Chapter 9 Elec Flashcards
Cell battery?
One cell , battery is multiple
Longer terminal is positive end , conventional current goes from positive to negative
Circuit symbols to remmeber
Diode = arrow and line tells you direction
Resistor is rectangle
Variable is rectangle + arrow through it
Fuse is rectangle with line through it
LDR is arrows (light) pointing to resistor
LED is diode + arrows leaving (light)
Thermistor is rectangle with line and straight through
CAPACITOR = Two gaps of vertical lines
What is PD
The difference of energy (J) per coulomb between points
So V=E/C
1V= 1JC-1
What is voltage in terms of PD and EMF
What does this prove
Voltage is both
EMF is the energy (voltage) added to a circuit per coloumb
PD is the energy OUT per coloumb
Thus total PD=Total WMF
Conversation of energy (Kirchhofs second law)
Sources of EMF
Solar cells, dynamos etc
As it is energy being added to it, work done to the change carriers
What is EMF
Energy added to a circuit per coloumb
Equation is also emf = E/C
Basically when is DMF and PD used
So like batteries is EMF, anything thst takes the voltage is PD
What is charge in electron
1.6 x 10^-19
What is mass of electron
9.11x10-31 kg
What is an electron gun?
A small metal filament which is heated by an electric current, that creates a beam of electrons with specific kinetic energy
How does an electron gun work?
What is the mechanism that does it
How do electrons gain KE and have a velocity
1) Small filament with electrons like a metal is heated with an electric current
2) electrons in wire gain kinetic energy
3) if not enough, they gain so much KE they can escape from the surface of the metal
- this process is called THERMIONIC EMISSION
4) if place in a vacuum and high PD is applied between it an anode, the filament Will act as a cathode , where free electrons ACCELERATE towards the anode with KE
5) if there is a whole through the anode (positive), then electrons will pass through it as a beam with a specific kinetic energy and thus velocity
How can you work out KE and thus velocity if these electrons
Work done on electron = QxV (as V= E/Q) so for the accelerating voltage and charge of electron you have its KE
then 1/2mv2 can rearrange if you know mass of electron
= 9.11 x 10^-31
What assumption do you make of calculating KE from electron in electron gun?
That it had no kinetic energy at the start
What conclusion can you make about pd, energy and speed?
Increase PD = greater kinetic energy = faster
What is 1 electron volt
The energy of an electron accelerated by 1v, as energy so small represented b 1 eV
In equation veQ =1/2 mv2 what to remember about Charge
It’s just magnitude, so whether positive for proton or negative you don’t use as energy can’t be negative
What is resistance
Something that resists the flow of charge
The ratio of PD in a circuit to current
Derive resistance base units
R=V/I V=E/Q = JC-1 J=WD= Fxd F=MA = KgMS-2 WD= Kgm2s-2 Q= As So v= Kgm2s-3A-1
And R = Kgm2s-3A-2
What is OHMS law
When temperature is constant
- current in wire is directly proportional to the pd
Remember what resistance = V/I means
It means TOTAL OVER TOTAL, not change so not tangent
What happens if you increase current+ voltage in light bulb
As R=V/I , increasing bith increases resistance
- however at a while resistance will increase at a different rate
- this is because an increase of current eventually leads to an increase in temperature ,
- increase for temperature actually reduces current , so resistance increases
Why does increasing temperature, or like having plastic wrapped around etc increase resistance
Increase of current = increase temoertaure
- this increase of temperature means the positive metal ions increase in kinetic energy so move around more
- moving areoind More they act as an impairment to electron flow, so (work is done on these instead) and thus resistance increases
What is conduction in gross and resitsnce
Conduction is 1/ resistance, so I/V gradient
IV graph for a fixed resistor , wires
Does it obey ohms law ?
Polarity
- a straight line through the origin
- current as proportional to voltage forever, so OBEYS OHMS law
- = ohmic conductor
- resistances constant in both polarity
I V graph for a filament lamp
Does it obey ohm law
Why?
Polarity?
- curved (decreasing current )
- not proportional, doesn’t obey ohms law
- this is because temperature = moving = impediment = resistance
- polarity it’s the same
When measuring current and voltage what do you want each resistance and why
!!! Ask sir
Ammeter = 0 resistance
- I=V/R
Voltmeter = infinite resistance
- so that current going in doesn’t change
What circuit should you use to get different values of I and V to measure resistsnce for IV characteristic ? !!
Not the vsrbsible resistor but the three legged POTENTIOMETER
- THIS IS BECAUSE it gives a full range of voltage , other has a limit that it doesn’t go under
What determines the resistance of something
- the material of the wire (determines number density )
- the cross sectional area
- the length of the wire
Temperature too
Why is resistance dependent on these , what’s the equation and what is proportional to other
R= p L/A
Resistance is proportional to L, increase length = increase resistance
Resistant is inversely proportional to cross sectional area , increase = decrease resistance
Adding a constant links all together
What are the units for restivity
If R = p L/A
P= RA/ L = ohm meters
So what is resitivry and when does it change
Resitivity is the product of resistance made by the material and its cross section over length
This changes at DIFFERENT TEMPS
Is Restivity determined by shape?
Is resistance?
Resitivty is NOT DETERMINED BY SHAPE, if you make area and length 1 you get r = p so p not determined by shape
Resistance is proportional to length and area so changing these will change
How to do practical for determining p?
Why should current stay the same?!!!
Change lengths, record resitsnce by ammeter and voltmer
- keep current same
- plot graph, use gradient and multiply by cross sectional area
Use micrometer
2) using variable resistor to keep same current necessary so WIRE DOESNR INCREASE IN TEMPERATURE AND THIS EFFECTS RESISTSANCE AND RESITIVRY OF WIRE !
How does resitivry roughly change for materials
Metals need low resitivry for high current
Non metals have high resitivry
Semidconductors are in between
How does resitivity change with temperature in different materials?
(Similar concept dw)
For metals it increases (as ions impede)
But for non metals it will decrease (this is because covalent bond break release electrons etc)
What to do to percentage uncertainties when multiplying by each other
Roughly add them
In reality work out best snd worst case scenario divide by 2n= half range
Dividing uncertainties ?
What about powers
Add them again
Add (multiply by that power)
How to reduce uncertainties
- smaller scale
- use bugger measurements so a uncertainty doesn’t have that great of effect
- repeat to iron out
IV chacterisric for a diode?
- curved up , starts low , and negative is straight l
- not proportional = non ohmic
- resistance changes
How do diodes work for resistance and voltage in both directions
- Diodes take their cut of PD and then the rest goes to the other components
- past this point the resistance of the diode is 0 (1/ grad which is infinity ) and increase of pd gives less resistance as its total not instantaneous
- in reverse direction it has infinite resistance, does not let current and thus takes all the voltage (r is 1/0 so infinity ). It will take ALL THE VOLTAGE IN THE CIRCUIT
What voltage will diodes take
LED is 0.7V but different will take different for different colours , silicon
Led letting in one direction means it is either on or off, and this determines everything
What does having a negative temperature coefficient relate to and what does it mean?
Resistance
- as increase temperature, resistance decreases (becomes more negative…)
What are resistance vs temperature graphs for positive and negative
Positive straight lime
Negative curved line starting high downwards
Why does current increase and resistance decrease for NTC thermistors?
- made out of a semiconductor material which has covalent binds in
- as temperature increases these are broken
- this releases electrons into the circuit
- increase of electrons decreases resistance
Hey do you need different thermistors
Which more expensive
Different thermistors will change resistance with increase of temperature at different ranges. Some will at say 20°, good for incubators while others you need at 100° like an oven
Those that over a short change gives a change if resistsnce
IV charchterisitcs of thermistor
Why (number density)
Opposite of filament lamp
- as voltage increase or increase in temperature, number density increases
- thus resistance decreases
Still non ohmic etc
IV for LDR and why?
Exact as thermistors
As light in entirety in wm-2 shines covalent binds break= increase if electrons as number density = decrease if resistance
Low lights resistance is very high
Explain ldr and thermistor in sensing circuits
If high temperature, resistance drops , so more current goes through. Here voltage drops too as less R = less v
If r is constant fir fixed your v will increase due to increase current
How do guy account do an experiment for I V on thermistor
What would happen to graph
Ideally keep it same temp, then it won’t have an effect and it would be proportional actually
Deriving power equation “
P = e/t
V= e/c so e = vq
P=vq/t
Q/t = I
So P=VI
What equations should you use for power gained / lost
Power gained = P=VI (as gained from battery)
Power lost = P= I2R or V2 R
- anything with resistance means it’s being lost at the resistor
What does p= I2 R show you about power grids
Double the I and you quadruple power, thus they step down current and increase voltage so more efficient (current hotter goes to waste)
What is KWH
A unit for energy , = to 3.6Mj
Power = W/t so w= pxt
1kwh = 1000 x 3600 = 3.6million
It’s just another unit
What is power
Rate of Energy transfer
If cross sectional area increase by 4 what happen to lengty if volume same
Decrease by 4
Even for lower dissapated what happens if you don’t know r
Use p =I
In an NTC explain how resistance changes as soon as battery turns on
As soon as battery cinnected current turns on and thst increase temp
- thus increase temp decrease resistance and current increase for covalent bond etc
- finally it stabilises temp so stable resistance etc
