Chapter 4 — Heat

What is heat? Heat is a form of energy that flows from a hotter object to a colder object because of the temperature difference between them. When two bodies at different temperatures are placed in contact, energy transfers until thermal equilibrium (same temperature) is reached.

Units: The SI unit of heat is the joule (J). An older unit is the calorie (cal): 1 cal ≈ 4.186 J. Temperature is measured in degree Celsius (°C) or Kelvin (K), but temperature and heat are different physical quantities.

How heat is transferred — three main modes:

• Conduction: Transfer of heat through direct contact inside solids (and between solids in contact). Example: heat moving along a metal rod heated at one end. Energy is passed from particle to particle.
• Convection: Transfer of heat by the bulk movement of fluids (liquids or gases). Example: warm air rising from a heater sets up convection currents in a room.
• Radiation: Transfer of heat by electromagnetic waves (infrared). Does not need a medium. Example: heat from the Sun reaches Earth by radiation.

Effects of heating include:

• Rise in temperature.
• Change of state (melting, boiling, condensation, freezing) when enough heat is supplied or removed.
• Expansion of solids, liquids and gases on heating (thermal expansion).
• Change in properties such as electrical resistance of materials.

Heat vs Temperature (difference) — Heat is energy transferred between bodies; temperature is a measure of the average kinetic energy of particles in a body. A large amount of heat may cause a small temperature change in a material with high heat capacity.

Conservation idea: Heat energy lost by a hotter body ≈ heat energy gained by a colder body (neglecting losses), which is the basis for experiments like mixing hot and cold water to find final temperature.

What is temperature?
Temperature is a measure of how hot or cold a body is. It indicates the average kinetic energy of the particles in a substance — higher temperature means particles move faster on average.

Temperature vs Heat
Temperature is not the same as heat. Temperature is a measure (a property) of a body. Heat is energy transferred between bodies because of a temperature difference.

Thermal equilibrium
Two bodies are in thermal equilibrium when they have the same temperature and no net heat flows between them.

How temperature is measured
Temperature is measured using thermometers. Common types: mercury-in-glass (traditional), alcohol-in-glass, and digital thermometers (electronic sensors). A thermometer measures expansion (or electrical response) that changes with temperature and is marked on a temperature scale.

Temperature scales
- Celsius (°C): used commonly in daily life and science (0°C for ice melting, 100°C for water boiling at 1 atm).
- Fahrenheit (°F): used in some countries (32°F = 0°C, 212°F = 100°C).
- Kelvin (K): SI unit; K = °C + 273.15. Absolute zero (no particle motion) is 0 K = −273.15°C.

Practical points and precautions when using thermometers

• Allow the thermometer to reach the temperature of the object (wait for reading to stabilise).
• Do not expose mercury thermometers to sudden shocks (they can break).
• Ensure correct placement: e.g., for body temperature, use recommended positions (oral, axillary, etc.).
• Read scale at eye level to avoid parallax error.

Everyday significance
Temperature controls weather reports, cooking, refrigeration, health (fever), industrial processes, and scientific experiments.

What is temperature? Temperature is a measure of how hot or cold a body is. It indicates the average kinetic energy of the particles in a substance.

What is a thermometer? A thermometer is an instrument used to measure temperature. Most thermometers work on the principle of thermal expansion — substances expand when heated and contract when cooled.

Types of common thermometers (Class 7 level):

• Liquid-in-glass thermometers: A liquid (mercury or coloured alcohol) is enclosed in a glass tube with a narrow capillary. When temperature rises the liquid rises in the capillary proportionally to the temperature.
• Clinical (medical) thermometer: A special mercury-in-glass thermometer with a constriction in the tube so mercury does not fall back quickly; used for measuring human body temperature (range roughly 35 °C to 42 °C).
• Laboratory thermometer: Mercury or alcohol thermometers with larger ranges (for experiments) and finer scale divisions.

Fixed points and scales: To calibrate thermometers we use fixed points — reproducible temperatures:

• Ice point: Temperature of pure melting ice under standard pressure = 0 °C (Celsius scale).
• Steam (boiling) point: Temperature of boiling water under standard pressure = 100 °C on Celsius scale.

Other temperature scales: Fahrenheit (°F) and Kelvin (K). Conversions are given below.

How a liquid-in-glass thermometer works (brief): The bulb contains liquid. As temperature increases, liquid expands into the capillary. The length (or volume) change is proportional to change in temperature, so markings on the glass are used to read temperature.

Useful points about clinical thermometers: They have a narrow constriction so the mercury column remains above the constriction after removing from mouth/underarm, allowing easier reading. To reset, shake firmly to bring mercury back into the bulb.

Precautions and errors when using thermometers:

• Read at eye level to avoid parallax error.
• Allow sufficient time for thermometer to reach the temperature of the object (equilibration).
• Do not use mercury thermometers if broken — mercury is toxic.
• Clinical thermometer must be sanitized before and after use.
• Ensure thermometer range suits the measurement (e.g., don't use a clinical thermometer for very low/high temperatures).

Why different liquids? Mercury has a uniform thermal expansion and remains liquid over a wide range (−39 °C to 357 °C). Alcohol is coloured and safer at very low temperatures where mercury would freeze.

Summary: Thermometers convert temperature changes into measurable length or volume changes of a substance. They are calibrated using fixed points and read against a scale (°C, °F, or K) with care to avoid errors.

Heat is a form of energy that is transferred from one body (or region) to another because of a temperature difference. Heat flows spontaneously from a hotter object to a colder one and can change the temperature or the state (solid/liquid/gas) of a substance. Heat is an extensive quantity (depends on the amount of substance) and is measured in joules (J) or calories (cal). Modes of heat transfer include conduction, convection and radiation.

Temperature is a measure of how hot or cold a body is. It indicates the average kinetic energy of the particles of the substance. Temperature is a measure or property of the state of a body (not energy itself), and it does not depend on the amount of substance (intensive property). Temperature is measured in degrees Celsius (°C), Kelvin (K) or Fahrenheit (°F) using a thermometer.

Key differences (summary):

• Nature: Heat is energy in transfer; temperature is a measure of hotness (a physical property).
• Units: Heat: joule (J) or calorie (cal). Temperature: °C, K, °F.
• Dependence on mass: Heat depends on mass (more mass can store more heat for same temperature change); temperature does not depend on mass for a homogeneous body.
• Measurement: Heat is measured using calorimeters (indirectly); temperature is measured using thermometers.
• Effect: Heat transfer may change temperature or cause a change of state; temperature tells the direction of heat flow (heat flows from higher to lower temperature).

Understanding both concepts together: When heat Q is supplied to a substance, its temperature usually rises. The amount of temperature rise depends on the mass and the specific heat capacity of the substance. Sometimes supplied heat is used entirely for a phase change (melting/boiling) and temperature remains constant during that process.

What is it? Expansion on heating is the increase in size (length, area or volume) of a substance when its temperature rises. Contraction on cooling is the decrease in size when the temperature falls. These happen because heating increases the kinetic energy of particles, making them vibrate/move more and occupy more space; cooling reduces this motion and the particles come closer.

Types

• Linear (Length) expansion — change in length of solids on heating. Useful for long objects like rods, rails, wires.
• Area expansion — change in surface area of flat objects (sheets) on heating.
• Volume expansion — change in volume of solids, liquids and gases on heating. Liquids and gases show much larger volume changes than solids.

How we measure it

• Coefficient of linear expansion (α): fractional change in length per degree change in temperature.
• Coefficient of volume expansion (β): fractional change in volume per degree change in temperature. For solids, β ≈ 3α (approximately).

Important points

• Different materials expand by different amounts (different expansion coefficients). Metals usually expand more than glass for the same temperature change.
• If expansion is prevented (constrained), large stresses develop and the object can bend, crack or break (for example, bursting of pipes or cracks in roads).
• Bimetallic strips (two metals with different α joined together) bend on heating and are used in thermostats and electric switches.
• Water shows anomalous behaviour: it contracts on cooling down to 4 °C, but below 4 °C it expands as it approaches 0 °C (ice is less dense than water). This is why ice floats.

Practical uses and safety

• Expansion joints in bridges and railways allow safe expansion and contraction with temperature changes.
• Mercury or alcohol thermometers use volume expansion of liquids to measure temperature changes.
• Hot-fitting: a metal lid can be loosened by heating the jar (lid expands more than glass) or by pouring hot water to expand the metal.

What is thermal expansion? Thermal expansion is the increase in size (length, area or volume) of materials when their temperature rises. On heating, particles gain kinetic energy and move slightly farther apart, so the material expands. On cooling, it contracts.

Why does it happen? Increased temperature makes atoms/molecules vibrate more and their average separation increases. The expansion amount depends on the material and the temperature change.

Main types of expansion

• 1. Linear expansion (1-dimensional)

  Applies to rods, wires or any object where one dimension (length) change is important. If a rod of original length L is heated by ΔT, its length increases by ΔL = α L ΔT, where α is the coefficient of linear expansion for the material.

• 2. Area (surface) expansion (2-dimensional)

  Applies to plates or surfaces. If the original area is A, the increase in area is approximately ΔA = 2α A ΔT (because both dimensions expand), where 2α is approximately the coefficient of area expansion.

• 3. Volume (cubical) expansion (3-dimensional)

  Applies to solids, liquids and gases when their volume changes. For a body of volume V, ΔV = γ V ΔT, where γ is the coefficient of volume (cubical) expansion. For most solids γ ≈ 3α.

Special notes for states of matter

• Solids: Show linear, area and volume expansion. For solids, γ ≈ 3α (true for isotropic solids).
• Liquids: Do not have a fixed shape; we use coefficient of cubical expansion (commonly written β or γ_liquid). Liquids usually expand more than solids; ΔV = β V ΔT.
• Gases: Expand a lot. For ideal gases PV = nRT. At constant pressure, V ∝ T (Charles' law): V1/T1 = V2/T2 (temperatures in Kelvin).

Practical consequences and applications

• Gaps in railway tracks and expansion joints in bridges allow for thermal expansion to avoid buckling or damage.
• Thermometers (mercury or alcohol) use liquid volumetric expansion to measure temperature.
• Bimetallic strips in thermostats use two metals with different α to bend on heating and operate switches.
• Care is needed for lids, metal fittings and glassware: heating may loosen or tighten fits; sudden cooling of heated glass can cause breakage due to uneven contraction.

Important points to remember

• Expansion is proportional to the original size and temperature change for small ΔT.
• Different materials have different coefficients; metals generally expand more than ceramics.
• Coefficients are usually given per degree Celsius (or per Kelvin) and are approximately the same scale for both units (Δ°C = ΔK).

What is thermal expansion? Thermal expansion is the increase in size (length, area or volume) of a substance when its temperature rises. Solids, liquids and gases expand on heating; gases expand the most for a given rise in temperature.

Experimental evidence (simple experiments)

• Ball-and-ring experiment: A metal ball passes through a metal ring at room temperature. On heating the ball, it no longer passes through the same ring — showing that the ball has expanded. Cooling the ball restores the fit.
• Metal rod with pointer and scale: Clamp a metal rod horizontally, fix a pointer at one end and place a scale under the pointer. On heating the rod the pointer moves along the scale indicating increase in length (linear expansion).
• Liquid-in-glass thermometer: The rise of mercury or coloured alcohol in a narrow tube when the bulb is heated demonstrates expansion of liquids. The narrow bore converts small volume changes into visible large height changes.
• Air expansion (syringe/flask experiment): A flask partly filled with colored water and fitted with a tube shows the water level rising when the air in the flask is heated, indicating expansion of air.

Why do these occur? Heating increases the kinetic energy of particles; they vibrate or move more and tend to occupy more space, producing expansion. For solids the increase is mainly in the average separation between atoms; for liquids and gases, molecules move farther apart and occupy more volume.

Devices based on expansion

• Thermometers: Use liquid (mercury or alcohol) expansion in a narrow tube to indicate temperature.
• Bimetallic strip (thermostat, electric iron, geyser): Two metals with different coefficients of linear expansion are bonded together. On heating the strip bends toward the metal with lower expansion; this bending is used to make or break electrical contacts for temperature control.
• Steam engine / piston arrangement (simple model): Expansion of heated gas or steam pushes a piston and can do mechanical work.
• Railway tracks and bridges: Expansion joints and gaps are left to allow metal and concrete to expand; otherwise rails could buckle and bridges crack.
• Jars & lids: Heating a metal lid slightly can loosen it as the lid expands more than the glass jar.

Key points for students

• Thermal expansion is usually nearly proportional to the temperature change over moderate ranges.
• Different materials expand by different amounts — quantified by coefficients of expansion.
• Designs use expansion (thermometers) or accommodate it (expansion gaps).

Change of State due to Heating/Cooling

Matter exists in three common states: solid, liquid and gas. A change of state (or phase change) happens when heat is added to or removed from a substance. Heating usually changes a solid to a liquid (melting) and a liquid to a gas (evaporation/boiling). Cooling makes a gas condense to a liquid (condensation) and a liquid freeze to a solid (freezing). Some substances can change directly between solid and gas (sublimation and deposition).

Molecular Explanation

Temperature measures the average kinetic energy of particles. On heating, particle kinetic energy increases. During a phase change the added heat is used to overcome intermolecular forces rather than to raise temperature. Thus temperature remains constant during the actual phase change (a plateau on a temperature vs time graph) while the arrangement and freedom of motion of the molecules change:

• Solid: particles closely packed, fixed positions, vibrational motion only.
• Liquid: particles close but able to move/slide past each other.
• Gas: particles far apart, move independently at high speeds.

Types of Changes

• Melting (fusion): solid -> liquid on heating (e.g., ice -> water). Temperature stays at melting point while melting occurs.
• Freezing (solidification): liquid -> solid on cooling (e.g., water -> ice). Heat removed = latent heat of fusion.
• Evaporation: liquid -> gas from the surface at any temperature; faster with higher temperature, larger surface area, lower humidity.
• Boiling: rapid liquid -> gas throughout the bulk at the boiling point (depends on pressure).
• Condensation: gas -> liquid on cooling (e.g., water droplets on a cold surface).
• Sublimation: solid -> gas without passing through liquid (e.g., dry ice, camphor).

Latent Heat

Latent heat is the heat absorbed or released during a phase change per unit mass without a temperature change. Two common types are latent heat of fusion (melting/freezing) and latent heat of vaporization (boiling/condensation).

Factors Affecting Change of State

• Pressure: boiling point depends strongly on external pressure (lower at high altitude).
• Impurities: solutes lower freezing point (freezing point depression) and can raise boiling point.
• Surface area and airflow: affect evaporation rates.

Practical Notes

During heating, first temperature rises within one phase (Q = mcΔT). When a phase change begins temperature stays constant while energy goes into changing state (Q = mL). After phase change completes, temperature of the new phase rises again.

What is evaporation? Evaporation is the process by which molecules at the surface of a liquid escape into the air as vapour at temperatures below the boiling point. It is a surface phenomenon and can occur at any temperature.

How evaporation happens (microscopic view): In a liquid, molecules move with a range of kinetic energies. Some surface molecules have higher-than-average energy. When such molecules gain enough energy to overcome the attractive forces of neighbouring molecules, they leave the liquid as vapour. Only molecules at the surface can directly escape.

Evaporation versus boiling: Boiling is a bulk phenomenon that occurs throughout the liquid at a specific temperature (boiling point) when vapour pressure equals atmospheric pressure. Evaporation occurs only at the surface and at all temperatures below the boiling point.

Why evaporation causes cooling: When a molecule evaporates, it takes away energy in the form of latent heat of vaporization. The remaining liquid loses average kinetic energy, so its temperature falls. This is why evaporation causes cooling of the liquid and of objects in contact with it (for example, our skin).

Factors affecting the rate of evaporation:

• Temperature: Higher temperature increases the number of high-energy molecules, so evaporation is faster.
• Surface area: Larger surface area gives more molecules an opportunity to escape, increasing the rate.
• Humidity of surrounding air: Dry air (low humidity) accepts more vapour, so evaporation is faster. High humidity slows evaporation.
• Air movement (wind): Moving air carries away vapour molecules, speeding up evaporation.
• Nature of the liquid: Liquids with weaker intermolecular forces (e.g., alcohol) evaporate faster than those with stronger forces (e.g., water).

Everyday cooling examples: When sweat evaporates from our skin it removes heat and cools us. Similarly, a wet cloth placed on the forehead, a wet floor drying in sunlight, and the cooling felt when alcohol is applied to skin are all due to evaporation.

Importance: Evaporation plays a major role in weather (evaporation from oceans and lakes drives the water cycle), in cooling mechanisms (sweating, evaporative coolers), and in many industrial drying processes.

What is transfer of heat? Transfer of heat means the flow of thermal energy from a hotter object or region to a colder one. Heat always moves from higher temperature to lower temperature until temperatures become equal (thermal equilibrium).

Three modes of heat transfer

• Conduction: Transfer of heat through a solid or between solids in contact, without the movement of the material as a whole. Heat is passed from particle to particle by collisions and vibration. Good conductors (metals) transfer heat quickly; insulators (wood, wool) do so slowly.
• Convection: Transfer of heat by the bulk movement of fluids (liquids or gases). When a part of a fluid is heated it becomes lighter (less dense) and rises, while cooler, denser fluid sinks, creating convection currents that carry heat.
• Radiation: Transfer of heat by electromagnetic waves (infrared). Radiation does not need any medium — heat from the Sun reaches Earth through space by radiation.

Some important points

• Direction: Heat flows from hot to cold.
• Materials: Metals are good conductors; non-metals like wood, plastic and air are insulators.
• Rate depends on temperature difference, contact area, nature of material and thickness (for conduction), and on fluid motion (for convection).

Simple classroom demonstrations

• Conduction: Heat one end of a metal rod with a flame; notice how heat travels to the other end. Attach wax drops on the rod — they melt one by one.
• Convection: Heat coloured water at the bottom of a beaker and watch warm coloured fluid rise and cold sink, showing convection currents.
• Radiation: Hold your hand near (but not touching) a hot object or a lamp and feel warmth — that is radiation. Compare heating of a black vs white surface under a lamp.

Everyday examples are given below in the examples list.

Why learn this? Understanding heat transfer helps explain cooking, weather (winds and sea breezes), keeping homes warm/cool, designing clothing and devices, and many natural processes.

Conductors and Insulators

Definition: Conductors are materials that allow heat (thermal energy) to pass through them easily. Insulators are materials that do not allow heat to pass through them easily and so slow down heat transfer.

How heat is transferred in solids (conduction): In conduction, heat is transferred from the hotter part to the colder part by the vibration and collisions of particles (atoms, molecules, or free electrons). In metals, free electrons carry much of the heat, so metals are good conductors. In non-metals (like wood, plastic), particles are more tightly bound and there are few free electrons, so they are poor conductors and act as insulators.

Key characteristics:

• Conductors (examples: copper, aluminium, iron) — high thermal conductivity; heat spreads quickly.
• Insulators (examples: wood, plastic, rubber, glass, air, wool) — low thermal conductivity; they retain heat or keep cold in.

Everyday situations & simple experiment: If you hold a metal spoon and a wooden spoon whose ends are dipped in hot water, the metal spoon quickly feels hot — it conducts heat to your hand. The wooden spoon remains relatively cool because wood is an insulator. Thermos flasks use a vacuum or insulating material to reduce heat transfer and keep liquids hot or cold.

Factors affecting rate of heat conduction:

• Nature of the material (thermal conductivity).
• Temperature difference between two ends (larger difference → faster heat flow).
• Cross-sectional area (larger area → more heat flow).
• Length or thickness of material (greater length → less heat flow).

Importance: Conductors are useful where heat must be transferred quickly (cooking utensils, heat sinks); insulators are used where heat transfer must be prevented (cold-storage, house insulation, handles of cookware).

Overview: "Applications and Everyday Phenomena" shows how heat transfer and thermal effects appear in daily life and in simple devices. It links the basic processes — conduction, convection and radiation — and thermal expansion and change of state, to practical uses and phenomena you observe around you.

Modes of heat transfer and everyday examples:

• Conduction — transfer of heat through a material without bulk movement. Good in solids, especially metals. Example: a metal spoon becoming hot when left in a cup of hot tea. Kitchen tongs and insulated handles are designed to reduce conduction to your hand.
• Convection — heat transfer by the bulk motion of fluids (liquids or gases). Warmer fluid rises and cooler fluid sinks, creating convection currents. Example: boiling water in a pot (hot water rises from the bottom and cooler water descends), warm air circulating from a room heater.
• Radiation — transfer of heat by electromagnetic waves; does not require a medium. Example: feeling warmth from the Sun, or from a fire, even across a room.

Thermal expansion and its applications/problems:

• Most materials expand on heating and contract on cooling. Solids expand in length, area and volume; liquids mainly change volume. Example applications: gaps left between sections of railway tracks and bridges to allow expansion in hot weather; bimetallic strips in thermostats: two metals with different expansion rates bend on heating and operate a switch.
• Problems due to expansion: cracking of roads or distortion of machinery if no allowance is made; nails or screws loosening in wooden furniture as humidity and temperature change.

Change of state and daily uses:

• Melting and boiling are used in cooking, refrigeration and many industrial processes. Example: boiling water cooks food; melting butter or chocolate changes texture.
• Latent heat explains why temperature stays constant during melting or boiling until the entire substance has changed its state — this is visible as plateaus in heating/cooling graphs.

Devices that use thermal principles:

• Thermos (vacuum) flask — reduces heat transfer by conduction, convection and radiation: double walls with vacuum between, silvered surfaces to reduce radiation, and a tight stopper to limit convection.
• Electric iron — uses conduction (heat flows from the hot soleplate to clothes) and sometimes a thermostat (bimetallic strip) to control temperature.
• Pressure cooker — increases pressure so water boils at higher temperature; food cooks faster (application of boiling point dependence on pressure).
• Insulation in buildings — materials with low thermal conductivity (wood, wool, polystyrene) reduce heat loss by conduction and convection; reflective surfaces reduce radiation.

Everyday phenomena explained by heat concepts:

• Sea breeze and land breeze — local convection caused by differential heating of land and sea during day/night.
• Hotter pavements and black surfaces — darker colors absorb more radiation and become hotter; metal surfaces feel colder because they conduct heat away from your skin faster.
• Sweating and cooling — evaporation (a change of state) requires latent heat; evaporation of sweat absorbs heat from the body, cooling you down.

Practical tips based on heat principles:

• Use lids while cooking to reduce heat loss by convection and radiation, cooking faster and saving fuel.
• Polish or paint surfaces white/reflective to reduce heating from sunlight; insulate hot-water pipes to reduce heat loss.

What is being measured?
Temperature is a quantitative measure of how hot or cold a body is. Heat is energy transferred because of a temperature difference. In everyday situations we measure temperature to know the extent of warming or cooling.

Measuring temperature
We use thermometers (laboratory or clinical) to measure temperature. The commonly used unit is degree Celsius (°C). Absolute temperature in Kelvin (K) is related by K = °C + 273.15. Thermometers read temperature by showing expansion (or other physical change) of a substance with temperature.

Simple quantitative idea (introductory)
When heat is supplied to a body, its temperature changes. For a given material and for moderate temperature changes, the amount of heat (q) required to produce a temperature change (ΔT) depends on:

• the mass (m) of the body — more mass takes more heat to change temperature;
• the type of material — different materials require different amounts of heat (this idea leads to the concept of specific heat capacity);
• the desired temperature change (ΔT) — larger ΔT needs more heat.

These relations are summarized by the simple introductory formula (used later in detail): q = m c ΔT, where c is the specific heat capacity of the material. This means for the same heat supplied, different masses or different materials show different temperature rises.

Important units and conversions
Celsius <> Fahrenheit: °C = (°F - 32) × 5/9 and °F = °C × 9/5 + 32.
Celsius <> Kelvin: K = °C + 273.15.

Short worked example (illustrative)
Heat required to raise 200 g of water from 20 °C to 30 °C.
Take c (water) = 4.2 J/g°C. Here m = 200 g, ΔT = 10 °C.
q = m c ΔT = 200 × 4.2 × 10 = 8400 J.
So 8400 joules of heat are needed (approx.).

Limitations and notes
The relation q = m c ΔT is an introductory linear approximation valid for moderate temperature ranges and when no phase change occurs. During melting or boiling, temperature does not change while heat is used for the phase change.

Practical use
This simple quantitative idea helps predict how long or how much energy is needed to heat objects (cookware, water, rooms) and explains why materials with different specific heats behave differently when heated.