1. Ohm’s Law
Ohm’s Law provides a basic understanding of how voltage, current, and resistance interact in a closed circuit.

2. Guess where the return circuit is.
On-Off
Valve
Sprinkler
Water Tank
Water returns to the tank via the water cycle – evaporation, airflow, condensation, rain, and return to the water source to refill the tank.
Water Analogy to Electrical Circuit
Guess where the return circuit is.

3. Simple Electrical Circuit Equivalent to the Water System
L OA D
Switch
See notes below
Electron Flow
Replaces the water tank
Replaces on-off water valve
Replaces the sprinkler
Replaces water flow in pipe
Battery
The term load can include any electrical component. Components may consist of resistive loads like light bulbs, electric heaters, stoves. Also, it may be inductive such as an electric motor. Capacitive effects are typically used to balance loads to make them look resistive to the electrical source (important in AC systems). Because in more complex systems a load can have resistance [R] – inductive [L] – or capacitive [C] characteristics, it often is just referred to as an “impedance [Z].”

4. Voltage – The electromotive force (EMF) or potential difference between two points in a circuitSource - DC – Batteries (dry cells, Lead Acid, LiH, etc.) Expressed in “Volts.”
Physical example – The depth of water in a water tank.
Current – Electron flow in the circuit, from + to – side of the power source (battery). Expressed in Amperes.
Physical example – Water flowing through a hose.
Resistance -- An electrical component that operates on voltage applied across it. Because it does work and dissipates the energy as heat it’s simply called a “Load” and is expressed in “ohms.”
Physical example – The lawn sprinkler.
The Three Variables

5. Ohm’s Law
Ohm's Law is a formulation of the of voltage, current, and resistance, expressed as: 
                                     V = I x R                                    
Where:  V - is the Voltage measured in volts  (V) I - is the Current measured in amperes  (amps) R - is the resistance measured in Ohms  (Ω)
Therefore: Volts = Amps times Resistance
Some folks use the symbol “E” for voltage. Also, Resistance [R] is often replaced by Impedance [Z] in AC circuitry.

6. How to find the Unknown Variable Using Ohm’s Triangle
I R
Ohm’s Law Simplified
The Variables:
V = Voltage
I = Current
R = Resistance
Solution:
Cover the unknown variable and the remainder gives the solution.
Divide across here
Multiply across here
Basic multiplication a/o division required.
How to find the Unknown Variable Using Ohm’s Triangle

7. V I R An Example Using Ohm’s Triangle
Example: What happens to current (I) as the resistance (R) goes to zero?
Hold a finger over current (I) and observe what happens when the resistance (R) goes to zero.
********
Current (I) becomes infinite (actually it will be limited by the internal resistance of the voltage source).
An Example Using Ohm’s Triangle

8. If Voltage = 10 volts and the Resistance starts at 10 ohms
Example Calculations
If Voltage = 10 volts and the Resistance starts at 10 ohms
How does the current, I, change as the resistance goes to zero?
According to Ohm’s Law we divide voltage by the resistance
Voltage / Resistance = Current (I)
10 volts / 10 ohms = 1 amp
10 volts / 5 ohms = 2 amps
10 volts / 2 ohms = 5 amps
10 volts / 1 ohm = 10 amps
10 volts / ½ ohms = 20 amps
10 volts / zero ohms = Unlimited amps*
*Current is actually limited by the battery’s internal resistance (impedance).
Actually, with zero resistance in the load, the current then is limited by the internal source impedance (resistance).

9. Be sure to plot the results
Be sure to plot the results. This helps to explain why a short circuit can cause fires.