Chapters 4.1, 4.2, 4.3 - Electricity Flashcards
Electric Current
The rate of flow of charge
Coulomb
Unit of charge - 1 Coulomb of charge will pass a point in 1 second by a current of 1 Ampere
Charge on an electron and proton
Electron: -e
Proton: +e
What is the net charge on an object always a multiple of and what is this called
Net charge is always a multiple of e. This means it is quantised
Physically, what is current in a metal
Movement of electrons
Physically, what is current in an electrolyte
Movement of ions
Kirchoff’s first law and associated conservation
Sum of the currents entering a junction is equal to the sum of the currents leaving a junction (conservation of charge)
Charge carrier
What carries the charge in a current (electron/ion)
Mean drift velocity
The average velocity of all electrons in a section of a circuit
Current equation in terms of electron drift velocity
I=Anev where I is current A = cross sectional area n = number of charge carriers per unit volume e = charge on a charge carrier v = mean drift velocity
Distinction between conductors, semiconductors and insulators in terms of n
Conductors - high n
Insulator - very low n (perfect insulator would have n=0)
Semiconductor - low n
Resistance
Energy per coulomb needed to pass a current of 1A through
Ohms law
V=IR
Resistivity of a material
The resistance of a 1m length of the material with a cross sectional area of 1m^2
Equation for resistance in terms of resistivity
R = ρL/A where R is resistance ρ is resistivity L is length A is cross sectional area
Typical order of magnitude of resistivity
Very small e.g. 10^-8
Ohmic conductor
A conductor which obeys ohms law.
The potential difference across a conductor is directly proportional to the current in the component as long as its temperature remains constant
Experiment to determine resistivity of a wire
Determine cross sectional area of wire standard way. Set up a circuit with ammeter in series and voltmeter around the wire. Have one end of the wire connected via crocodile clip that can be moved up and down the wire and have the wire set up next to a ruler. Attach the crocodile clip at various places along and record the current, potential difference and length at each point. Calculate resistance as V/I. Plot R against L. Gradient will be ρ/A
How to improve accuracy of experiment to determine resisrtivity
Keep the wire at a constant temperature by only using a small current
Energy transfer in a circuit
Work done = potential difference * charge
W = VQ
How to calculate the kinetic energy of an electron that is accelerated over a potential difference
W=VQ
W=Ve
(1/2)mv^2=Ve
So the kinetic energy of an electron is equal to the potential difference * its charge
How to investigate I-V characteristics of a component
Set up a circuit with an ammeter in series and a voltmeter around the component. Also have a variable resistor in series. Vary the resistance to change the potential difference over the componenet
I-V characteristic for a normal resistor
Straight line through origin
What is the gradient of an I-V graph
1/Resistance
I-V characteristic for a filament lamp
Curved through origin in shape of a sigmoid function. Increasing the voltage increases the temperature and therefore the resistance, hence the gradient gets shallower
How temperature affects resistivity of a metal and why
High temperature increases resistivity. This is because the positive ions in the metal vibrate more and therefore collide more with electrons, which causes the electrons to lose energy
Explain resistivity of a semiconductor
Lower than that of a metal due to fewer charge carriers. When energy is supplied, electrons are released since they are given just enough energy, meaning that there are more charge carriers and therefore lower resistivity.
Resistance-Temperature characteristic of a thermistor (NTC)
y=1/x
Temperature increase results in resistance decrease, so it is the inverse of a normal resistor
I-V characteristic of a thermistor (NTC)
Looks like a x^3 graph
I-V Characteristics of a LDR
Essentially the same as a thermistor in all ways except the energy is provided by light instead of thermal energy
What does a diode do
Only allows current to flow in one direction and only allows the current to flow in that direction if it has a P.D higher than the threshold voltage (minimum voltage to start conducting)
I-V characteristic of a diode
Very shallow line in negative P.D. In positive P.D. it is 0 up until the threshold voltage where it starts increasing rapidly
Forward bias
The direction that current is allowed to flow in a diode
Equation for power
P=IV where P is power I is current V is voltage
Equation for total work done by a circuit
W=Pt or W=VIt or (many others from rearranging V=IR)
kWh
The work done by something with a power of 1 kilowatt in 1 hour = 3.6 million joules
One unit of energy
One kWh
Electromotive force
The amount of work a power source does on each coulomb of charge
Transformation of energy over a potential difference
Electric to another form
Transformation of energy by the electromotive force
Another form to electric
i.e chemical > electrical
Internal resistance
Resistance that is inherent to the power source
Why is terminal p.d. different to the e.m.f.
Some of the potential difference is lost overcoming the internal resistance
Lost volts
The energy wasted per coulomb overcoming the internal resistance (e.m.f. - terminal p.d.)
What is the equation ε = V + v referring to
ε is the e.m.f.
V is the terminal p.d.
v is the lost volts
What is r and R
r is the internal resistance
R is the load resistance (external resistance)
effective e.m.f. of cells in series
ε + ε + ε…
effective e.m.f. of identical cells in parallel
ε (no effect)
Experiment to determine internal resistance
Set up a circuit with a variable resistor, ammeter and voltmeter around the resistor. Vary the resistance and record the p.d. and current and plot them on a graph (with p.d as the vertical axis).
ε = V +Ir
V = ε - Ir
therefore y intercept is ε and gradient is -r
Kirchoff’s second law and associated conservation
Sum of p.d. in a closed loop (conservation of energy) equals sum of e.m.f in a closed circuit .
hopefully u can analyse circuits
i hope so
Explain potential divider
Two resistors in parallel divide the potential among them, proportional to their resistance. Decreasing the resistance of one will decrease the p.d. across it and therefore increase the p.d. across the other resistor (by kirchoff’s 2nd)
How a potentiometer works
It is just a resistor in series but with a wire running parallel to it. Whats the catch you say? Well the catch is that one end of the wire is just touching the resistor at some point along it and this distance can be change by a slider, essentially simulating a potential divider
How to investigate a potential divider
Vary the resistance of one of the resistors and record the p.d. across the other resistor as well as the current. Resistance could be varied by the resistor being a thermistor, LDR e.t.c
