Which of the following obeys Ohm's law?

Analysis of the Question

The question asks us to identify which of the listed materials follows Ohm's Law. Ohm's Law states that the current flowing through a conductor is directly proportional to the potential difference across its ends, provided that the temperature and other physical conditions remain constant ($V = IR$).

Concept Explanation

• Ohmic Conductors: These are materials that obey Ohm's law. Their current-voltage ($I-V$) graph is a straight line passing through the origin. Examples include metallic conductors (like copper, silver, and aluminum) at a constant temperature.
• Non-Ohmic Conductors: These do not obey Ohm's law. Their $I-V$ graph is not a straight line. Examples include: • Diodes: These are semiconductors that allow current to flow in only one direction. • Electrolytes: These often show non-linear behavior due to chemical changes and polarization. • Glass: This is an insulator; it does not conduct electricity under normal conditions.

Step-by-Step Solution

1. Evaluate Option A (All metals): Metals are the classic examples of ohmic conductors. As long as the temperature is kept constant, the resistance remains constant, and the relationship between voltage and current is linear.
2. Evaluate Option B (Diode only): A diode is a semiconductor device. Its $I-V$ characteristic curve is exponential, not linear. Therefore, it does not obey Ohm's law.
3. Evaluate Option C (All electrolytes): Electrolytes are ionic conductors. Their resistance changes with temperature and concentration, and they do not maintain a linear $V/I$ ratio.
4. Evaluate Option D (Glass): Glass is an insulator. It has an extremely high resistance and does not conduct current in the way required to demonstrate Ohm's law.

Conclusion

Since metals maintain a constant resistance under constant temperature, they are the only category listed that obeys Ohm's law.

The correct option is A.

Exam Tip

When you see questions about Ohm's Law, remember the acronym "M.I.T.": Metals In Temperature-constant conditions obey Ohm's Law. If the temperature changes, even a metal stops obeying the law!