Electricity Chapters 8/9/10 Flashcards
Charge
Q = IT
Charge = current x time
Number of electrons from charge
Total charge/ 1.6x10^-19
Mean drift velocity equation
I = nAev
Current = charge carrier density x area of wire x charge on an electron x mean drift velocity
Work done/ energy transferred (4)
W = VQ
Work done = voltage x charge
W = EQ
Work done = EMF x charge
W = VIt
Work done = voltage x current x time
W= PT
Resistance from resistivity
R= pL/A
Resistance = reisitivity x length/ area
Resistors in series
Resistors in parallel
R = R1 + R2
1/R = 1/R1 + 1/R2 etc
Power (3)
P = IV Power = current x voltage
P = I^2 R Power = current squared x resistance
P= V^2/R Power = voltage squared / resistance
Internal resistance
Actual and factorised
EMF = V + Ir EMF = I(R+r)
Voltage (2)
V = IR
(VIR tree)
V= W/Q
Work done/ charge
Charge equation
Q= IT
eV equation for an electron gun
eV = 1/2 x mass x velocity^2
Potential dividers
Do as a fraction of total p.d.
Where a greater resistance gives a greater P.D.
Kirchhoff’s first law
And what it shows
Sum of current in = sum of current out
Shows conservation of charge
Kirchhoff’s second law
What does this show
Sum of EMF in a in a loop = sum of voltages in the same loop
Conservation of energy
Define potential difference
And define the volt
Potential difference is an energy transfer per unit charge
V = w/q
And the volt is equal to one joule per coulomb
State ohms law
For a metallic conductor at a constant temperature, current is directly proportional to potential difference
Resistance is constant
Define resistivity
Resistivity at a given temperature is the equal to the product of resistance and area per unit length
P = RA/L
Define the kilowatt hour
The kilowatt hour is the energy transferred when 1000w is run for an hour
Define current
Current is the rate of flow of charge, or the charge per unit time and is defined by
I = Q/t
1 coulomb per second
How to investigate internal resistance
Set up a cell in series with a variable resistor an ammeter and a voltmeter in parallel over the cell
Take reading of current and voltage for varying resistances
Plot an V-I characteristic graph
The emf is the y intercept
The gradient is -r
What is the charge on an electron
1.6 x 10^-19
What is the coulomb
The unit of charge equal to the charge flowing past a point in one second with a current of one amp
Calculating net charge
Positive charges - negative charges
Should be a multiple of e as charge is quantised
What was Milikin’s experiment
Tiny Droplets of negatively charged oil were sprayed between two oppositely charged plates
When these droplets were stationary he knew
Weight = electrostatic charge
From here the charge could be calculated
What is the difference between charge in wires/metals and charge in electrolytes
In metals electrical charge is the movement of electrons
In electrolytes electrical charge is the movement of ions
2 ways to increase electrical current
Faster moving electrons
Or more electrons
Difference between conventional current and electron flow
Conventional current flows +ve to -ve
Electrons flow -ve to +ve
What is number density and what are typical values for
Conductors
Semiconductors
Insulators
The number of charge carriers per m^-3
Conductors = around 10^28 Semiconductors = around 10^17 Insulators = very few
Difference in circuit symbols of thermistor and variable resistor
Thermistor had a bent line
Variable resistor has an arrow
Difference between p.d. And emf
Potential difference is a measure of work done by charge carriers
EMF is a measure of work done to charge carriers
What should an ideal voltmeter and ammeter have as resistances
A voltmeter would ideally have infinite resistance
An ammeter would ideally have no resistance
Describe the electron gun experiment
1) a metal filament (which acts as a cathode -ve) is heated using a p.d.
2) some electrons gain enough energy to escape from thermionic emission
3) they accelerate towards the anode
4) it is assumed all the work done on the electrons is converted to K.E.
5) W = VQ and Q for an electron is 1.6x10^19 (e)
6) thus, eV = 1/2 x mass x velocity ^2
Define resistance and define the ohm
Resistance is the force opposing electrical current
Defined as the ratio between potential difference and current where
1 ohm = 1 VA^-1
What happens to most components’ resistances when temperature increases and why
Resistance increases because the positive ions of the metal vibrate about their fixed postings more causing more collisions with the electrons thus increasing resistance as the charge carriers have to do more work
I-V characteristic of a resistor
Straight line where a
low gradient = high resistance
High gradient = low resistance
I-V characteristic for a filament lamp
Curved where it’s like a semi -n^2 on positive v and a semi n^2 on negative v
What is the unique property of a diode and how is this possible
They only allow current in one direction by having a very large resistance in one direction and in the other (above a threshold value) the resistance is very low.
This is because above a certain P.D. the number of charge carriers increases
I-V characteristic of a diode
Flat line then after Y axis curving upwards
Resistance-temperature
characteristic of an NCT thermistor and why
Decreasing curve (concave) This is because in greater temperatures there is enough energy to release a greater number of charge carriers from the structure of the thermistor, thus allowing more charge to flow and decreasing resistance
Resistance - light intensity curve for an LDR
Resistance decreases with light intensity because the energy releases more charge carriers
Concave curve down
Methods of determining resistivity
1) measure the diameter of a wire using micrometers at different points and take an average
2) calculate cross-sectional area using A= pi x (d/2)^2
3) record the length of the wire
4) connect in series to a cell, with an ammeter in series and a voltmeter in parallel
5) calculate resistance using R = V/I
6) calculate resistivity using p = RA/L
OR
do 1,2,3,4
repeat steps 4,5 for different lengths
plot a graph of resistance against length
gradient is p/a
I-V characteristic of a thermistor
Concave curve up in positive V
Convex curve down in negative v
Why is there internal resistance and what is the only point when terminal pd = emf
When there is a current in a power source work is done so volts are “lost”
When current = 0
Producing a variable PDout using different components
Thermistors and LDRs in series
For example if the temperature increases then the resistance of a thermistor will decrease this gives a lower p.d. Over the thermistor or a greater P.D over the other resistor which could be monitored
What is the purpose of a potentiometer
It varies resistance and thus varies potential difference
define mean drift velocity
the average change in displacement of an electron per unit time through a wire, in the opposite direction to conventional current. The electrons constantly collide with the lattice of metal ions
what happens to the resistance of a parallel system when the resistance of one of its components decreases
the overall resistance decreases but not linearly
