Basics

Be sure to revise the Electricity unit of G.C.E. (A/L) Physics.

Charge

Quantized & conserved. Measured in Coulomb ().

Time invariant charge is denoted as . And time varying charge is denoted as .

Current

Amount of charge flowing through a point in unit time. Conventional current (opposite to electron flow) flows from positive to negative potentials.

Time invariant current (DC) is denoted as . And time varying current (AC) is denoted as .

Voltage

Voltage at a point is the work that must be done against the electric field to move a unit positive charge from infinity to that point.

volt is the potential difference between points when joule of energy is used to move coulomb of charge from one point to the other.

Time invariant voltage is denoted as . And time varying voltage is denoted as .

Voltage difference between 2 points is the work that must be done against the electric field to move a unit positive charge from one point to another.

Double subscript notation

-	Current	Voltage
Double subscript
Equation
Description	Current is flowing from point to point	Voltage is higher at point and lower at point

Electric Circuit

Types of circuits

• Closed circuit - the electricity flows
• Open circuit - the electricity doesn’t flow. current = . resistance.
• Short circuit - very large current. resistance.

Power

Total Work

When v and i are constant

Electrical Load

Something that consumes electrical energy.

Linear loads

Loads that have a linear relationship between the applied voltage and the current. Can be expressed using a combination of resistors, capacitors and inductors only.

Note

If a AC sinusoidal voltage is applied across a load, current through the load will also be sinusoidal iff the load is linear.

Non-linear loads

• Diodes
• Superconductors
• Varistors (voltage-dependent resistors)
• Non-linear inductors