334 



HEAT. 



The Pressure Of Radiation. We shall assume Maxwell's value 

 for the pressure of radiation. He showed that a pencil of electro- 

 magnetic waves exerts pressure each way in the line of propagation 

 numerically equal to the energy per c.c. or energy density in the pencil. 

 On his theory there is no pressure at right angles to the line of propa- 



gation. If, then, E is the energy density 

 of the radiation falling normally on a 

 receiving surface, it exerts pressure E per 

 sq. cm. on that surface. If the radiation 

 is incident at 6 (Fig. 189), let AB be the 

 trace of 1 sq. cm. of the surface, BC the 

 trace of area AB cos x 1 perpendicular 

 to the pencil. The force on BO is E cos 6, 

 and resolving, this gives a normal pressure 

 on AB equal to E cos 2 0. There is also a 

 tangential force on AB equal to E cos 

 sin 0, but this does not concern us here. 

 If the pencil is totally reflected, the reflected 

 ray also exerts pressure E cos 2 0. It may be 

 noted that the tangential force in this 

 case is equal and opposite to that due to 

 the incident pencil, but that if there is 

 absorption the tangential force is not wholly neutralised. 



The Normal Stream of Radiation, the Total Stream, and 



the Energy Density. We can describe the stream of energy from a 

 full radiator by three quantities, between which there are necessary 

 relations. These are 



1. The Normal Stream of Radiation N. 



If a sq. cm. is placed parallel to an emitting sq. cm., at a distance 

 r from it along the normal, then if the energy 

 from the emitting sq. cm. passing through the 



N 

 other sq. cm. is -^ per second, N is the normal 



stream of radiation from the radiator. Or if 

 cZS is an element of a radiator, d& an element 

 along the normal parallel to dS and r from it 

 the energy from ofS passing through d& is 



^ per second. 



'9 



2. The Total Stream of Radiation R. 



The total stream of radiation is the energy 

 emitted from 1 sq. cm. per second round a hemi- FIG. 190. 



sphere. Let <2S be an element of the radiating 



surface (Fig. 190). Draw a sphere radius r round dS and let ON be the 

 normal radius. The stream from rfS passing through an element rfS' of 



the sphere at N is 5 per second. If d& is at the end of a radius 



r 2 



making with ON, the stream through it is, by the law of inclined 



., 



pencils, 



j 



T .. ., 



Divide the hemisphere above c/S into rings round 



