B-unit IB physics 2025

Density

The ratio of the mass of a body to its volume (mass per volume)

Internal energy

(Of a system:) The total intermolecular potential energy arising from the forces between the molecules plus the total random kinetic energy of the molecules arising from their random motion.

Thermal energy

Internal energy transfer through contact (heat)

Thermal equilibrium

When two substances in contact with each other no longer exchange any heat energy and both reach an equal temperature.

Specific heat capacity

The amount of heat required to raise the temperature of a unit mass of material by one (temperature) unit - (1 kg by 1 K/C)

Specific latent heat

The amount of energy required to change the state of 1 kg of a substance without changing its temperature.

Specific latent heat of fusion

The energy released when 1 kg of liquid freezes to become solid at constant temperature. The energy released when 1 kg of solid melts to become liquid at constant temperature.

Specific latent heat of vaporization

The energy released when 1 kg of gas condenses to become liquid at constant temperature. The energy released when 1 kg of liquid evaporates to become gas at constant temperature.

Thermal capacity

The amount of heat needed to raise the temperature of a body by one unit.

Conduction

Main means of thermal energy transfer in solids. Occurs when two solids of different temperatures come in contact with one another, thermal energy is transferred from the hotter object to the cooler object (until equilibrium is reached).

Convection

Only in fluids (liquids and gases). Occurs when a fluid is heated causing the movement of groups of atoms or molecules due to variations in density. Heated part of fluid expands and rises because it's less dense. Eventually it cools down, contracts and sinks down again. This is called a convection current.

Thermal radiation

Heat transfer by means of electromagnetic radiation normally in the infrared region. Only means of thermal energy transfer through vacuum. All bodies emit thermal radiation.

Black body

A perfect radiator of thermal energy. Does not reflect, only absorb energy from outer sources.

Wien's displacement law

The peak wavelength and absolute temperature of a black body are inversely proportional to each other.

Ideal gas (assumptions)

The collisions between molecules are perfectly elastic (Ek is conserved), the molecules are spheres, the molecules are identical, there are no intermolecular forces (except in collisions) -> move with constant velocity between collisions, the molecules are very small; their total volume is much smaller than volume of container.

Pressure

Force per unit area where the force acts perpendicular to the surface.

Temperature

The temperature of a gas is directly proportional to its kinetic energy.

Boyle-Mariotte's law

States that the pressure if a mixed mass of gas at constant temperature is inversely proportional to its volume.

Gay-Lussac's law

States that the pressure of a fixed mass of gas with constant volume is directly proportional to its temperature in kelvin.

Charles' law

States that the volume of a fixed mass of gas at a constant pressure is directly proportional to its temperature in kelvin.

Isothermal

Constant temperature (see on graph).

Isobaric

Constant pressure (see on graph).

Isovolumetric

Constant volume (see on graph).

Internal energy (of a gas)

The total kinetic energy of all the molecules (no potential energy).

Current

The rate of flow of charge.

Potential difference (voltage)

The work done per unit charge in passing between two points.

Direct current (DC)

When the current (flow of electrons) is in one direction.

Alternating current (AC)

When the current (flow of electrons) changes direction.

Resistance

Measure of the opposition to the flow of current in an electric circuit.

Resistivity

Electrical resistance of a conductor of unit cross-sectional area and unit length. Characteristic property of a material.

Ohm's law

The current through an ohmic conductor is directly proportional to the potential difference across it provided that the temperature remains constant.

Insulator

A material in which electric current does not flow freely, e.g. plastic, wood, glass, etc.

Conductor

Substance or material that allows electric current to flow through it, e.g. metals -> copper.

Emf (electromotive force)

The emf of a cell is the work done per unit charge taking the charges from the low potential to the high potential. This is equivalent to the amount of chemical energy transferred to electrical potential energy per unit charge.

Ideal voltmeter

Has infinite resistance.

Ideal ammeter

Has zero resistance.