Voltage and Current Sources

Voltage Sources

A voltage source adds energy per charge (voltage ($V$)) to the charge flowing through it.

• Unit: $[V]=\left[\frac{J}{C}\right]$ Voltage is defined as energy per unit charge added as current flows through it.
• Can be thought of as “force” pushing current.

Voltages can be added in series; they cannot be placed in parallel unless they are the same voltage.

Current Sources

A current source moves a constant amount of charge per second at a constant rate, called current ($I$).

• Unit: $[A]=\left[\frac{C}{s}\right]$

Current sources can be added in parallel, such that both sources add current flow to the conductor; they cannot be placed in series as they are fighting different currents through the same wire.

Dependent Sources

These perform the same function as the independent sources above, but their value is some function of some other state in the system.

They have units of $\left[\frac{\text{output type}}{\text{input type}}\right]$.

• Output voltage depends on some other voltage: $\left[\frac{V}{V}\right]$
• Output voltage depends on some current: $\left[\frac{V}{A}\right]$
• Output current depends on some voltage: $\left[\frac{A}{V}\right]$
• Output current depends on some other current: $\left[\frac{A}{A}\right]$