Electrical Equations Reference

Beginner Time: 10 min Type: Reference Focus: Controls / Panel Design

After this module: Quick-reference sheet for Ohm's law, power, dividers, KCL/KVL, and capacitor/inductor energy — formatted for fast lookup.

Purpose

This page is a compact reference card for the most-used relationships from the fundamentals training set.

Ohm’s law

V = I × R
I = V / R
R = V / I

Power

P = V × I
P = I² × R
P = V² / R

Series resistance

R_eq = R1 + R2 + R3 + ...

Parallel resistance

1 / R_eq = 1 / R1 + 1 / R2 + 1 / R3 + ...

For two resistors:

R_eq = (R1 × R2) / (R1 + R2)

Voltage divider

V_out = V_s × (R_x / R_total)

Current divider

I_x = I_s × (R_other / (R_x + R_other))

Kirchhoff’s laws

Sum of currents at a node = 0
Sum of voltages around a closed loop = 0

Branch current in nodal form

I = (V_a - V_b) / R

Source transformation

V_s = I_s × R
I_s = V_s / R

Thevenin and Norton relationship

V_th = I_n × R_th
I_n = V_th / R_th

Capacitor energy

E = ½ × C × V²

← Equivalent Circuit Methods ↑ Electrical Fundamentals Passive Components →

Trust Boundary — Engineering Judgment Required

This site is a personal-use paraphrase and navigation reference for industrial automation standards. It is not a substitute for authoritative standards documents, professional engineering judgment, or legal review. All content is sourced from a local RAG corpus and has not been independently verified against current published editions.

Items marked TO VERIFY have limited or unconfirmed local coverage. Items marked NOT IN CORPUS are not covered in the local repository. Do not rely on this site for compliance determinations, safety-critical design decisions, or legal interpretation.