While ohm’s law calculations can be intimidating, understanding how to read and interpret exam questions that require the use of ohm’s law formulas can result in achieving a passing score on your electrical exam.
Can you answer the following question with ease:
A 120 volt single-phase source supplies four paralleled branch loads of 10 ohms, 20 ohms, 30 ohms, and 40 ohms respectively. What is the total amperage of the circuit?
A. 67.0 amps
B. 40.0 amps
C. 25.0 amps
D. None of the above
Solution:
C. 25 amps
Step #1
Use the reciprocal method formula to determine the total resistance of the circuit.
Reciprocal Method Formula:
= 1/(1/R1) + (1/R2) + (1/R3)
= 1/(0.1) + (0.05) + (0.033) + (0.025)
= 1/0.2083
= 4.80 ohms
Step #2
Use the ohm’s law equation for current to determine total amperage.
I = E/R
=120 volts/4.80 ohms
= 25 amps
To answer this question correctly you’d need to know the answers to the following questions:
Why isn’t the resistance of the circuit additive?
How would I know that the resistances are in parallel?
Why is the total resistance of the circuit smaller numerically than the ohmic value of the least resistive branch?
What formula should be used to determine total resistance?
If there is more than one ohm’s law equation for current, how would I know which one to use?
Understanding the rules that govern the way resistance and current operate in a parallel circuit is the key to answering ohm’s law questions correctly.
Guesswork won’t get you a license, but careful preparation will. We specialize in explaining formulas in a way that’s easy to understand and remember. Call 888-919-3926 or click on the “Register Now” tab to begin preparing to pass your exam today.