# Voltage, Current, Resistance, and Power

> Turn formulas into intuition: voltage pushes, current flows, resistance limits, and power becomes heat or work.

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Learning reference only: electrical codes, permits, inspection bodies, and licensing rules differ by region. This course is not a substitute for local code, licensed training, permits, or authority requirements. Before any hands-on work, de-energize, verify absence of voltage, and follow local regulators/AHJ and employer procedures.

## Practise the relationships with two known values

Use the free calculator for ideal positive-value DC resistive exercises, then explain which two values were entered and which two were derived.

- [Open the Ohm's law calculator](<https://elect-learn.org/en/tools/ohms-law-calculator/>)

## Lesson information

Circuit Fundamentals · Basic · 20 min

Circuits

Interactive

## Voltage drives. Resistance limits. Power tells the rate.

The source provides voltage. Closing the switch completes the path so current can flow through the load.

- At fixed voltage, more resistance means less current.
- For an ideal resistive load: I = U ÷ R.
- Power P = U × I; the values below share one model.

## Goals

- Use V=I×R and P=V×I to explain loads.
- Understand why current and heating are tightly related.
- Connect meter readings with circuit state.

## Too many heaters on one power strip

In winter, two heaters and a computer share one power strip, and the plug feels hot.

Wrong call: Assuming it is safe because the breaker has not tripped.

Stop point: Stop use and de-energize immediately if there is heat, odor, or discoloration.

Reasoning path: Calculate total power/current and check circuit rating, power strip rating, and duration.

Review: Common exam topics: Ohm's law, power, current, and conductor/equipment ratings.

## Core concepts

- Voltage is the pushing difference; current is what actually flows through the load.
- More power means more energy per unit time, so heating matters.
- A load is not abstract; it is a lamp, heater, motor, or charger.

## Digital Multimeter

Measures voltage, continuity, and resistance so you verify instead of guess.

Misuse and limits: Wrong jack or mode selection can damage the meter or create danger.

## NM Cable / Conductors

Carries circuit current; color and markings help identify use.

Misuse and limits: Wrong gauge, jacket, environment, or color use may violate code.

## Common Pitfalls

- Looking only at voltage and ignoring load current.
- Treating a power strip as a way to increase circuit capacity.
- Ignoring continuous loads and heat dissipation.

## Safe Workflow

1. List wattage or amperage for every load.
2. Estimate current with P=V×I.
3. Compare with device, strip, and circuit ratings; stop if heating appears.

## At 120V, about how much current does a 1500W heater draw?

About 12.5A because I=P/V=1500/120.

## If a plug feels hot, should you keep watching it?

No. Stop use, de-energize, and investigate ratings, connections, and load.

## Look Back

- [Review Lesson 3: voltage is measured across two points.](<https://elect-learn.org/en/lesson/03-multimeter-basics/>)