4.2 | energy, power, and resistance

what is potential difference (p.d.)?

a potential difference of 1 volt is defined as 1 joule of energy transferred per coulomb.

how is potential difference measured?

it is measured in volts and represents the work done by charge carriers as they lose energy through components in a circuit.

what is electromotive force (e.m.f.)?

the work done to charge carriers when they gain energy as they pass through a cell or power supply - the energy transferred from chemical energy to electrical energy per unit charge.

what is thermionic emission?

the process where electrons gain enough kinetic energy to escape from a heated metal filament.

what is the function of an electron gun?

it produces a thin beam of electrons accelerated to high speeds through thermionic emission.

how is the velocity of electrons in an electron gun determined?

using the conservation of energy, where the work done on the electron is equal to the charge of the electron multiplied by the potential difference.

what is resistance?

the potential difference across a component divided by the current through it, measured in ohms.

what does ohm's law state?

for a metallic conductor at constant temperature, the current is directly proportional to the potential difference across it.

what are ohmic components?

components that follow ohm's law, exhibiting a constant linear gradient in their I-V characteristic graph.

what are non-ohmic components?

components that do not follow ohm's law, where resistance changes with current due to increased collisions of electrons with vibrating metal ions.

what is the I-V characteristic of a filament lamp?

it shows that at low currents it behaves like an ohmic component, but as current increases, resistance also increases due to heating.

what is the behavior of diodes in circuits?

they allow current to flow in one direction only and do not conduct until a threshold potential difference is reached.

what is a negative temperature coefficient (NCT) thermistor?

a thermistor that decreases in resistance as temperature increases, useful in temperature sensing circuits.

what are light-dependent resistors (LDRs)?

non-ohmic components made from semiconductors that decrease in resistance with increased light intensity.

what is resistivity?

a physical property of a material that links resistance with its area and length, measured in ohm-meters.

how does temperature affect the resistivity of metals?

increasing temperature causes fixed metal ions to vibrate more, increasing collisions with electrons and thus increasing resistance.

how does temperature affect the resistivity of semiconductors?

increasing temperature increases the number density of charge carriers, leading to decreased resistance.

what is the formula for calculating resistivity?

rho = ra/L

how can resistivity be determined experimentally?

by measuring the cross-sectional area and length of a wire, and using a circuit to measure voltage and current to find resistance.

what is power?

the rate of energy transfer, measured in watts.

what is the equation for power in terms of work done?

P = W/t

how can power be expressed in terms of potential difference and current?

P = IV

what are the alternative equations for power using resistance?

P = I^2R and P = V^2/R

how is energy transferred calculated in electrical circuits?

W = Pt

what is the significance of kilowatt-hours (kWh)?

a unit of energy used for measuring energy consumption - the energy used by a device with a power of 1 kilowatt over 1 hour.