T5C: Ohm's Law and the concepts of energy and power; concepts of frequency, including AC vs. DC, frequency units, and wavelength

Direct current is the name of a current that flows only in one direction.

Alternating current is the name of a current that flows back and forth, first in one direction, then in the opposite direction.


This simple drawing shows Ohm's law:

Just cover the value you need to know with your finger and the remaining letters show the equation you need to use to find it.


The current (I) in a DC circuit is calculated from the voltage and resistance by:

The resistance (R) in a DC circuit is calculated from the voltage and current by:

The voltage (E) in a DC circuit is calculated from the current and resistance by:


If a current of 2 amperes flows through a 50-ohm resistor, the voltage across the resistor is 100 volts.

If a 100-ohm resistor is connected to 200 volts, the current through the resistor is 2 amperes.

If a current of 3 amperes flows through a resistor connected to 90 volts, the resistance is 30 ohms.


The basic unit of electrical power is the watt. Power is the term used to describe how fast electrical energy is used.
What happens to a signal's wavelength as its frequency increases? It gets shorter!