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Effects of heat

Chapter: Heat and Its Transmission

Question 1 of 5 NDA MCQ

 Evaporation from the surface of a given liquid takes place more rapidly when

Detailed Explanation

Correct Answer

✓ Option A is correct. Evaporation occurs more rapidly at higher temperatures and larger surface areas.

📖 Governing Law / Core Concept

Evaporation is governed by the principles of thermodynamics, specifically, the rate of change of phase between liquid and gas states, which is influenced by factors like temperature and surface area.

🧪 Step-by-Step Breakdown

Evaporation involves molecules at the surface of a liquid gaining enough energy to escape into the gas phase. Key factors influencing this process include:

  • Temperature: Higher temperatures increase kinetic energy, allowing more molecules to overcome intermolecular forces.
  • Surface Area: More surface area exposes more molecules to the air, enhancing evaporation.

Therefore, the rate of evaporation can be expressed as:

\[ Rate \propto A \cdot e^{-\frac{E}{kT}} \]

where:

  • Rate: The amount of liquid evaporated per unit time.
  • A: Surface area of the liquid.
  • E: Activation energy for the phase change.
  • k: Boltzmann's constant.
  • T: Absolute temperature (Kelvin).

🔍 Option Analysis

  • Option B: Low temperature reduces kinetic energy, decreasing evaporation.
  • Option C: Low temperature and small surface area hinder evaporation.
  • Option D: Higher temperature benefits evaporation; small surface area does not.

⚡ Mnemonic / Speed-Run

Remember: High temperature + Large area = Rapid evaporation (H + A = R).

6. Visual Suggestion

6. Visual Suggestion

A schematic diagram illustrating increasing evaporation with large surface area and high temperature, perhaps with arrows indicating increased molecular energy on the surface.

📖 Factual Verification & Reference

📖 Factual Verification & Reference:

Verified against NCERT Class XI Physics, Chapter 12 (Thermodynamics).

Question 2 of 5 NDA MCQ

Water is heated with a coil of resistance R connected to domestic supply. The rise of temperature of water will depend on

(1) Supply voltage.

(2) Current passing through the coil.

(3) Time for which voltage is supplied. Select the correct answer from among the following : 

Detailed Explanation

Correct Answer

✓ Option (a) is correct. The temperature rise depends on all three factors: supply voltage, current through the coil, and time.

📖 Governing Law / Core Concept

The temperature rise in water due to the heating coil can be described by the formula:

\[ Q = V \cdot I \cdot t \text{, where } Q \text{ is heat energy (in J), } V \text{ is voltage (in V), } I \text{ is current (in A), and } t \text{ is time (in s).} \]

The heat energy transferred to the water leads to a temperature increase.

🧪 Step-by-Step Breakdown

The total heat energy supplied is given by:

\[ Q = m \cdot c \cdot \Delta T \text{ (where } m \text{ is mass of water, } c \text{ is specific heat capacity, and } \Delta T \text{ is temperature rise).} \]

From Ohm's Law, we have:

\[ V = I \cdot R \]

Substituting for \(I\) gives:

\[ Q = (V/R) \cdot V \cdot t = \frac{V^2 \cdot t}{R} \]

This shows temperature increase depends on voltage \(V\), current \(I\), and time \(t\).

🔍 Option Analysis

Option (b): Incorrect, only mentions voltage and current but omits time.

Option (c): Incorrect, only addresses voltage.

Option (d): Incorrect, combines only current and time but ignores voltage.

⚡ Mnemonic / Speed-Run

Remember: "VIT" = Voltage, Current, Time - all are crucial for heating water!

📊 Visual Suggestion

📊 Visual Suggestion

A flowchart displaying the relationship between heat energy (Q), voltage (V), current (I), resistance (R), and time (t) in heating processes. Include an illustration of a heating coil in a container of water to visually represent energy transfer.

📖 Factual Verification & Reference

📖 Factual Verification & Reference:

Verified against NCERT Class XII Physics, Chapter 12 (Thermodynamics).

Question 3 of 5 NDA MCQ

10 g of ice at -10 °C is is mixed with 10 g of water at 0 °C. The amount of heat required to raise the temperature of mixture to 10 °C is

Detailed Explanation

Solution

Option (c) is correct.

Explanation:

\[ Q = m \cdot c \cdot \Delta T \]
\[ Q_{ice} = m_{ice} \cdot c_{ice} \cdot (0 - (-10)) + m_{ice} \cdot L_f + m_{water} \cdot c_{water} \cdot (10 - 0) \]
\[ Q_{ice} = 10 \cdot 0.5 \cdot (10) + 10 \cdot 80 + 10 \cdot 1 \cdot (10) \]
\[ = 50 + 800 + 100 \]
\[ = 950 \text{ cal} \]
\[ Q_{total} = Q_{ice} + Q_{water} \]
\[ Q_{total} = 950 + 100 = 1050 \text{ cal} \]
📌 Hints / Properties Used:
  • Specific heat of ice: \(c_{ice} = 0.5 \text{ cal/g°C}\)
  • Latent heat of fusion: \(L_f = 80 \text{ cal/g}\)
  • Specific heat of water: \(c_{water} = 1 \text{ cal/g°C}\)
📊 Visual Diagram Suggestion:

A schematic showing the phase change of ice to water, including temperature changes, highlighting energy transfer at each step from ice at -10°C to ice at 0°C, water heating from 0°C to 10°C, and heat values involved.

📖 Factual Verification & Reference:

Verified against NCERT Class XII Physics, Chapter 11 (Thermodynamics).

Question 4 of 5 NDA MCQ

The temperature at which “vapour pressure of the liquid in an open vessel becomes equal tothe atmospheric pressure” is called

Detailed Explanation

Correct Answer

✓ Option B is correct. The boiling point is the temperature at which the vapor pressure of a liquid equals atmospheric pressure.

📖 Governing Law / Core Concept

The temperature at which the vapor pressure of a liquid equals the atmospheric pressure is defined as the boiling point.

🧪 Step-by-Step Breakdown

At boiling point:

\[ P_{vapor} = P_{atmospheric} \]

Where:

  • Pvapor = Vapor pressure of the liquid (measured in pascals, Pa)
  • Patmospheric = Atmospheric pressure (assumed standard value = 101.3 kPa)

When \( P_{vapor} \) reaches \( P_{atmospheric} \), bubbles of vapor form within the liquid, resulting in boiling.

🔍 Option Analysis

  • A: Melting point - The temperature at which a solid becomes a liquid.
  • C: Liquid point - Not a standard term in thermodynamics.
  • D: None of the above - This is incorrect as option B accurately identifies the boiling point.

⚡ Mnemonic / Speed-Run

To remember boiling point concepts: "Bubbles Boil at Boiling Point".

6. Visual Suggestion

6. Visual Suggestion

A diagram illustrating a boiling liquid in a pot, showing temperature arrows, vapor pressure, and atmospheric pressure lines, helping visualize how boiling occurs when vapor pressure equals atmospheric pressure.

📖 Factual Verification & Reference

📖 Factual Verification & Reference:

Verified against NCERT Class 11 Physics, Chapter 1 (Thermodynamics).

Question 5 of 5 NDA MCQ

A system that does NOT allow exchange of heat with its surrounding is called

Detailed Explanation
✓

1. Correct Answer

Option A is correct. A system that does not allow the exchange of heat with its surroundings is called an adiabatic system.

📖 Core Concept

An adiabatic system isolates itself thermally from its environment, meaning that no heat is transferred in or out, thus enabling internal energy changes from work done directly.

💡 3. Explanation

An adiabatic process is defined as one where the heat transfer (\(Q\)) equals zero. Therefore:
\[ Q = 0 \]
It follows that any work done on or by the system alters its internal energy without heat exchange with the environment. The first law of thermodynamics expresses this as:
\[ \Delta U = W \]
Thus, for an adiabatic system: - If the system does work on the surroundings, it loses internal energy. - If work is done on the system, it gains internal energy, effectively illustrating the principles of energy conservation in thermodynamic processes.

❌ 4. Option Analysis

  • •Option B: A non-adiabatic system allows heat exchange with its surroundings, which contradicts the definition of an adiabatic system.
  • •Option C: An equilibrium system describes a state where all processes occur at equal rates, and does not specifically entail heat exchange characteristics.
  • •Option D: A non-equilibrium system is characterized by ongoing changes, and it does not directly correlate with heat exchange properties like those defined in adiabatic systems.
⚡

NDA Speed-Run / Mnemonic Shortcut

Remember the mnemonic "Adi-bag" where "Adi" stands for Adiabatic, indicating a closed system with no heat exchange.

🎯

5. Key Takeaways

An adiabatic system describes a process with no heat exchange, focusing purely on work-related changes in internal energy.

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