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 (when at a constant temperature)
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
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