Thermal Radiation Flashcards
All 19 cards in this deck
Which bodies emit thermal radiation?
All bodies emit radiation, at every temperature. They also absorb radiation from their surroundings.
Which two features of the radiation emitted by a body depend on its temperature?
The intensity of the radiation and its wavelength distribution (the range of wavelengths emitted).
On an intensity–wavelength graph, what happens to the peak intensity as a body gets hotter?
The peak intensity increases (the whole curve is higher), so more radiation is emitted every second.
On an intensity–wavelength graph, what happens to the wavelength of peak intensity as a body gets hotter?
It becomes shorter — the peak moves towards shorter wavelengths.
True or false? Only objects hot enough to glow emit thermal radiation.
False. All bodies emit radiation at all temperatures; cooler bodies emit less, and mainly at longer (infrared) wavelengths.
True or false? A hot body emits radiation at one single wavelength.
False. It emits a continuous range of wavelengths, with the intensity reaching a peak at one wavelength.
What must be true of the radiation for a body to stay at a constant temperature?
It must radiate the same average power that it absorbs (power out = power in).
What happens to a body if the average power it absorbs is greater than the average power it radiates?
e.g. a lamp just after being switched on
Its temperature rises. The lamp heats up until it radiates power at the same rate as it is supplied, then stays at a constant temperature.
What happens to a body if the average power it radiates is greater than the average power it absorbs?
Its temperature falls (the body cools).
True or false? A body sitting at a constant temperature is emitting no radiation.
False. It is radiating, but at the same average power as it absorbs, so its temperature does not change.
What controls the average temperature of the Earth?
The balance between the average power of radiation absorbed from the Sun and the average power radiated back out into space.
What effect do greenhouse gases in the atmosphere have on the Earth's temperature?
They absorb infrared radiation emitted by the Earth and re-radiate some back to the surface, so less power escapes and the temperature rises.
What effect does a pale, reflective surface such as ice or cloud have on the Earth's temperature?
It reflects more of the incoming solar radiation, so less is absorbed and the temperature is lower.
What are the steps to investigate how the nature of a surface affects the thermal energy radiated?
e.g. comparing a matt black can with a shiny silver can
- Fill each can with the same volume of water at the same starting temperature, e.g. cm at °C.
- Measure the temperature of each can at regular intervals with a thermometer or temperature sensor, e.g. every s for minutes.
- Compare the cooling: the matt black can cools fastest, so it is the better emitter.
In the cooling-cans experiment on thermal radiation, name three control variables.
Volume of water, starting temperature of the water, and the size/shape of the containers (also same surroundings and same room temperature).
What is the main safety precaution when investigating cooling using hot water?
Take care with the hot water and kettle to avoid scalds — stand containers on a stable surface away from the edge.
True or false? A shiny silver surface is a better absorber of thermal radiation than a matt black surface.
False. Matt black surfaces are the best absorbers and emitters; shiny silver surfaces are the worst.
Give two improvements to the cooling-cans thermal radiation experiment.
Use lids on the cans (so energy is lost mainly by radiation) and repeat the experiment to find a mean; a datalogger gives more frequent, reliable readings.
True or false? A surface that is a good absorber of thermal radiation is also a good emitter of thermal radiation.
True. A matt black surface both absorbs and emits thermal radiation better than a shiny silver one.