free electrons
81
What are the properties of drift velocity for free electrons?
* They only travel small distances before colliding with a metal ion
* They only have a relatively slow drift velocity of roughly 0.001ms-1 (10-3 ms-1)
82
Which direction is the drift velocity of **positive** charge carriers in relation to current?
The drift velocity of positive charge carriers is in the same direction as the current
83
Which direction is the drift velocity of **negative** charge carriers in relation to current?
The drift velocity of negative charge carriers is in the opposite direction to the current
84
In terms of charge carriers, what does density (n) represent?
The number of free charge carriers (electrons) per unit volume
* Conductors (such as metals) have a high value of n
* Insulators (such as plastics) have a low value of n
85
If the flow of current appears to happen instantaneously (when electric current travels through a conductor almost immediately), what does that say about the density of the charge carriers in the conductor?
There is a very large density
86
What is the transport equation?
I = nqvA
Where:
* I = current (A)
* n = number density (m-3)
* q = the charge of the charge carrier (C)
* v = drift velocity (ms-1)
* A = cross-sectional area of the wire (m2)
87
In the transportation equation, what would it mean if there was a negative value for v?
There is a negative drift velocity, and therefore current is moving in the opposite direction to the charge carriers, meaning that electrons have to move towards the higher potential (against the electric field)
88
What does the transportation equation show about the relationship between **drift velocity** (v) and the **density number** (n)?
v ∝ 1/n
They are inversely proportional (the more charge carriers available per unit volume, the more the density will slow down their speed through the conductor)
89
What does the transportation equation show about the relationship between **current** (I) and the **density number** (n)?
I ∝ n
They are directly proportional (greater n means a greater charge is flowing and therefore a larger current)
90
When the value of the density number (n) is lower, how do charge carriers change?
The charge carriers must travel faster in order to carry the same current
91
What is meant by a semiconductor?
A material with electrical conductivity between that of a conductor and an insulator (e.g. silicon).
Semi-conductors have their own conditions for whether electricity can be conducted or not, such as voltage, temperature and light.
92
How many charge carriers should a conductor have?
Conductors should have very large number of charge carriers
93
What effect does having a very large amount of charge carriers have on an conductor?
They have a low resistivity
94
What effect does having a negligible amount of charge carriers have on an insulatior?
They have a very high resistivity (to an extent that virtually no current will flow through them)
95
What would the properties of a perfect insulator be?
* Contains no charge carriers (n = 0)
* Remains at a current of zero, regardless of the voltage applied
96
What effect does having a small amount of charge carriers have on an semiconductor?
They require higher temperatures to provide a greater number of free electrons, and due to variation in temperature, resistivity changes
97
Based on the logic of the electrical voltages rule (Kirchhoff's 2nd Law), what would happen to the potential difference across a power source when two resistors are connected in series?
The potential difference across the power source would be divided across the two resistors
98
What is meant by a potential divider circuit?
A circuit which uses two resistors in series to split or divide the voltage of a supply in a chosen ratio, in order to produce an output voltage as a fraction of the input voltage
99
What are the three main purposes of a potential divider circuit?
* To provide a variable potential difference (an electrical voltage between two points in a circuit that can be changed, rather than remaining constant)
* To enable a specific potential diference to be chosen
* To split the potential difference of a power source between two or more components
100
What is the potential divider equation?
Vout = (R2/(R1 + R2)) x Vin
101
In potential divider circuits, which voltage is applied across both resistors in series?
The input voltage (Vin)
102
In potential divider circuits, which voltage is measured across only one of the resistors?
The output voltage (Vout)
103
In potential divider circuits, what does the potential difference across each resistor depend on and why?
The potential difference across each resistor depends on its **resistance** as the resistor with the **largest resistance** will have the **greater** potential difference, or a greater Vout (V = IR)
104
In potential divider circuits, if the resistance of one of the resistors (R1) is increased, what happens to the share of the potential difference between the two resistors?
If the resistance of R1 is increased:
* R1 will get a greater share of the potential difference
* R2 will therefore get a smaller share of the potential difference
105
How can the ratios of the potential differences and resistance across each resistor in a potential divider circuit be linked?
V1/V2 = R1/R2
Where:
* R1 = resistor 1
* R2 = resistor 2
* V1 = potential difference of R1
* V2 = potential difference of R2
106
What is a potentiometer?
A type of variable resistor which can act as a potential divider, consisting of a coil of wire with a sliding contact that can be used to vary the output voltage by being moved along the component
107
On a potentiometer, why does it matter that the sliding contact has the effect of seperating the component into two parts?
Each part will have different resistances, and therefore the output voltage will change
108
On a circuit diagram, if the arrow on a potentiometer symbol is drawn closest to the side of the positive cell terminal, what would this mean about the values of resistance and potential difference?
The resistance of the other part will increase, and so the potential difference across it will also increase. Therefore, the potentiometer obtains a maximum or minimum value for the output voltage.
If the resistance is 3Ω:
* Maximum voltage is when the resistance is 3Ω
* Minimum voltage is when the resistance is 0Ω
109
What is meant by a sensory resistor?
A type of sensor that changes its electrical resistance based on the amount of light or heat detected on the surface
110
What are some examples of variable **sensory** resistors?
* Thermistors
* Light-dependent resistors (LDRs)
111
If an LDR or thermistor's resistance decreases, what happens to the potential difference through it?
It also decreases
112
In potential divider circuits, why must the potential difference of across a resistor increase if the potential difference across the other resistor decreases?
Because the total potential difference of the components must be equal to Vin
113
If the resistance of an LDR is high, what does that mean about the light intensity present?
There is a low light intensity
114
If the resistance of an LDR is low, what does that mean about the light intensity present?
There is a high light intensity
115
What would happen to the output voltage in a potential dividers circuit (with an LDR) if the light intensity increases?
Vout would decrease because resistance would decrease
116
If the resistance of a thermistor is low, what does that mean about the temperature present?
There is a high temperature
117
If the resistance of a thermistor is high, what does that mean about the light intensity present?
There is a colder temperature
118
What would happen to the output voltage in a potential dividers circuit (with a thermistor) if the temperature decreases?
Vout would increase because resistance would increase
119
What energy store does a power supply gain as charge passes through it?
Electrical energy
120
What is the definition of the electromotive force (e.m.f.)?
The amount of chemical energy converted to electrical energy per unit charge when charge passes through a power supply
121
What unit does e.m.f. have?
Volts (V)
**OR**
Joules per coulomb (J C-1)
122
How can e.m.f. be expressed as an equation?
ε = E/Q
Where:
* ε = electromotive force (V)
* E = energy transferred to electrical energy (J)
* Q = charge (C)
123
Is e.m.f. a force?
No
It's a measure of energy transferred per coulomb of charge
124
What is the name for the potential difference across a cell when no current is flowing?
The e.m.f
125
How can e.m.f be measured?
By connecting a high-resistance voltmeter around the terminals of the cell in an open circuit
126
What is meant by internal resistance?
The resistance that takes place between the terminals of a power supply
127
What effect does internal resistance have on energy?
It causes some electrical energy to be transformed to heat energy within the power supply itself (which explains why a cell becomes warm after a period of time) and therefore causes energy loss in a power supply
128
How can a cell be thought of as its own smaller circuit?
A cell can be thought of as a source of e.m.f. with an internal resistance connected in series.
129
What is the name given to the amount of voltage lost in a cell due to internal resistance and what does it mean?
**Lost volts**: The energy per unit charge (voltage) wasted on overcoming the internal resistance
130
What is meant by **terminal** potential difference?
The potential difference across the terminals of a cell (not through the cell, but what the difference is in the reading of the positive terminal voltage, and the negative terminal voltage)
131
If there was no internal resistance in a cell, what would be shown about the values of the terminal p.d. and e.m.f?
The terminal p.d. would be equal to the e.m.f. value
132
If a cell has internal resistance, what would be shown about the values of the terminal p.d. and e.m.f?
The terminal p.d. would be lower than the e.m.f.
133
What is meant by a **load** resistor?
A component placed at the output of a circuit to consume power, simulate an actual device, and control voltage/current levels
134
How is terminal potential difference calculated?
VR = I x R
Where:
* R = load resistance (Ω)
* I = current in the circuit (A)
* VR = terminal p.d. (V)
135
How is the internal resistance of a cell affected as a current flows through it and by there being a load resistor in a circuit?
When a load resistor is connected, **potential difference develops** across the internal resistance, and therefore is classed as 'lost volts' as it is used lost overcoming the internal resistance
136
What is the equation for calculating the lost volts?
Vr = I x r
Where:
* r = internal resistance (Ω)
* I = current in the circuit (A)
* Vr = lost volts (V)
137
How do you calculate e.m.f. from the terminal p.d. and lost volts?
E = IR + Ir
**OR**
E = I(R + r)
Where:
E = e.m.f. (V)
IR = terminal p.d. (V)
Ir = lost volts across cell (V)
I = current (A)
R = load resistance (Ω)
r = internal resistance (Ω)
138
What is the difference between potential difference and e.m.f?
**The type of energy transfer per unit charge**
* Potential difference describes the loss of energy from charges
* E.M.F. describes the transfer of energy from the power supply to electrical charges within the circuit
139
As the temperature in a metal rises, what happens to its ions?
The ions vibrate with a greater frequency and amplitude
140
What happens to current if electrons collide with vibrating ions when temperature is high?
Current decreases
141
If cross-sectional area (A) and length (L) are constant, what is the relationship between **resistivity** and **resistance**?
ρ ∝ R
They are directly proportional
142
What happens to the resistance and resistivity if an ohmic-metallic conductor experiences an increase in temperature?
The resistance and resistivity increase
143
In a filament lamp, what is the relationship of current and the collisions between the free electrons and the lattice of ions?
As the current increases, the number of collisions increases
144
In reference to metals, how does the resistivity of a thermistor behave?
As a thermistor is a type of **semiconductor**, its resistance behaves in the opposite way to metals
145
What happens to the resistance and resistivity if a non-ohmic conductor (e.g. a thermistor) experiences an increase in temperature?
The resistance and resistivity decrease
146
What are two examples of uses for **thermistors**?
* Thermometers (measures temperature)
* Thermostats (controls temperature)
147
What types of conductor is a thermistor?
* A **non-ohmic** conductor
* A **semiconductor**
148
What **type** of resistor is a thermistor?
* A sensory resistor
149
How does the relationship between voltage and current change look for a **thermistor** on a resistance-temperature graph?
A curve that starts high on resistance and decays exponentially as temperature increases
(high resistance at low temperature, low resistance at high temperature)
150
How can light cause a change in conductivity of some semiconductors?
When light (through the form of photons) is absorbed by a semiconductor material, it provides enough energy to excite more electrons from a bound state to a higher energy state of 'free electrons'. This causes and increase in the number of conduction electrions, and therefore reduces resistance.
151
What is meant by a negative temperature coefficient (ntc) component?
If temperature increases in the component, the resistance of the component decreases (and vice versa)
152
What types of conductor is a light-dependent resistor (LDR)?
* A non-ohmic conductor
* A semiconductor
153
How does the relationship between voltage and current change look for an **LDR** on a resistance-light intensity graph?
A curve that starts high on resistance and decays exponentially as light intensity increases
(high resistance at low light intensity, low resistance at high light intensity)
A-Level Physics Edexcel
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