Heat Transfer Concepts

• Woollen clothes are from animal fibres, while cotton clothes are from plant fibres
• Woollen clothes keep us warm in winter
• Light coloured cotton clothes keep us cool in summer

Hot And Cold

• Some objects are hot while some are cold
• Some objects are hotter or colder than others
• The sense of touch can sometimes be unreliable when deciding if something is hot or cold

Activity 3.1

• Fill three containers with cold water, hot water, and a mix of both
• Dip one hand in the cold water, and the other in the hot water
• Then dip both hands in the mixed water to see how temperature perception differs

Temperature

• Temperature is a reliable measure of the hotness of an object
• A thermometer measures temperature

Clinical Thermometer

• A clinical thermometer measures body temperature, and contains mercury
• Ensure you do not touch objects that are too hot, like a candle flame or a stove
• Clinical thermometers range from 35°C to 42°C

Reading a Thermometer

• Note the temperature difference between the bigger marks
• Note the number of divisions between them
• Thermometers use the celsius scale, indicated by °C
• India has adopted the celsius scale for thermometers
• The fahrenheit scale (°F) was previously used

Precautions For Clinical Thermometers

• Wash before and after use, using antiseptic solution
• Ensure mercury level is below 35°C before use
• Read the thermometer keeping the mercury level along the line of sight
• Handle with care because it can break easily
• Avoid holding the bulb while reading

Taking Temperature

• Hold the thermometer firmly and give it a few jerks to bring the mercury down
• Place the bulb of the thermometer under your tongue for one minute
• Take the thermometer out and note the reading, stating the units as °C
• The normal human body temperature is 37°C

Body Temperature

• Body temperature varies slightly from person to person
• Normal temperature is the average body temperature of many healthy people
• Clinical thermometers designed to measure human body temperature
• This is because the human body normally stays between 35°C and 42°C

Laboratory Thermometer

• Do not use clinical thermometers to measure other object's temperatures
• Take some tap water in a beaker or mug
• Dip the thermometer in the water, ensuring the bulb is immersed but not touching the container
• One type is laboratory thermometer

Reading A Laboratory Thermometer

• Laboratory thermometers generally range from -10°C to 110°C
• The laboratory thermometer should be kept upright, not tilted
• The bulb should be supported by the substance being measured, not the container

Laboratory Thermometer Vs Clinical Thermometer

• Cannot take the thermometer out of the water when reading
• Mercury levels will fall as soon as removed from the liquid
• Clinical thermometer cannot be used to measure high temperatures
• A laboratory thermometer cannot measure body temperature

Clinical Thermometers

• Clinical thermometers have a kink near the bulb
• This prevents the mercury level from falling on its own when reading
• Digital thermometers are available that do not use mercury

Transfer of Heat

• A frying pan heats up because heat passes from the flame to the utensil
• The pan cools down when removed from the fire, as the heat transfers to the surroundings
• Heat flows from a hotter object to a colder object

Conduction

• Heat is transferred from a flame through a metal strip
• This process is known as conduction
• The process by which heat transfers from the hotter end to the colder end of an object
• Solid materials are generally conductive
• Materials allowing heat to pass through easily are conductors of heat
• Aluminum, iron, and copper are conductors of heat
• Materials that do not allow heat to pass through easily are insulators
• Plastic and wood are insulators
• Water and air are poor conductors of heat

Convection

• When water is heated, hot water rises and cold water moves towards the source of heat
• This process continues until all the water is heated
• This mode of heat transfer is convection

Air Convection

• Air near a heat source gets hot and rises, and air from the sides replaces it
• The air at the top of a flame feels hotter due to convection
• There is no convection on the sides, so it does not feel as hot

Coastal Areas

• During the day, the land heats faster than the water
• Hot air rises, and cooler air from the sea replaces it
• This air from the sea is called the sea breeze
• Houses in coastal areas often face the sea to receive the sea breeze
• At night, the water cools slower than the land, reversing the effect
• Cool air from the land moves toward the sea, called the land breeze

Radiation

• We feel warm in the sun from radiation
• Radiation does not require any medium for heat transfer
• When we sit near a heater we feel the heat from radiation
• Hot items cool through radiation, transferring heat to their surroundings
• All hot bodies radiate heat
• When the sun shines on an object, some heat is reflected, absorbed, and transmitted
• The absorbed heat raises the object's temperature

Kinds Of Clothes

• In summer, light-colored clothes are preferred, while dark clothes are worn in winter
• Dark surfaces absorb more heat, making dark clothes comfortable in winter
• Light-colored clothes reflect heat, making them comfortable in summer

Activity 3.10

• Paint two identical cans, one black and one white
• Fill with equal amounts of water then leave in sunlight
• Measure the temperature of the water
• Dark surfaces absorb more heat

Activity 3.11

• Fill two cans with equal amounts of hot water
• Leave the cans in a room or shade before measuring the temperature
• The temperature of the water will decrease more in the light coloured can

Clothes In Winter

• Woollen clothes are worn in winter
• Wool is a poor conductor of heat
• Air trapped between wool fibres prevents heat flow from our body to the surroundings
• A thick blanket vs two thin blankets creates an extra layer of air
• This provides more warmth than one thick blanket