Does Ohm's Law apply to all materials?

No. Ohm's Law applies to ohmic conductors — materials where resistance stays constant regardless of voltage or current, like most metals at constant temperature. Non-ohmic components include diodes (resistance depends on direction of current) and light bulbs (resistance increases with temperature). Semiconductors are also non-ohmic.

What is the difference between voltage, current, and resistance?

Voltage (V) is the electrical pressure driving charge through a circuit — like water pressure in a pipe. Current (I) is the flow rate of charge — like water flow. Resistance (R) is the opposition to that flow — like the pipe diameter. Ohm's Law V = IR relates all three.

Why does resistance increase with temperature in metals?

In metals, electrons carry current by moving through a lattice of atoms. As temperature rises, atoms vibrate more, increasing collisions with electrons and impeding their flow — resistance increases. In semiconductors the opposite happens: more charge carriers are freed at higher temperatures, so resistance decreases.

What units are used in Ohm's Law?

Voltage in Volts (V), current in Amperes (A), and resistance in Ohms (Ω). In practical electronics you often see milliamps (mA = 0.001 A), kilohms (kΩ = 1000 Ω), and megaohms (MΩ = 1,000,000 Ω).

Ohm's Law Calculator — Voltage, Current and Resistance

Ohm's Law is one of the most fundamental relationships in electronics and electromagnetism. It states that the current flowing through a conductor is directly proportional to the voltage across it and inversely proportional to its resistance. First formulated by Georg Ohm in 1827, it underlies the design of every electrical circuit from a torch to a data centre.

The law is expressed as a simple triangle: V = IR. Know any two of the three quantities and you can always find the third.

Solve for:

Voltage

20.00 V

Current

2.00 A

Resistance

10.00 Ω

Power

40.00 W

Current I (A)2

Resistance R (Ω)10

The Formula

$V = IR \quad \Leftrightarrow \quad I = \frac{V}{R} \quad \Leftrightarrow \quad R = \frac{V}{I}$

Symbol	Quantity	SI Unit
V	Voltage (potential difference)	Volt (V)
I	Current	Ampere (A)
R	Resistance	Ohm (Ω)

Power Dissipation

A resistor dissipates electrical energy as heat. The power consumed is:

$P = VI = I^2 R = \frac{V^2}{R}$

Symbol	Quantity	SI Unit
P	Power	Watt (W)

Worked Examples

Worked Example

Example 1 — Household appliance

A toaster is connected to a 230 V mains supply and draws a current of 4 A. What is its resistance, and how much power does it consume?

Resistance: $R = \frac{V}{I} = \frac{230}{4} = 57.5 \text{ Ω}$

Power: $P = VI = 230 \times 4 = 920 \text{ W}$

The toaster has a resistance of 57.5 Ω and consumes 920 W — consistent with a typical 900 W–1 kW toaster.

Worked Example

Example 2 — LED current limiting

An LED requires 2 V across it and 20 mA (0.02 A) through it to operate correctly. You are powering it from a 5 V supply. What resistor value do you need in series?

Voltage across resistor: 5 − 2 = 3 V

Required resistance: $R = \frac{V_R}{I} = \frac{3}{0.02} = 150 \text{ Ω}$

Place a 150 Ω resistor in series with the LED to limit the current to 20 mA.

Frequently Asked Questions

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