Chapter 9: Heat

Everyday precautions related to heat: not touching steam, not pouring hot water into a glass tumbler in winter, not keeping a full water bottle in the deep freeze, not removing a hot lid with bare hands.

Matter is made up of atoms or molecules that are always in motion. The kinetic energy of molecules in hot water is greater than in cold water. The faster the molecules move, the higher their kinetic energy.

The sum of the kinetic energy of all the molecules of a substance is called thermal energy. Thermal energy depends on both the mass of the substance and the average speed of its molecules.

Very Important for SEE: Even if boiling water has higher average kinetic energy per molecule, a large amount of lukewarm water can have more total thermal energy than a small amount of boiling water, because it has far more molecules.

The average kinetic energy of the molecules of a substance determines its temperature. Temperature is an index (measure) of the average kinetic energy of molecules. The SI unit of temperature is kelvin (K), though degree Celsius (°C) is more commonly used. It is measured using a thermometer.

The amount of thermal energy transmitted from one place (or object) to another due to a difference in temperature is called heat. The SI unit of heat is joule (J), like other forms of energy. It can be measured using a calorimeter.

Heat is not stored in an object — it is only the quantity of thermal energy transferred from one object to another. When heat is transmitted, the thermal energy of one object decreases while that of the other increases.

Absolute zero is a theoretical state of zero thermal energy. It is equal to 0 K, which is -273.15°C.

A hot body and a cold body with molecules moving at different average speeds, shown reaching the same average speed after thermal equilibrium.

Heat always flows from a body at higher temperature to a body at lower temperature. This flow continues until both bodies reach the same temperature, a state called thermal equilibrium.

When we touch an object, if heat enters our body from the object, it feels hot. If heat leaves our body into the object, it feels cold.

Methods of heat transfer: conduction, convection and radiation.

Comparison of molecular arrangement and spacing in solids, liquids and gases, showing increased vibration and spacing on heating.

Molecules of a solid are bound in a fixed pattern by forces of attraction, but they are always vibrating, not stationary. When a solid is heated, its molecules gain energy and vibrate faster.

As heating continues, rapid vibration weakens the force of attraction between molecules, so they move farther apart — the volume increases. When an object releases thermal energy (cools), molecular speed decreases, temperature decreases, and the object contracts.

Graphs showing the relationship between volume of 1 kg of water and temperature, and between density of water and temperature, both showing a turning point at 4°C.

Most substances expand continuously when heated. Water behaves differently: as it is heated from 0°C to 4°C, its volume actually decreases; only above 4°C does its volume increase normally. Similarly, on cooling from a higher temperature down to 4°C, volume decreases, but cooling further from 4°C to 0°C causes volume to increase again.

Very Important for SEE: Water has its minimum volume and maximum density at 4°C. This unusual behaviour is called the anomalous expansion of water.

Fish and aquatic organisms surviving in liquid water beneath a layer of surface ice on a frozen pond.

In very cold places, as the temperature of a water body drops, water at 4°C (being densest) sinks to the bottom, while layers of water at 3°C, 2°C, 1°C and 0°C form above it. When the surface water reaches 0°C, it freezes and floats on top since ice is less dense than water. This is why fish and other aquatic creatures can survive in the water below the ice layer.

In cold places, when water inside a pipe gradually cools and freezes into ice, its volume increases and exerts very high pressure on the pipe walls, which can cause the pipe to burst. For the same reason, a water-filled glass bottle kept in a deep freeze can crack.

Different materials (steel plate vs ceramic plate, wooden chair vs plastic chair, sand vs soil, steel vessel vs glass) heating at different rates under the same sunlight.

Even objects made of the same material but in different states or amounts may absorb heat at different rates. The quantity of heat absorbed by a substance (Q) is directly proportional to its mass (m), keeping temperature constant: Q ∝ m.

The quantity of heat absorbed (Q) is also directly proportional to the change in temperature (T2 - T1), keeping mass constant: Q ∝ (T2 - T1).

Combining both relations: Q ∝ m(T2 - T1), so Q = ms(T2 - T1), where s is a constant called the specific heat capacity of the substance.

Therefore, the heat absorbed or released by a substance equals the product of its mass (m), specific heat capacity (s), and temperature change (T2 - T1). This is called the heat equation.

If mass m = 1 kg and temperature change = 1°C, then s = Q. So, specific heat capacity is the amount of heat required to change the temperature of 1 kg of a substance by 1°C. Its SI unit is joule per kilogram per degree Celsius (J/kg°C).

Specific heat capacity varies according to the nature of the material. Even different states of the same substance (like ice and water) have different specific heat capacities.

Given: mass of water m = 5 kg, initial temperature T1 = 10°C, final temperature T2 = 100°C, specific heat capacity of water s = 4200 J/kg°C

Required: Heat energy consumed (Q)

Formula: Q = ms(T2 - T1)

Calculation: Change in temperature = 100 - 10 = 90°C. Q = 5 x 4200 x 90 = 1890000 J = 1.89 x 10^6 J

Answer: The heat consumed is 1.89 x 10^6 J.

Given: 5 kg of water at 100°C is mixed with 15 kg of water at 15°C. Heat lost to the surroundings is neglected.

Required: Final temperature (T) of the mixture

Formula: Heat released by hot water = Heat absorbed by cold water, i.e., m1 s (T - T1) = m2 s (T2 - T), where m1 = 15 kg (cold water mass), T1 = 15°C, m2 = 5 kg (hot water mass), T2 = 100°C

Calculation: 15(T - 15) = 5(100 - T), so 3(T - 15) = (100 - T), 3T - 45 = 100 - T, 4T = 145, T = 36.25°C

Answer: The final temperature of the mixture is 36.25°C.

Water has a very high specific heat capacity (4200 J/kg°C). Every kilogram of water absorbs or releases 4200 J of heat for a 1°C temperature change. This makes water an excellent coolant, because it can absorb large amounts of heat without a big rise in its own temperature.

Water used as a coolant in a car radiator, circulating through the engine to absorb heat.

A wet cloth placed on the forehead of a person with fever, and a hot water bag being used to relieve muscular pain.

Daytime sea breeze blowing from sea to land, and nighttime land breeze blowing from land to sea, due to differing specific heat capacities of land and water.

In coastal areas, the specific heat capacity of land (sand) is about five times less than that of seawater. During the day, the sun heats both equally, but land heats up faster than the sea. Warm air above the land rises, creating low pressure, and cool air flows in from the sea toward the land — this is called sea breeze.

At night, both land and sea release heat and cool down, but land cools faster than the sea (again due to its lower specific heat capacity). Warm air above the sea rises, creating low pressure over the sea, and cooler air flows from the land toward the sea — this is called land breeze.

Because of this daily wind reversal, coastal areas experience a smaller difference in temperature between day and night compared to places far from the sea.

A liquid-in-glass thermometer showing a mercury-filled bulb, a capillary tube and a marked temperature scale.

This thermometer contains a thermometric liquid, usually mercury, inside a heat-sensitive bulb. Mercury is a good conductor of heat and has high density. When the bulb touches a hot object, heat transfers to the mercury, which expands and moves up the capillary tube. The point where mercury stops indicates the temperature, read on the scale marked on the outer wall.

A digital thermometer with a heat-sensitive thermistor probe and a digital display panel showing the measured temperature.

A digital thermometer uses a heat-sensitive thermistor connected to an electric circuit. When the thermistor touches the body, heat transfer changes its electrical resistance, which changes the current in the circuit. This change is converted and displayed as a number on the display panel.

A radiation thermometer being pointed at a person's forehead without contact, showing infrared rays being detected by its sensor.

This thermometer measures temperature without touching the body, based on the intensity of infrared radiation produced by the body. A lens focuses incoming infrared radiation onto a sensor, which generates electrical signals that are converted into a temperature reading. It is very quick and easy to use.

Calibration process of a thermometer, marking the lower fixed point in melting ice and the upper fixed point in steam above boiling water, then dividing the scale into equal parts.

Calibration is the process of determining the scale of a thermometer. First, two fixed points (lower and upper) are determined, then the distance between them is divided into equal parts.

For a thermometer, the lower fixed point is the temperature of melting ice (0°C) and the upper fixed point is the temperature of steam just above water boiling at one atmospheric pressure (100°C). The distance between these two points is divided into 100 equal parts, so each part represents 1°C.