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Electricity and Electronics > Circuit Theory > Flashcards

Circuit Theory Flashcards

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1
Q

What is a capacitor?

A

A device used for storing electricity in the form of static electrical charges.

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2
Q

What is capacitance (C)?

A

The measure of the ability of a capacitor to store electrical charges. Measured in Farads.

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3
Q

What are the applications of capacitors?

A
  • Act as a storage bank of an electrical charge
  • Keeping the voltage at the same level across certain components
  • Remove noise from components
  • MRI, filters, oscillators and radio receivers
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4
Q

What is the composition of a capacitor?

A

Consists of two parallel metal plates separated by some insulating medium called a dielectric.

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5
Q

What is the simplest dielectric?

A

Air.

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6
Q

What is a dielectric?

A

Insulating materials which will polarise when in touch with an electric field. Ceramic and air are its examples.

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7
Q

What does a fully charged capacitor have?

A

Has the same voltage as the power supply.

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8
Q

How is energy stored in a capacitor?

A

Is stored as electric potential energy.

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9
Q

Where are capacitors used?

A

A variety of devices including defibrillators, microelectronics such as calculators, and flash lamps, to supply energy.

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10
Q

What are the characteristics of capacitors in series?

A
  • The current is common to each resistor
  • The total charge held by the circuit is equal to charge on each capacitor
  • The supply voltage is equal to the sum of the voltages across each capacitor
  • Capacitor formula similar to resistors in parallel
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11
Q

What are the characteristics of capacitors in parallel?

A
  • The pd across each capacitor is equal and same to the supply voltage Vs
  • The total charge of the circuit is equal to the sum of individual charges on each capacitor
  • The equation for total capacitance is similar to the equation for resistors in series
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12
Q

What is the time constant of a capacitor?

A

Refers to how long it takes to fully charge a capacitor. T=CR.

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13
Q

What happens when charging a capacitor through a resistor?

A

The capacitor is fully discharged. At the instant the switch is closed the capacitor acts as a short circuit and the current is at a maximum and Vc=0. As the capacitor charges, the pd across the capacitor increases and the current taken from the supply falls. When Vc=E, the current falls to zero.

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14
Q

How long does it take for a capacitor to charge?

A

Theoretically the capacitor would take an infinite time to charge. While in practice the capacitor can be assumed to be fully charged after a time equal to 5 time constants.

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15
Q

What happens when discharging a capacitor through a resistor?

A

The capacitor is now acting as the source of supply. The voltage across the capacitor, Vc, will fall exponentially to zero. The current supplied by the capacitor will also fall exponentially to zero from its initial value Io.

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16
Q

What is an inductor?

A

Passive circuit element that opposes sudden changes in current. They do so by storing energy in electromagnetic fields and releasing it back to the circuit when certain conditions are met.

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17
Q

How do inductors differ from resistors?

A

Inductors store kinetic energy of moving electrons in the form of a magnetic field, while resistors dissipate energy in the form of heat.

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18
Q

What do Faraday’s Law and Lenz’s Law explain?

A

They explain the behaviour of inductors.

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19
Q

What is back EMF?

A

Occurs when the current through an inductor is increased, dropping a voltage opposing the direction of the current flow.

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20
Q

What happens when discharging an inductor?

A

When the current through an inductor is decreased, it drops a voltage aiding the direction of the current flow.

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21
Q

Why is the inductor said to be discharging?

A

Because its store of energy is decreasing as it releases energy from its magnetic field to the rest of the circuit.

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22
Q

Why is the inductor said to be charging?

A

Because an increasing amount of energy is being stored in the magnetic field.

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23
Q

What is inductance?

A
  • Ratio of voltage to the rate of change of current
  • Quantifies how much energy an inductor is capable of storing.
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24
Q

What is self-inductance?

A

When the emf is induced in the same circuit which the current is changing.

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25
What factors affect the inductance of an inductor?
- The number of turns of the wire - The cross-sectional area of the coil of wire - Presence of a magnetic core - The way the turns are arranged
26
What are the applications of inductors?
- Choking, blocking, attenuating or filtering/smoothing high frequency noise in electrical circuits - Storing and transferring energy in power converters - Creating tuned oscillators or LC 'tank' circuits - Impedance matching
27
Why do inductors have to be shielded?
If they are not shielded they will be affected not only by their own magnetic fields, but also by the magnetic fields of other inductors around them.
28
What is the formula for inductors in series?
Ls=L1+L2+L3.
29
What is the formula for inductors in parallel?
Lp=1/(1/L1+1/L2+1/L3).
30
What is mutual induction?
A scenario where two coils are adjacently connected, and when the EMF is induced magnetically.
31
What is mutual inductance?
When two or more coils are magnetically linked together by a common magnetic flux.
32
What is nodal analysis?
Is based on the application of Kirchoff's current law at each junction of a circuit, to find the node voltages.
33
What is Thevenin's Theorem?
States that any network supplying a load can be replaced by a constant voltage source in series with an internal resistance.
34
What is electrical charge?
Charge possessed by a single charge.
35
What is a coulomb?
The quantity of charge which passes any section of a conductor in 1s, when a current of 1A is flowing.
36
What is electric current (I)?
The rate of flow of electrical charges.
37
What is 1 Ampere?
Steady current which, when flowing in two infinitely long straight, parallel conductors of negligible cross-sectional area, 1 metre apart in a vacuum, causes each conductor to exert a force of 2*10^-7 N on each metre of the other.
38
What is electricity?
Movement of electrons between atoms.
39
What is voltage?
Also known as 'potential difference', is the measure of electrical potential energy between two points.
40
What is electrical potential?
Energy required to move each Coulomb of electricity.
41
What is electromotive force (emf)?
Characteristic of any energy source capable of driving electric charge around a circuit.
42
What is resistance (R)?
Opposition to the flow of electrons.
43
What is ground (GND) or common (COM)?
The process of removing excess charge by providing a path for the current to return either to the source or physically into the ground.
44
What is the difference between emf and voltage?
Emf is the measure of energy supply to each coulomb of charge, whereas the voltage is the energy used by one coulomb of charge to move from one point to another.
45
How does emf differ from voltage?
Emf is measured between the end points of the source, when no current flows through it, whereas, the voltage is measured between any two points of the closed circuit.
46
What is power (P)?
The rate of energy use or energy conversion.
47
What is Ohm's Law?
The current in an electrical circuit is directly proportional to the applied potential and inversely proportional to the resistance provided the temperature remains constant.
48
What is a potential divider?
Two resistors connected in series, which enables the supply to be split in the same proportion as the values of resistance.
49
What are the applications of a potential divider?
"Tapping" off a lower voltage from a fixed supply voltage - transistor biasing circuits.
50
What is direct current?
Flow of electricity in one direction. Used in most electronics.
51
What is alternating current?
Describes the flow of charge that changes direction periodically.
52
What is conventional current?
Assumes that current flows out of the positive terminal, through the circuit and into the negative terminal of the source.
53
What is electron flow?
Is what actually happens, where electrons flow out of the negative terminal, through the circuit and into the positive terminal of the source.
54
What are resistors?
Electrical component that restricts the flow of electrical charge.
55
What is a short circuit?
Occurs when an electrical current flows through an unintended path with very little (or zero) resistance.
56
What are the effects of a short circuit?
Often leads to excessive current flow, which can generate heat, damage components, pose safety risks or in extreme cases cause fires.
57
What causes a short circuit?
Happens when two points in a circuit that should not be directly connected come into contact, allowing current to bypass the intended load.
58
What is a current divider?
Parallel circuits in which the source (or supply) current divides into several parallel paths.
59
What are insulators?
Prevent the flow of electrons or electricity.
60
What are conductors?
Allow the flow of electrons or electricity.
61
What are semiconductors?
Can be altered to function as either a conductor or an insulator.
62
What does it mean for atoms to be electrically neutral?
The number of protons equals the number of electrons, which cancel each other out, being therefore, neutral.
63
What is a source?
Supplies energy to a system.
64
What is a load?
Accepts energy from a system.
65
Series circuits
The current must pass through each and every one of the components so connected.
66
What is a network?
Consists of two or more circuits.
67
Parallel circuits
The electron has the choice of which load unit through which to pass.
68
What happens in a parallel lamp arrangement when one lamp fails?
It doesn't affect the operating of other lamps; the current continues to pass through the remaining branches as before.
69
What happens when one lamp fails in a series circuit?
If one fails then all the lamps are extinguished.
70
Which arrangement is most preferred in practical terms?
Parallel arrangement.
71
How do resistors protect circuits from damage?
By dissipating the kinetic energy of electrons in current as thermal energy.
72
What is a potentiometer?
Manually adjustable, variable resistor with three terminals.
73
What is the maximum power transfer theorem?
States that the DC voltage source will deliver maximum power to the variable load resistor only when the load resistance is equal to the source resistance.
74
What are the advantages of digital instruments?
- More precise - More robust - Easier to read either visually or by a computer.
75
What is a multimeter?
Measuring instrument that can measure multiple electrical properties.
76
What are the components of a multimeter?
Voltmeter (voltage), Ohmmeter (resistance) and Ammeter (current).
77
What are the parts of a multimeter?
Display, selection dial and ports.
78
What does the display of a multimeter do?
Usually has 3 to 4 digits and the ability to display a negative sign.
79
What does the selection dial of a multimeter do?
Allows you to select which measurement you want to record.
80
What are the ports of a multimeter?
COM-stands for common and is connected to the ground of a circuit. mAVΩ- allows the measurement of current (up to 200mA), voltage and resistance.
81
What does an ammeter do?
Measures electric current and must be connected in series.
82
What do fuses do?
Break the circuit if a fault in an appliance causes too much current to flow.
83
What does a voltmeter do?
Measures voltage and the connection must be made in parallel with that part of a circuit across which a voltage measurement is required.
84
What is a Wheatstone Bridge?
Alternative of measuring resistance (when a resistance is unknown) and is a bridge that comprises two potential dividers.
85
What are the applications of Wheatstone bridges?
- Often used with transducers to measure physical quantities like temperature, pressure, strain - Used in applications where small changes in resistance are to be measured in sensors.
86
What is Kirchoff's Current Law (KCL)?
The total current entering a circuit's junction is equal to the current leaving the same junction.
87
What is Kirchoff's Voltage Law (KVL)?
The sum of the voltage drops around a closed loop is equal to the resultant emf.
88
What is the Superposition Theorem?
States that in a circuit containing more than one source, the current in, or the pd across, any branch can be found by considering each source separately and adding their effects.
89
What is the maximum self-induced emf?
This occurs at the instant of switch off, when the maximum current flows through maximum resistance.
90
Ideal Voltemeter Characteristic
Infinite Resistance ## Footnote This prevents current from flowing through it, preveting any disturbance to the circuit to which its attached
91
Ideal Ammeter Characteristic
Zero Resistance ## Footnote So that it doesn't change the current that flows in the circuit in its absence

Electricity and Electronics (5 decks)

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