Chapter 3 – Heat
- What is Heat? (What is Heat?)
- Heat is a form of energy that sensations the warmth or coolness of an object.
- Heat always flows from an object with a higher temperature to an object at a lower temperature .
- We can tell by our sense perception whether something is hot or cold, but it is not always reliable.
- Temperature : This is the measure of the degree of heat of an object.
- Measuring Temperature
- A thermometer is used to measure temperature.
- The common units of temperature are Celsius (°C) and Fahrenheit (°F).
- Types of Thermometers :
- Clinical Thermometer:
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- It is used to measure the temperature of the human body.
- It usually ranges from 35°C to 42°C or 94°F to 108°F.
- It has a kink or dissolution that prevents the mercury level from falling down so as to get enough time to take the readings.
- It should not be used to measure the temperature of other objects.
- It should be washed with an antiseptic solution before and after use.
- The mercury level should be below 35°C.
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- Laboratory Thermometer:
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- It is used to measure the temperature of other objects or substances.
- Its range is usually from -10°C to 110°C.
- It doesn’t have the kink of a medical thermometer.
- The thermometer must be kept upright when using it and the bulb should not be taken out of the substance whose temperature is being measured when taking the reading.
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iii. Digital Thermometer:
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- These are mercury-free and safe to use.
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- Normal human body temperature: 37°C or 98.6°F.
- Transfer of Heat
Heat is transferred from one place to another in three ways:
- Conduction:
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- It is a method of heat transfer in which heat is transferred by direct contact with particles.
- It mainly occurs in solids.
- The heat travels from the hot end to the cold end.
- Conductors: Materials that allow heat to pass through them easily.
- Examples: metals (e.g. iron, copper, aluminum).
- Misconductor Insulators: Substances that do not allow heat to pass through them easily.
- Examples: wood, plastic, air, wool.
- Convection:
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- It is a method of heat transfer in which heat is transferred by the actual motion of particles of matter.
- It occurs mainly in liquids and gases.
- When a liquid or gas is heated, the hot particles rise up and the cold particles come down and heat up, creating a convection current.
- Example:
- Sea Breeze: During the day, the land warms faster than the sea. The air above the land warms up and rises, and the cold air from the sea flows towards the land.
- Land Breeze: At night, the sea cools slower than the land. The air over the ocean warms up and rises, and cold air from the land flows towards the sea.
- Working a heater or air conditioner in the room.
iii. Radiation :
- It is a method of heat transfer in which no medium is required to transfer heat.
- Heat is transmitted in the form of electromagnetic waves.
- Example:
- Heat from the sun to the earth is brought by radiation (because there is no medium in space).
- The warmth we feel when standing near a fire.
- Hot objects emit heat by radiation.
- Clothes in Winter and Summer
- Woollen Clothes in Winter:
- Wool is a poor conductor of heat.
- Air is trapped between the woollen fibres, and air is also a poor conductor of heat.
- It prevents heat from escaping from our body, making us feel hot.
- It is better to wear dark-coloured clothes in winters as they absorb heat more.
- Light-Coloured Clothes in Summer:
- Light-colored clothes absorb less heat and reflect more.
- This makes us feel cool in the summer.