Energy, Power and Resistance

What is potential difference?

work done by charge carriers as they lose energy passing components in a circuit

(energy transferred from electrical to other forms)

What is the equation for potential difference and emf?

W/Q

What is electromotive force? emf

work done to charge carriers when they gain energy passing through a call or battery

(energy transfer from chemical to electrical energy)

How to derive energy transfer equation for charged particles?

V = W/Q W = 1/2mv²

VQ = 1/2mv² Q = e

eV = 1/2mv²

What is resistance?

potential difference across the component divided by its current in ohms

R = V/I

How to determine the resistance of a component?

set up a circuit of variable power supply, and ammeter and voltmeter. vary V, and record I, and resistance is the reciprocal of the gradient

What is Ohm’s law?

for a metallic conductor in constant temp, current in the wire is directly proportional to the potential difference across it. where this is true, IV graph has linear gradient

What happens in components where ohm’s law is not followed?

as current increases, the ions are heated gaining kinetic energy, increasing the frequency of collisions, meaning more work is done increasing resistance

What are IV characteristics of a fixed resistor?

constant resistance, following ohm’s law

What are IV characteristics of a filament lamp?

non-ohmic, as current increases, heating effect increases

(beginning of sin graph)

What are the IV characteristics of a diode?

made from semiconductors, current only flows one direction, so no negative current. does not conduct until the threshold is reached. number density increases

(e^x)

What is a negative temperature coefficient thermistor?

semiconductor with a negative temperature coefficient - as temp increases, resistance decreases

What are the IV characteristics of a thermistor?

non-ohmic

(tan graph)

How does resistance vary with intensity of an LDR?

increased intensity decreases resistance (non-ohmic)

What is resistivity and its formula?

a physical property linking resistance with area and length

ρ = RA/L (resistance, area, length)

ohm-meters

How does resistivity of a metal and semiconductor change with temperature?

metals - increased temperature means greater vibrations, more collisions of electrons with ions, causing increased resistance/resistivity

semiconductors - number density increases with temperature, so resistance/resistivity decreases

How does number density relate to resistance?

n is inversely proportional to resistance

Describe techniques to determine resistivity

measure cross-sectional area using vernier callipers, record the length of the wire, with a voltmeter and ammeter. measure pd and current to find resistance and calculate with A and L

What is electrical power?

rate of energy transfer in J

Derive power formulae for circuits

P = W/T W = VQ

P = IV V = IR

P = I²R or P = V²/R