How is heat (thermal

energy) transferred?

Journal WorkA hot cup of cocoa is left on the kitchen counter. A bowl of ice cream is left on the dining room table. Describe what happens to both, and explain in terms of “thermal energy” and the motion of atoms.

How is Heat Transferred?

Heat is transferred in three ways:

Thermal Energy Transfer

CONDUCTION: heat transferred when two objects touch. (works best in solids)

Thermal Energy Transfer: Conduction

Conduction works better in solids (such as land and metals), but not as well in liquids and gases (air and water do not conduct heat as well.)

Examples of conduction where heat is transferred from particle to particle.

How is the thermal energy being transferred in each picture?

Conduction Ex. #1

the heat is transferred from the frying pan to the cooking egg

Conduction Ex. #2

the heat is transferred from the hot coffee to the spoon

Conduction Ex. #3

the heat is transferred from the feet to the cold tile floor

Thermal Energy Transfer

CONVECTION: heat transferred by the circulation or movement of a liquid or gas. (fluids)

A fluid is a liquid or gas and has the ability to flow.

Solids, however, maintain their position

Thermal Energy Transfer: Convection

Thermal Energy Transfer: Convection

When you look at the road in the summertime on a hot day, you may notice that the air above the road looks “blurry” – this is convection taking place as the hot air directly over the road absorbs the heat from the road and rises.

Although the explanation can be quite complex, convection can help explain why mirages are seen in the desert.

Thermal Energy Transfer: Convection

Examples of convection where heat is transferred in fluids.

Convection Ex. #1

Convection occurring in water inside a pot that is on a stove

Convection Ex. #2

Convection occurring in a room with a radiator.

Convection Ex. #3

Convection occurring in the atmosphere.

Convection CurrentsThe upward movement of warm air and downward movement of cool air forms convection currents.Heated fluid (liquid or gas) expands, becoming less dense. Because it is less dense, it rises. Cooler fluids rushes in to replace the air that lifted up. As the warm fluid rises and cool fluid falls, a circular pattern is created known as a convection currentEventually the warmer air cools and begins to fall again.

COLDHOT

Thermal Energy TransferIf conduction deals with solids and convection deals with liquids and gases, how does heat arrive to Earth from the Sun?

There is very little matter in the 93 million miles between the Sun and the Earth.

• Heat can travel in waves without a medium (a liquid, solid, or gas).

• These waves are called electromagnetic waves.

Thermal Energy Transfer

Electromagnetic Wave Spectrum

Thermal Energy Transfer: Radiation

The heat you feel from a fireplace, campfire or the sun travels directly to you as infrared radiation. This is part of the Electromagnetic Wave Spectrum.

Thermal Energy Transfer: Radiation

You can’t see infrared radiation, but you can feel it as heat.

The Electromagnetic Wave SpectrumThe light we see with our eyes is really a very small portion of what is called the "Electromagnetic Spectrum." The Electromagnetic Spectrum includes all types of radiation - from the X-rays used at hospitals, to radio waves used for communication, and even the microwaves you cook food with.

Thermal Energy Transfer: Radiation

All of the forms of electromagnetic energy in the spectrum can travel through the universe as waves – we call this radiation.

Thermal Energy Transfer: Radiation

RADIATION: heat transferred through matter or empty space (no direct contact) by electromagnetic waves.

Energy in the Atmosphere

Heat transfer by radiation

Heat transfer by conduction

Heat transfer by convection

Heat transfer by convection

Heat energy is transferred by a number of ways.

Conduction Video clipshttp://www.youtube.com/watch?v=9joLYfayee8http://www.youtube.com/watch?v=w_IbPRNZ6ho

Conduction Video Clips

Conduction Video clip (great lab idea)http://www.youtube.com/watch?v=0cydb3TNEqk

Convection Current Video cliphttp://www.youtube.com/watch?v=7xWWowXtuvA&feature=related

Convection Video Clips

Convection Current Video clip (great lab idea)http://www.youtube.com/watch?v=IpnHAj4R-Z8

Radiation Video Clips

Radiation Video cliphttp://www.youtube.com/watch?v=GYUN8o4eJ24http://www.youtube.com/watch?v=gY9BpJmhIrE

How to Avoid Radiation Video cliphttp://www.youtube.com/watch?v=6sIeKRhclQI

Review Rap

Review of conduction, convection, and radiation rap videohttp://www.youtube.com/watch?v=7Y3mfAGVn1c

Which materials assist or limit thermal energy transfer?

In your journal….

• What do you think causes pot holes in the road?

Think about our last unit on Matter and our discussions of expansion…

• Potholes form because asphalt road surfaces eventually crack under the heat of the day and the constant stresses of traffic. These cracks allow snow and rainwater to seep into the underlying dirt and gravel. During cold nights, the water freezes and expands, pushing out some of the dirt and gravel, leaving a hole when the water eventually melts. This whole process repeats itself over and over.

• Even in places where the air temperature rarely falls below freezing, excessive rainfall or flooding can eat away at the road.

Some materials are good conductors of thermal energy – they let heat pass through them easily.

THERMAL CONDUCTORS

METALS, such as aluminium, copper, and steel are good THERMAL CONDUCTORS

...because heat passes through them quickly – metal normally feels COLD

THERMAL CONDUCTORS

THERMAL CONDUCTORSConductor – a material that transfers thermal or electrical energy very well.

* Conductors allow energy (thermal and electrical) to easily flow through them

Some Materials Do Not let Heat pass through them

Wooden handle

Oven Mitt

Wooden handle

THERMAL INSULATORS

Wood, plastic, glass, wool and rubber are all insulators.

*insulators are poor conductors of both thermal and electrical energy

Wooden handle

THERMAL INSULATORS

Wooden handle

THERMAL INSULATORS

Insulator – a material that does not transfer thermal or electrical energy very well.

Insulators and Conductors both have important Uses

Wires are coated in plastic – electricity cant flow through it so you don’t get a shock if you touch it

Copper wire allows electricity to flow through

Insulators and Conductors both have important Uses

A plug has metal pins – conduct electricity, and a plastic cover to hold with your hands

Thermal Expansion: an increase in the size of a substance in response to an increase in the temperature of the substance.

Can energy transform (change)?

Energy TransformingThe law of conservation of energy states that energy can neither be created nor destroyed.

It can only be converted from one form to another.

Energy Transforming

Energy Transforming

Some thermal energy in all materials is transformed into light energy

Energy TransformingFor example, a light bulb works by transforming electric energy into thermal energy. The thermal energy is then transformed into light energy.

ELECTRICENERGY

THERMALENERGY

LIGHTENERGY

Light energy is radiated into the environment by electromagnetic waves.

Energy Transforming

Energy TransformingOnce the electromagnetic waves strike another material they are transformed back into thermal energy.

if you put your hand near a lit light bulb, you can FEEL the heat, without touching it