Ohm’s Law Ohm’s Law provides a basic understanding of how voltage, current, and resistance interact in a closed circuit. Assembled By Ken Mitchell Livermore TOPScience

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.

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].”

The Three Control Variables Voltage – Source - 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 Control Variables

Therefore: Volts = Amps times Resistance Ohm's Law is a formulation of the of voltage, current, and resistance, expressed as:                                                                            Where:  V - is the Voltage measured in volts  I - is the Current measured in amperes  R - is the resistance measured in Ohms  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.

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

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

Given: Voltage = 10 volts and the Resistance = 10 ohms Example Calculation Given: Voltage = 10 volts and the Resistance = 10 ohms How does the current, I, change as the resistance goes to zero? Place finger over I in the triangle, Voltage is over Resistance so we divide voltage by the resistance. Make small table: 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. Actually, with zero resistance in the load, the current then is limited by the internal source impedance (resistance).

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