Questions: Ohm's Law

Question 1

A resistor and a diode are each connected to a 5V power supply. You measure 50 mA through the resistor. You then double the voltage to 10V. What do you predict about each device?

• A. Both devices will double their current to 100 mA, since all conductors follow V = IR
• B. The resistor will double to 100 mA; the diode's current will change in a nonlinear way that cannot be predicted from V = IR alone
• C. The resistor current is unpredictable because resistance changes with temperature; the diode will double
• D. Neither device follows a simple rule because real circuits are always more complex than the ideal equation

Answer: (B)

An ohmic resistor has a constant R — doubling V doubles I exactly. A diode is non-ohmic: its V-I relationship is exponential, not linear, and current changes dramatically near a threshold voltage. You cannot calculate the diode's current from V = IR with a fixed R, because R is not fixed. The existence of non-ohmic devices is not a complication to work around — it is precisely what makes diodes useful for rectification, signal processing, and logic circuits.

Question 2

The resistance of a metal wire is measured at room temperature and again when it is heated to 200°C. What does Ohm's law and the microscopic picture predict?

• A. Resistance stays the same — Ohm's law says R is a constant property of the material, independent of conditions
• B. Resistance increases — hotter ions vibrate more, causing more frequent electron scattering and reducing average drift speed
• C. Resistance decreases — higher temperature gives electrons more kinetic energy to overcome resistance
• D. The wire stops obeying Ohm's law at high temperatures, so no prediction is possible

Answer: (B)

While R is constant for ohmic materials at a fixed temperature, resistance does depend on temperature. Hotter ions vibrate more vigorously, scattering conduction electrons more frequently and reducing their average drift speed — which raises resistance. This is why incandescent bulb resistance changes dramatically from cold to operating temperature. The wire remains ohmic (its V-I graph is still a straight line) across a temperature range, but R itself is temperature-dependent. Option A confuses 'R is constant at a given temperature' with 'R is constant under all conditions.'

Question 3

In a circuit with a fixed resistor, increasing the current through the resistor causes the voltage across it to increase.

• T. True
• F. False

Answer: False

This reverses the causal direction. Voltage (potential difference, maintained by a power supply or battery) drives current through the resistance. Writing R = V/I is a definition used to measure resistance from applied voltage and resulting current — not a causal statement that current causes voltage. For a fixed ohmic resistor, the ratio V/I equals R, but voltage is the cause and current is the effect. You set the voltage; the current follows from it and the resistance.

Question 4

Ohm's law is a fundamental law of physics, like conservation of energy, and applies to most electrical conductors.

• T. True
• F. False

Answer: False

Ohm's law is an empirical relationship — it describes the behavior of certain materials (metals, carbon resistors) under certain conditions, not a universal physical law. Semiconductors, diodes, transistors, electrolytes, and plasmas do not obey V = IR. Even for ohmic materials, the relationship is approximate: resistance depends on temperature, so extreme conditions can cause nonlinearity. The distinction matters because assuming universal applicability leads to incorrect analysis of real-world non-ohmic devices — which include nearly all the components that make modern electronics functional.

Question 5

Diodes and transistors violate Ohm's law — their V-I relationships are not linear. Why is this a feature of these devices rather than a flaw?

Model answer: A device that obeys Ohm's law with a fixed resistance can only scale current proportionally to voltage — it is a passive element with no ability to rectify, amplify, or switch. Diodes and transistors are useful precisely because they violate this linearity. A diode's threshold behavior allows it to pass current in only one direction, converting AC to DC (rectification). A transistor's nonlinear response lets a small control voltage modulate a large current — the basis of amplification and digital switching. All of modern electronics depends on non-ohmic devices; if everything were ohmic, logic gates, amplifiers, and rectifiers would be impossible.

The key conceptual move is recognizing that Ohm's law describes passive resistive elements, and that violation of it is precisely what enables active, functional electronic behavior. Understanding where the law applies — and why it fails where it does — is more useful than treating it as a universal rule.