Flashcards
Electricity Notes
Cards in this deck (37)
- 1C = 3r 3o A / d - explain C & formulaCapacitor = two plates that hold charge (stores & releases electric charge)
3o = Permittivity of free space = 8.85x10-12
3r = dielectric constant
d = d between plates/thickness in m
A = area in m2
- A, d & 3r determine how good a capacitor is
- measured in Farads - 2Formulas involving C:C = 3r 3o A / d
E = 1/2 Q V
E = 1/2 C V^2
V = IR - 3C in Parallel & Series:C in series = 1/C1 + 1/C2...
- same Q
- VT = V1 + V2...
- Lowers Capacitance
C in parallel = C1 + C2...
- same V
- QT = Q1 + Q2...
- Increases Capacitance - 4Charging a capacitor: (I & V graphs)V = growth graph
I = decay graph
Formula: ymax - (0.37^n x ymax)
- At any instant the V across Capacitor & resistor = V supply. (Vs = Vr + Vc)
- V across Capacitor increases, Voltage across Resistor decreases so Current decreases.
Explanation: When a voltage is applied, a large current flows initially. As the charge accumulates, the currrent becomes smaller and it will be zero when Q = CV.
When Capacitor is fully charged, current = 0 & the resistor V = 0 so the Capacitor V = Battery V
- Needs 5 time constants to full charge - 5Discharging a capacitor: (I & V graphs)V & I = decay graphs
- No V supply
Formula: yn = ymax x 0.37^n
- Capacitor is the only source of Voltage, as Vc decreases, I also decreases 63% (x0.37) voltage drop = 1 time constant.
- Needs 5 time constants to full discharge
Explanation: When a charged capacitor is discharged, a large current flows instantaneously, which decreases till 0. - 6Extra notes on Capacitors:E in Capacitor = 1/2 of E in Battery
so E Battery / 2 = E in Capacitor
- The rest of E is converted/changed into heat due to resistance or light.
- change in t determines how quickly a capacitor charges (dependent on R total or C total).
- strength of capacitor is also relative to electric field strength - 7Time Constant =T = RC
- Time measured in seconds for the value of I or V to change by 63%.
or time taken for capacitor to charge/discharge.
- Takes 5 time constants to charge/discharge a Capacitor. - 8Kirchoff's Law:I : The current entering a
junction, must equal
the current leaving a
junction (so I total = 0)
V : Voltages around a closed loop must add to zero - 9emf formulas:emf = Ir + r
or emf = I (R + r)
or V terminal = emf - IR - 10Explanation of current flow:Current flows from the + ve terminal of the battery (usually) as it is at a higher potential than the + ve terminal of the other battery.
(So from higher potential to lower potential) - 11What is an Inductor? & what are the factors affecting it?An inductor opposes change
- measured in H (Henry's)
-Factors that affect inductance are:
- Number of turns/coils
- Coil Area
- Coil Length
- Core material - 12Explain InductorsThey produce a change in current so a change in magnetic field, this creates an oppposing Voltage (back emf) = an opposing current = opposing magnetic field. V = 0 in the end after 5 time constants.
- V is largest when the switch is open as I falls quickly so there is a high rate of change of I, which explains sparks etc. - 13Faraday's Law definition & formula:induced emf = - change in magnetic flux
/ change in time
- States that the size of the induced Voltage = the rate of change of magnetic flux - 14Magnetic Flux formula:magnetic flux = BA
= the amount of magnetic field
- measured in WB (Webers)
A = Area & B = magnetic field strength (T)
- Can be changed by; B, A & angle - 15Lenz's Law definition & formula:= the (-) in Faraday's law: induced emf = - change in magnetic flux
/ change in time
- Stating that the induced emf itself creates magnetic flux that will oppose the change that has caused the emf. - 16Induced Voltage & Current formula:V = BvL
- B = magnetic field strength (T)
- V = Voltage
- v = velocity
- L = length of wire - 17Explain The Generator Effect:If a force moves a wire through a magnetic field then a voltage is induced across the ends. Electricity is generated through the magnetic field.
- 18Mutual Inductance formula:Np / Ns = Vp / Vs
N = number of turns
V = Voltage
p = primary & s = secondary - 19Time Constant formula for Inductance:T = L / R
decay graph- at 5 T's, V = 0 & I is max
growth graph - at 5 T's, I is max - 20Mutual Inductance explanation:A changing current in 1st coil creates a changing magnetic field, hence, a changing magnetic flux in the first coil. This flux passes through the 2nd coil & so there is a changing flux in the 2nd coil. This changing flux induces a voltage to oppose the change that created it.
- 21What is a Transformer?An electrical device that uses mutual inductance to change the Voltage of a power supply.
- 22Transformer & Power formula:Vp Ip = Vs Is
Power in = Power out
as P = VI - 23What are the Transformer Functions?- Step Down Transformer:
Fewer windings in secondary coil = lower V = higher I
- Step Up Transformer:
- More windings in secondary coil = higher V = lower I
- Isolating Transformer:
same V & I - 24Formula for RMS:"like an average"
RMS = ( 1 / square root of 2 ) x peak
& Vmax = square root of 2 x Vrms
& Imax = square root of 2 x Irms - 25AC Current / Resistor Voltage & Capacitor Voltage info:Capacitor Voltage = 90 degrees / one quarter cycle behind
- 26Angular Frequency formulas:w = 2 pi f
or w = 2 pi / T - 27Reactance of Capacitor formulas:= the resistance of the Capacitor measured in ohms
Xc = 1 / wc or. Xc = 1 / 2 pi fc
- no current - 28AC Circuit V = IR formula for Capacitor:V rms = Irms x C
- 29Reactance of Capacitor explanations of frequencies:High frequency:
- Current oscillates faster = less time for Capacitor to charge
- Less charge (Q) = current flowing around circuit is impeded less by Capacitor
= Xc will decrease
Low frequency:
- Current oscialltes slowly, = more time to charge
- Less charge (Q) on capacitor = current flowing around circuit is impeded more by Capacitor
= Xc will increase - 30AC Current / Resistor Voltage & Inductor Voltage info:Inductor Voltage = 90 degrees / one quarter cycle ahead
- 31Reactance of Inductor formulas:XL = wL = 2 pi f L
= resistance of Inductor
- no current - 32AC Circuit V = IR formula for Inductor:Vrms = Irms XL
- 33Phasor info:Phase:
• Inductor and capacitor reactance (and voltage) are 180 0 out of phase (opposite).
• This means they cancel each other out.
- In the order of XL, R, XC
and VL, VR, VC
(Inductance always ahead/at the top) - 34Impedance info:Impedance:
• Impedance is “overall resistance” of the circuit
• Impedance = Resistance (R) + Reactance (X)
• Use SOH CAH TOA and Pythagoras to find - where z = square root of x^2 + y^2 and angle = tan-1 y/x - 35Resonance info:Resonance:
• When XL and Xc are equal sizes, they cancel each other FULLY. This means NO reactance (Z = R)
= a resonant frequency = f0
- where XL = Xc (so their formulas equal each other too, you can rearrange to find w or f)
• Minimum impedance (Z) means a much higher current (V = IZ) - 36Impedance formulas:V = IZ
Z^2 = (XL - Xc)^2 - 37Reactance formulas:XL = wL & Xc = 1/ wC
at resonance: XL = XC & Z =R
you can make Xc or XL= V / I
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- NCEA Level 3 physics level 3 (Electricity!!!! :) ) Flashcards on Electricity Notes, created by Humayra Azam on 03/12/2021.
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- Humayra Azam
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