Resistance & Ohm's Law

Understand resistance and the relationship between voltage, current, and resistance.

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Resistance

Learn what resistance means and why it limits current flow.

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Ohm's Law

Connect voltage, current, and resistance with one simple relationship.

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Interactive practice

Change voltage and resistance yourself and watch the calculated current respond.

Click Next to begin.

What Is Resistance?

opposition to current

Resistance describes how strongly a component or material opposes electric current. It is measured in ohms, written with the symbol Ω.

Low resistance → current can flow more easily.

High resistance → less current flows for the same voltage.

Unit → ohm (Ω). 1 kΩ = 1,000 Ω and 1 MΩ = 1,000,000 Ω.

The Three Quantities

V • I • R

Ohm's Law connects three electrical quantities. Changing one can change the relationship among the others.

V

Voltage

Electrical potential difference — the electrical “push.” Measured in volts.

I

Current

Rate of electric charge flow. Measured in amperes (amps).

R

Resistance

Opposition to current flow. Measured in ohms.

Ohm's Law

V = I × R

If you know any two of the three quantities, Ohm's Law lets you calculate the third.

V = I × Rvoltage = current × resistance

Find currentI = V ÷ R

Find resistanceR = V ÷ I

Worked Example

12 V and 1 kΩ

A 12 V source is applied across a 1,000 Ω resistor. How much current flows?

1. Start with: I = V ÷ R

2. Substitute: I = 12 V ÷ 1,000 Ω

3. Calculate: I = 0.012 A

4. Convert: 0.012 A = 12 mA

What Happens When Resistance Changes?

same 12 V

Keep voltage fixed and change only resistance. Notice what happens to current.

Voltage	Resistance	Current
12 V	500 Ω	24 mA
12 V	1 kΩ	12 mA
12 V	2 kΩ	6 mA

At the same voltage, doubling resistance halves the current.

Interactive Voltage, Current & Resistance

change one thing

Use the actual visualizer. Change voltage and resistance and watch current, charge flow, carrier drift, and the resistor visualization respond.

Voltage, Current & Resistance

Change voltage and resistance to see their effect on current, charge flow, carrier drift, and resistance.

Voltage — electrical push

12 V

Resistance — opposition to flow

20 kΩ

VOLTAGE / PUSH

12 V

CURRENT / CHARGE FLOW

600 µA

RESISTANCE

20 kΩ

Voltage

Voltage represents electrical potential difference — the electrical "push."

Current / Charge Flow

More same-size dots represent more charge passing through the circuit per second.

Resistance

Higher resistance makes the resistor visually thicker and more intensely red.

I = V / R

Why Resistors Matter

real circuits

A resistor provides a known resistance so the circuit behaves within a desired range.

Protect components by limiting current, such as current through an LED.

Create voltage relationships in divider and sensing circuits.

Control transistor current, such as the base resistor in a transistor switching circuit.

Set operating conditions so parts remain within safe electrical limits.

Resistance and Power

heat matters

Resistors dissipate electrical energy as heat, so resistance is not the only rating that matters.

P = V × Ipower in watts = voltage × current

Important: A resistor must have a sufficient power rating for the circuit. A correct resistance value can still overheat if the resistor's wattage rating is too low.

Knowledge Check

four questions

Select an answer for each question.

1. What unit measures resistance?

2. If voltage stays constant and resistance increases, what happens to current?

3. What is the correct formula for current?

4. 12 V across 1 kΩ produces approximately:

Choose an answer to receive immediate feedback.

Summary

lesson complete

✅ Resistance opposes current and is measured in ohms (Ω).

✅ Ohm's Law connects voltage, current, and resistance: V = I × R.

✅ At fixed voltage, increasing resistance decreases current.

✅ Resistors limit current and help establish useful circuit conditions.

✅ Real resistors also need an adequate power rating.

🎉 Congratulations!

lesson complete

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You completed Resistance & Ohm's Law!

✅ You can explain what resistance does.

✅ You can use Ohm's Law to calculate V, I, or R.

✅ You can predict how changing resistance affects current.