K323 / 2027
Kinetic particle model of matter overview

Topic 2 of 4

Temperature and particle motion

A rise in temperature corresponds to an increase in the average kinetic energy associated with the particles' random motion.

Kinetic energy is associated with motion. The particle model concerns random motion within a substance, including vibration in a solid. It is different from the whole object travelling across a room.

Consider the same gas first at 20°C and then at 50°C. In the warmer sample, the particles have greater average random kinetic energy. Their sizes and number do not increase just because the temperature rises.

Same gas, different temperatures

The particle count, symbol size and gas state are unchanged. The warmer sample has greater average random kinetic energy.

Blue arrows show particle motion. Lengths give a qualitative indication of speed; they do not give a numerical speed or temperature ratio.

20 °C

Gas at 20 degrees CelsiusTwelve equal-size particles are spread through a container of unchanged volume. Their blue motion arrows have varied directions and lengths. The particles at 20 degrees Celsius are already moving. Arrow lengths are qualitative; these are model snapshots, not measurements or tracked particles.

50 °C

Gas at 50 degrees CelsiusTwelve equal-size particles are spread through a container of unchanged volume. Their blue motion arrows have varied directions and lengths. At 50 degrees Celsius, the typical motion arrows are longer, although some are shorter. The warmer gas has greater average random kinetic energy, not a requirement that every particle always moves faster. Arrow lengths are qualitative; these are model snapshots, not measurements or tracked particles.

Individual speeds vary. Higher temperature describes the average random kinetic energy of all the particles.

These models show the same gas at 20°C and 50°C. The same number of equally sized particle symbols is used. Motion arrows have varied directions and lengths in both panels; the warmer sample has greater typical speeds, not one identical speed for every particle.

Average does not mean every particle moves alike

Particles have a range of kinetic energies and continually exchange energy in collisions. A temperature rise concerns the average over all the particles, not a requirement that every individual particle speeds up at every instant.

For a solid, particles continue vibrating about their positions. Warming makes the average kinetic energy of that vibration greater; it does not require particles to stream freely through the solid.

The Celsius readings establish which temperature is higher, but their ratio is not an energy ratio. For example, 50°C is not evidence for 2.5 times the average kinetic energy at 20°C. A cooler sample at these temperatures still contains moving particles.

Measure the sample's temperature

Temperature may be represented by θ (theta) or T. The symbol θ commonly denotes a Celsius temperature in °C, while T commonly denotes a temperature in kelvin (K). These letters name the quantity; °C and K name the units. Always retain the stated scale and unit with a reading.

A thermometer measures temperature through a property that changes as its sensing region responds to the sample. Place and read it so that it represents the sample you intend to measure.

  1. Choose a suitable range and resolution. A thermometer covering -10°C to 110°C can include the 20°C and 50°C readings above. With divisions every 1°C, the scale does not justify reporting many decimal places.
  2. Immerse the sensing region correctly. Follow the instrument's immersion marking or instructions. A liquid-in-glass thermometer needs its bulb in the sample; do not leave the bulb mainly in the air above it.
  3. Avoid contact with the vessel. Keep the sensing region clear of the base and sides, especially while the vessel is being heated. Gentle stirring, where suitable, can reduce temperature differences within a liquid.
  4. Allow time for the response. For a sample held at a steady temperature, wait until the reading settles. If the sample is continuously warming, a slow thermometer can lag behind its changing temperature.
  5. Read the scale properly. View the top of a liquid-in-glass thermometer's column at eye level to reduce parallax. Record the unit and the relevant time when following a change.

Measurement reasoning

Two readings of the same warming liquid

A thermometer touches the heated base of a beaker. That region may be hotter than the bulk liquid, so its reading can be too high for the intended liquid temperature. Moving the sensor into the liquid, away from the base, addresses the placement problem.

A second thermometer is correctly placed but responds slowly. While the liquid warms, its reading can be below the liquid's current temperature. Repeating the same timing does not remove this response lag; use a suitable sensor and account for its response time.

Repeated readings can reveal variation, but they do not correct a sensor touching the wrong object or an unsuitable response time. A finely divided display also does not guarantee that the measurement is accurate.

Compare this temperature relationship with internal energy, which includes total random kinetic energy and particle potential energy.

Optional check The same gas warms from 20 degrees C to 50 degrees C. What does this establish about its particles?
The same gas warms from 20 degrees C to 50 degrees C. What does this establish about its particles?