Heat is associated with (a) kinetic energy of random motion of molecules. (b) kinetic energy of orderly motion of molecules. (c) total kinetic energy of random and orderly motion of molecules. (d) kinetic energy of random motion in some cases and kinetic energy of orderly motion in other.

> [!TLDR] > Heat is the energy transferred due to temperature difference, which arises from the **random** (thermal) motion of molecules — not their orderly bulk motion. The correct answer is **(A)**. The question tests a fundamental distinction in thermodynamics: what exactly is "heat"? Many students confuse heat with the total internal energy of a system, or with any form of kinetic energy. Let's clear that up. Heat is not a property stored in a body. It is energy **in transit** — specifically, energy that flows from a hotter object to a cooler one because of a temperature difference. Temperature itself is a measure of the average random kinetic energy of molecules (their translational, rotational, and vibrational motion). That random motion is what we call **thermal motion**. Now look at the options: 1. **Option (A)** — "kinetic energy of random motion of molecules." This is exactly what heat transfer is linked to. When you heat a gas, you increase the random motion of its molecules. The energy transferred as heat goes into this random kinetic energy (and also potential energy if the substance changes phase, but the core idea is randomness). This is correct. 2. **Option (B)** — "kinetic energy of orderly motion of molecules." Orderly motion means the entire object moving as a whole, like a bullet flying or a rotating fan. That's **bulk kinetic energy**, not heat. A hot stationary object has no orderly motion but plenty of heat content. So this is wrong. 3. **Option (C)** — "total kinetic energy of random and orderly motion." This would be the sum of thermal energy and mechanical energy. Heat is specifically about the random part, not the total. A moving hot object has both, but heat transfer only concerns the random part. So this is wrong. …

Heat is associated with (a) kinetic energy of random motion of molecules. (b) kinetic energy of orderly motion

NCERT Exemplar · Q6

Q. Heat is associated with

(a) kinetic energy of random motion of molecules.

(b) kinetic energy of orderly motion of molecules.

(d) kinetic energy of random motion in some cases and kinetic energy of orderly motion in other.

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Concept understanding — Thermal Radiation Properties

Thermal Radiation Properties

Imagine holding your hand near a hot iron — you feel warmth without touching it. That warmth travels through empty space, not through air or metal. This is thermal radiation: energy emitted by any object purely because it has a temperature.

Unlike conduction (touching) or convection (fluid flow), radiation doesn't need a medium. It's electromagnetic waves — mostly infrared, but at high temperatures, visible light too. A glowing red-hot steel rod is radiating; so is your own body, though you can't see it.

The Core Idea: Every Object Radiates

Every object above absolute zero ( $0\ \text{K}$ ) emits radiation. The amount and type depend on two things:

Temperature — hotter objects radiate more energy, and at shorter wavelengths.
Surface properties — some surfaces are good emitters, others are poor.

This leads to three key properties that describe how a real surface behaves compared to an ideal "perfect" radiator.

1. Emissivity ( $\varepsilon$ )

Intuition: A black matte surface feels hotter in sunlight than a shiny white one. Why? The black surface emits radiation more efficiently.

Definition: Emissivity is the ratio of radiation emitted by a real surface to the radiation emitted by an ideal blackbody at the same temperature.

\varepsilon = \frac{E_{\text{real}}}{E_{\text{blackbody}}}

$\varepsilon = 1$ for a perfect blackbody (ideal emitter).
$0 < \varepsilon < 1$ for all real surfaces.
$\varepsilon$ depends on material, surface finish, and wavelength.

Note

A good emitter is also a good absorber. This is Kirchhoff's law: at thermal equilibrium, $\varepsilon = \alpha$ (absorptivity) for the same wavelength and direction.

2. Absorptivity ( $\alpha$ )

Intuition: A black car roof gets hotter in summer than a white one. It absorbs more sunlight.

Definition: Absorptivity is the fraction of incident radiation that a surface absorbs.

\alpha = \frac{\text{absorbed radiation}}{\text{incident radiation}}

$\alpha = 1$ for a perfect blackbody (absorbs everything).
$\alpha = 0$ for a perfect reflector.
For opaque surfaces: $\alpha + \rho = 1$ , where $\rho$ is reflectivity.

Watch out

Don't confuse absorptivity with emissivity. A shiny metal has low $\alpha$ (reflects most light) and low $\varepsilon$ (emits poorly). A black surface has high $\alpha$ and high $\varepsilon$ .

3. Reflectivity ( $\rho$ ) and Transmissivity ( $\tau$ )

Intuition: A mirror reflects; a glass window transmits; a brick wall does neither.

Definition: For any surface, incident radiation is either absorbed, reflected, or transmitted:

\alpha + \rho + \tau = 1

Reflectivity $\rho$ : fraction reflected.
Transmissivity $\tau$ : fraction transmitted through.
For opaque solids: $\tau = 0$ , so $\alpha + \rho = 1$ .

The Blackbody: The Ideal Reference

A blackbody is a theoretical surface that:

Absorbs all incident radiation ( $\alpha = 1$ ).
Emits the maximum possible radiation at any temperature.
Follows Planck's law, Stefan-Boltzmann law, and Wien's displacement law.

Real surfaces are compared to this ideal. The blackbody is a standard — like comparing a real engine to a Carnot engine.

E_b = \sigma T^4

Putting It Together: A Real Surface

A hot metal plate at $500\ \text{K}$ with $\varepsilon = 0.3$ emits:

$E = 0.3 \times 5.67 \times 10^{-8} \times (500)^4 \approx 1063\ \text{W/m}^2$ …

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Heat is the energy transferred due to temperature difference, which arises from the random (thermal) motion of molecules — not their orderly bulk motion. The correct answer is (A).

The question tests a fundamental distinction in thermodynamics: what exactly is "heat"? Many students confuse heat with the total internal energy of a system, or with any form of kinetic energy. Let's clear that up.

Heat is not a property stored in a body. It is energy in transit — specifically, energy that flows from a hotter object to a cooler one because of a temperature difference. Temperature itself is a measure of the average random kinetic energy of molecules (their translational, rotational, and vibrational motion). That random motion is what we call thermal motion.

Now look at the options:

Option (A) — "kinetic energy of random motion of molecules." This is exactly what heat transfer is linked to. When you heat a gas, you increase the random motion of its molecules. The energy transferred as heat goes into this random kinetic energy (and also potential energy if the substance changes phase, but the core idea is randomness). This is correct.
Option (B) — "kinetic energy of orderly motion of molecules." Orderly motion means the entire object moving as a whole, like a bullet flying or a rotating fan. That's bulk kinetic energy, not heat. A hot stationary object has no orderly motion but plenty of heat content. So this is wrong.
Option (C) — "total kinetic energy of random and orderly motion." This would be the sum of thermal energy and mechanical energy. Heat is specifically about the random part, not the total. A moving hot object has both, but heat transfer only concerns the random part. So this is wrong. …

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