Electricity Flashcards
(102 cards)
1
Q
What is AC current and how does it vary?
A
Alternating current that varies sinusoidally.
2
Q
What is the effective voltage/DC voltage of AC voltage? What about current?
A
0.71 x maximum AC voltage. 0.71 x maximum AC current.
3
Q
What is the relationship between voltage and current in a resistor in AC? What is current when voltage is maximum?
A
They are in phase, so they are proportional. Current is also maximum.
4
Q
What is the relationship between voltage and current in a capacitor in AC? What is current when voltage is maximum?
A
They are out of phase. Current leads the voltage by 90 degrees in the phasor. Current is 0.
5
Q
What is reactance? Units? Symbol?
A
A measure of how much a capacitor or inductor opposes the flow of current. Ohms. X.
6
Q
Vc =
A
Xc x I
7
Q
What does capacitance reactance depend on?
A
Capacitance and frequency of AC current.
8
Q
What happens to capacitance reactance when we increase capacitance?
A
This decreases reactance as Xc = 1/omega C.
9
Q
What happens to capacitance reactance when we increase frequency of the AC current?
A
This decreases reactance as Xc = 1/2pif(C).
10
Q
What is an RC circuit?
A
Resistor and capacitor circuit.
11
Q
What is the relationship among resistor voltage, supply voltage and capacitor voltage?
A
Resistor voltage leads supply voltage by 45 degrees. Supply voltage leads capacitor voltage by 45 degrees.
12
Q
What is impedance? Symbol? Unit?
A
Impedance is the total of resistance and reactance. Z. Ohms.
13
Q
What theorem can be used to solve RC voltages to calculate impedance? What is the equations?
A
Pythagoras. So Z = √[(R)^2 + (Xc)^2)]
14
Q
What is the relationship between voltage and current in an inductor in AC? What is voltage when current is maximum?
A
Voltage of inductor leads current by 90 degrees. Voltage is 0 when current is maximum (not changing for that moment so no induced back emf).
15
Q
What does inductor reactance depend on?
A
Inductance and frequency of AC current.
16
Q
What happens to inductor reactance when we increase inductance?
A
Increasing inductance decreases current. This Increases reactance.
17
Q
What happens to inductor reactance when we increase frequency of AC current?
A
Increasing frequency of AC current decreases current. This increases reactance.
18
Q
VL =
A
XL x I
19
Q
What is AC frequency?
A
AC frequency is the number of cycles per second in an alternating current that varies sinusoidally.
20
Q
What is an RLC circuit?
A
Resistor, inductor and capacitor circuit.
21
Q
The reactance of capacitors and inductors are always in ___.
A
Opposite directions.
22
Q
What is the resonant frequency?
A
When the current is maximum. This is because the voltage across the capacitor and inductor is equal and opposite, cancelling out. This means only the resistance affects the current, there is no reactance. Impedance is small.
23
Q
RLC circuit: Describe the current size at low frequency. Why?
A
Current is small. This is because at low frequency the capacitance has a large reactance, causing a large impedance.
24
Q
RLC circuit: Describe the current size at high frequency. Why?
A
Current is small. This is because at high frequency the inductor has a high reactance, causing a large impedance.
25
What theorem is useful in RLC circuits?
Pythagoras
26
How are RLC circuits used in metal detectors?
Metal detectors that detect iron contact RLC circuits that are initially resonating. The current is maximum as capacitor and inductor reactance cancel out. When iron passes through the metal detector, the inductance increases so the circuit no longer resonates. This changes the current.
27
When the frequency of the power supply matches the natural frequency, _____ is produced.
Maximum current
28
How are RLC circuits used in radio tuning circuits?
Radio stations use different frequency waves. When the radio waves match the resonant frequency of the circuit, current will be maximum. So we increase or decrease the resonant frequency of the tuning circuit to match different radio wave frequencies to listen to the different radio stations. Using the resonant frequency equation: to do this we increase capacitance to decrease the resonant frequency or we decrease capacitance to increase the resonant frequency.
29
Resistance for components in series
Rs = R1 + R2...
30
Resistance for components in parallel
Rp = 1/R1 + 1/R2...
31
An ideal battery has
No resistance
32
A real battery has
Its own internal resistance
33
Kirchoff's current law
Total current entering a junction is equal to total current leaving the junction.
34
Kirchoff's voltage law
The total voltage around a closed loop in a circuit equals zero.
35
A capacitor stores ___ then ___ it quickly
Charge. Releases.
36
Capacitor description
Capacitors consists of two parallel metal plates separated by a distance, d, usually with an insulating material between them called a dielectric.
37
When connected to a ___, electrons flow from the ___ of the power supply to the
___ near the ___, and so this ___ becomes ___. Meanwhile electrons are
___ from the other plate towards the ___ of the ___, making that plate ___. The capacitor continues to build up ___,___, until the voltage across the capacitor is equal to
the ___, and the plates have an ___charge, Q.
Power supply. Negative terminal. Plate. Negative terminal. Plate. Negatively charged. Drawn. Positive terminal. Power supply. Positively charged. Charge, Q. Battery voltage. Equal and opposite.
38
Voltage vs time graph explanation for charging capacitor.
Initially the voltage on the capacitor is zero so it is easy for charge to get pushed onto the capacitor. Thus, the voltage of the capacitor builds up quickly, hence the steep gradient. As charge builds up on the capacitor, the voltage of the capacitor opposes the flow of current and so it is more difficult to push charge onto the capacitor. The increase in voltage slows and the gradient is less. This occurs until the gradient of the graph approaches zero and the capacitor is effectively fully charged.
39
Voltage vs Time Charging Capacitor is an ___ curve.
Projectile curve
40
Current vs Time charging capacitor is an ___ curve.
Exponential decay.
41
Voltage vs time graph discharging capacitor
When a capacitor discharges, the capacitor starts off at max voltage, pushing charge off the plates quickly. As charge moves off quickly, the voltage across the capacitor also decreases quickly. As the capacitor voltage decreases, charge is being pushed off the plates more slowly, so the decrease in voltage gradient is less. The voltage vs time gradient decreases until it approaches zero where the capacitor is fully discharged.
42
Dielectric
```
The dielectric (an insulating layer between the plates) is used to keep the plates separated and increase
the capacitance.
```
43
Eo
Permittivity of free space. A constant.
44
Er
Dielectric constant.
45
Energy in capacitor =
1/2QV or 1/2C(V^2)
46
Why is the electrical potential energy stored in a charged capacitor only half of the electrical energy
supplied by the power source
The other half of electrical energy was turned into heat as the capacitor charged due to the resistance of the circuit.
47
τ
Time constant
48
How long to effectively charge or discharge a capacitor?
Approximately 5 time constants
49
The time constant, τ, is the time for the voltage of the capacitor, or the current in the circuit, to change by ___
0.63 of the remaining voltage or current.
50
Capacitors in parallel ___ total circuit capacitance. Equation for capacitance? Charge equation?
Increases. CT = C1 + C2 + ..... Q = CT(V).
51
Adding capacitors in series will ___ the ___. Equation for capacitance? Why?
Reduce. Total circuit capacitance. 1/CT = 1/C1 + 1/C2 + ...
The capacitors share the supply voltage so each capacitor has less voltage across them and so they store less charge. Q = CV.
52
Increasing capacitance resistance ___ time constant. Increasing capacitance capacitance, ___ time constant. Equation?
Increases. Increases. τ = RC
53
What is Q?
Amount of charge stored.
54
The direction of the ___ is given by the right hand
grip rule for ___ current, (so if it is the flow of electrons use the ___ instead).
```
Fingers = Magnetic field direction
Thumb = Positive charge or negative charge movement direction
```
Magnetic field. Conventional. Left.
55
Field lines into the page are represented with ___.
| Field lines out of the page are represented with ___.
Crosses. Dots.
56
For a coiled conductor e.g. a loop or a solenoid,
use the ___ for the direction of the ___.
```
Fingers = Conventional current direction
Thumb = Magnetic field direction
```
The north pole is where the magnetic field lines ___ the coil/magnet. The south pole is where the magnetic field lines ___ the coil/magnet.
Modified right hand grip rule. Magnetic field. Leave. Enter.
57
Right hand slap rule
```
Fingers = ___
Thumb = ___
Palm = ___
```
Magnetic field direction. Conventional current direction. Magnetic force direction.
58
If the area is ___ to the magnetic field then the magnetic flux is given by ___, and the magnetic flux is reduced.
Not perpendicular. ɸ = BA x cosθ.
59
Faraday noticed that a ___ was produced across a conductor the ___ it was moved in or out of
a ___. This is because the ___ of the conductor in the magnetic field changes at a ___, and
therefore the ___ changes at a ___ (because ___). Since there is a ___ in ___, a ___ is induced. This is ___ Law. The ___ induced will ___ the change that induced it by ___ the magnetic flux (___ Law). Therefore, the current flows through the loop in a direction that produces a ___ that is in the ___ to the magnetic field that the loop is moving through, and so the strength of the
magnetic field, B, ___. Therefore, the ___ inside the loop is ___, as ___. This opposes the ___ in flux.
Greater voltage. Faster. Magnetic field. Area. Greater rate. Magnetic flux. Greater rate. ɸ = BA. Change in magnetic flux. Voltage. Faraday's. Voltage and current. Oppose. Lenz's. Magnetic field. Opposite direction. Is reduced. Magnetic flux. Reduced. ɸ = BA. Change.
60
Inductor
Made up of an inductance and resistance. A coiled wire, often around an iron core. This is the inductance. There is a resistance connected in series.
61
When a current, I, flows through the coils of an inductor a ___ is produced in the inductor, and therefore there is a ___ in the inductor. The ___ in flux induces a ___ that ___ the change that produced it, which was the change in current. This induced voltage is called a ___ and is in the ___ to the flow of current and so it ___ the increase in current. Once the current reaches its ___, there is ___ change in current so there is no more change in the strength of the ___. This means there is no more change in ___ and there is no more ___. Therefore, an inductor only affects the circuit when current is ___.
Magnetic field. Change in magnetic flux. Change. Voltage. Opposes. Back voltage. Opposite direction. Slows. Max value. No more. Magnetic field. Magnetic flux. Induced voltage. Changing.
62
Lenz's Law
Induced voltage or current will oppose the change that caused it.
63
Faraday's Law
Change in magnetic flux will induce a voltage.
64
The inductance, L, of an inductor is a measure of ___. The induced back voltage depends on the ___ it can produce. The bigger its ___, the bigger the change in ___ that will occur, and so the bigger the ___. The magnetic flux it can produce depends on the strength of the ___, in the inductor and its
___ (ɸ = BA).
The inductor’s ability to produce an induced voltage to
| oppose the change in current. Flux. Flux. Induced voltage. Magnetic field. Area.
65
What are three factors that can increase inductance? What do they do?
Adding or increasing iron core. Increases B.
Increasing the number of coils. Increases A.
Decreasing resistance in wires. Increases current, which increases B.
66
Magnetic field strength is proportional to the size of the ___.
Current.
67
Inductance symbol and unit.
L and H.
68
Energy equation for inductor.
E = 1/2L(I)^2
69
What is the pattern in energy equations.
Always the "storage" of the object, e.g. mass, capacitance, inductance. Plus the "action thing" that is squared, e.g. velocity, voltage, current.
70
Graph shape for current when switching on inductor. Why?
Square root graph (half of negative parabola). Initially, the current is high, but then the inductor induces a back voltage that slows the increase in current. This means the gradient of the graph decreases. The gradient of the graph continues to decrease until the current approaches its maximum value where the current stops changing.
71
Graph shape for voltage when switching on inductor. Why?
Exponential decay. Initially, the induced voltage is high because the current is changing the most. The change in current begins to decrease as the induced back voltage itself slows the increase in current. This decreases the induced voltage slowly as well until zero where current stops changing as it is at its maximum value.
72
Graph shape for current when switching OFF inductor. Why? This graph shape is the same for ___.
Exponential decay. Initially, the current is high and it begins to decrease rapidly. The inductor induces a back voltage due to the rapid decrease and change in current, slowing the decrease in current. The current decreases slowly until it approaches zero. The graph shape for voltage of inductor.
73
Time constant equation for inductor.
τ = L/R
74
When the current is switched on for an inductor, the inductor voltage is initially equal to the ___. Why?
Supply voltage. Kirchoff's voltage law: Vs - VL - VR = 0.
| Vs - VL - IR = 0. Vs - VL = IR. Initially, there is no resistance in the circuit, so IR = 0. So Vs - VL = 0. So Vs = VL.
75
Transformer
Transformers are made from a primary coil connected to the power supply and a secondary coil wound onto one iron core. Primary and secondary coil are part of two different circuits.
76
Transformers only work with ___. How does it work?
AC circuits. When the primary coil of a transformer is connected to an AC power supply, the changing current creates a changing magnetic flux in the primary coil. This changing magnetic flux is passed through the shared iron
core to the secondary coil, where it induces an alternating voltage in the secondary coil (which produces
an alternating current in the secondary circuit).
77
What is the purpose of transformers?
To change (transform) the voltage in an alternating current (AC) circuit through the secondary coil having more turns (increases voltage) or less turns (decreases voltage) than the primary coil.
78
Vp/Vs = Np/Ns. What are these symbols?
Vp and Vs = voltage of primary and secondary coil.
| Np and Ns = Number of turns of primary and secondary coil.
79
For an ideal transformer, the power input is equal to the ___. So ___. However, in reality, the power input is ___ to the power output as some energy is ___ as heat energy due to the ___.
Power output. VpIp = VsIs. Not equal. Lost. Circuit's resistance.
80
For an ideal inductor, the energy input is ___ to the energy output. However, in reality, the energy input is ___ to the energy output as some energy is ___ as heat energy due to the ___.
Equal. Not equal. Lost. Circuit's resistance.
81
Current vs time graph charging capacitor
Initially the current is maximum because there is no charge and so no voltage on the capacitor. As charge gets pushed and stored onto the capacitor, the voltage of the capacitor increases and it opposes the flow of current so the current decreases and so the graph's gradient decreases. The voltage continues to build up on the capacitor, so opposes the flow of current more so the continues to decrease until it approaches zero where the capacitor is effectively fully charged.
82
Current vs time graph discharging capacitor
The capacitor starts off at max voltage initially, pushing the charge off the capacitor fast. This means there is max current in the circuit. V = IR. As charge gets pushed off, the voltage across the capacitor decreases so the charge is pushed off slower and the current decreases. At first, the gradient is steep because charge is being pushed off fast. As charge gets pushed off, the capacitor's voltage decreases, so it pushes charge off more slowly. The gradient of the current vs time graph decreases and it continues to decrease until it approaches zero where the capacitor has fully discharged.
83
CIVIL
Capacitor: Current leads voltage.
Inductor: Voltage leads current.
84
What do the RC voltage phasors look like?
Vresistor leads Vsupply by 45 degrees which leads Vcapacitor by 45 degrees.
85
What happens to current when we increase capacitance?
Xc = 1/wC. Increasing capacitance will decrease reactance. This means current increases.
86
Why is the current low when something's frequency is above the resonant frequency of the circuit?
This is because the inductor reactance is high and so impedance is high so the current is low.
87
What happens to current when the frequency is below the resonant frequency.
The current is low because the capacitor reactance is high so the impedance is high so the current is low.
88
Capacitance unit
F
89
Inductance unit
H
90
Reactance unit
Ohms
91
What is Emf?
The voltage of the battery when no current flows.
92
A battery is connected in series with a resistor. What happens to the battery voltage as we increase the resistance of the resistor?
The circuit's total resistance increases so the circuit's current decreases. This means there is a smaller voltage drop across the battery's internal resistance so the battery voltage increases.
93
Describe how a capacitor charges up in terms of the flow of electrons.
Electrons flow from the negative terminal of the power supply onto the plate closest to it. Electrons from the other plate are attracted towards the positive terminal, leaving that plate positively charged. Charge continues to build up on the capacitor until its two plates are equally and oppositely charged. Now, the capacitor voltage is equal to the supply voltage.
94
What is rms voltage?
Rms voltage is the average of AC voltage that is equivalent to the DC voltage that produces the same power output as the AC voltage.
95
What happens to magnetic flux when AC current frequency is higher? Why?
When the frequency of current is higher, the rate of change of current is greater so there is greater change in magnetic flux.
96
Current flows from the ___ side of the battery
Longer/positive
97
From the shorter to longer side of the battery, you ___ the voltage of the battery.
Add
98
From the positive terminal to the negative terminal of the battery, you ___.
Minus the voltage of the battery
99
P =
VI
100
A capacitor is marked 220 μF 6.0 V. What does this mean?
The capacitor can store 220μC for every 1 volt. 6 volts is the maximum voltage it can be charged to.
101
What happens to light bulb brightness when we increase capacitance?
The brightness stays the same because the voltage it is connected to remains constant.
102
A bulb a capacitor is in series in AC. What happens to bulb voltage when we decrease frequency of current?
The capacitor reactance increases, so the capacitor voltage increases. This causes the bulb voltage to decrease as they are connected in series and share the voltage.
