6. Work, Energy, Power

Law of conservation of energy
The total energy of an isolated system cannot change—it is conserved over time. Energy can be neither created nor destroyed, but can change form e.g. from g.p.e to k.e.

Work Done

Work done by a force: the product of the force and displacement in the direction of the force
W=Fs
Work done by an expanding gas: the product of the force and the change in volume of gas
W=P⋅ΔV
- Condition for formula: temperature of gas is constant
- The change in distance of the piston, \text{δx}Δx, is very small therefore it is assumed that PP remains constant.

W = Fs

Deriving Kinetic Energy

Gravitational Potential Energy
arises in a system of masses where there are attractive gravitational forces between them. The g.p.e of an object is the energy it possesses by virtue of its position in a gravitational field.
Elastic potential energy
this arises in a system of atoms where there are either attractive or repulsive short-range inter-atomic forces between them.
Electric potential energy
arises in a system of charges where there are either attractive or repulsive electric forces between them.

Internal energy: sum of the K.E. of molecules due to its random motion & the P.E. of the molecules due to the intermolecular forces.
Gases: k.e. > p.e.k.e.>p.e.
- Molecules far apart and in continuous motion = k.e=k.e
- Weak intermolecular forces so very little \text{p.e.}p.e.
Liquids: k.e. \approx p.e.k.e.≈p.e.
- Molecules able to slide to past each other = k.e.=k.e.
- Intermolecular force present and keep shape = p.e.=p.e.
Solids: k.e. < p.e.k.e.<p.e.
- Molecules can only vibrate \therefore k.e.\∴k.e. very little
- Strong intermolecular forces \therefore∴ high \text{p.e.}p.e.