CBSE Class 7 Science Advanced Physics Heat Notes Set 01

Heat

Heat as a Form of Energy

When a pressure cooker sits on a burning stove, the steam that builds up lifts the weight on its nozzle. Lifting the weight is work, and the capacity to do work is energy. Since the work comes from the heat supplied by the stove, heat must be a form of energy.

Heat itself cannot be seen, but it gives us the feeling of hot or cold. How hot or cold a body is tells us its temperature. Giving heat to a body raises its temperature, and taking heat away lowers it.

Heat is always produced from some other kind of energy. Rubbing two stones produces heat from the work we do, an electric iron gets its heat from electrical energy, and a burning candle gives heat from the chemical energy stored in wax. In the opposite direction, a steam engine turns heat into mechanical energy, and the filament of an electric bulb turns heat into light once it becomes white hot. Heat can therefore change a body's size, its state (solid, liquid or gas) and even its chemical nature.

Hot and Cold

Touch is not a dependable way of judging how hot something is. The reliable way is to measure its temperature. The SI unit of temperature is the kelvin (K), while the Celsius degree (°C) is used in the CGS system. The two scales are linked by

\[ K = {}^\circ C + 273 \]

If two bodies at different temperatures touch each other, heat passes from the hotter body to the colder one, and this goes on until both reach the same temperature.

Difference between Heat and Temperature

Measurement of Temperature

A thermometer measures temperature. It works because substances expand when heated and shrink when cooled. Most thermometers use mercury.

Why Mercury is a Good Thermometric Liquid

1. It expands evenly over a large range of temperature.
2. It conducts heat well, so it quickly takes the temperature of the body.
3. Its specific heat capacity is low, so it needs very little heat to warm up.
4. It is shiny and opaque, so the thread is easy to see.
5. It does not stick to (wet) glass.
6. It can be easily obtained in a pure form.
7. It boils at a high temperature and melts at a low one.
8. Its vapour pressure is very small.

Mercury also has drawbacks. Its expansion for each degree is small, so tiny changes in temperature are hard to read. It also freezes at about −40 °C, so it cannot be used for very cold surroundings.

Construction of a Mercury Thermometer

The starting piece is a very fine glass tube (capillary) of the same bore throughout, with a thin-walled bulb at one end and a funnel at the other. The tube is enclosed in a thicker glass stem, as in Fig. 3.1.

Mercury is poured through the funnel and trickles into the tube, but small air bubbles often get trapped. To drive them out, the bulb is warmed in a water bath; the air escapes through the mercury. This is repeated until the bulb and tube are full of mercury with no air in them. The funnel end is then cut off and sealed.

The scale is marked in two steps. With the bulb in pure melting ice, the mercury level is marked as the lower fixed point (0 °C). With the bulb in steam above boiling water, the new level is marked as the upper fixed point (100 °C). The gap between the marks is divided into 100 equal parts, each one standing for 1 °C.

Clinical Thermometer

A clinical thermometer is the one used to take body temperature. It has a long, narrow glass tube with a mercury bulb at one end and a kink (constriction) just above the bulb. The constriction stops the mercury thread from sliding back into the bulb once the thermometer is taken out of the mouth or armpit, so the reading can be noted.

It carries both a Celsius scale (°C) and a Fahrenheit scale (°F). Normal human body temperature, 37 °C or 98.4 °F, is shown by a red arrow. Because the body temperature of a living person stays between about 35 °C and 43 °C, the scale is marked only over this range.

Measuring the Temperature of a Furnace

A furnace is hotter than 1000 °C, which is too high for a laboratory thermometer. A device called a pyrometer is used. It is made by joining two wires of different metals; the metals are chosen to suit the temperature range. When one end is placed in the furnace, an electric current flows, and the size of this current tells the temperature.

The highest and lowest temperatures reached during a day are recorded with Six's maximum-minimum thermometer.

Transmission of Heat

Hot food left alone slowly cools because its heat flows into the surroundings. Heat always travels from a hotter object to a colder one, and it does so in three ways: conduction, convection and radiation.

Conduction

Stick some iron nails with wax along an aluminium rod fixed to a stand, and heat the free end with a spirit lamp (Fig. 3.3). The nails drop one after another, beginning with the one nearest the flame. This shows that heat moves along the rod from the hot end to the cold end.

This passing of heat from the hotter part to the colder part of a body, without the body itself moving, is conduction. It is the usual way heat travels in solids.

In a solid the particles stay in their places and only vibrate. The particles at the hot end vibrate more strongly and pass this extra energy on to their neighbours, which pass it on to theirs.

Activity: Take two identical rods, one of copper and one of aluminium, fix nails on both with wax at equal spacing, and heat both with the same kind of lamp (Fig. 3.4). The nails on the copper rod fall earlier, so copper conducts heat faster than aluminium.

When the particles of a material are packed close together conduction is fast; when they are far apart it is slow. Materials that let heat pass easily are called conductors (aluminium, silver, copper). Materials that do not let heat pass easily are called bad conductors or insulators (plastic, wood).

You can hold a plastic stick by one end and burn the other end without feeling the heat, but a copper rod heated at one end soon becomes too hot to hold. Both kinds of material are useful: cooking vessels are metal so that heat reaches the food quickly, while their handles are made of insulators.

Uses of Good Conductors

1. Cooking vessels are metallic so heat reaches the food. Vessels with a copper bottom save time and fuel.
2. Mercury is chosen for thermometers partly because it conducts heat well.
3. The radiator tubes of vehicles, and the tubes in air conditioners and refrigerators, are copper. Copper takes heat from the hot fluid inside and passes it quickly to the surroundings.
4. The tip of a soldering iron is copper so that heat reaches the solder.

Uses of Bad Conductors

Glass, wool, cotton, felt, asbestos, wood and similar materials are used where heat flow must be stopped.

1. Woollen clothes and blankets contain a lot of trapped air, which is a bad conductor, so body heat cannot escape and we stay warm in winter. The fur of animals in cold regions works the same way. Mud houses with thatched roofs stay cool in summer and warm in winter because mud and the air trapped in thatch both resist heat flow. Cold stores have an air gap between double walls so that heat does not enter.
2. The gap between the double walls of an ice box is filled with glass wool so that heat does not reach the ice and melt it.
3. Handles of pressure cookers, electric irons and ovens are wood, plastic or ebonite so that our hands are not burnt.
4. Steam pipes from a boiler are wrapped in asbestos or glass wool to cut the heat lost by conduction.

Convection

Water and air conduct heat very poorly, yet liquids and gases do get heated. Half fill a beaker with water, drop in a few crystals of potassium permanganate and heat the beaker from below (Fig. 3.5). Coloured streams are seen rising from the bottom and then sinking again.

The water at the bottom gets hot and expands, so its density falls and it rises. The cooler, denser water at the top sinks to take its place. These circulating streams are called convection currents. Heat carried in this way by the movement of a liquid or a gas is called convection. The experiment shows that (1) convection needs a medium, and (2) in convection the particles of the medium themselves travel from one place to another.

Convection in Gases

Take a wooden box with two glass chimneys A and B on its top, and place a burning candle under chimney B (Fig. 3.6). A smouldering incense stick held over chimney A shows its smoke being drawn down A and coming out of B.

The candle warms the air near B. This lighter air rises out through B, lowering the pressure inside. Heavier cold air rushes in through A and carries the smoke with it. In this way hot air moves away from the source of heat and cold air moves towards it, forming a convection current.

Water is a Bad Conductor of Heat

Put a few pieces of ice, wrapped in copper gauze so that they stay at the bottom, into a hard glass tube. Fill the tube about three-quarters full with ice-cold water and clamp it in a slanting position (Fig. 3.7). Heat the tube near its mouth with a burner.

The water at the top soon boils, yet the ice at the bottom does not melt. So heat is not conducted down through the water; water is a bad conductor.

Air is a Bad Conductor of Heat

Place bits of wax at the bottom of a hard glass tube, close the mouth with a cork, and clamp the tube (Fig. 3.8). Heat the tube near the mouth.

After a while the cork pops out, but the wax at the bottom stays solid. The hot air near the mouth builds up pressure and pushes the cork out, but its heat has not travelled down through the air to the wax. Hence air is a bad conductor.

Uses of Convection in Gases

1. Ventilation: Warm, damp air in a room rises and leaves through ventilators near the ceiling. Fresh cooler air enters through doors and windows, so the air keeps circulating.
2. Sea breeze and land breeze: By day the land warms faster than the sea. Air over the land becomes hot and rises, and cooler air from the sea moves in to replace it; this is the sea breeze. At night the land cools faster than the sea, so the flow reverses and the land breeze blows from land to sea.

Radiation

We feel the Sun's warmth even though the space between the Sun and the Earth has no medium, so the heat cannot have reached us by conduction or convection. The way of passing heat without any medium is called radiation.

Every hot body, whether solid, liquid or gas, radiates heat in all directions. When this heat falls on an object, part is reflected and part is absorbed, and the absorbed part raises the temperature of the object.

Experiment: Paint the outside of one vessel black and that of an identical vessel white, fill both with equal amounts of water and leave them in sunlight for half an hour. The water in the black vessel becomes warmer, so a black surface absorbs more heat than a white one.

Uses of Radiation

1. The bottoms of cooking vessels are given a coat of mud or dull paint so that they absorb more heat.
2. White clothes are worn in summer because white reflects most of the heat falling on it. Dark clothes are better in winter because black absorbs and radiates heat well and so keeps the body warm.
3. Tankers carrying petrol, LPG and other highly inflammable materials are painted white so that they reflect heat and stay cooler.

Thermos Flask

A thermos flask keeps hot liquids hot and cold liquids cold.

Construction: It has a double-walled glass bottle in which the air between the two walls has been pumped out and the gap sealed. The shiny bottle rests on a rubber cork inside a plastic or metal case, and the space between the bottle and the case is packed with cork, which is a bad conductor. The mouth is closed with a plastic stopper and a screw-on cover (Fig. 3.9).

All three ways of losing heat are reduced:
1. Conduction: A vacuum cannot conduct heat, and the cork and glass wool are bad conductors too.
2. Convection: With no air between the walls there are no convection currents.
3. Radiation: The shiny surface of the glass reflects heat back, so little is radiated.

Effects of Heat

On heating, a substance may rise in temperature, change in volume (expand), or change its state.

Expansion of Solids

Most substances expand when heated and shrink when cooled. To see this in a solid, an aluminium rod is clamped at one end and its other part rests on a glass rod lying across a wooden block. Two weights W₁ and W₂ keep the rods in firm contact, and a light pointer is fixed to the glass rod so that it sweeps over a paper scale. When the aluminium rod is heated it lengthens, rolls the glass rod, and the pointer moves over the scale (Fig. 3.10).

Gravesand's Ring and Ball Experiment

The apparatus has a metal ball and a ring whose inner diameter equals the outer diameter of the ball, so the cold ball just passes through the ring (Fig. 3.11(a)). After heating the ball in a spirit-lamp flame it no longer passes through (Fig. 3.11(b)), proving that it has expanded. When the ball cools to room temperature it again slides through, proving that solids contract on cooling.

Uses of Expansion of Solids

1. Iron rims are fitted on wooden cart wheels in the hot state.
2. Metal plates are joined by riveting.
3. A small gap is left between rails when a track is laid.
4. Girders of bridges rest on rollers with some room to move.
5. Telephone and electric wires are not stretched tight but left with a sag.
6. Small gaps are left between slabs when concrete roads are built.
7. Borosil or pyrex glass, which expands very little, is used so that it does not crack when hot water is poured in.

The Bimetallic Strip

A bimetallic strip is made by riveting together two strips of different metals of the same size, for example brass and invar (Fig. 3.12). It works because different metals expand and contract by different amounts.

On heating, brass expands more than invar, so the strip bends with the longer brass on the outside of the curve (Fig. 3.13(a)). On cooling, brass shrinks more, so it comes to the inside of the curve (Fig. 3.13(b)).

Bimetallic strips are used in fire alarms, in thermal switches such as the thermostat of a refrigerator, and in bimetallic thermometers.

Expansion of Liquids

Fill a test tube with water right to the top, close it with a one-holed cork and push a glass tube through the hole so that the water stands a little way up the tube (Fig. 3.14). Note this level, then warm the test tube. The water level climbs slowly; when heating stops, the level comes back down. So liquids expand on heating and contract on cooling.

Anomalous Expansion of Water

Most liquids keep expanding as the temperature rises, but water behaves differently between 0 °C and 4 °C. When water at 0 °C is heated, its volume decreases until 4 °C. At 4 °C water therefore has its smallest volume and greatest density. Above 4 °C it expands like other liquids. This unusual behaviour is called the anomalous expansion of water.

Hope's apparatus demonstrates it. A tall cylinder of water is surrounded in its middle by a trough holding a freezing mixture (common salt and ice mixed in the ratio 1 : 3). The cylinder has a thermometer through a hole near the top and one near the bottom (Fig. 3.15). The cylinder is filled with water at 10 °C, and at first both thermometers read the same.

The water next to the freezing mixture cools and becomes denser, so it sinks. When it reaches 4 °C it is at its densest and settles at the bottom, where the lower thermometer steadies at 4 °C. The water in the middle keeps cooling below 4 °C, becomes lighter and rises, so the upper thermometer falls to 0 °C and a thin layer of ice forms at the top while the bottom remains at 4 °C. This is why fish and other aquatic life survive below the frozen surface of ponds in cold countries.

Expansion of Gases

Fit a cork with a hole in the mouth of an empty test tube, and push through it a narrow glass tube holding a drop of coloured liquid (Fig. 3.16). When the test tube is warmed, the drop moves up because the air inside expands. When heating stops the drop slides back. Solids, liquids and gases in general expand on heating and contract on cooling.

Effect of Heat on Living Organisms

The human body works best at 37 °C (98.4 °F), and every living thing has its own suitable temperature. At very high or very low temperatures the body does not function properly. Hotter water holds less dissolved oxygen, which can kill water animals. Plants too cannot survive extreme heat or cold. Milk and water are boiled to kill germs, and food is kept in a refrigerator to keep bacteria from growing.

Life exists on Earth because its distance from the Sun gives a suitable temperature. Mercury, the planet nearest the Sun, ranges from about 370 °C to −240 °C. Venus reaches about 480 °C. The Moon ranges from 110 °C to −150 °C. The farther a planet is from the Sun, the lower its maximum temperature, and the conditions on other planets make life unlikely; scientists think Mars has little chance of life.

Polar regions are extremely cold, and only well-adapted living things survive there, such as lichens among plants and polar bears, polar pigs and reindeer among animals.

Change of State

Heated water eventually boils and turns into vapour; heated ice turns into water; cooled water turns into ice. These changes (solid to liquid, liquid to solid, liquid to gas and gas to liquid) are called changes of state. A substance changes state only when heat is given to it or taken from it.

Melting Point and Boiling Point

Place crushed ice in a beaker with a thermometer standing in it and heat it with a burner (Fig. 3.17). The temperature climbs to 0 °C and then stops rising while the ice melts, even though heat keeps flowing in. Only after all the ice has melted does the temperature of the water rise again, up to 100 °C. There it stays constant while the water boils away.

A graph of temperature (Y-axis) against time of heating (X-axis) gives the curve of Fig. 3.18, with two horizontal stretches.

The heat supplied during the flat parts is used up in changing the state and does not raise the temperature.

Definitions:
Melting point: the fixed temperature at which a solid turns into a liquid.
Boiling point: the fixed temperature at which a liquid starts to boil.
Freezing point: the temperature at which a liquid turns to a solid by losing heat. It is the same as the melting point of that substance.
Condensation: the change from gas to liquid at a fixed temperature, with heat given out; that temperature is the condensation point.
Sublimation: the direct change of a solid into a gas without becoming a liquid in between. The solid is called the sublimate and its vapour the sublime.
Evaporation: the change of a liquid to a gas at any temperature below its boiling point. It is a slow process, while boiling is quick.

Different Substances Absorb Heat Differently

Put 100 g of water in beaker A and 100 g of coconut oil in beaker B, note both temperatures, and heat them for 5 minutes with identical spirit lamps (Fig. 3.19).

The oil ends up hotter than the water although both received the same heat. So equal heat does not give equal rise of temperature in different substances; to get the same rise, different substances need different amounts of heat.

Heat capacity: the heat needed to raise the temperature of a given body by 1 °C.

Specific Heat

Heat capacity depends on how much of the substance is taken, so a quantity independent of the amount is used instead. The specific heat (specific heat capacity) of a substance is the heat required to raise the temperature of unit mass of it by 1 °C. For a given material it is a fixed value. Water has a higher specific heat than almost any other substance, 1 cal g⁻¹ °C⁻¹, which is why water is used as a coolant.

The heat gained or lost by a body depends on its mass (m), its specific heat (s) and its change of temperature (Δt):

\[ Q = m s \Delta t, \qquad s = \frac{Q}{m \Delta t} \]

Example. Find the heat needed to raise the temperature of 250 g of water from 27 °C to 67 °C (specific heat of water = 1 cal g⁻¹ °C⁻¹).

Δt = 67 − 27 = 40 °C
Q = m s Δt = 250 × 1 × 40 = 10,000 cal = 10 kcal

Units of Heat

The calorie is the heat required to raise the temperature of 1 g of water from 14.5 °C to 15.5 °C, that is by 1 °C. Because the calorie is small, the kilocalorie (kcal), the heat needed to warm 1 kg of water by 1 °C, is also used: 1 kcal = 1000 cal.

The SI unit of heat, as for all energy, is the joule. Experiments show that 4.2 J of work produces 1 calorie of heat, so 1 cal = 4.2 J and 1 kcal = 4200 J.

Calorific Value of Fuels

The calorific value of a fuel is the total heat produced when a unit mass of the fuel burns completely.

\[ \text{Calorific value}\ (C) = \frac{Q}{M} \]

Its units are J kg⁻¹ or cal g⁻¹. Gaseous fuels generally have higher calorific values than solid or liquid fuels.

Example. Find the heat produced by 150 g of a fuel whose calorific value is 8000 cal g⁻¹.

Q = C × M = 8000 × 150 = 12,00,000 cal = 12 × 10⁵ cal

Calorimeter

A calorimeter is a polished copper vessel used to measure quantities of heat. The vessel sits inside a wooden box, with the space between them filled with a bad conductor such as cork or sawdust. The box has a lid of ebonite (or another bad conductor) with two holes, one for a stirrer and one for a thermometer (Fig. 3.20).

Its working rests on the fact that when a hot body is put in contact with a colder one, the hot body loses heat and the cold one gains it. To find the specific heat of a substance, a known mass of water is placed in the calorimeter and its temperature noted. The substance is heated to a known temperature and dropped into the water. After stirring, the final steady temperature is read, and by equating heat lost with heat gained the specific heat of the substance is calculated.

Points to Remember

1. Temperature is the degree of hotness or coldness of a body; it is measured in °C, °F or K with a thermometer.
2. A clinical thermometer measures body temperature.
3. Heat flows from a body at higher temperature to one at lower temperature until both have the same temperature.
4. Heat travels by conduction, convection and radiation. Conduction and convection need a material medium; radiation does not and is the fastest.
5. Conductors pass heat easily; bad conductors do not.
6. Heat can change the dimensions, state and chemical nature of a substance.
7. A bimetallic strip works on the unequal expansion or contraction of two metals.
8. A change of state takes place at a constant temperature.
9. The boiling point is the constant temperature at which a liquid boils. The freezing point equals the melting point, and heat is released during freezing.
10. Heat capacity is the heat needed to raise the temperature of a body by 1 °C; specific heat is the heat needed for unit mass.
11. Heat gained or lost: Q = m s Δt.
12. 1 cal = 4.2 J and 1 kcal = 1000 cal = 4200 J.
13. A calorimeter is used to find specific heats, using the principle that heat lost by the hot body equals heat gained by the cold body.