Temperature (1) - Oxford University Press

Brownian motion: evidence for moving particles
Smoke is made up of millions of tiny bits of ash or oil droplets. If you look at
smoke through a microscope, as on the right, you can see the bits of smoke
glinting in the light. As they drift through the air, they wobble about in zig-zag
paths. This effect is called Brownian motion, after the scientist Robert Brown
who first noticed the wobbling, wandering motion of pollen grains in water, in
1827.
The kinetic theory explains Brownian motion as follows. The bits of smoke are
just big enough to be seen, but have so little mass that they are jostled about
as thousands of particles (gas molecules) in the surrounding air bump into
them at random.
View through
microscope
zig-zag paths
of smoke bits
Energy of particles
The particles (atoms or molecules) in solids, liquids, and gases have kinetic energy
because they are moving. They also have potential energy because their motion
keeps them separated and opposes the bonds trying to pull them together. The
particles in gases have the most potential energy because they are furthest apart.
The total kinetic and potential energies of all the atoms or molecules in a
material is called its internal energy. The hotter a material is, the faster its
particles move, and the more internal energy it has.
If a hot material is in contact with a cold one, the hot one cools down and
loses internal energy, while the cold one heats up and gains internal energy.
The energy transferred is known as heat.
The term thermal energy is often used for both internal energy and heat.
Q
1 Say whether each of the following describes a solid, a
liquid, or a gas:
a Particles move about freely at high speed.
b Particles vibrate and cannot change positions.
c Fixed shape and volume.
d Particles vibrate but can change positions.
e No fixed shape or volume.
f Fixed volume but no fixed shape.
g Virtually no attractions between particles.
Related topics: energy 4.01; fusion and vaporization 5.11; atoms and elements 11.01
THERMAL EFFECTS
microscope
lamp smoke glass
cell
Kinetic energy
Energy because of motion.
Potential energy
Energy stored because of a
change in position or shape.
glass
cover
2 Smoke is made up of millions of tiny bits of ash or oil
droplets.
a What do you see when you use a microscope to
study illuminated smoke floating in air?
b What is the effect called?
c How does the kinetic theory explain the effect?
3 If a gas is heated up, how does this affect the motion of
its particles?
4 What is meant by the internal energy of an object?
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