I have mentioned in lecture that all objects emit electromagnetic waves. This is
ID: 1428044 • Letter: I
Question
I have mentioned in lecture that all objects emit electromagnetic waves. This is because the protons and electrons in the objects are always moving all the time due to their temperature. Jiggling charges mean changing electric and magnetic fields (which is all an electromagnetic wave is). A reasonable question that physicists may ask, then, is "what wavelenth of light do these jiggling electrons and protons emit?". The answer is given by another law I will never ask you about again: Average wavelength of EM waves emitted by object = -b/Temperature (in Kelvin) This is known as Wieni's displacement law, and 6 is (appropriately) called Weill's displacement coefficient: b 2.90 x 10^-3 K m. (a) Suppose you have an iron bair at room temperature (20degreeC). What is the average wavelength of EM waves emitted by the ball? What part of the electromagnetic spectrum does this wavelength belong to? (b) Suppose you heat the ball to "red hot" (red light is about 700 nm, or 7 x 10^-7 m). What temperature is the ball? (c)Explanation / Answer
Temp. in Kelvin = temp in degree Celsius + 273
Therefore, Temp. of the iron ball = 20 + 273 = 293 K
Now b = 2.90 * 10-3 K.m, T = 293 K. We have to find average wavelength
Therefore = (2.90 * 10-3)/293
= 0.009897 * 10-3
= 9.8*10-6 metre = 9.8 micrometre
The part of the electromagnetic spectrum does this wavelength belongs to INFRARED region
We have to find T.
Therefore,
T = b/
T =(2.90 * 10-3 )/( 7 * 10-7)
T = 0.414 * 104 = 4140 K
The temperature of Sun’s surface is,
T = b/
T =(2.90 * 10-3 )/( 5 * 10-7)
T = 0.58 * 104 = 5800 K