Electrical Quantities

Formulas for Q (charge)

Q = It

Q = ±Ne (± can be optional sometimes)

Q = CV

Define Coulomb

amount of charge when 1 amp flows for 1 second

Define potential difference

the ​energy transferred per unit charge​ between two points in a circuit

Define Volt

1 joule per coulomb

V = W/Q

state Ohms law

current is directly proportional to voltage and inversely proportional to resistance

V = IR

Power equations

P = IV

P = I²R

P = V²/R

P = E/t

Define resistivity

An intrinsic material property measuring how strongly a substance opposes electric current flow. Equal to the resistance R of a specimen, multiplied by its cross-sectional area A, and divided by its length l

rho = RA/ l

Resistivity vs Resistance

Resistance - Depends on shape, size, and material (e.g., a long wire has high resistance).

Resistivity - Depends only on the material (e.g., copper has low resistivity regardless of its size).

EMF vs Potential

(EMF) is the total energy provided by a source (like a battery) per unit charge, acting as the "cause" of current. V = E -Ir

Potential difference (PD) is the energy dissipated by a unit charge passing between two points in a circuit

Define drift velocity

Drift velocity is the average velocity of charged particles due to an electric field

I = nevA

Where I = current, n = number of charge carriers per unit volume (m^-3) , v = the mean drift velocity of the electrons /m s^-1, A = area (m²)

Derive I = nqvA

Explain how terminal potential difference , internal resistance and ideal source are related in a practical source of electromotive force (EMF) such as an alkaline battery.

A practical source has internal resistance r, so some energy is lost inside the source when current flows. The e.m.f. ε represents the total energy supplied per unit charge, while the terminal potential difference V is the energy available to the external circuit.

When current flows, a voltage drop Ir occurs inside the source. Therefore,

V=ε−Ir

For an ideal source, r=0 so V=ε