ALPHY6: TRANSFER OF HEAT

This unit explains transfer of heat

Heat Transfer

Heat can be transferred by conduction, convection and radiation. It is only transferred from hotter things to cooler things.

Heat is transferred in order to equalise the temperatures of the object and its environment

Examples:

  • A cup of coffee will cool down because it is giving out heat energy into the surroundings
  • A cold drinks can (taken out of the fridge) will warm up because it is taking in heat energy from the surroundings

Conduction

In a hot solid, particles vibrate more. They collide with the particles next to them and set them vibrating. The kinetic energy is transferred from particle to particle. Metals are the best conductors. Solids are better than liquids. Gases are very poor conductors. They are insulators.

Atoms in a substance are always vibrating.If the substance gets hotter, the atoms vibrate more. The heat energy is given to the atoms, which makes them move about faster

Every time they collide with another atom, the heat energy is transferred. This is how heat travels through a solid

The video below shows how heat transfer occurs through conduction

Convection

In a hot fluid (gas or liquid) the particles have more kinetic energy so they move more. They spread out and the fluid becomes less dense. The hot fluid rises above the denser cold fluid forming a convection current.

  • Hot air rises in cold air
  • Hot water rises in cold water
  • This is called convection

When hot air rises, colder air has to move in to replace it. Convection cannot happen in solids, as the atoms aren’t able to move around

 convection

The video below explains how heat transfer occurs by convection

 

Radiation

All objects emit and absorb infrared radiation. The higher the temperature the more they emit.

When objects absorb this energy their temperature increases.

Radiation will travel through a vacuum – it does not need a medium (material) to pass through:

  • Dark and matt surfaces are good absorbers and emitters of infrared radiation.
  • Light and shiny surfaces are poor absorbers and emitters of infrared radiation.
  • Light and shiny surfaces are good reflectors of infrared radiation.

The video below explains about how heat transfer occurs by Radiation

Insulation

When we insulate our homes we reduce the heat lost, we use less fuel and it costs less. Still air is a good insulator, so materials with air trapped in them are often used:

  • In cavity walls the air gap between the walls stops conduction.
  • In cavity wall insulation the cavity is filled with foam or mineral wool.
  • Loft insulation using layers of fibreglass or mineral wool
  • Reflective foil on walls reflects infrared radiation.
  • Draught-proofing stops hot air leaving and cold air entering the house.

All these improvements cost money to buy and install, but they save money on fuel costs. You can work out the payback timewhich is the time it takes before the money spent on improvements is balanced by the fuel savings, and you begin to save money:

Payback time (in years) = cost of insulation ÷ cost of fuel saved each year

If the price of the fuel increases, the payback time will be less.

Conductors and insulators

Equally as interesting as the effects of heat transfer on a system are the methods by which this occurs. Whenever there is a temperature difference, heat transfer occurs. Heat transfer may occur rapidly, such as through a cooking pan, or slowly, such as through the walls of a picnic ice chest. We can control rates of heat transfer by choosing materials (such as thick wool clothing for the winter), controlling air movement (such as the use of weather stripping around doors), or by choice of color (such as a white roof to reflect summer sunlight). So many processes involve heat transfer, so that it is hard to imagine a situation where no heat transfer occurs. Yet every process involving heat transfer takes place by only three methods:

  1. Conduction is heat transfer through stationary matter by physical contact. (The matter is stationary on a macroscopic scale—we know there is thermal motion of the atoms and molecules at any temperature above absolute zero.) Heat transferred between the electric burner of a stove and the bottom of a pan is transferred by conduction.
  2. Convection is the heat transfer by the macroscopic movement of a fluid. This type of transfer takes place in a forced-air furnace and in weather systems, for example.
  3. Heat transfer by radiation occurs when microwaves, infrared radiation, visible light, or another form of electromagnetic radiation is emitted or absorbed. An obvious example is the warming of the Earth by the Sun. A less obvious example is thermal radiation from the human body.
In a fireplace, heat transfer occurs by all three methods: conduction, convection, and radiation. Radiation is responsible for most of the heat transferred into the room. Heat transfer also occurs through conduction into the room, but at a much slower rate. Heat transfer by convection also occurs through cold air entering the room around windows and hot air leaving the room by rising up the chimney.

The figure shows a fireplace in a room. The fireplace is at the lower left side of the figure. There is a window at the right side of the room. From the window cold air enters into the room, and follows some curved blue arrows labeled convection to the fireplace. The air heated by the fire rises up the chimney following some red curved arrows, which are also labeled convection. Yellow wavy lines emanate from the flames of the fire into the room and are labeled radiation. Finally, a black curved line labeled conduction goes from beneath the logs of the fire and points into the floor under the room.

We examine these methods in some detail in the three following modules. Each method has unique and interesting characteristics, but all three do have one thing in common: they transfer heat solely because of a temperature difference.

Summary

  • Heat is transferred by three different methods: conduction, convection, and radiation.

Conceptual Questions

What are the main methods of heat transfer from the hot core of Earth to its surface? From Earth’s surface to outer space?

When our bodies get too warm, they respond by sweating and increasing blood circulation to the surface to transfer thermal energy away from the core. What effect will this have on a person in a \text{40}\text{.}\text{0ºC} hot tub?

 shows a cut-away drawing of a thermos bottle (also known as a Dewar flask), which is a device designed specifically to slow down all forms of heat transfer. Explain the functions of the various parts, such as the vacuum, the silvering of the walls, the thin-walled long glass neck, the rubber support, the air layer, and the stopper.

The construction of a thermos bottle is designed to inhibit all methods of heat transfer.

The figure shows a cutaway drawing of a thermos bottle, with various parts labeled.

EXAMPLE

Calculating Heat Transfer by Convection: Convection of Air Through the Walls of a House.

Most houses are not airtight: air goes in and out around doors and windows, through cracks and crevices, following wiring to switches and outlets, and so on. The air in a typical house is completely replaced in less than an hour.

Suppose that a moderately-sized house has inside dimensions 12.0m×18.0m×3.00m high, and that all air is replaced in 30.0 min. Calculate the heat transfer per unit time in watts needed to warm the incoming cold air by 10.0 ºC, thus replacing the heat transferred by convection alone.

Strategy:

Heat is used to raise the temperature of air so that 

Q=mcΔT. The rate of heat transfer is then 

Q/t, where 

t is the time for air turnover. We are given that 

ΔT is 10.0ºC, but we must still find values for the mass of air and its specific heat before we can calculate 

Q. The specific heat of air is a weighted average of the specific heats of nitrogen and oxygen, which is 

c=cp≅1000J/kg⋅C (note that the specific heat at constant pressure must be used for this process).

Solution:

(1) Determine the mass of air from its density and the given volume of the house. The density is given from the density 

ρ and the volume 

m=ρV=(1.29kg/m3)(12.0m×18.0m×3.00m)=836kg 

(2) Calculate the heat transferred from the change in air temperature: 

Q=mcΔT so that 

Q=(836kg)(1000J/kg⋅∘C)(10∘C)=8.36×106J 

(3) Calculate the heat transfer from the heat 

Q and the turnover time 

t. Since air is turned over in 

t=0.500h=1800s, the heat transferred per unit time is 

Qt=8.36×106J1800s=4.64kW.

This rate of heat transfer is equal to the power consumed by about forty-six 100-W light bulbs.

Newly constructed homes are designed for a turnover time of 2 hours or more, rather than 30 minutes for the house of this example. Weather stripping, caulking, and improved window seals are commonly employed. More extreme measures are sometimes taken in very cold (or hot) climates to achieve a tight standard of more than 6 hours for one air turnover. Still longer turnover times are unhealthy, because a minimum amount of fresh air is necessary to supply oxygen for breathing and to dilute household pollutants. The term used for the process by which outside air leaks into the house from cracks around windows, doors, and the foundation is called “air infiltration.”

ASSIGNMENT : TRANSFER OF HEAT ASSIGNMENT MARKS : 100  DURATION : 1 week, 3 days

 

Welcome to FAWE

STEM Elearning

We at FAWE have built this platform to aid learners, trainers and mentors get practical help with content, an interactive platform and tools to power their teaching and learning of STEM subjects, more

How to find your voice as a woman in Africa

top
© FAWE, Powered by: Yaaka DN.
X