Basics

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 is denoted as . And time varying current 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.

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

Voltage is heigher at node . Current is flowing from to .

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