Energy Transfer Notes

Energy Transfer

Learning Objectives

  • Describe how energy is transferred by particles in conduction and convection.

  • Explain how an insulator can reduce energy transfer.

  • Explain how energy is transferred by radiation.

  • Describe some sources of infrared radiation.

Keywords

  • Conductor

  • Conduction

  • Convection

  • Radiation

  • Insulator

  • Convection current

  • Infrared radiation

  • Thermal imaging camera

Flashback Questions

  1. What is the difference between thermal energy and temperature?

  2. What 3 factors affect the energy needed to increase the temperature of an object?

  3. Which state of matter can be compressed and why?

Flashback Answers

  1. Thermal energy is measured in Joules and temperature is measured in Degrees Celsius.

  2. Mass of object, what the object is made of, and the temperature rise that you want.

  3. Gases can be compressed due to the large spaces between their particles.

Learning Journey

  • Today: Energy transfer: particles and radiation.

Starter Activity

  • Energy is always transferred from a hot object to a cooler object, never the other way around.

Keyword Definitions

  • Conductor: A material that conducts charge or energy well, such as a metal or graphite.

  • Conduction: The way in which energy is transferred through solids, and to a much lesser extent in liquids and gases.

  • Convection: The transfer of energy by the movement of gases or liquids.

  • Radiation: The transfer of energy as a wave.

  • Insulator: A material that doesn’t conduct electricity or transfer energy well.

  • Convection current: The movement of heated fluids (liquids or gases).

  • Infrared radiation: Radiation given off by the sun and other objects that brings about energy transfer.

  • Thermal imaging camera: A camera that absorbs infrared and produces a (false-colour) image.

Conduction

  • The transfer of thermal energy through a material by direct contact.

  • Energy in the hot part of the bar is transferred along the bar, making particles vibrate more.

  • Vibrating particles transfer some of their energy to the next particles in the bar.

  • As energy is transferred to the metal bar, its particles vibrate faster.

Conduction - Fill in the Blanks

  • Thermal energy is given to some of the particles causing them to VIBRATE. This causes the neighbouring PARTICLES to vibrate, passing on the energy. This happens mainly in SOLIDS because the particles are packed really CLOSE to each other.

Convection

  • The rising and falling of liquids and gases due to differences in density (which are caused by differences in temperature).

  • Warm air transfers energy to the air around it and cools down.

  • Air sinks to replace cooler air near the floor.

  • Air is pushed out of the way by rising warm air.

  • Air becomes warmer and rises.

Convection - Complete the Sentence

  • Convection is the transfer of heat energy from HOT places to COOLER places.

How Convection Works

  • Convection occurs in liquids and gases. When molecules are heated, they vibrate. These vibrating molecules bump into the neighboring molecules causing them to move apart. This decreases the density which results in hot molecules rising.

Convection in a Lava Lamp

  • In a lava lamp, there is a heater at the bottom. This heats the lava making it less dense. This lower density causes it to rise. When it reaches the top, it cools down. This causes the density to increase and thus the lava falls down again.

Everyday Examples of Convection

  • Heaters in a house, water in a kettle, ice in a drink.

Can Convection Happen in Solids?

  • No, convection requires rising and falling to transfer energy. As there is no movement of atoms in a solid, convection cannot occur.

Particles and Sound

  • Sound energy is transferred to the surroundings, causing the air and walls to get a bit warmer. The particles in the air move a bit faster and the particles in the walls vibrate more.

Do All Solids and Liquids Conduct Energy?

Solid Conductor vs. Solid Insulator

  • Energy is not easily transferred through materials like wood. Wood and many non-metals are poor conductors and are insulators. This does not mean that they do not conduct at all, but that energy is transferred very slowly through them.

Insulating Liquids and Gases

  • Liquids are poor conductors. Divers wear wetsuits, which use a thin layer of water against the skin as an insulator to keep them warm.

Explain Thermal Energy Transfer - Convection

  • When a substance is heated, the molecules get more energy and start to RISE. When they cool down and lose their energy, they move closer to each other, making the substance heavier so it begins to SINK. This process is called CONVECTION. It can only happen in FLUIDS. This means substances that can flow so LIQUIDS and GASES.

Soup Pan Heating Sequence

  1. The base of the pan heats up.

  2. Soup at the bottom of the pan warms up.

  3. Particles in warm soup move faster than particles in cool soup.

  4. This means particles in warm soup move further apart.

  5. This means the warm soup is less dense than cool soup.

  6. The warm soup rises to the top of the liquid and the cool soup falls to the bottom of the liquid.

  7. The soup continues to circulate until all the soup is hot.

Radiation

  • All objects emit infrared radiation.

  • The hotter an object is, the more infrared radiation it gives off.

  • Infrared cannot be seen, only felt as heat.

  • Radiation is energy transmitted by waves.

Emitting Infrared Radiation

  • All objects give out or emit radiation.

  • The type of radiation that they emit depends on their temperature.

  • How much radiation they emit per second depends on the type of surface.

  • Infrared can be transmitted, absorbed, or reflected just like light.

Infrared Images

  • Infrared images show the amount of heat being radiated from an object.

Absorption of Radiation

  • Dull, dark colours are good absorbers of radiation.

Reflection of Radiation

  • Light, shiny surfaces will reflect radiation.

Questions

  1. Why do you feel hotter wearing a black top than any other colour?

  2. Why do people paint their houses white in hot countries?

  3. Why are shiny foil blankets wrapped around runners after a race?

Answers

  1. Matt black absorbs the most radiation.

  2. White reflects heat radiation and keeps the house cooler.

  3. The shiny metal reflects the heat radiation from the runner back in, this stops the runner getting cold.

Key Knowledge - Radiation

  • Heat can be transferred by conduction, convection, and radiation.

  • Radiation is transmitted by waves, NOT particles.

  • Radiation can work through the vacuum of space because no particles are involved.

Particles and Sound Reminder

  • When you play music, energy is transferred to the surroundings. The air and walls get a bit warmer. The particles in the air move a bit faster and the particles in the walls vibrate more.

Vacuum Flask

  • A vacuum has no particles; therefore, it stops thermal energy from being lost by conduction and convection.

  • The plastic stopper is a good insulator, and it also stops convection currents forming at the top of the flask.

  • Glass container: glass is a poor conductor, reducing energy transfer by conduction.

  • The shiny walls reflect the infrared radiation so heat cannot escape via radiation.

  • Conduction, convection, and radiation are reduced to help keep your tea hot or your drink cool.

Questions

  1. Define “conductor” and “insulator.”

  2. How does the thickness of walls affect how warm a house is?

  3. What does the thermal conductivity tell us about these materials (ie. best conductors/ insulators)?

  4. Explain which of the materials above would be best to insulate a house.

Answers

  1. Conductors let thermal energy through. Insulators do not let thermal energy through.

  2. The thicker the walls of a house, the less thermal energy can transfer through them.

  3. The best conductors are copper (385 W/mK) and lead (34.7 W/mK), as they have a high conductivity and are metals. The best insulators are fibreglass (0.04 W/mK) and polystyrene (0.03 W/mK). Glass (0.8 W/mK) and brick (0.6 W/mK) are in the middle.

  4. A good house could be insulated with fibreglass and polystyrene as these are good insulators with low thermal conductivities and this will reduce heat loss. However, brick and glass are good as they are also strong.

Plenary - True or False

  1. When particles in a solid vibrate, they cause neighboring particles to vibrate TRUE

  2. Convection only occurs in gases and liquids TRUE

  3. When a gas gets hot its molecules move further apart from one another TRUE

  4. Conduction only occurs in solids and gases FALSE – conduction occurs in solids

  5. Metals are good conductors TRUE

  6. Plastic is a good conductor FALSE – plastic is a bad conductor

  7. Wood is a bad conductor TRUE

  8. Radiators warm our room through convection TRUE

Exam Question

  • Compare and contrast the differences between conduction and convection

Exam Question - Answers

  • Convection occurs in fluids, liquids, gases

  • Convection has moving molecules

  • Convection causes density to change

  • Convection causes hot fluids to rise/cold fluids to sink

  • Conduction occurs in solids

  • Conduction causes particles to vibrate

  • Conduction doesn’t cause particles to move

  • Conduction occurs best in metals and worst in insulators

Summary of Key Knowledge

  1. Energy is transferred by conduction, convection, or radiation

  2. Conduction: Particles collide with each other when they vibrate, transferring energy through the material.

    • Solids and metals are the best conductors. Materials that are poor conductors are called insulators.

  3. Convection: If a heat source is on the bottom of a material, hotter particles spread out and move faster, decreasing density. This causes heated particles to rise. Colder particles fall onto the heat source and begin to be heated as well. This causes convection currents.

    • Examples - saucepan, hurricane, Earth’s crust

  4. Radiation: Does not require particles (a medium) as it is transferred by waves e.g. infrared or light. When a surface absorbs infrared it feels hotter.

    • Dark colours absorb radiation, shiny surfaces reflect it, see-through objects transmit it.

    • Examples: space (vacuum), thermal imaging, foil

Types of Heat Transfer - Summary

  • Conduction – Only happens in solids

  • Convection – Only happens in fluids (liquids & gases)

  • Radiation – Does not use particles – Can occur anywhere, even in a vacuum – Transmitted as Infrared radiation waves

Conductors vs. Insulators

  • Substances that allow thermal energy to move easily through them are called conductors. Metals are good conductors of thermal energy.

  • Substances that do not allow thermal energy to move through them easily are called insulators. Air and plastics are insulators.

  • Conduction → the process through which energy is transferred.

  • Air is an insulator because its molecules are far apart.