Q.Distinguish between Ohmic and non-ohmic substances; explain with the help of example.

Ohm's law requires physical conditions (especially temperature) to stay constant.

Ohm's law, $V = IR$ with R constant, is obeyed only when the conductor's physical state — chiefly its temperature — remains unchanged. If temperature changes, the resistivity (and hence R) changes, so the current is no longer proportional to the applied voltage and the V–I graph departs from a straight line.

Thus Ohm's law does not apply when the temperature changes.

A p-n junction diode is a non-ohmic device — its current does not vary linearly with the applied voltage.

Ohm's law states that $V \propto I$ (i.e. $V/I$ = constant = resistance) for a conductor kept at constant physical conditions. A p-n junction diode does not obey this: in forward bias, the current stays negligibly small until a threshold (knee) voltage is crossed, after which it rises very steeply and non-linearly with voltage; in reverse bias, the current stays almost constant (a small saturation current) over a wide range of reverse voltage. Since the $V$–$I$ graph of a diode is not a straight line through the origin, it is a non-ohmic device.

Ohmic conductor → straight line through origin ($V\propto I$); non-ohmic conductor → curved $V$–$I$ graph.

Concept. A conductor is ohmic if it obeys Ohm's law, $V=IR$ with $R$ constant, so $V$ is directly proportional to $I$. A non-ohmic conductor (e.g. a semiconductor diode, a filament lamp, a gas discharge tube) does not obey Ohm's law — $R$ changes with the applied voltage or current.

Graph (description).

• Ohmic: a straight line passing through the origin; its constant slope $\dfrac{V}{I}=R$.
• Non-ohmic: a curve (not a straight line); the slope $\dfrac{dV}{dI}$ varies from point to point.

 V |        /  (ohmic: straight line)
   |      /
   |    /      __--  (non-ohmic: curve)
   |  /   __--
   |/__--
   +-----------------> I

Resistance R = tan θ when the V–I line makes angle θ with the I-axis.

Here the y-axis represents current I and the x-axis represents voltage V. The graph is a straight line making angle θ with the y-axis.

The slope of the line measured from the x-axis is (90° − θ), so

$\text{slope} = \frac{dI}{dV} = \tan(90^\circ-\theta) = \cot\theta$

Since slope dI/dV = 1/R (conductance),

$R = \frac{1}{\cot\theta} = \tan\theta$

Ohm's law (V proportional to I, i.e. a constant V/I ratio) is an empirical rule that holds for metallic conductors at constant temperature, but many important conducting devices are non-ohmic and do not obey it.

Ohm's law states that the current through a conductor is directly proportional to the potential difference across it, provided physical conditions like temperature remain constant - giving a constant resistance R = V/I.

This holds reasonably well for pure metallic conductors (like a nichrome or copper wire) at constant temperature. However, many devices show a non-linear V-I relationship and are therefore called non-ohmic: examples include semiconductor diodes (current flows easily only one way), transistors, electrolytes/gas discharge tubes, and even a metal filament bulb once self-heating significantly changes its resistance with current.

So Ohm's law is a special case, not a universal law of nature - it applies only to certain (ohmic) conductors under limited conditions.

Ohm's law states V = IR at constant temperature, so a V-I graph is a straight line through the origin.

Ohm's law states that for a conductor at constant physical conditions (especially constant temperature), the potential difference V across it is directly proportional to the current I flowing through it:

$V = IR$

where R (the resistance) is constant. Plotting V (y-axis) against I (x-axis) therefore gives a straight line passing through the origin, whose slope equals the resistance R. (Equivalently, plotting I against V gives a straight line through the origin with slope 1/R.)

Apply Ohm's law, I = V/R.

Given: V = 12 V, R = 4 kΩ = 4 × 10³ Ω.

By Ohm's law:

I = V/R = 12 / (4 × 10³) = 3 × 10⁻³ A

Resistance R = V/I, so its SI unit is volt per ampere, called the ohm (Ω).

Resistance is defined through Ohm's law: $R = \dfrac{V}{I}$, where V is the potential difference across a conductor (in volt) and I is the current through it (in ampere).

So the unit of resistance is volt/ampere, which is given the special name ohm, symbol $\Omega$. A conductor has a resistance of 1 ohm if a potential difference of 1 volt across it drives a current of 1 ampere through it.

At constant temperature, V ∝ I, so V = IR.

An ohmic resistance is one whose V-I graph is a straight line through the origin; copper wire (a metal) is ohmic.

Ohm's law states that for a conductor at constant temperature, $V = IR$, i.e. the V-I graph is a straight line through the origin with slope R. Metals such as copper obey this law over a wide range of currents because their resistance depends only on temperature, not on the direction or magnitude of current.

A transistor, a light emitting diode, and a junction diode are all non-ohmic (non-linear) devices — their V-I curves are not straight lines, and current does not flow equally in both directions (in the diode/LED case) or the relation is far from linear (transistor).

By Ohm's law, $I = V/R = 4\text{ V}/2000\,\Omega = 2\text{ mA}$.

Given $V = 4\ \text{V}$, $R = 2\ \text{k}\Omega = 2000\ \Omega$.

$I = \dfrac{V}{R} = \dfrac{4}{2000} = 2\times10^{-3}\ \text{A} = 2\ \text{mA}$

By Ohm's law, $I = V/R = 2\text{ V}/1000\ \Omega = 2\ \text{mA}$.

Given $V = 2$ V and $R = 1\ \text{k}\Omega = 1000\ \Omega$. By Ohm's law,

$I = \dfrac{V}{R} = \dfrac{2}{1000} = 2\times10^{-3}\ \text{A} = 2\ \text{mA}$