IB Physics Glossary

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Ideal gas

is a gas in which the molecules do not interact or the intermolecular forces can be neglected. (d)
The ideal gas equation is PV=nRT. Here we use absolute temperature, T, with unit Kelvin.


is the change in momentum or the product of force and time: \Delta p = F \times \Delta t. (d)
Its unit is N s (or kg m s-1).

Inclined plane

is used to show the forces acting on a block which rests, or moves, along the plane:

The component of the weight down the plane is

 m g \sin\theta.

Inelastic collisions

occur when the total kinetic energy of a system decreases.


a material that does not allow the flow of electric charge through it.

Internal energy

of a substance is the total potential energy and random kinetic energy of the molecules of the substance. (d)

Internal resistance

of a cell/battery is the effective additional resistance that is added to a circuit by the cell/battery. This is caused by the electrical energy generated by the chemical reactions inside the cell/ battery which is dissipated within the battery when a current flows:

\epsilon = I (R + r),

where \epsilon is the emf of the cell/battery, I is the current flowing through the circuit, R is the resistance of the circuit component(s) and r is the internal resistance of the cell/battery.

A graph of voltage/potential difference across external resistor (y-axis) against current flowing (x-axis) can be used to find
  • the internal resistance, r = - gradient
  • emf of cell/battery, \epsilon= y-intercept

Ionization radiation

is radiation composed of particles that individually carry enough energy to liberate an electron from an atom thus ionizing it.

Taken from wikipedia.

Isobaric process

takes place at constant pressure.


The 1st law of thermodynamics is

Q=\Delta U + W, and W=P \Delta V.

Isochoric process

occurs at constant volume.

The 1st law of thermodynamics

Q=\Delta U + W
Q=\Delta U (all heat converted into internal energy)
\Delta V = 0 \Rightarrow W = 0 (no work is done by gas).

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