Electric charge
Electric charge is a fundamental property of matter. There are two types of charge: positive and negative.
Coulomb's law
The electric force between two point charges depends on the magnitude of the charges and the distance separating them.
F = k|q₁q₂| / r²
k is Coulomb's constant, q₁ and q₂ are the charges, and r is their separation.
Electric current
Electric current measures the rate at which electric charge passes through a cross-section of a conductor.
I = Q / t
I is current, Q is charge transferred, and t is time.
Current is measured in amperes (A), where one ampere corresponds to one coulomb of charge passing a point each second.
Potential difference
Potential difference describes the energy transferred per unit charge between two points in an electrical system.
V = W / Q
V is potential difference, W is energy transferred, and Q is charge.
Resistance
Resistance describes how strongly a component opposes the flow of electric current.
V = IR
For an ohmic conductor under suitable conditions, voltage is proportional to current.
Electric circuits
A circuit provides a complete path through which charge can move. Components connected in different arrangements behave differently.
Kirchhoff's laws
Kirchhoff's laws provide powerful tools for analysing more complicated electrical circuits.
Electrical power
Electrical power describes the rate at which electrical energy is transferred.
P = VI
Using Ohm's law, this can also be written as P = I²R or P = V²/R for suitable resistive circuits.
Electrical energy
If electrical power remains constant for a period of time, the energy transferred can be calculated directly.
E = Pt
E is energy transferred, P is power, and t is time.
Quick example
A resistor has a resistance of 6 Ω and carries a current of 2 A. What potential difference is across it?
Using Ohm's law:
V = IR
V = 2 × 6