IB Physics Glossary



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W

Wavelength

\lambda, is the distance moved by a wavefront during one oscillation of the wave source or the distance between consecutive neighboring successive points which are in phase. (d)

Weight

is the force of gravity acting on a mass: W=m g, m is the mass, g is the gravitational field strength \equiv acceleration due to gravity.
Its value does change with the strength of gravity. Identical objects on Earth and on the Moon have different weight values. Its SI unit is the Newton (N).

Work

is force × distance (moved) in the direction of the force. (d)

Work done by a gas

is given by W = P \Delta V.

For an expansion, \Delta V > 0 \Rightarrow W > 0, work is done by the gas.

For a compression, \Delta V < 0 \Rightarrow W < 0, work is done on the gas.


Derivation


Consider a gas in a closed container with a piston of area A in contact with the gas.

1. The gas expands, via some thermodynamic process, and the piston is displaced by \Delta x.
piston

2. The gas does work on the piston (heat is converted into mechanical energy).

3. The work done is W=F \times s, ie. force x displacement:

W = F \Delta x.

4. Recall the pressure exerted by the gas is given by P=\frac{F}{A}, force per unit area. Therefore F = P A.

5. W = F \times s = (P A) \times \Delta x = P (A \Delta x) = P \Delta V, since A \Delta x is the change in volume of the gas.

Work function

\phi, is the minimum energy required to liberate an electron from the surface of a metal:

\phi = h f_0

where f0is the minimum or threshold frequency required to produce the photoelectric effect.



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