Heat, Work and Internal Energy
Class 11 Physics chapter covering heat transfer, thermodynamic work, internal energy, sign convention, mechanical equivalent of heat, numericals and PYQs.
Thermodynamics becomes easy when heat, work and internal energy are understood as three different ways of discussing energy. Heat is energy transfer because of temperature difference. Work is energy transfer because of a macroscopic force acting through displacement, especially a moving boundary in gas problems. Internal energy is the energy stored inside the system at molecular level. This chapter connects these ideas with sign convention, P-V work, Joule's mechanical equivalent of heat, conduction, convection, radiation and exam-level problem solving.
Introduction: The Energy Language of Thermodynamics
Heat, work and internal energy are not three names for the same quantity. They are three connected ideas that describe how energy enters a system, leaves a system, and remains stored inside a system.
In mechanics, energy transfer is often visible: a force pulls a block, a spring stretches, a ball rises, or a body gains speed. In thermodynamics, energy transfer may happen without visible motion of the whole body. A cup of tea cools, a gas expands, steam pushes a piston, a metal rod conducts heat, and a thermometer reading changes. These situations are handled by separating the selected part of the universe as the system and then tracking energy crossing its boundary.
Heat and work are boundary phenomena. A system does not contain heat in the same way that it contains internal energy. A system receives or loses heat. Similarly, work is done by the system or on the system during an interaction. Internal energy, on the other hand, is a property of the system. It belongs to the state of the system and depends on its microscopic molecular condition.
Heat
Energy transferred due to temperature difference. Heat flows naturally from a body at higher temperature to a body at lower temperature.
Work
Energy transferred due to macroscopic force and displacement. In thermodynamics, expansion and compression work are extremely important.
Internal Energy
Energy stored in the microscopic motion and interaction of molecules. For an ideal gas, internal energy depends only on temperature.
Heat
Heat is energy transferred between a system and surroundings because of temperature difference.
If a hot iron ball is placed in cold water, energy flows from the hot iron to the cold water. This energy in transit is called heat. The flow continues until thermal equilibrium is reached. Heat is therefore not a substance and not a fluid stored inside a body. Modern physics treats heat as a mode of energy transfer.
Older language sometimes says "a body contains heat", but in precise thermodynamics a body contains internal energy. Heat exists only during transfer across a boundary. Once energy enters the body, it becomes part of internal energy or may be used to do work.
Nature and Physical Meaning of Heat
- Heat transfer requires temperature difference.
- Heat flows naturally from higher temperature to lower temperature.
- Heat is a scalar quantity, but heat flow has a direction from hot to cold.
- Heat is not a state function; it depends on the path of the process.
- The symbol for heat is usually Q.
- SI unit of heat is joule, written as J.
Units of Heat and Conversions
SI unit of heat and all forms of energy. 1 J is the work done by 1 N force through 1 m displacement.
1 calorie is the heat required to raise the temperature of 1 g water by 1 °C near room temperature.
1 kcal = 1000 cal. Food energy is often expressed in kilocalories.
1 cal = 4.186 J and 1 kcal = 4186 J.
Heat Capacity
Heat capacity is the amount of heat required to raise the temperature of a body by 1 K or 1 °C. It depends on the mass and material of the body.
A bucket of water has greater heat capacity than a cup of water because the bucket contains more mass. A large heat capacity means the body needs more heat for the same temperature rise.
Specific Heat Capacity
Specific heat capacity is the heat required to raise the temperature of unit mass of a substance by 1 K or 1 °C. It is a property of the material, not the total body.
Water has high specific heat capacity, so it warms slowly and cools slowly. This is why coastal regions have moderate climate and why water is useful as a coolant.
Latent Heat
Latent heat is heat absorbed or released during change of state at constant temperature. During melting, boiling, condensation or freezing, the supplied or released energy changes molecular arrangement rather than changing temperature.
When ice at 0 °C melts into water at 0 °C, heat is absorbed without temperature rise. This heat breaks the rigid molecular arrangement of ice. Similarly, boiling water at 100 °C absorbs heat to become steam at 100 °C, and that energy separates molecules further apart.
Heat Flow
Work in Thermodynamics
Thermodynamic work is energy transfer that occurs when the system boundary moves under pressure or when a macroscopic force acts through displacement.
The most important form in Class 11 thermodynamics is pressure-volume work. Consider gas enclosed in a cylinder with a movable piston. When the gas expands, it pushes the piston outward. The gas does work on the surroundings. When the surroundings compress the gas, work is done on the gas.
Expansion Work
During expansion, volume increases. The gas pushes the piston outward. In the commonly used chemistry convention, work done by the system may be negative; in the physics convention used here, work done by the system is taken positive. Always check the convention used in the question.
Compression Work
During compression, volume decreases. The piston moves inward. The surroundings do work on the gas. For work done by the gas, ΔV is negative; for work done on the gas, the magnitude is positive.
Work Done from P-V Graph
On a pressure-volume graph, the work done by a gas is equal to the area under the P-V curve between the initial and final volumes. This is a favorite JEE and NEET idea because students often try to use only W = PΔV even when pressure is not constant.
Exam Perspective
- If pressure is constant, use rectangle area: W = PΔV.
- If pressure varies linearly with volume, use trapezium area.
- If a cyclic process is shown, net work equals area enclosed by the loop.
- Clockwise P-V cycle gives positive work by system; anticlockwise cycle gives negative work by system.
Expansion Work
Compression Work
P-V Graph
Internal Energy
Internal energy is the total microscopic energy possessed by the molecules of a system due to their random motion and mutual interactions.
A gas in a container may be at rest as a whole, but its molecules are moving randomly at high speeds. They translate, rotate, vibrate and collide. Molecules may also exert attractive or repulsive forces on each other. The energy associated with all these microscopic motions and interactions is internal energy.
Microscopic Meaning
- Molecular kinetic energy comes from translational, rotational and vibrational motion.
- Molecular potential energy comes from intermolecular forces and separation.
- Internal energy does not include kinetic energy of the whole system moving as one object.
- Internal energy does not include gravitational potential energy of the entire system due to height.
Internal Energy of Ideal Gas
For an ideal gas, intermolecular forces are neglected except during collisions. Therefore molecular potential energy is taken as zero or constant. The internal energy of an ideal gas is purely kinetic and depends only on temperature.
This idea is crucial in isothermal processes. During isothermal expansion of an ideal gas, temperature remains constant, so internal energy change is zero, even though heat may be supplied and work may be done.
Internal Energy Representation
Sign Convention
Sign convention tells us whether heat and work are counted as positive or negative in a thermodynamic equation.
Students lose marks not because the concept is difficult, but because they mix conventions. Some books use work done by the system as positive. Some chemistry texts use work done on the system as positive. The safest approach is to read the question carefully and state the convention before substitution.
Physics Convention Used in These Notes
| Process | Meaning | Sign | Memory Tip |
|---|---|---|---|
| Heat supplied to system | Energy enters as heat | Q positive | System gains heat |
| Heat rejected by system | Energy leaves as heat | Q negative | System loses heat |
| Work done by system | Gas expands and pushes surroundings | W positive | System gives work output |
| Work done on system | Gas is compressed by surroundings | W negative for work by system | System receives work input |
Difficult Conceptual Examples
Expansion with Cooling
A gas may expand and do work while losing heat. Work sign and heat sign must be assigned separately. Expansion does not automatically mean heat is supplied.
Compression with Heat Loss
When a gas is compressed, work is done on it. But if the cylinder is in contact with ice, heat may leave at the same time. Both energy transfers can occur together.
Isothermal Ideal Gas
Temperature constant means ΔU = 0 for ideal gas. If gas expands isothermally, heat supplied equals work done by gas.
Adiabatic Process
No heat exchange means Q = 0. Internal energy may still change because work can be done by or on the gas.
Mechanical Equivalent of Heat
The mechanical equivalent of heat establishes that heat and mechanical work are different forms of energy and can be converted into each other.
Before the energy concept became clear, heat was sometimes imagined as a material-like fluid. Joule's experiments showed that mechanical work can produce a definite amount of heat. In his paddle-wheel experiment, falling weights rotated paddles inside water. The paddles stirred water, mechanical energy was dissipated, and the temperature of water increased.
If m kg of water is stirred and its temperature rises by ΔT, the heat gained is Q = mcΔT. If falling weights lose gravitational potential energy mgh, that mechanical energy is converted into heat, after accounting for losses. Joule's careful measurements led to the conversion factor between calorie and joule.
Joule's Paddle Wheel Experiment
Energy Transfer: Conduction, Convection and Radiation
Heat transfer takes place mainly by conduction, convection and radiation. The correct mechanism depends on the medium and physical situation.
Conduction
Conduction is heat transfer through a material without bulk motion of the material. It is common in solids. When one end of a metal rod is heated, particles at the hot end vibrate more vigorously and transfer energy to neighbouring particles. Metals are good conductors because free electrons also carry energy.
Convection
Convection is heat transfer by bulk motion of a fluid. When water is heated from below, warmer water becomes less dense and rises, while cooler water sinks. This circulation transfers energy. Sea breeze, land breeze and boiling water currents are examples.
Radiation
Radiation is heat transfer by electromagnetic waves. It does not require a material medium. Heat from the Sun reaches Earth through space by radiation. Black and dull surfaces are good absorbers and emitters of thermal radiation, while shiny surfaces are poor absorbers and emitters.
Conduction
Convection
Radiation
Conceptual Traps
- Radiation can occur through vacuum; conduction and convection need material medium.
- Convection is not possible in solids because bulk motion is not possible.
- A shiny surface reduces heat loss by radiation, not by completely stopping conduction.
- Heat transfer direction depends on temperature difference, not on total internal energy.
Formula Summary and Comparison Tables
Heat required for temperature change without phase change.
Heat required or released during phase change at constant temperature.
Work done by gas at constant pressure.
First Law form when W means work done by the system.
| Quantity | Symbol | Unit | Nature | Exam Note |
|---|---|---|---|---|
| Heat | Q | J | Energy transfer due to temperature difference | Path dependent, not stored as heat |
| Work | W | J | Energy transfer due to force and displacement | Area under P-V curve for gas work |
| Internal energy | U | J | Microscopic energy of system | State function; for ideal gas depends only on T |
| Specific heat | c | J kg-1 K-1 | Material property | Use with mass and temperature change |
| Latent heat | L | J kg-1 | Phase-change energy per unit mass | Temperature remains constant during phase change |
Deep Coaching Notes: How to Think Like an Examiner
Most exam questions on heat, work and internal energy do not test memorisation alone. They test whether you can identify the system, recognise the path, assign signs correctly and connect macroscopic observations with microscopic energy changes.
1. Heat Is Not Automatically Temperature Rise
A very common mistake is to assume that whenever heat is supplied, temperature must increase. This is true only when the heat supplied increases the average kinetic energy of molecules. During phase change, heat is used to change molecular arrangement and separation. Ice at 0 °C can absorb heat and remain at 0 °C while melting. Water at 100 °C can absorb heat and remain at 100 °C while boiling. Therefore, the first question is: is there a temperature change or a phase change?
For temperature change, use Q = mcΔT. For phase change, use Q = mL. In multi-step heating problems, split the process into stages. For example, ice at -10 °C becoming steam at 100 °C involves heating ice, melting ice, heating water and vaporising water. Students often lose marks by applying only one formula to the whole process.
2. Work Depends on the Path, Not Only the End Points
Suppose a gas goes from volume V1 to V2. You cannot find work unless you know how pressure varies during the process. If pressure is constant, W = PΔV. If pressure changes linearly, the work is area of a trapezium on the P-V graph. If pressure follows a curve, the exact work is the area under that curve. This is why P-V graphs are powerful: they show the path, not just the initial and final states.
Internal energy is different. It is a state function. If the initial and final states are fixed, change in internal energy is fixed, regardless of path. Heat and work adjust according to the path so that energy conservation remains valid.
3. Internal Energy Is Not the Same as Temperature, But They Are Related
Temperature is closely related to average molecular kinetic energy. Internal energy is total microscopic energy. A large vessel of water at 30 °C may have far greater internal energy than a small hot nail at 200 °C because the vessel contains many more molecules. But heat flows from the nail to the water because heat flow direction depends on temperature, not total internal energy.
For an ideal gas, internal energy depends only on temperature because molecular potential energy is neglected. For real substances, internal energy may also depend on molecular separations and interactions. At Class 11 level, the key exam idea is: ideal gas isothermal process means ΔU = 0.
4. Sign Convention Is a Language
Sign convention is not a new law of physics; it is a bookkeeping language. If energy enters the system as heat, we write Q positive. If heat leaves, Q is negative. If the system expands and does work on surroundings, W is positive in the physics convention used here. If surroundings compress the system, W for the system is negative. Once the convention is fixed, use it consistently from first line to final answer.
A strong student writes a short sign statement before solving: "Using W positive for work done by system." This habit prevents confusion, especially when a question includes both compression and heat rejection.
5. Area Under P-V Graph: Visual Method
On a P-V graph, pressure is plotted on the vertical axis and volume on the horizontal axis. Work done by a gas is the area under the curve. If the gas expands, the process moves to the right and work by the gas is positive. If the gas is compressed, the process moves to the left and work by the gas is negative. In a cycle, the enclosed area represents net work. Clockwise cycles represent positive net work by the system; anticlockwise cycles represent negative net work by the system.
In JEE-style problems, the graph may be a rectangle, triangle, trapezium or curved path. Do not rush into W = PΔV. First ask: is P constant? If not, calculate graph area.
6. Energy Transfer Mechanisms in Daily Life
Conduction dominates in solids, especially metals. A steel spoon in hot tea becomes warm because energy passes from molecule to molecule and through free electrons. Convection dominates in fluids when heated fluid moves bodily. Radiation can travel through vacuum and explains solar heating. A thermos flask reduces all three: shiny walls reduce radiation, vacuum reduces conduction and convection, and the stopper reduces heat exchange through air movement.
Memory Tip for Heat
Heat needs temperature difference. No temperature difference means no net heat flow, even if the bodies have different sizes or different internal energies.
Memory Tip for Work
For gas work, watch the boundary. If the piston moves outward, gas does work. If it moves inward, work is done on gas.
Memory Tip for Internal Energy
For ideal gas, temperature is the key. Same temperature means same internal energy for the same amount and type of ideal gas.
Common Mistakes and Conceptual Traps
Mistake 1: Saying Heat Is Stored
Correct language: energy is stored as internal energy. Heat is energy crossing the boundary because of temperature difference.
Mistake 2: Ignoring Units
Litres must be converted to cubic metres before using W = PΔV. 1 L = 10-3 m3. Calories must be converted to joules if SI units are required.
Mistake 3: Using Q = mcΔT During Melting
During phase change at constant temperature, use Q = mL. Use Q = mcΔT only when temperature changes within the same phase.
Mistake 4: Treating Work as Always Positive
Expansion and compression have opposite signs under a fixed convention. A negative value often carries important physical meaning.
Mistake 5: Forgetting Graph Area
If pressure varies, the work is not simply final pressure times volume change. Work is area under the actual path.
Mistake 6: Confusing Adiabatic and Isothermal
Adiabatic means Q = 0. Isothermal means ΔT = 0. For ideal gas, isothermal means ΔU = 0, but heat exchange may occur.
High-Yield Conceptual Comparisons
| Pair | First Concept | Second Concept | Trap |
|---|---|---|---|
| Heat vs Internal Energy | Heat is energy transfer across boundary. | Internal energy is energy stored microscopically. | Do not say heat is contained in a body. |
| Temperature vs Internal Energy | Temperature relates to average molecular kinetic energy. | Internal energy is total microscopic energy. | A larger cooler body can have more internal energy than a smaller hotter body. |
| Heat Capacity vs Specific Heat | Heat capacity belongs to a particular body. | Specific heat belongs to material per unit mass. | Heat capacity changes with mass; specific heat does not for the same material. |
| Latent Heat vs Specific Heat | Latent heat is for phase change. | Specific heat is for temperature change. | Do not mix Q = mL and Q = mcΔT in the same step. |
| Conduction vs Convection | Conduction has no bulk motion. | Convection has bulk fluid motion. | Convection cannot occur in solids. |
| Isothermal vs Adiabatic | Isothermal means constant temperature. | Adiabatic means no heat exchange. | They are not the same condition. |
Process-Wise Exam Guide
Different thermodynamic processes simplify the first law in different ways. Recognising the process quickly is one of the biggest scoring skills in thermodynamics.
| Process | Condition | First Law Result | Physical Meaning | Exam Clue |
|---|---|---|---|---|
| Isochoric | Volume constant | W = 0, so ΔU = Q | Heat supplied changes internal energy only. | Rigid container, fixed volume, no piston motion. |
| Isobaric | Pressure constant | W = PΔV | Heat partly changes internal energy and partly does work. | Movable piston under constant external pressure. |
| Isothermal ideal gas | Temperature constant | ΔU = 0, so Q = W | Heat supplied becomes work output during expansion. | Slow process with thermal contact, ideal gas. |
| Adiabatic | Q = 0 | ΔU = -W | Work changes internal energy directly. | Insulated cylinder, rapid expansion/compression. |
| Cyclic | Final state equals initial state | ΔU = 0 over one cycle | Net heat equals net work over complete cycle. | Closed loop on P-V graph. |
How to Approach a Numerical
- Step 1: Write the system. Example: gas inside cylinder, water in calorimeter, or metal block.
- Step 2: Identify whether heat is supplied or rejected. Assign sign to Q.
- Step 3: Identify whether the system expands or compresses. Assign sign to W.
- Step 4: Check whether temperature changes, phase changes, or graph area is given.
- Step 5: Use the correct formula and convert all units to SI before substitution.
- Step 6: Interpret the final sign. A negative answer is often meaningful, not automatically wrong.
15 Solved Numericals
Each solution follows the exam pattern: question, given, formula, solution and final answer.
Show solution
Given: m = 2 kg, c = 4200 J kg-1 K-1, ΔT = 10 K.
Formula: Q = mcΔT.
Solution: Q = 2 × 4200 × 10 = 84000 J.
Final Answer: 8.4 × 104 J.
Show solution
Given: Heat = 500 cal.
Formula: 1 cal = 4.186 J.
Solution: Q = 500 × 4.186 = 2093 J.
Final Answer: 2093 J.
Show solution
Given: P = 2 × 105 Pa, V1 = 0.01 m3, V2 = 0.04 m3.
Formula: W = P(V2 - V1).
Solution: ΔV = 0.03 m3. W = 2 × 105 × 0.03 = 6000 J.
Final Answer: 6000 J, positive for expansion.
Show solution
Given: Q = +700 J, W = +250 J.
Formula: ΔU = Q - W.
Solution: ΔU = 700 - 250 = 450 J.
Final Answer: Internal energy increases by 450 J.
Show solution
Given: m = 0.5 kg, L = 3.36 × 105 J kg-1.
Formula: Q = mL.
Solution: Q = 0.5 × 3.36 × 105 = 1.68 × 105 J.
Final Answer: 1.68 × 105 J.
Show solution
Given: V1 = 5 L = 5 × 10-3 m3, V2 = 2 × 10-3 m3, P = 1.5 × 105 Pa.
Formula: W = PΔV.
Solution: ΔV = -3 × 10-3 m3. W = 1.5 × 105 × (-3 × 10-3) = -450 J.
Final Answer: Work done by gas = -450 J; work done on gas = 450 J.
Show solution
Given: Heat rejected means Q = -300 J. Work done on system means W by system = -500 J.
Formula: ΔU = Q - W.
Solution: ΔU = -300 - (-500) = +200 J.
Final Answer: Internal energy increases by 200 J.
Show solution
Given: m = 0.1 kg, c = 390 J kg-1 K-1, ΔT = 40 - 90 = -50 K.
Formula: Q = mcΔT.
Solution: Q = 0.1 × 390 × (-50) = -1950 J.
Final Answer: Heat lost = 1950 J.
Show solution
Given: Ideal gas, isothermal process, Q = 1200 J.
Concept: For ideal gas, internal energy depends only on temperature. Isothermal means ΔT = 0, so ΔU = 0.
Formula: ΔU = Q - W.
Solution: 0 = 1200 - W, so W = 1200 J.
Final Answer: W = 1200 J and ΔU = 0.
Show solution
Given: Adiabatic process, W = 800 J.
Concept: Adiabatic means Q = 0.
Formula: ΔU = Q - W.
Solution: ΔU = 0 - 800 = -800 J.
Final Answer: Q = 0, ΔU = -800 J.
Show solution
Given: P1 = 2 × 105 Pa, P2 = 5 × 105 Pa, ΔV = 3 L = 3 × 10-3 m3.
Formula: Work = area under P-V graph = average pressure × volume change for linear variation.
Solution: Pavg = (2 + 5)×105/2 = 3.5 × 105 Pa. W = 3.5 × 105 × 3 × 10-3 = 1050 J.
Final Answer: 1050 J.
Show solution
Given: Clockwise cycle area = 250 J.
Concept: Clockwise cycle gives positive net work by system. Internal energy is a state function and returns to same value after a cycle.
Solution: Wnet = +250 J, ΔUcycle = 0.
Final Answer: Net work = 250 J; change in internal energy = 0.
Show solution
Given: Q = 1000 J, ΔU = 350 J.
Formula: ΔU = Q - W.
Solution: 350 = 1000 - W, so W = 650 J.
Final Answer: Work done by gas = 650 J.
Show solution
Given: m = 2 kg, c = 2500 J kg-1 K-1, ΔT = 60 K, vaporised mass = 0.4 kg, L = 2 × 105 J kg-1.
Formula: Q = mcΔT + mL for vaporised part.
Solution: Heating heat = 2 × 2500 × 60 = 300000 J. Latent heat = 0.4 × 2 × 105 = 80000 J. Total = 380000 J.
Final Answer: 3.8 × 105 J.
Show solution
Given: Q = -150 J means heat is rejected. W = -400 J means work is done on the system.
Formula: ΔU = Q - W.
Solution: ΔU = -150 - (-400) = +250 J.
Interpretation: Even though heat leaves, more energy enters as work, so internal energy increases.
Final Answer: ΔU = +250 J.
30 PYQs and Exam-Style Questions
These questions include CBSE, NEET, JEE, IB, ICSE, IGCSE, A-Level, assertion-reason, case-study, conceptual and difficult styles. Each answer is inside a toggle for self-testing.
Answer and explanation
Heat is energy transferred due to temperature difference. It is not a state function because the heat exchanged depends on the path of the process, not only on initial and final states.
Answer: Heat is path dependent energy transfer.
Answer and explanation
Internal energy is the microscopic energy of molecules in a system. It consists of molecular kinetic energy and molecular potential energy.
Answer: Molecular kinetic energy plus molecular potential energy.
Answer and explanation
The mechanical equivalent of heat gives the conversion between calorie and joule.
Answer: 1 calorie = 4.186 joule.
Answer and explanation
If no phase change occurs, heat supplied is proportional to mass, specific heat and temperature rise.
Answer: Q = mcΔT.
Answer and explanation
Internal energy depends only on the state of the system. Heat and work depend on path.
Answer: (C) Internal energy.
Answer and explanation
For an ideal gas, intermolecular potential energy is neglected and internal energy is molecular kinetic energy, which depends on temperature.
Answer: (C) Temperature only.
Answer and explanation
Adiabatic means no heat exchange between system and surroundings.
Answer: (B) Q.
Answer and explanation
For a gas process, work done by gas is integral of P dV, which is area under the P-V curve.
Answer: (B) Work.
Answer and explanation
On P-V graph, constant pressure process is a horizontal line. Area under the line is a rectangle of height P and width ΔV.
Answer: Rectangular area PΔV.
Answer and explanation
Using ΔU = Q - W, if ΔU = 0 then Q = W. Therefore W is positive when Q is positive.
Answer: W is positive; heat supplied becomes work output.
Answer and explanation
Internal energy is a state function. In a cycle, final state equals initial state.
Answer: ΔU = 0.
Answer and explanation
If work done on the system is greater than heat lost, net energy stored internally can increase. For example, compression with cooling may still increase temperature and internal energy.
Answer: Work input can exceed heat rejected.
Answer and explanation
Adiabatic compression means Q = 0 and work is done on the gas. Internal energy increases, so temperature rises for an ideal gas.
Answer: Internal energy and temperature increase.
Answer and explanation
Heat is energy transfer due to temperature difference, measured in joules. Temperature measures thermal state and average molecular kinetic energy, measured in kelvin or degree Celsius.
Answer: Heat is transfer; temperature is state measure.
Answer and explanation
Water has high specific heat capacity, so it absorbs or releases large heat with small temperature change.
Answer: Due to high specific heat capacity of water.
Answer and explanation
Latent heat is heat absorbed or released during change of state at constant temperature.
Answer: Q = mL during phase change.
Answer and explanation
Steam releases latent heat of condensation when it turns into water on skin, giving additional energy.
Answer: Steam carries latent heat of vaporisation.
Answer and explanation
Radiation transfers energy by electromagnetic waves and can occur through vacuum.
Answer: Radiation.
Answer and explanation
Metals are good conductors of heat, so they transfer heat quickly to food.
Answer: Because metals have high thermal conductivity.
Answer and explanation
Heat and work describe energy crossing the system boundary during a process. They are not stored in the system and depend on the path taken.
Answer: They are process quantities, not state properties.
Answer and explanation
It expresses conservation of energy for thermodynamic systems. Heat supplied is used to increase internal energy and to do work.
Answer: ΔU = Q - W with work done by system positive.
Answer and explanation
The reason is true, but assertion is false. During phase change, heat may be supplied without temperature rise.
Answer: Assertion false, Reason true.
Answer and explanation
Both statements are true, and the reason correctly explains the assertion.
Answer: Both true; Reason is correct explanation.
Answer and explanation
In expansion against vacuum, external pressure is zero, so W = PextΔV = 0.
Answer: Both true; Reason is correct explanation.
Answer and explanation
Heat enters the gas. Since volume is fixed, boundary work is zero. Internal energy increases because temperature rises.
Answer: Q positive, W = 0, ΔU positive.
Answer and explanation
Mechanical work is done on water by stirring. Since container is insulated, little heat escapes. Internal energy and temperature increase.
Answer: Temperature rises due to conversion of work into internal energy.
Answer and explanation
Yes. Heat flow depends on temperature, not total internal energy. A small hot body can transfer heat to a large cooler body with greater total internal energy.
Answer: Yes, if the smaller body has higher temperature.
Answer and explanation
Wool traps air, and air is a poor conductor. The blanket reduces heat loss from the body by conduction and convection.
Answer: It acts as thermal insulation.
Answer and explanation
Yes. In an adiabatic process, Q = 0, but work can change internal energy. For an ideal gas, change in internal energy changes temperature.
Answer: Yes, adiabatic compression or expansion.
Answer and explanation
P-V area gives boundary work for compressible systems. If other types of work occur, such as electrical work, shaft work or surface work, total work may include more than P-V work.
Answer: P-V area gives only pressure-volume boundary work.
Quick Revision Notes
Important Definitions
- Heat: Energy transfer due to temperature difference.
- Work: Energy transfer due to force and displacement.
- Internal energy: Microscopic kinetic plus potential energy of molecules.
- Heat capacity: Heat required to raise body temperature by 1 K.
- Specific heat capacity: Heat required per unit mass per kelvin rise.
- Latent heat: Heat required per unit mass during phase change at constant temperature.
Important Formulas
- Q = mcΔT
- Q = mL
- C = Q/ΔT
- W = PΔV for constant pressure
- Work = area under P-V curve
- ΔU = Q - W when W is work done by system
- 1 cal = 4.186 J
Exam Tips
- Never say a body contains heat; say it contains internal energy.
- Check sign convention before applying the first law.
- During phase change, temperature remains constant but heat is exchanged.
- For ideal gas, internal energy depends only on temperature.
- For a cycle, change in internal energy is zero.
- In an adiabatic process, Q = 0 but temperature can change.
- Radiation does not need a medium; conduction and convection do.
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