Heat (energy) from the Sun

Overview

This part six of a two week unit on heat and energy. For many students this will be their first exposure to a quantitative study of heat. The focus is on a conceptual understanding of heat as a form of energy as well as mastery of simple quantitative problems, including changes of state.

Topics

  1. Heat transfer: review radiation, conduction, convection
  2. Thermal vs visible radiation: greenhouses
  3. Energy from the sun: the solar constant
  4. Green-house gases: climate and the environment

Energy from the Sun

The Earth receives 1.37 kJ per square metre every second from the sun*. To be in thermodynamic equilibrium the Earth must radiate energy back into space at the same rate.

Greenhouse gases

All atmospheric gases inhibit the escape of infrared radiation away from the Earth's surface. Some gases trap infrared radiation more efficiently than others; these are the so called "greenhouse gases". The main gases in this group are carbon dioxide, methane, and water vapour. If it were not for the presence of these gases in the Earth's atmosphere, the earth would be a frozen wasteland, because the global equilibrium temperature would be below the freezing point of water. Investigate,for example, the planet Mars.

On the other hand, too high a concentration of these gases has the possibility of leading to what scientists call " a run-a-way greenhouse effect", turning the Earth into an inferno. For example, the planet Venus.

Energy from Space

* The watt is defined as one joule per second , therefore the solar constant is frequently given as 1.37kW/m2.

Scientists have proposed putting large solar panels into orbit to collect solar energy. The energy would be converted into microwaves, beamed to a ground station in a remote part of the country, converted to electricity and then sent to large cities via high voltage power lines. This promises to be a very clean and reliable source of energy. Canada's participation in the space station gives Canadians an opportunity to participate as pioneers in the development of this amazing resource

Discussion Questions

  1. Make a list of the primary green-house gases
  2. For each of these gases, list
    1. their natural sources.
    2. their artificial sources.
  3. Using resources in the library, have students find out how ice-core samples from glaciers are used by scientists to help trace the history of the Earth's atmosphere. Have students write a brief report or give an oral presentation to the class summarizing their research.
  4. The Earth casts a shadow in space as if it were a disc with a radius of 3500 Kim. How much energy is intercepted by the Earth each second?
  5. Have students call their local utility company to find out the (approximate) power requirements of their community. Using the solar constant as 1.37kW/m2,
    1. Calculate the area of a space -based solar panel required to meet their community power requirements.
    2. List the energy conversions required from the incident solar radiation(on the panel) to the consumers home.
    3. If the maximum combined efficiency of all energy transformations is 50%, how large would the space-based solar array need to be?