Heat transfer


Topic 6 Heat Transfer

General Heat Transfer

  1. What is heat transfer?
Answer: The movement of heat energy from a hotter place to a colder place.
  1. What are the three processes by which heat can be transferred?
Answer: Conduction, convection, and radiation.


Conduction

  1. What is conduction?
Answer: The transfer of heat energy through solids.
  1. What happens when a hotter substance is in direct contact with a cooler substance?
Answer: Heat energy is transferred through the bumping of molecules.
  1. What substances are good conductors of heat?
Answer: Metals, such as silver and copper.
  1. What substances are poor conductors of heat?
Answer: Non-metals, such as plastic, foam, wood, and rubber.
  1. Why are poor conductors of heat used as insulators?
Answer: To prevent the movement of heat.

Convection

  1. What is convection?
Answer: The transfer of heat energy through liquids and gases by the movement of molecules.
  1. What happens when a liquid or gas is heated?
Answer: The molecules move faster and further apart, decreasing the density of the heated part.
  1. How does heat travel through gases and liquids?
Answer: By convection, through the movement of molecules.
  1. How do convection currents warm the air in a room with a fireplace?
Answer: By rising and sinking of warm and cool air, respectively.
  1. What is the role of density in convection currents?
Answer: The heated part becomes less dense and rises, while the cooler part becomes denser and sinks.
  1. What is an example of convection in a liquid?
Answer: Boiling water.
  1. How does convection help in the formation of clouds?
Answer: Rising air helps form clouds.
  1. What is the relationship between convection currents and the temperature of the land and water?
Answer: Convection currents are affected by the temperature difference between the land and water.


Radiation

  1. How does heat from the sun reach the earth?
Answer: By radiation.
  1. Why can't heat be transferred through space by conduction and convection?
Answer: Because there is almost no matter in space.
  1. What is an example of radiation?
Answer: The heating effect of sunshine on our bodies on a clear day.
  1. How can you observe radiation in your home?
Answer: By sitting near a fire or electric heater and feeling the heat on your hands without direct contact.
  1. What is transferred by radiation without passing through matter?
Answer: Heat energy.
  1. What is the purpose of Activity 24?
Answer: To show the movement of heat through solids.
  1. What happens when a metal spoon is placed in a heat source?
Answer: The handle of the spoon becomes warm.
  1. Why were you able to feel the sensation of heat after the spoon had been heated?
Answer: Because heat is transferred through the metal spoon by conduction.
  1. What is the result of heat being transferred through the metal spoon?
Answer: The handle of the spoon becomes warm.
  1. Why is it important to be cautious when handling hot objects?
Answer: To avoid burning your hands.
  1. What is the explanation for the sensation of heat felt after the spoon had been heated?
Answer: Heat is transferred through the metal spoon by conduction.
  1. What is the purpose of the experiment with the metal spoon?
Answer: To demonstrate the transfer of heat through solids.
  1. What type of energy is transferred by radiation?
Answer: Heat energy.
  1. What is an example of radiation in everyday life?
Answer: The heating effect of a fire or electric heater.
  1. How does radiation differ from conduction and convection?
Answer: Radiation transfers heat without passing through matter, while conduction and convection require a medium to transfer heat.
  1. What is required for conduction and convection to transfer heat?
Answer: A medium, such as matter.
  1. How does radiation enable heat transfer in a vacuum?
Answer: Radiation can transfer heat without passing through matter.
  1. What is an example of radiation enabling heat transfer in a vacuum?
Answer: The sun's heat reaching the earth through space.
  1. How does the temperature of an object affect its ability to radiate heat?
Answer: Hotter objects radiate more heat than cooler objects.
  1. What is the relationship between radiation and the color of an object?
Answer: Dark-colored objects tend to absorb and radiate more heat than light-colored objects.
  1. What happens to the heat energy radiated by an object?
Answer: It is transferred to surrounding objects or space.
  1. Can radiation occur through a medium, such as air or water?
Answer: Yes, but it is more effective in a vacuum.
  1. How does radiation play a role in the Earth's climate?
Answer: It helps regulate the planet's temperature by transferring heat from the sun to the Earth.
  1. What is the difference between radiation and conduction in terms of heat transfer?
Answer: Radiation transfers heat without physical contact, while conduction requires direct contact between objects.
  1. Can you think of an example where radiation is used in everyday life?
Answer: A microwave oven uses radiation to heat food.


Activity 25

  1. What is the purpose of Activity 25?
Answer: To show the movement of heat through liquids.
  1. What substance is used to demonstrate the movement of heat through liquids?
Answer: Potassium permanganate.
  1. What safety precautions should be taken when working with potassium permanganate?
Answer: Wearing latex gloves, as it is poisonous and can cause stains.
  1. What is observed when the potassium permanganate is heated at the bottom of the beaker?
Answer: The color pattern of the potassium permanganate moves upward through the water.
  1. What does the movement of the color pattern indicate?
Answer: The transfer of heat through the liquid (water).

How a Vacuum Flask Works

  1. What is the purpose of a vacuum flask?
Answer: To keep liquids hot or cold by preventing heat transfer.
  1. How does a vacuum flask prevent heat transfer?
Answer: By having a vacuum between the inner and outer walls, which prevents conduction and convection.
  1. What feature of the vacuum flask prevents radiant heat transfer?
Answer: The silver-coated walls of the outer housing, which reflect radiant heat.
  1. What would happen if the flask is left open?
Answer: Heat would escape by convection, conduction, and evaporation.

Activity 26

  1. What is the aim of Activity 26?
Answer: To observe the movement of heat in gases by observing the movement of smoke.
  1. What materials are needed for Activity 26?
Answer: Cardboard box, candle, paper, matches or lighter.
  1. What happens to the smoke from the paper when the candle is lit?
Answer: The smoke moves from the paper towards the candle.
  1. Why does the smoke move in that direction?
Answer: Because heat from the candle causes the air near it to rise, creating a convection current that pulls the smoke towards the candle.


Activity 27

  1. Name three methods of heat transfer.
Answer: Conduction, convection, and radiation.
  1. In which state of matter does convection take place?
Answer: Liquids and gases.
  1. Give an example of convection currents in everyday life.
Answer: Boiling water or air rising from a heater.
  1. Explain how heat transfers from the sun to earth.
Answer: Through radiation.
  1. Name the device that can keep liquids at a constant temperature.
Answer: A vacuum flask (thermos).
  1. Name the part of the device that helps reduce heat loss by radiation.
Answer: The silver-coated walls of the outer housing.
  1. Which types of heat transfer can the vacuum in this device prevent?
Answer: Conduction and convection.

Heat and Expansion of Substances

  1. What happens to substances when they are heated?
Answer: They expand and take up more space.
  1. Why do substances expand when heated?
Answer: Because the particles move faster and further apart.
  1. What happens to substances when they are cooled?
Answer: They contract and take up less space.
  1. Why do substances contract when cooled?
Answer: Because the particles move slower and closer together.
  1. Do the particles of a substance change size or number when it expands or contracts?
Answer: No, the particles stay the same size and number, but the space between them changes.
  1. How do the particles of a solid behave when it is heated?
Answer: They vibrate more and take up more space.
  1. How do the particles of a liquid behave when it is heated?
Answer: They move around more and take up more space.
  1. How do the particles of a gas behave when it is heated?
Answer: They move more quickly in all directions and take up a lot of space.

Expansion of Solids

  1. What happens to most solids when they are heated?
Answer: They expand.
  1. What happens to most solids when they are cooled?
Answer: They contract.
  1. How does temperature affect the size of objects?
Answer: When temperature is raised, dimensions (length, width, height) increase.
  1. Why do different solids have different expansion rates?
Answer: Because the particles of different solids behave differently when heated.
  1. Which solid expands the most when heated?
Answer: Aluminium.
  1. Which solid expands the least when heated?
Answer: Steel.


Activity 28

  1. What is the aim of Activity 28?
Answer: To show that solids expand when heated.
  1. What materials are needed for Activity 28?
Answer: Metal ball and ring.
  1. What happens to the metal ball when it is heated?
Answer: It expands and no longer fits through the ring.


Expansion of Liquids

  1. What happens to the volume of a liquid when it is heated?
Answer: It increases.
  1. Why is it necessary to heat the container along with the liquid?
Answer: To account for the expansion of the container itself.
  1. How can you determine if the liquid has expanded more than the container?
Answer: If the liquid level in the container has risen after heating.


Activity 29

  1. What is the aim of Activity 29?
Answer: To show that liquids expand when heated.
  1. What materials are needed for Activity 29?
Answer: Flat-bottomed flask, coloured water, rubber stopper, glass tube, tripod, and source of heat.
  1. What happens to the level of the coloured water when the flask is heated?
Answer: It rises.
  1. What happens to the level of the coloured water when the flask is cooled?
Answer: It falls.
  1. What does the experiment demonstrate?
Answer: That liquids expand when heated and contract when cooled.


Observations/Results

  1. What happens to the water level in the experiment?
Answer: It falls at first, and then rises steadily.
  1. Why does the water level fall initially?
Answer: Because the flask expands first before the heat reaches the water.
  1. Why does the water level rise afterwards?
Answer: Because the volume of the water increases as it is heated.


Expansion of Gases

  1. Why can the expansion of the container be ignored when studying the expansion of gases?
Answer: Because the expansion of gases is much larger than the expansion of the container.


Activity 30

  1. What is the aim of Activity 30?
Answer: To show that gases expand when heated.
  1. What materials are needed for Activity 30?
Answer: Source of heat, round-bottomed flask, delivery tube, rubber stopper, water bath, and beaker.
  1. What happens to the air in the round-bottomed flask when it is heated?
Answer: It expands.
  1. How is the expansion of the gas demonstrated in the experiment?
Answer: By the rise in the water level in the beaker.

  1. What happens to the air molecules in the round-bottomed flask when it is heated?
Answer: They gain energy, move faster and further apart, and increase in volume.
  1. What is the result of the air expanding in the flask?
Answer: The air escapes from the flask through the delivery tube and forms bubbles in the beaker of water.




  1. What happens to the water and ethanol as they become warmer?
Answer: They occupy more space and their levels rise in the tubes.
  1. Why do the liquid levels rise?
Answer: Because the volume of the liquids increases due to heating and thermal expansion.
  1. What can be concluded from the experiment about the expansion of different substances?
Answer: Different substances expand by different amounts when heated by the same heat source.


Expansion and Contraction

  1. What happens to the volumes of solids, liquids, and gases when they are heated or cooled?
Answer: Their volumes change.
  1. What happens to the movement of particles when heat is added to a substance?
Answer: The movement of particles increases.
  1. What is the result of the increase in movement of particles?
Answer: An increase in the space occupied by the substance, known as expansion.
  1. What happens to the particles of a substance when it is cooled?
Answer: They lose energy, move less, and are closer together, resulting in a decrease in volume, known as contraction.


Uses of Expansion of Different Substances in Everyday Life

  1. What is a common use of the expansion of liquids?
Answer: In thermometers, where the expansion of the liquid is used to measure temperature.
  1. How does a thermometer work?
Answer: The liquid in the bulb expands and rises up the tube when heated, indicating the temperature.

Thermometers

  1. What substances are used in thermometers used by doctors and scientists?
Answer: Mercury or alcohol.
  1. Why are these substances used?
Answer: Because they expand quickly.


Thermostats

  1. What is the purpose of a thermostat?
Answer: To control the temperature of indoor areas and electrical appliances.
  1. How do thermostats maintain a steady temperature?
Answer: By switching appliances on and off.
  1. What devices use thermostats?
Answer: Air conditioners, refrigerators, ovens, irons, and fire alarms.


Bimetallic Strips

  1. What is a bimetallic strip?
Answer: A device that makes use of different rates of expansion of solids, consisting of two different metals joined together.
  1. How does a bimetallic strip work?
Answer: When heated or cooled, one metal expands or contracts faster than the other, causing the strip to bend.
  1. What happens to a brass and steel bimetallic strip when it is heated?
Answer: It bends towards the steel because brass expands more than steel.
  1. What happens to a brass and steel bimetallic strip when it is cooled?
Answer: It bends towards the brass because brass contracts more than steel.

  1. How does the bimetallic strip in a thermostat control the temperature?
Answer: By bending and breaking contact when heated, and reconnecting when cooled.


Internal Combustion Engines

  1. What is an internal combustion engine?
Answer: A device that uses contraction and expansion to generate power.
  1. What are the four stages of an internal combustion engine?
Answer: Intake stroke, compression stroke, power stroke, and exhaust stroke.
  1. What happens during the intake stroke?
Answer: The piston moves down, and air and fuel are drawn into the cylinder.
  1. What happens during the compression stroke?
Answer: The piston moves up, and the air-fuel mixture is compressed into a smaller volume.
  1. What happens during the power stroke?
Answer: A spark ignites the compressed air-fuel mixture, causing it to explode and expand, which forces the piston down.
  1. What happens during the exhaust stroke?
Answer: The piston moves up, and the waste exhaust gases are forced out of the exhaust cylinder.

  1. What are the main parts of an internal combustion engine?
Answer: Cylinders, spark plugs, pistons, and crankshaft.
  1. What happens to the fuel-air mixture in the cylinder?
Answer: It is ignited by a spark from the spark plug, burns at a high temperature, and expands rapidly.
  1. How does the expansion of the hot air affect the piston?
Answer: It pushes the piston down.
  1. What happens when the piston moves?
Answer: It pushes the crankshaft, which turns the wheels.


Activity 32

  1. What happens to the bimetallic strip when it is heated evenly?
Answer: One metal expands more than the other, causing the strip to bend.
  1. If the metals are iron and copper, which metal expands more?
Answer: Copper.
  1. What device uses a bimetallic strip to control temperature?
Answer: A thermostat.
  1. What electrical appliance uses a thermostat?
Answer: A refrigerator or air conditioner.


Effects of Expansion and Contraction of Substances

  1. Why must the expansion and contraction of solids be considered when designing and building structures?
Answer: To prevent damage from changes in temperature.
  1. How are bridges designed to accommodate expansion and contraction?
Answer: With steel rollers or sliding expansion joints to allow for movement.

  1. Why is it necessary to leave a gap between the glass pane and the window frame?
Answer: To allow the glass to expand when it gets hot and not press against the sides of the window frame.
  1. What happens if the glass pane is too big for the window frame?
Answer: The glass would crack with expansion.
  1. How is the glass pane secured in the window frame?
Answer: With putty, which keeps it in the correct position and allows the glass to expand.
  1. What happens to the metal rails of railway tracks when the temperature increases?
Answer: They expand.
  1. How are the metal rails of railway tracks laid to accommodate expansion?
Answer: In sections with expansion gaps between them.
  1. What is the purpose of fish plates on railway tracks?
Answer: To join sections of railway track and allow for expansion due to heating.

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