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What is Ohm's law?

Class 10 ScienceNCERT chapter: Electricityइसे हिंदी में पढ़ें

Exam-ready answer

Ohm's law states that the current through a conductor is directly proportional to the potential difference across its ends, provided its temperature remains constant. So V ∝ I, or V = IR, where R is the resistance. The SI unit of resistance is the ohm (Ω); 1 Ω = 1 volt per ampere.

Explanation

Ohm's law connects three quantities of a circuit: potential difference (V), current (I) and resistance (R). It says that if you double the potential difference across a metallic conductor kept at constant temperature, the current through it also doubles. This direct proportionality is written as V ∝ I, which becomes V = IR when the constant of proportionality R, the resistance, is introduced.

Resistance is the property of a conductor that opposes the flow of electric current. A conductor with greater resistance allows less current for the same potential difference. The SI unit of resistance is the ohm (Ω). One ohm is the resistance of a conductor through which a current of 1 ampere flows when a potential difference of 1 volt is applied across its ends.

In NCERT problems you often use the same relation in the form I = V/R or R = V/I. For example, if a 6 V battery is connected across a resistor and a current of 2 A flows, then R = V/I = 6/2 = 3 Ω. Remember that Ohm's law is valid only for metallic conductors at constant temperature; it is not obeyed by devices such as diodes, filament bulbs and semiconductors.

Points to remember

  1. Ohm's law: at constant temperature, current is directly proportional to potential difference, V ∝ I, so V = IR.
  2. Resistance R = V/I; its SI unit is the ohm (Ω), where 1 Ω = 1 V A⁻¹.
  3. Current I = V/R and potential difference V = IR are the rearranged forms used in numericals.
  4. Ohm's law holds for metallic conductors at constant temperature; it fails for diodes, LEDs and filament bulbs.
  5. A V–I graph for a conductor obeying Ohm's law is a straight line through the origin; its slope gives 1/R (or R, depending on the axes).

Exam tip

Always write Ohm's law first as V = IR, then state the condition 'at constant temperature'. In numericals, write the formula, substitute values with SI units (V, A, Ω) and box the final answer with its unit.

Questions students also ask

What is the formula of Ohm's law?
The formula is V = IR, where V is the potential difference in volts, I is the current in amperes and R is the resistance in ohms. It can also be written as I = V/R or R = V/I, depending on the quantity you need to find.
Under what conditions is Ohm's law valid?
Ohm's law is valid only when the temperature of the conductor remains constant and the conductor is metallic. It is not obeyed by non-ohmic devices such as diodes, LEDs, transistors and filament bulbs, where the V–I graph is not a straight line.
What is the SI unit of resistance and how is it defined?
The SI unit of resistance is the ohm (Ω). One ohm is the resistance of a conductor when a potential difference of 1 volt across its ends produces a current of 1 ampere through it, i.e. 1 Ω = 1 V/1 A.

Chapter labels help you find the topic in your textbook. Check your own edition: chapter names and order can change. These are Know.it explanations, not official NCERT solutions.