Download Ohm`s Law and the Power Triangle

Ohm’s Law and the Power Triangle
Accurate for D.C. circuits.
E
I R
RR
P
I
E
P = watts I = amps R = ohms E = volts
Example: You have measured resistance with an Ohm meter and found you have 35 Ohms
resistance. You also have measured the DC voltage and know it to be 12 volts.
You want to find out how big a fuse you need to run the circuit; you need to know how many
amps it will draw.
Using Ohm’s Law, we find that voltage (E) divided by resistance (R) equals amperage (I).
In this circuit, 12 volts ÷ 35 ohms = about .343 amps. So you might try a 5-amp fuse to start
with.
Now let’s say you want to know how many watts this circuit will draw; this time we use the
power triangle. Now that we have calculated the amperage (I), we can plug the answer into the
formula to find watts (P).
In the power triangle, we find that amps (I) times volts (E) = watts (P).
Therefore: .343 amps x 12 volts = 4.116 watts of power.
*NOTE:
Ohm’s Law works all the time with DC (direct current) circuits, but only works with AC
(alternating circuits) when the circuit is purely resistive. An electric stove heating element, an
electric hot-water heater element, or electric toasters/space heaters are examples of purely
resistive applications.
The familiar Ohm's Law used for DC circuits can only be used for AC if the load is purely
resistive. Most AC circuits however, contain series or parallel combinations of resistance,
capacitance and inductance. This leads to the voltage and currents being out of phase and the
load becomes complex. Special formulas apply for alternating circuit calculations.