Electricity and Magnetism Exam Flashcards
State Ohms Law
V = IR (V = volatge, I = amps, R = resistance)
What is voltage
A simple chemical cell. A chemical reaction between two electrodes, sepereated by an electrolyte. It is the reactivity of a metal which dictates the voltage.
Voltage is the electromotive force which provides the enery to push electrons around a circuit.
Nominal cell voltage of Lead acid
2V
Nominal cell voltage of nickel based
1.2V
Nominal cell voltage of LiB
3.6V
What is the electromotive force?
Voltage measured when no current flows (open circuit) written as Voc
3 factors of voltage in series cells?
“Measure using a voltmeter
Goes across the component (in parallel)
Takes no current flow from the circuit”
What do up arrows represent on a circuit diagram?
Rising voltage
4 factors of current?
“Measure using ammeter
MUST be in series
Theoretically can be placed anywhere in the circuit
Ideally has 0 resistance”
What is i always shown as in a circuit diagram?
“I is always shown as conventional current and flows from positive to negative, when you say ‘current’ this is always taken to mean conventional current.
Negative electrons always flow from negative to positive.
Positive holes always flow from positive to negative”
Why do electrons move? (conductors)
“Copper is a very good conductor
It has low resistance
Because the outermost electrons is far away from the positive nucleus.
The distance means it is ‘easy’ to persuade it to leave the atom and become part of the current flow”
Why do electrons move? (insulators)
“Pure silicone is an insulator as it is normally found in a crystal lattice structure
It has high resistance
As a lattice, the outer electron shell is full (8) and therefore it is difficult to get an electron to leave the lattice (covalent bonding)”
Name another insulator and an example of how it is used?
Pure Water, used in high voltage transformers as a coolant
What is the charge of one electron?
1.602*10^-19 C (coulombs)
What is the current flow when 1 C of charge passes a point in a circuit in 1s?
state the formula for this
A current of 1A flows when 1 C of charge passes a point in a circuit in 1s (charge = amps * time)
State the charge flow formula
“Q=it
Q = charge in coulombs C
i = current in amps (A)
t = time in seconds (s) “
For ohms law, which component do you vary and which do you measure?
“To measure a characteristic, vary V and measure I
V is the independent variable (x axis)
I is the dependent variable (y axis)”
Draw the IV graph/characteristics of a filament lamp
Describe the IV graph/characteristics of a filament lamp
As V increases, I increases, temperature increases, P = I^2 * R is non linear, power creates heat and light, gradient decreases, so resistance increases and limits the current
Draw the IV graph/characteristics of a signal diode
Draw the IV graph/characteristics of an LED
Draw the resitance vs temperature graph of an NTC thermistor
Draw the NTC thermistor symbol
Draw the PTC thermistor symbol
Difference between the NTC and PTC thermistor symbols
NTC has a ‘-t’ and PTC has ‘+t’
Vs =
V1 + V2
Vs =
iR1 + iR2 = i(R1 + R2)
Rtotal =
R1 + R2 + …… + Rn
Vs =
i(Rtotal)
Vs/Rtotal =
i
V1 =
iR1 = Vs(R1/Rtotal)
V1 =
Vs(R1/(R1+R2))
V1 =
Vs(R1/R1+R2+R3+R4+R5)
V4 =
Vs(R4/R1+R2+R3+R4+R5)
V3+V4 =
Vs(R3+R4/R1+R2+R3+R4+R5)
Factors of resistors in parallel
“In a parallel circuit, the same voltage is across each resistor. So, Vs = V1 + V2, but the current divides between two resistors.
The sum of the currents into and out of a node must equal zero (kirchkoff’s current law)”
Vs =
V1 + V2
V1 =
i1*R1
V2 =
i2*R2
Vs =
itotalRtotal = (i1+i2)Rtotal
Upside of using aluminium cables over copper cables
Cheaper
Downside of using aluminium cables over copper cables
“Needs a bigger cross section/radius
Would expand more in hot weather
Connections can be more problematic due to dissimilar metals”
Resistivity of Copper
1.68*10^-8
Resistivity of Aluminium
2.65*10^-8
Power equation
“Power = Current * Voltage
P = I(A) *V(V)”
Using P = IV and V = IR come up with the other equations
P = IV = (I^2)R = (V^2)/R
A 240V shower is rated at 11kw, what fuse is needed?
“To calculate fuse size you need to calculate the current
Power = Current * Voltage
Current = Power/Voltage
Fuse = 11000/240 = 45.83, truncate to 45A”
1kg of water needs 4200kJ to boil, how long will it take to boil the 2kW kettle
“Convert the power of the kettle to the same unit as the energy required.
1kW = 1kJ/s, therefore the kettle produces 2kJ/s
Time (s) = Energy (J) / Power (kJ/s)
Time = 4200/2 = 2100s”
Three reasons why a cicuit would lose efficiency
“Conduction
Convection
Radiation”
Making a cup of tea, kettle has an input power of 750W what is the useful output power
450W
“A 90 ohm load is connected to a 120V battery, how much power is delievered by the battery?
Work this out using 2 different formulas”
“Method 1:
P = V^2 / R
P = 120^2 / 90 = 160 watts
Method 2:
I = 120/90 = 1.33A
Power = 1.33(120) = 160 watts”
How much current is ‘drawn’ by a 100W lightbulb connected y a 120V outlet?
“P = IV
I = P/V
I = 100/120 = 0.833A”
“The circuit has two resistors with R1 > R2. Which of the two reistors dissipates
the larger amount of power? Explain”
“They both have the same voltage
Power in R1 = V^2 / R1^2
Power in R2 = V^2 / R2^2
Difference = P1/P2
= (V^2 / R1^2) / (V^2 /R2^2)
= R2/R1, therefore P1 > P2”
“Bulbs A, B and C are all identical and are all glowing
Rank in order from most to least, the brightness of the three bulbs. Explain
A wire is connected betwewn points 1 and 2. What happens to each bulb?”
“The current flows through A and then divides itself equally between B and C. Therefore A must be the brightest as it takes the most current. B and C are equal but half of A.
A will get brighter and B/C will go out as the circuit has been short circuited”