Physics Class 10 Chapter 10: Heat Capacity and Modes of Heat Transfer

A heat wave is a period of abnormally hot weather lasting more than two days. Global warming raises the average temperature, so the hot days become hotter and more frequent. Cities are affected most because roads, concrete and dark roofs absorb more solar radiation than grass or water and emit it slowly at night (urban heat island); there are fewer trees for shade and evaporation, and vehicles, air-conditioners and factories add heat and greenhouse gases. So the extra warming pushes city temperatures above the heat-wave level more often.

Hurricanes get their energy from warm ocean water. Warmer oceans mean more evaporation and more latent heat released when the vapour condenses, so the storms become stronger, with faster winds and heavier rain. Warm air also holds more moisture, so hurricanes will drop even more rain and cause more flooding. Future hurricanes are therefore expected to be more intense.

Both let the short-wave radiation of the Sun pass in, but do not let the long-wave infrared radiation emitted by the warm surface inside escape easily. The glass roof traps the infrared radiation inside the greenhouse so plants stay warm; carbon dioxide (and other greenhouse gases) absorbs the infrared radiation emitted by the Earth and re-emits it back, keeping the Earth's surface warmer than it would otherwise be. In both cases heat is trapped and the temperature rises.

Iron (the metal). In 60 minutes iron rises by 40 °C (25 → 65 °C) while water rises by only 5 °C (25 → 30 °C). The iron line is much steeper. Reason: specific heat of iron (452 J kg⁻¹ K⁻¹) is about nine times smaller than that of water (4200 J kg⁻¹ K⁻¹), so the same heat gives it a bigger temperature rise.

Water. Because of its large specific heat, water stores a large amount of heat for every degree of temperature; to cool by 1 °C it must lose 4200 J per kg, while iron loses only 452 J per kg. So water takes much longer to cool – a hot cup of tea stays hot longer than a hot spoon.

The iron pot heats faster. It is in direct contact with the flame, it is a good conductor, and its specific heat is small, so its temperature rises quickly; the water needs about nine times more heat per kilogram for the same rise, and it receives the heat only from the pot. (This is why an empty pot on the stove becomes red hot very quickly.)

Dark (black) surfaces are good absorbers of radiation. In winter dark clothes absorb the Sun's radiation and keep us warm. White surfaces are bad absorbers and good reflectors, so in summer white clothes reflect most of the sunlight and keep the body cool.

By convection. The water heated in the geyser expands, becomes less dense and rises through the pipes to the upper floor, while the colder, denser water flows down to the geyser to be heated. This natural convection current keeps hot water flowing upward without any pump.

One metre above the fire. In front of the fire we receive heat only by radiation. Above the fire we receive the radiation plus the hot air and gases rising by convection, so the heat received is more.

(i) Roads, concrete buildings and dark roofs absorb more solar radiation than soil, grass and trees and have low specific heat, so they heat up quickly. (ii) There are few trees and water bodies, so there is less cooling by shade and evaporation. (iii) Tall buildings block the wind and stop convection currents. (iv) Vehicles, air-conditioners and factories release extra heat and greenhouse gases. All this makes the city an "urban heat island".

Both are at the same room temperature, but metal is a good conductor of heat (free electrons) while wood is a bad conductor. When touched, the metal conducts heat away from the hand very quickly, so the hand loses heat fast and the metal feels cold. Wood conducts heat very slowly, so it does not feel cold.

Trees absorb carbon dioxide (a greenhouse gas) from the air during photosynthesis and store carbon; they also give shade, release water vapour that cools the air, and hold the soil. If forests are cut down, less CO₂ is absorbed and the carbon stored in the trees is released, so greenhouse gases increase, global warming speeds up, and floods, droughts, heat waves and soil erosion become worse.

Gases such as water vapour, carbon dioxide and methane have molecules that can absorb infrared (heat) radiation emitted by the Earth and re-emit it in all directions, including back towards the Earth. Gases like nitrogen and oxygen do not absorb infrared radiation, so they let it escape. The absorbing gases are the greenhouse gases and they trap heat.

Metals with low specific heat (copper, aluminium, steel) heat up quickly with little heat, so pans and tawas are made from them and cooking starts fast. Materials with high specific heat or low conductivity (water, clay, wood, plastic) heat slowly and keep heat longer – so clay handis keep food warm, water is used for even cooking, and handles are made of plastic or wood so they stay cool.

Water has a very large specific heat capacity (4200 J kg⁻¹ K⁻¹) compared with soil and rock (about 800 J kg⁻¹ K⁻¹). The sea therefore warms up slowly in the day and cools slowly at night, so it keeps the nearby land from becoming too hot or too cold.

Also, during the day cool air moves from the sea to the land (sea breeze), and at night air moves from the land to the sea (land breeze). Inland areas have no such large store of heat, so their temperature swings are large.

Sea breeze (day): Land has a small specific heat and gets hot quickly. The hot air above it rises and cool, moist air from the sea moves in to replace it. This cools the coast.

Land breeze (night): Land cools faster than the sea. The air above the warm sea rises and cooler air from the land moves out to sea. This keeps the night temperature mild.

Land heats and cools faster than water. In the day the land is hotter, so air above it rises and creates a low-pressure area; cooler air from the sea (high pressure) flows in: a sea breeze. At night the sea is warmer, air above it rises, and air flows from the land to the sea: a land breeze.

The temperature difference between land and sea causes the density and pressure difference that drives the wind.

Convection currents in the sea are caused by differences in temperature and salinity. Upwelling lifts cold, nutrient-rich water from the depths to the surface, where phytoplankton use it with sunlight and become food for fish. Downwelling carries oxygen-rich surface water to the deep sea so that deep-water animals can breathe. Convection also spreads heat around the oceans and keeps the temperature suitable for marine life.

Specific heat capacity of a substance is the amount of heat required to change the temperature of 1 kg of the substance by 1 K (or 1 °C).

Its SI unit is J kg⁻¹ K⁻¹.

Specific heat capacity is the heat absorbed or released by 1 kg of a substance for a temperature change of 1 K.

Heat capacity is the heat needed to raise the temperature of the whole object by 1 K: C = Q/ΔT = m c, unit J K⁻¹.

Specific heat capacity is for 1 kg of the material: c = Q/(mΔT), unit J kg⁻¹ K⁻¹. Heat capacity depends on the mass of the object; specific heat does not.

Heat from the flame passes through the metal pan to the handle by conduction. Wood and plastic are insulators, so the handle stays cool enough to hold, while the metal pan conducts heat quickly to the food.

Aluminium is a good conductor, so heat flows quickly from the processor into the sink. The many fins give a large surface area, so more heat can be given to the air by convection and radiation. A fan can increase convection. Painting it black increases radiation.

Water has a large specific heat capacity (4200 J kg⁻¹ K⁻¹), more than oil. For the same mass and temperature it stores more heat energy and cools slowly, so the bottle keeps the bed warm for hours.

Fill a beaker with water and drop a few crystals of potassium permanganate at the bottom. Heat the beaker gently from below, just under the crystals. The purple colour rises in a stream above the flame, spreads across the top and sinks at the sides. This shows that the heated water, which is less dense, rises and the cooler, denser water sinks: a convection current.

Land has a smaller specific heat than water, so it cools faster after sunset. The air above the sea is warmer, less dense and rises. This makes a low-pressure area over the sea, and cooler air from the land flows out to take its place: a land breeze.

The Sun heats dark ground more than grass or water, and the air above it becomes less dense and rises as a thermal. Eagles spread their wings and circle inside the rising air, which lifts them without effort. They glide to the next thermal when they need more height.

A surface that reflects most of the radiation falling on it absorbs little. Emission and absorption go together, so it is also a poor emitter. A dull black surface absorbs most radiation and emits it equally well.

The hotter an object is, the more infrared radiation it emits every second. At the same temperature, a body with a larger surface area emits more radiation than one with a smaller surface area. This is why radiators have fins.

A Leslie cube is a hollow metal cube with four faces: dull black, white, dark colour and shiny metal. It is filled with boiling water. An infrared thermometer is held at the same distance from each face. Result: the dull black face gives the highest reading (best emitter) and the shiny face gives the lowest (poorest emitter).

Sunlight passes through the atmosphere and warms the ground. The ground emits infrared radiation. Greenhouse gases such as carbon dioxide and methane absorb part of this infrared and send it back, so the Earth stays warmer than it would otherwise be.

A thermos has two glass walls with a vacuum between them. The vacuum has no particles, so there is no conduction or convection across the gap. The inner surfaces are silvered to reflect heat radiation. The stopper and plastic outer case are poor conductors and prevent heat loss through the top.

Water has a large specific heat, so it takes up a large amount of heat in the day for a small rise in temperature and gives it out slowly at night. Sea and land breezes also move heat.

Over warm ocean water, evaporation makes warm, moist air that rises by convection. As it rises it cools and the vapour condenses into clouds, releasing latent heat that warms the air and keeps it rising. The pressure near the sea surface falls, so more warm moist air rushes in. The Earth’s rotation makes the air spiral, forming a cyclone.

The lamp emits infrared radiation in all directions, which reaches the person directly (radiation). The air around the hot bulb rises by convection. Conduction occurs only through the lamp fittings and any air in contact.

Both rods have been in the room long enough to be at the same temperature. When touched, heat flows from the warmer hand into each rod. Copper has many free electrons and conducts this heat away very quickly, so the skin cools fast and the rod feels cold. Wood is a poor conductor, so the skin loses heat slowly and the rod feels warm. The sensation depends on the rate of heat flow, not on the actual temperature.

Air near a floor heater expands, becomes less dense and rises; cooler, denser air sinks to take its place and is heated in turn, so a convection current carries warmth through the whole room. An air-conditioner near the ceiling releases cold, dense air that sinks and pushes warmer air upward to be cooled, again setting up circulation. If the heater were high or the cooler low, the treated air would stay where it was and most of the room would be unaffected.

The Earth absorbs short-wave radiation from the Sun and, being much cooler, emits long-wave infrared radiation to space. When the rate of emission equals the rate of absorption, the average temperature stays constant. Greenhouse gases such as CO₂ let sunlight pass but absorb much of the outgoing infrared and re-radiate part of it back to the surface. Adding CO₂ reduces the net energy escaping, so the Earth warms until its higher temperature increases emission enough to restore the balance: this is global warming.

Specific heat capacity of a material is the amount of heat required to raise the temperature of 1 kg of the substance by 1 K (1 °C): c = Q/(mΔT), SI unit J kg⁻¹ K⁻¹. Different materials need different amounts of heat: metals have low c (copper 387 J kg⁻¹ K⁻¹) and heat up or cool quickly; water has a very high c (4200 J kg⁻¹ K⁻¹) and heats and cools slowly, storing a large amount of heat. The heat capacity of a body is C = mc (J K⁻¹).

Applications of water's high specific heat: (1) Moderate coastal climate – the sea absorbs a lot of heat in summer and releases it slowly in winter, so Karachi's temperature varies only from about 19 °C to 32 °C while inland areas go from 0 °C to 50 °C. (2) Stable lakes and oceans – slow temperature changes protect aquatic life. (3) Body temperature – our body is 60 % water, which helps keep the temperature near 37 °C. (4) Coolant – water in car radiators, power plants and industrial heat exchangers removes large amounts of heat and prevents overheating. (5) Cooking and hot drinks – water heats evenly without burning food, and tea stays hot for a long time. (6) Hot-water bottles and central heating carry heat for hours.

Radiation is the transfer of heat in the form of electromagnetic waves, mainly infrared radiation. It needs no material medium – heat from the Sun crosses the vacuum of space to reach the Earth, and we feel the warmth of a fire from a distance. Every object above absolute zero emits infrared radiation; hotter objects emit more. Fastest method: conduction and convection depend on the slow movement of particles, but radiation is an electromagnetic wave travelling at the speed of light, 3 × 10⁸ m/s, so heat reaches the receiver almost instantly (sunlight takes only about 8 minutes to cover 150 million km).

Factors affecting the rate of radiation: (1) Temperature of the surface – the hotter the object, the more radiation it emits per second (the Sun radiates far more than a stove). (2) Surface area – at the same temperature a larger surface emits more; radiators and heat sinks have fins. (3) Colour and texture (nature) of the surface – dull black and rough surfaces are the best absorbers and emitters; white and shiny silver surfaces are poor emitters but good reflectors (Leslie cube and two-cans experiments). A body whose emission equals absorption stays at constant temperature (thermal equilibrium).

Greenhouse effect: Sunlight passes through the atmosphere and warms the Earth. The warm Earth emits long-wavelength infrared radiation. Greenhouse gases (carbon dioxide, methane, water vapour) absorb part of it and send it back to the ground. This trapping of heat is the greenhouse effect.

Good or bad? The natural greenhouse effect is good: without it the Earth would be about 33 °C colder and life would be difficult. But the extra greenhouse gases from burning fuels and cutting forests make the effect stronger. This enhanced greenhouse effect is bad: it causes global warming.

Consequences: more heat waves; melting glaciers and polar ice; rise in sea level and coastal flooding; stronger hurricanes and heavier rainfall; droughts and wildfires; loss of habitats and damage to crops.

Causes: Burning coal, oil and gas, and cutting forests, increase carbon dioxide and methane. These gases trap more of the infrared radiation from the Earth (enhanced greenhouse effect), so the average temperature rises.

Extreme weather: Higher temperature causes heat waves; the warmer air holds more water vapour, which gives heavy rainfall and flooding; hot, dry weather causes wildfires; warmer seas supply more energy to hurricanes and cyclones; melting ice and expansion of sea water cause sea-level rise and coastal flooding.

Solutions: use renewable energy, plant trees, save energy.