Module 4: Chapter 10 - Electrical Circuits Flashcards
1
Q
What is Kirchoff’s second law?
A
In any circuit, the sum of the electromotive forces is equal to the sum of the potential differences around a closed loop
2
Q
What is the equation for kirchoff’s second law?
A
Σε = Σv in a closed loop
3
Q
What physical quantity is conserved by Kirchoff’s first law?
A
Charge
4
Q
What physical quantity is conserved by Kirchoff’s second law?
A
Energy
5
Q
What is the rule for PD in parallel circuits?
A
PD is the same in parallel circuits
6
Q
What is the rule for PD in series circuits?
A
PD is shared between components in a series circuit
7
Q
What is the rule for current in parallel circuits?
A
Current is shared between parallel circuits
8
Q
What is the rule for current in series circuits?
A
Current is the same for all components in a series circuit
9
Q
What is the value of B and C?
A
B = 12V
C = 9V
10
Q
What is the value of D and E?
A
D = 8V
E = 4V
11
Q
What is the value of F and G?
A
F = 4V
G = 3V
12
Q
What is the equation for total PD in a series circuit?
A
Vₜ = V₁ + V₂ + V₃
13
Q
What is the equation for total current in a series circuit?
A
Iₜ = I₁ = I₂ = I₃
14
Q
What is the equation for total PD in a parallel circuit?
A
Vₜ = V₁ = V₂ = V₃
15
Q
What is the equation for total current in a parallel circuit
A
Iₜ = I₁ + I₂ + I₃
16
Q
What is the equation for total resistance in a series circuit?
A
Rₜ = R₁ + R₂ + R₃
17
Q
Show how the equation for total resistance in a series circuit is found:
A
Vₜ = V₁ + V₂ + V₃
Iₜ = I₁ = I₂ = I₃
RₜIₜ = R₁I₁ + R₂I₂ + R₃I₃
RₜIₜ = R₁Iₜ + R₂Iₜ + R₃Iₜ
Rₜ = R₁ + R₂ + R₃
18
Q
What is the equation for resistance in a parallel circuit?
A
1/Rₜ = 1/R₁ + 1/R₂ + 1/R₃
19
Q
How is the equation for resistance in a parallel circuit found?
A
Iₜ = I₁ + I₂ + I₃
Vₜ = V₁ = V₂ = V₃
Vₜ/Rₜ = V₁/R₁ + V₂/R₂ + V₃/R₃
Vₜ/Rₜ = Vₜ/R₁ + Vₜ/R₂ + Vₜ/R₃
1/Rₜ = 1/R₁ + 1/R₂ + 1/R₃
20
Q
Kirchoffs first law states that ΣIᵢₙ = ΣIₒᵤₜ, what determines how much current splits into each branch?
A
The greater the resistance of the branch, the lower the current that passes through it. If one branch has half the resistance of the other, it will have twice the current through it
21
Q
Which type of circuits do kirchoff’s first and second laws apply?
A
All circuits
22
Q
What is internal resistance?
A
The resistance of a source of e.m.f due to its construction, causing a loss in energy as the charge passes through the source
23
Q
What is terminal p.d?
A
The potential difference across a power source
24
Q
What are lost volts?
A
Energy lost as heat within a power source due to internal resistance
25
What is the equation for terminal p.d?
terminal p.d = e.m.f - lost volts
26
A device runs of 2 1.5V cells, yet contains a lamp rated at 2.5V and 0.5A, what is the internal resistance of each identical cell?
3 = 2.5 + Ir
0.5 = 0.5r
1 = r
1/2 = 0.5
**0.5 Ω**
27
What is the equation for lost volts?
lost volts = Ir (r is the internal resistance)
28
What does it mean if there is a dotted rectangle around the cell and a resistor?
The dotted rectangle indicates the cell, containing a source of resistnace (internal resistance) and a source of e.m.f
29
What is the equation for e.m.f?
ε = V + Ir
30
What is the e.m.f when 2 cells are connected in series?
The sum of the e.m.f of the 2 cells
31
What is the e.m.f when 2 identical cells are connected in parallel?
The e.m.f is the same as one of the cells alone
32
2 identical 1.5 V cells, each with an internal resistance of 0.75Ω, are connected in series. Calculate the terminal p.d when the cells are connected to a resistor of 10Ω
2.6V
33
What is the equation for e.m.f?
ε = V + Ir
ε = IR + Ir
ε = I(R+r)
34
2 identical 1.5 V cells, each with an internal resistance of 0.75Ω, are connected in parallel. Calculate the terminal p.d when the cells are connected to a resistor of 10Ω
1.4 V
35
Calculate the pd lost over the internal resistance when emf = 12V, R1 = 4Ω, R2 = 6Ω, R3 = 2Ω. The reading on the ammeter says 3A
4V
36
Calculate the voltages over R2 and R3 when emf = 10V, r = 2Ω, R1 = 5Ω, R2 = 6Ω, and R3 = 9Ω
V2 = 2.61V
V3 = 3.91V
37
If a power source needs to provide a large current, what internal resistance is required?
A small internal resistance is required to provide a large current
38
What happens to "lost volts"?
Energy is trasferred to a source of internal ressistance in the form of heat
39
Why can you not connect many AA batteries in order to substitute for a car battery?
Car batteries have a very low internal resistance in order to provide the large current required. Even if you connect enough AA batteries together in order to produce an equal emf of that of the car battery, they would not provide enough current due to their higher internal resistance
40
What happens to terminal pd when you increase the current (with a constant emf and internal resistance) and why?
Terminal pd decreases as current increases as terminal pd is given by the equation: terminal pd = ε - Ir. By increasing current, lost volts are increased and therefore terminal pd decreases
41
When is terminal pd equal to emf?
When the current is 0
42
What is the y intercept in a graph of **terminal** V against I?
ε (emf)
43
What is the gradient in a graph of **terminal** V against I?
-r (-1 x internal resistance)
44
What is the equation of the line in a graph of **terminal** V against I?
terminal V = -rI + ε
45
Draw a graph of **terminal** V against I?
46
Why is the current at x twice the current at y or z?
Due to kirchoff's second law, the p.d. across each parallel branch is the same. The resistance in branch X is 6Ω, whereas it is double in branch YZ - 12Ω. Therefore the current in branch X is double that of branch YZ as V = IR
47
How do you determine how much current exits each branch at a parallel junction?
Using kirchoff's second law and V = IR
48
Why is the pd between x and z 0?
Due to kirchoff's second law, the pd in each branch is equal to the terminal pd and each otehr. At branch x there is a total resistance of 6Ω, with 3Ω (half the total resitance) before x, therefore the pd at x is half the terminal pd. Similarly, at branch z, there is a total resistance of 12Ω, with 6Ω (half the total resistance) before z, therefore the pd at z is half the terminal pd - the same as at x. Therefore the pd between x and z is 0
49
What electrode is the positive terminal in a simple electrolyte battery?
Negative electrode
50
What electrode is the negative terminal in a simple electrolyte battery?
Positive electrode
51
What is a potential divider circuit?
A potential divider circuit is a simple circuit that uses resistors (or thermistors/LDR's) to supply a variable output potential difference
52
What is the potential divider equation?
53
What is loading a potential divider circuit?
Loading refers to connecting a component or circuit to Vout, placing it in parallel with R2.
54
How does loading a potential divider circuit affect Vout?
Loading a potential divider circuit lowers the resistance of this part of the circuit and therefore lowers Vout.
55
How can variable resistors be used in a potential divider circuit?
A variable resistor can be used to vary the output voltage in a circuit
56
How can thermistors be used in a potential divider circuit for a heating element?
Thermistors have high resistance at low temperatures, therefore the voltage will be high at 0°C and will switch the heater on. As the thermistor heats up, the resistance decreases and will cause the heater to turn off
57
Design a potential divider circuit that will turn on a street lamp when the sun sets
58
Design a potential divider circuit that will turn off a kettle when the water has boiled
59
Design a potential divider circuit that will increase Vout as temperature increases
60
What is a potentiometer?
A potentiometer works in the same way as a potential divider circuit but rather than using 2 resistors in series it uses a length of resistive wire with a sliding contact
61
How can you increase Vout in a potentiometer?
When the contact is moved towards A, Vout increases until at A when Vout = Vin. At B, Vout = 0
62
What are the advantages of a potentionmeter over a potential divider circuit?
* More compact
* Can be made even more compact by use of a rotary dial
63
Why may a variable resistor be connected with a thermistor as part of a potential divider rather than a fixed resistor?
It allows you to vary the output at a constant temperature
64
65
What do you do if a circuiot has more than 1 source of emf?
* If in the same direction, add them
* If in opposite directions, find the difference
66
What is the ratio of resistances in a potential divider circuit?
V1 / V2 = R1 / R2
67
How do you analyse a circuit?
Draw a table for all values needed and work systematically through
68
How does an electron gun work?
1. A small metal filament is heated, known as a cathode
2. The electrons in the wire gain kinetic energy
3. SOme gain enough kinetic energy to escape the surface of the metal
4. This process is known as thermionic emission
5. A high potential difference is applied between the filament and the anode inside a vacuum
6. THe free electrons accelerate towards the anode gaining kinetic energy
7. A small hole in the anode allows a line of electrons to pass through, creaing a beam with specific kinetic energy
69
What is the equation for the kinetic energy of an electron emitted from an electron gun?
Elementary charge x voltage = Kinetic energy
eV = 0.5mv²
70
What is the actual mass of an electron?
9.11 x 10⁻³¹ kg
71
What is the speed of light?
3x10⁸ ms⁻¹
72
What is thermionic emission?
The emission of electrons through the action of heat
73
What is the IV graph for a fixed resistor?
74
What is the IV graph for a filament lamp?
75
How can you collect data for an IV graph?
1. Set up a circuit with a dc variable supply (0-6V) with the component. Make sure there is an ammeter connected in series and a voltmeter connected in parallel
2. Take readings of the current and potential difference over the range 0-6V
3. Reverse the connections and repeat step 2
4. Plot results in an I-V graph
76
What is an ohmic conductor?
A conductor that obeys ohm's law (such as a resistor)
77
Describe the I-V characteristics of a resistor:
A fixed resistor is designed to ensure that the resistance remains constant regardless of typical changes in temperature as current varies. Therefore the potential difference is directly proportional to the current
78
What is a non-ohmic conductor?
A component that does non obey ohms law (such as a filament lamp)
79
Describe and explain the I-V characteristics of a filament lamp:
The potential difference across a filament lamp is not directly proportional to the current as the resistance is not constant. The resistance of the filament increases as the p.d across it increases. This is because as current increases, the rate of flow of charge increases, therefore there are more collisions between the electrons and positive metal ions per second. When the electrons collide with the ions they transfer energy to the ions, causing the ions to vibrate more causing the resistance to increase
80
What is a diode?
An electrical component that only allows current in one particular direction
81
What makes Light emitting diodes so useful?
* They emit light of a single specific wavelength
* They are extremely energy efficient
* They last a very long time
82
What component's behaviour is affected by polarity?
All diodes are affected by polarity, all other components are not
83
Describe and explain the I-V characteristics of a diode:
At A, the resistance is extremely high (treated as infinite for practical purposes). With the PD in this direction the diode cannot conduct. At B, as the p.d increases the resistance gradually begins to drop and the current increases. This point is known as the "threshold pd". Above this value, the resistance drops sharply for every small increase in pd. As the resistance is constantly changing, a diode is a non-ohmic component
84
Why do different LED's have different threshold p.d's?
the threshold p.d is related to the colour of the light they emit
85
What is a thermistor made from?
A semiconducting material with a negative temperature coefficient
86
What does a negative temperature coefficient mean?
The resistance decreases as temperature increases. As the temperature increases, the number density of the charge carriers increases, causing resistance to decrease
87
What are thermistors used for?
To monitor and regulate temperature
88
How does resistance change as temperature increases in a thermistor?
Resistance decreases as temperature increases
89
Describe and explain the I-V characteristics of a thermistor
The thermistor is a non-ohmic component, as the current increases the temperature increases, however this causes a decrease in resistance as the number density of charge carriers increases. Therefore as current increases the resistance decreases
90
Describe how an LDR works:
An LDR is made from a semiconductiing material in which when the incident light increases, the number density of charge carriers increases causing the resistance to decrease
91
Describe how resitance changes as light intensity changes in an LDR
Resistance decreases as light intensity increases
92
What is the I-V graph of an LDR?
93
What does resistance depend on?
* Temperature
* Material (Resistivity)
* Length of wire
* Cross sectional area of the wire
94
How is resistance affected by length?
Resistance is directly proportional to length
95
How is resistance affected by cross sectional area?
Resistance is inversly proportional to cross sectional area
96
What is the equation for resistance (considering resistivity)?
R = ρl / A
Resistance = (Resistivity x length) / Cross sectional area
97
What is resistivity?
The measure of how much a particular material opposes electron flow
98
What are the units for resistivity?
Ωm (ohm metre)
99
Describe the effect of temperature on resistivity in a metal:
The resistivity of a metal is directly proportional to the temperature
100
How can you increase conductivity in a semi-conductor?
Impurities in semiconductors such as silicon significantly increase conduction
101
Describe superconductivity:
When certain materials are cooled, their resistivity drops (as expected), however once they reach a "critical temperature" the resistivity suddenly drops to 0. This is superconductivity
102
Explain why superconductors do not increase in temperature as current passes through them
Superconductors have no electrical resistance. Therefore no energy is lost when there is a current in the material. Therefore huge amounts of charge can pass through a superconductor without it getting warm
103
What is the difference between resistance and resistivity?
Resistivity is an intrinsic property of a material whereas resistance can be changed by a number of factors
104
What is the equation for resistivity?
ρ = RA / l
105
Describe the relative resistivity of conductors, semiconductors, and insulators
Insulators have a very high resistivity, Conductors a very high low resistivity and semiconductors between the 2
106
What is electrical power?
The rate of energy transfer by each electrical component
107
What are the 3 equations for electrical power?
* **P = VI**
* **P = V² / R**
* **P = I²R**
108
What is the equation for electrical energy transferred?
W = ItV
Work done = Current x Time x Potential Difference
109
What factors affect how much energy is transferred by a device?
* The power of the device
* How long the device is used for
110
What unit is used to measure the energy used in homes?
kilowatt-hour (kWh)
111
What is a kilowatt-hour?
The energy transferred by a device with a power of 1kW used for 1 hour
112
What is 1kWh in joules?
3.6 MJ
113
What does an electricity meter do?
Records the transfer of energy from the national grid to the house
