The Ohm's Law Wheel with Power shown above provides a graphical representation of the relationships between voltage, current, resistance and power in a direct current electrical circuit.

George (Georg) Simon Ohm (1789 -1854), a German physicist, discovered the relationship between applied voltage, current flow and various lengths of wire (resistance).

Ohm's Law expresses these relationships as follows:

*The current flowing in a circuit is directly proportional to the applied EMF and is inversely proportional to the resistance.*

When expressed as an equation it takes the form: I = E/R (I = E divided by R).

Where:

I = current in amperes

E = EMF (Electromotive Force) in volts

R = resistance in ohms

The equation above solves for the value of current flowing in a circuit when voltage and resistance values are known. This equation can be transposed allowing any of the three quantities to be determined if the remaining two are known:

E = IR (E = I times R) solves for the value of the voltage applied to a circuit when the current and resistance values are known.

R = E/I (R = E divided by I) solves for circuit resistance when applied voltage and current flow are known.

It is important to remember that the units of measurement used in the expression are amperes, volts and ohms. Other units such as milliamperes (1/1000th of an ampere), kilohms (K ohms) or kilovolts (1000 volts) must be converted before using the equation.

Example:

10 ma (milliamperes) is flowing in a circuit with 12 volts applied, what is the circuit resistance?

10 ma = .01 ampere

R = E/I

R = 12/.01 = 1200 ohms (1.2k ohms)

The Ohm's Law Triangle shown above is a memory aid used to help remember the formula required to solve for an unknown circuit value.

Simply cover the unknown quantity (the value that you are trying to find) and the remaining values and their relationship to each other will indicate mathematical operation required to solve for the unknown quantity.

For example, to solve for voltage (E) cover the E, the remaining values I and R are side-by-side indicating multiplication. If solving for current (I), cover the I and the remaining value E is over R indicating division.

Power, the rate of doing work, in an electrical circuit is equal to the applied voltage multiplied by current. The basic unit of electrical power, the watt, is named after James Watt (1736 - 1819) in honor of his work contributing to the development of the steam engine. One watt is equal to one volt multiplied by one ampere.

When expressed as an equation it takes the form: P = IE (P = I times E).

Where:

P = power in watts

I = current in amperes

E = EMF (Electromotive Force) in volts

The equation above solves for the value of the power dissipated in a circuit when voltage and current values are known. This equation can be transposed allowing any of the three quantities to be determined if the remaining two are known:

I = P/E (I = P divided by E) solves for the value of the current flowing in the circuit when the power and voltage values are known.

E = P/I (E = P divided by I) solves for applied voltage when the power and current values are known.

The Power Triangle shown above, like the Ohm's Law Triangle, is a memory aid and is used as described above.

All of the formulas on the Ohm's Law Wheel can be derived by substituting values in the basic formulas represented by the triangles.

- DIY Alarm Forum\'s Glossary of Security Related Terms
- NBFAA\'s Minimum Installation Standards
- Industry Links
- State Alarm Associations
- Find An Alarm Company In Your Area
- Online Alarm Dealers