IB Physics Short Note Temperature and Density

A question from Thermodynamics exam

A sample of an ideal gas is compressed by a piston from 10 m3 to 5 m3 and simultaneously cooled from 273 degrees C to 0 degrees C. As a result there is:
A. an increase in pressure
B. a decrease in pressure
C. a decrease in density
D. no change in density
E. an increase in density

Density refers to the mass per unit volume. First condition describes the decrease in volume, therefore density increases. Second condition describes the decrease in temperature. Base on kinetic molecular theory, molecules are closer together as energy is removed from the matter. Again, the volume decreases.

E. is the correct answer, stating “an increase in density”.

IB Physics Short Note Emissivity vs Albedo

Currently we are doing environmental physics in Physic HL and mainly talking about the energy received by Earth, for example, from the Sun.

Power radiating from an object is calculated by
P=\sigma \varepsilon AT^{4}

σ is the Stefan-Boltzmann constant, which is approximately 5.68×10-8
ɛ is the emissivity of the object’s surface

Freeman also talked albedo (e), and after doing some research online, I found that there is a difference between these two terms.

ɛ (emissivity) is the emissivity of the object’s surface
e (albedo) is the percentage of light energy being reflected away

So if the body is not transparent, ɛ+e should equal to 100% or 1.0.