What is the primary difference between an Ohmic and a non-Ohmic conductor?

An Ohmic conductor has a constant resistance and a linear $I-V$ relationship (straight line through the origin). A non-Ohmic conductor has a resistance that changes with voltage or current, resulting in a curved $I-V$ graph.

How does the gradient of an $I-V$ graph differ from a $V-I$ graph in terms of resistance?

In an $I-V$ graph (Current on y-axis), the gradient is $\frac{1}{R}$. In a $V-I$ graph (Voltage on y-axis), the gradient is directly equal to the resistance $R$.

Why is a semiconductor diode considered a non-Ohmic component?

A diode only allows current to flow in one direction and has a resistance that changes drastically depending on the applied voltage, failing the requirement for a constant $V/I$ ratio.

What is a common mistake when calculating resistance from an $I-V$ graph?

Students often assume the gradient is always $R$. If Current is on the y-axis, the gradient is actually the conductance ($\frac{1}{R}$), so you must take the reciprocal of the gradient to find resistance.

Why might a metal wire stop obeying Ohm's Law at very high currents?

High currents generate significant heat due to electron collisions. This increase in temperature increases the resistance of the wire, breaking the proportionality required by Ohm's Law.

What error occurs if units like $mA$ or $k\Omega$ are used directly in the $V=IR$ formula?

The formula requires standard SI units (Volts, Amperes, Ohms). Using $mA$ without converting to $A$ (multiplying by $10^{-3}$) will result in a resistance value that is off by a factor of 1,000.

Define the 'Ohm' in terms of other SI units.

One Ohm ($\Omega$) is defined as the resistance of a conductor such that a potential difference of one Volt ($V$) results in a current of one Ampere ($A$). Thus, $1\Omega = 1 V/A$.

What is 'Potential Difference' in the context of Ohm's Law?

Potential difference is the work done per unit charge to move electrical energy between two points in a circuit, acting as the 'driving force' for the current.

What does the term 'Directly Proportional' imply for a $V-I$ relationship?

It implies that the ratio of $V$ to $I$ is constant ($R$), and the graph of $V$ against $I$ is a straight line that must pass through the origin $(0,0)$.

If the voltage across a fixed resistor is tripled, what happens to the current?

According to Ohm's Law ($V=IR$), if resistance $R$ is constant, the current $I$ is directly proportional to $V$. Therefore, tripling the voltage will triple the current.

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Physics

9. Electricity

Ohm's Law

Summary

Ohm's Law describes the fundamental linear relationship between voltage, current, and resistance in an electrical circuit. It establishes that for many materials, the current flowing through a conductor is directly proportional to the potential difference applied across it, provided physical conditions like temperature remain constant.

1. Definition & Core Concepts

Ohm's Law states that the current ( $I$ ) flowing through a conductor is directly proportional to the potential difference ( $V$ ) across it, provided the temperature remains constant.
Potential Difference ( $V$ ): Measured in Volts (V), it represents the energy transferred per unit charge between two points in a circuit.
Current ( $I$ ): Measured in Amperes (A), it is the rate of flow of electrical charge through a cross-section of the conductor.
Resistance ( $R$ ): Measured in Ohms ( $\Omega$ ), it is the property of a component that opposes the flow of electric current, defined by the ratio $R = \frac{V}{I}$ .

A basic circuit diagram showing a voltage source, a resistor (R), and the direction of current flow (I).

2. Underlying Principles

3. Methods & Techniques

4. Key Distinctions

5. Exam Strategy & Tips

6. Common Pitfalls & Misconceptions