Voltage, Current, Resistance, and Ohm's Law

What Is Voltage, Current, and Resistance?

Every electrical circuit depends on three quantities working together: voltage, current, and resistance. Understanding each one—and the relationship among them—is the foundation of every task you will perform as an electrician.

Voltage (V) is the electrical pressure that pushes electrons through a conductor. Think of it like water pressure in a pipe. Voltage is measured in volts (V) using a voltmeter placed in parallel across the component you are measuring.

Current (I) is the flow of electrons through a conductor. Returning to the water analogy, current is the rate of flow. Current is measured in amperes (amps, A) using an ammeter placed in series with the circuit—meaning the current must flow through the meter.

Resistance (R) is the opposition to current flow. Every conductor, every component, and every load has some resistance. Resistance is measured in ohms (Ω) using an ohmmeter. Always de-energize and isolate a circuit before measuring resistance. Measuring resistance on an energized circuit will damage your meter and can injure you.

Ohm's Law

In 1827, Georg Simon Ohm demonstrated that voltage, current, and resistance are linked by a simple, reliable relationship:

V = I × R

This single equation has three forms depending on which quantity is unknown:

• Solve for voltage: V = I × R
• Solve for current: I = V ÷ R
• Solve for resistance: R = V ÷ I

A common memory aid is the Ohm's Law triangle: write V on top, I and R side by side on the bottom. Cover the unknown with your finger and the remaining two quantities show the operation.

Ohm's Law applies to resistive (linear) loads—resistors, heating elements, and incandescent lamps—under steady-state DC conditions. It is equally valid in AC resistive circuits. It does not directly describe reactive components such as capacitors and inductors, which you will study in later modules.

Setting Up a Simple Resistive Circuit

In today's lab you will build four single-resistor circuits on terminal-strip trainers using a DC regulated bench power supply. Each circuit has one resistor connected between the positive and negative terminals of the supply. This forms a complete loop: electrons leave the negative terminal of the supply, flow through the resistor, and return to the positive terminal.

Before you energize any circuit:

1. Have your lab partner inspect the wiring.

2. Set the power supply to 0 V before connecting leads.

3. Confirm the resistor value using the color-code chart in your NCCER Level 1 textbook.

4. Record your predicted V, I, and R values in your lab log before measuring.

Measuring Voltage and Current

To measure voltage: Set your digital multimeter (DMM) to DC volts, select a range higher than the expected reading, and touch the probes across (in parallel with) the resistor. Red probe to the higher-potential side, black to the lower.

To measure current: Set your DMM to DC amps (mA range for small currents), break the circuit at one point, and insert the meter in series so all current flows through it. Re-check your range—an overloaded ammeter can blow the meter's internal fuse.

To measure resistance: Power supply OFF and leads disconnected from the supply. Set DMM to ohms, touch probes across the resistor. If the resistor is still in-circuit, other parallel paths can cause a false reading; when in doubt, remove one lead of the resistor before measuring.

Percent Error and Error Analysis

No measurement is perfect. Meters have a specified accuracy (typically ±0.5 % to ±2 % for a quality DMM), resistors have a tolerance (usually ±5 % for carbon-film), and connection resistance adds a small amount. You calculate percent error to judge whether a discrepancy is within acceptable limits:

% Error = |Measured − Calculated| ÷ Calculated × 100

If your percent error is within ±5 %, your circuit and measurement technique are correct. If it exceeds ±5 %, check: wrong resistor value, loose terminal connection, incorrect meter range, or meter probes in wrong jacks.

Safety Rules for This Lab

• Maximum supply voltage: Your instructor will set a safe maximum (commonly 12 V or less for this exercise). Never exceed the assigned limit.
• Power down before rewiring: Turn the supply to 0 V and switch it off before changing any connection.
• Lockout/Tagout (LOTO): In the field, any time you work on de-energized equipment you must apply LOTO per OSHA requirements. Practice the habit now—turn off and lock out the supply whenever your hands are in the circuit.
• PPE: Safety glasses are required at all times in the lab. Although voltages in this lab are low, the habit of wearing PPE must be automatic before you work in the field.
• Resistor wattage: Use only the 1/4 W resistors provided. Calculate expected power (P = V²÷R or P = I²×R) before energizing to confirm the resistor is not overloaded. If calculated power exceeds 1/4 W, report it to your instructor before proceeding.
• Damaged components: Never use a burned or cracked resistor. Inspect components before every use.