When you cook on an electric stove, the pots and pans are heated by conduction. In this process, heat is transferred from the burner to the bottom of the pot. The particles (both the molecules and electrons) at the bottom of the pot move faster and have more collisions with neighboring particles, causing them to move faster. Eventually, the entire pot is hot because of transmission of thermal energy through these collisions.

Materials that are good conductors of electricity are also good conductors of heat. This is because the transmission of heat depends on the outermost electrons, just as the ability to conduct electricity does. Solids with weakly-bonded electrons are good conductors of both heat and electricity. Materials that are poor conductors of heat and electricity are insulators. In general, both gases and liquids are good insulators.

Engineers use good insulator materials to maintain an object's temperature, whether it is hot or cold. For example, many types of materials are used to insulate buildings, including fiberglass and rigid foam. Much of the insulation value of these materials is provided by tiny air pockets in the material. Insulation works both ways; it slows the rate of heat transfer between a building and the environment. If it is warm in the building and cold outside, insulation slows the rate of heat transfer to the outside. If it is cold in the building and warm outside, insulation slows the rate of heat transfer to the inside.

Heat is transferred by convection in liquids and gases. When a fluid, either a liquid or gas, is in contact with a hot object, warmed fluid rises and cooler fluid sinks to take the place of the warmed fluid. This produces currents in the fluid. When a pan of water is heated on the stove, heat is transferred to the pan and then to molecules of water that are in contact with the pan by conduction. As a packet of water gains heat, and its molecules speed up, it expands, becoming less dense than the surrounding water. This less dense packet of water rises like a balloon through the surrounding liquid. Colder, denser water packets sink to take its place.

Heat transfer can also occur between objects that are not touching and that are separated by empty space. This is exactly how energy from the sun reaches Earth across 150 million kilometers (93 million miles) of nearly empty space. Similarly, energy from a fire or stove burner reaches our bodies through empty space. In fact, all objects transfer energy in this way at all times. This phenomenon is known as radiation. Besides heat, radiation transmits other forms of energy, such as visible light, x-rays, radio, which are all forms of electromagnetic radiation. The energy in x-rays is greater than the energy in visible light, which is greater than the energy of heat (infrared radiation), which is greater than the energy of radio waves. In a vacuum, energy is transmitted at the speed of light by radiation. In an object that absorbs infrared radiation, the motion of its particles increases, thus increasing its temperature.

Everything is continually radiating energy and absorbing energy by radiation. An object's temperature increases if it absorbs more energy than it radiates and vice versa. If an object is warmer than its surrounding it transfers heat to its surroundings by radiation (and by conduction, if it is touching another object, and by convection if there is a fluid in the environment).

Black objects are good absorbers and radiators, while shiny or white objects are poor absorbers and radiators. For example, dark-colored cars get much hotter than white cars on hot days. White and shiny surfaces reflect radiation. This is why engineers, architects and builders sometimes paint roofs or houses white in hot climates to help reduce the need for air conditioning.

The atoms and molecules in all matter (solids, liquids, gases and plasmas) are constantly vibrating. There is more movement of the particles in a warmer object and less movement of the particles in a cooler object. Temperature is a quantitative measurement of how warm or cool an object is. Measuring the temperature of an object gives the average kinetic energy — the energy due to motion — of the particles in the object.

Engineers use many types of thermometers to measure and monitor temperatures. click for copyright

Materials that are good conductors of heat feel hotter to the touch than materials than are insulators even if they are at the same temperature. Two objects at the same temperature that are identical in all respects but size have different amounts of thermal energy. The larger object has a greater amount of thermal energy than the smaller object due to its greater mass.

As the temperature of matter changes, its characteristic properties, such as electrical conductivity, may change. Almost all materials also change volume as their temperature changes. Most materials expand as their temperature increases and contract as their temperature decreases. Materials expand when their temperature increases because the particles are moving about a higher average speed and therefore have more energetic collisions. The energetic collisions force the particles to move farther apart.

The expansion and contraction of gases is dramatic. For example, the volume of a balloon decreases greatly if it is dunked in liquid nitrogen. The balloon returns to its original size as it warms back up to room temperature. The volume change of solids with a change in temperature is less noticeable. However, most people have experienced the result of the expansion of a metal lid on a glass jar (making it easier to open) when it is held under hot water. The metal lid expands more than the glass jar, so the lid can be removed. In general, expansion and contraction of liquids is greater than expansion and contraction of solids. Different materials expand and contract at different rates.

How long it takes a material to heat up or cool down depends on the amount of the material and its specific heat. Specific heat is the amount of heat required to raise the temperature of one unit mass of a material by one degree. Specific heat is unique for every type of material. Diamonds have high specific heat and thermal conductivity, which is why they are cold to the touch. The specific heat of water is much higher than for most materials. So, for example, it takes much longer to heat up or cool down a given mass of water than an equal mass of aluminum or iron. The high specific heat of water makes it useful for both cooling and warming. This characteristic of water explains why locations next to large bodies of water tend to have more moderate climates than those further inland. Most people have experienced the effects of varying specific heats when eating fruit pie right out of the oven. The fruit filling, which is made mostly of water, can still burn your mouth even though the crust is no longer hot.

A pie's fruit filling has a different specific heat than its crust. click for copyright

Understanding heat transfer, whether the goal is to maximize or minimize it, is important to engineers. Civil and architectural engineers use insulation and passive solar design to construct buildings that neither lose nor gain too much heat. Mechanical engineers design appliances, such as air conditioners, refrigerators, heaters and ventilation systems. Civil engineers design heating, cooling and ventilation systems for buildings. Aerospace engineers design thermal control systems for spacecraft and study how airplane designs affect aerodynamic heating.

Sometimes engineers need to maximize heat transfer of devices that tend to overheat or work less efficiently at higher temperatures. For example, the waste heat from an electric generator, a computer CPU, or a refrigerator compressor must be transferred away from the devices as much as possible. In other applications, engineers must minimize heat transfer to keep a device working efficiently. For instance, the storage space of a refrigerator must be well insulated to minimize the heat transfer from the outside to the inside.

Brainstorming: As a class, have the students engage in open discussion. Remind them that in brainstorming, no idea or suggestion is "silly." All ideas should be respectfully heard. Take an uncritical position, encourage wild ideas and discourage criticism of ideas. Have students raise their hands to respond. Write their ideas on the board. Ask the students:

• One type of energy is heat energy. What object can you think of that use heat to work?

Question/Answer: Ask students the following questions and have them raise their hands to respond. Write their answers on the board.

• If an engineer is looking for a material to minimize heat transfer by conduction in a device she is building, what type of material should she choose? (Answer: An insulator, such as Styrofoam, plastic or wood.)
• Name some devices that engineers have designed in which heat transfer is an important part of their function. (Possible answers: Oven, stove, hair dryer, air conditioner, convection oven, toaster, clothes dryer, dish washer, water heater and furnace.)

Numbered Heads: Divide the class into teams of three to five students each. Have the students each team number off so each member has a different number. Ask the students a question (give them a time frame for solving it, if desired). The members of each team work together to answer the question. Everyone on the team must know the answer. Call a number at random. Students with that number should raise their hands to give the answer. If not all the students with that number raise their hands, allow the teams to work a little longer. Ask the students:

• Heat transfer between two objects is always from the (colder or hotter) object to the (colder or hotter) object. (Answer: Hotter to colder.)
• True or False: Heat transfer occurs between objects at the same temperature. (Answer: False)
• A shiny surface (absorbs, transmits or reflects) most of the light that shines on it. (Answer: Reflects)
• Heat transfer in liquids is by (conduction, convection or radiation). (Answer: Convection)
• Name a material that would be a good conductor of heat. (Possible answer: Any metal)
• Heat transfer over a distance is by (conduction, convection or radiation). (Answer: Radiation)
• What is another name for heat energy? (Answer: Thermal energy.)
• Challenge Question: What types of engineers use thermal energy in their work? (Answer: Thermal energy is considered in almost every engineering application! For example, civil and architectural engineers take thermal energy into consideration when planning the insulation and passive solar design of buildings so they neither lose nor gain too much heat, as well as solar-paneled stop lights, and power generators. Mechanical engineers design appliances, such as air conditioners, refrigerators, heaters and ventilation systems. Civil engineers design heating, cooling and ventilation systems for buildings. Aerospace engineers design thermal control systems for spacecraft, and study how airplane designs affect aerodynamic heating. Electrical and computer engineers use their understanding of thermal energy and heat transfer when designing electrical and computer components.