Physics 2. Thermal physics

What is convection and where does it occur?

Convection is the transfer of heat through the movement of fluids (liquids or gases). It does not occur in solids.

Why does hot air or liquid rise?

Heating causes particles to vibrate and spread out, reducing density. The less dense fluid rises, and cooler, denser fluid moves in to replace it.

How can you demonstrate convection in water?

Using potassium permanganate crystals in water. When heated, the purple dissolved substance rises with warm water, showing the convection current.

True or False: "Heat rises."

False. Hot fluids rise due to decreased density, not heat itself.

What is thermal radiation and how does it travel?

Thermal radiation is infrared radiation emitted by all objects. It can travel through a vacuum (unlike conduction and convection).

How do surface color and texture affect thermal radiation?

Black/dull surfaces: Good absorbers and emitters. White/shiny surfaces: Poor absorbers and emitters (they reflect radiation).

What is thermal equilibrium?

When an object absorbs and emits radiation at the same rate, its temperature remains constant.

If an object absorbs more energy than it emits what happens to its temperature?

Its temperature increases until it reaches thermal equilibrium.

What would Earth's average temperature be without an atmosphere?

About -18°C.

What are greenhouse gases and what do they do?

Gases like CO2, methane, and water vapor absorb and re-emit infrared radiation from Earth, trapping heat and warming the planet.

How does increasing CO₂ affect Earth's temperature?

More CO2 reflects more radiation back to Earth, increasing the absorption rate at the surface and raising the temperature.

Which color cools fastest and why?

Black cools fastest because it is the best emitter of infrared radiation.

Give two examples of conduction in everyday life.

1. Metal pans heating food quickly. 2. Radiators transferring heat to air.

How do double-glazed windows reduce heat loss?

They trap a layer of air between two panes. Air is a poor conductor, so it insulates and slows heat transfer.

Why is a radiator actually a "convection heater"?

It heats air, which rises and creates convection currents, warming the room. Radiation plays only a small part.

What is thermal expansion and why does it occur?

Thermal expansion is the increase in volume of a material when heated at constant pressure. It occurs because molecules gain kinetic energy, vibrate or move faster, and push each other apart, increasing the average distance between particles.

Compare the thermal expansion of solids, liquids, and gases.

Solids expand slightly, liquids expand more than solids, and gases expand the most. This is due to differences in intermolecular forces and particle energy in each state.

Describe one useful application and one consequence of thermal expansion.

Application: Liquid-in-glass thermometers use expansion of liquid to measure temperature. Consequence: Railway tracks, roads, or bridges can buckle if expansion gaps are not provided.

How does a bimetallic strip work in a temperature-activated switch?

It consists of two metals with different expansion rates. When heated, the strip bends predictably, closing or opening an electrical circuit.

What is internal energy and what two components make it up?

Internal energy is the total energy stored in a system by its particles, due to their motion and positions. It is the sum of kinetic energy (motion) and potential energy (positions).

Define specific heat capacity and its units.

Specific heat capacity is the energy required to raise the temperature of 1 kg of a substance by 1 °C. Units: J/kg°C.

How does specific heat capacity affect how quickly a substance heats up or cools down?

A low specific heat capacity means the substance heats up and cools down quickly. A high specific heat capacity means it heats up and cools down slowly.

Why does temperature remain constant during melting or boiling?

Energy is used to overcome intermolecular forces during the change of state, not to increase kinetic energy, so temperature stays constant.

What happens during condensation?

A gas cools to its boiling point, loses potential energy, intermolecular forces re-form, and it becomes a liquid. Temperature remains constant during the change.

How does evaporation differ from boiling?

Evaporation occurs at any temperature and only at the liquid's surface. Boiling occurs only at the boiling point and throughout the liquid.

Explain how evaporation causes cooling.

The most energetic molecules escape the liquid, lowering the average kinetic energy of the remaining molecules, thus reducing temperature.

List three factors that affect the rate of evaporation.

Temperature (higher temperature increases rate), surface area (larger area increases rate), and air movement (more airflow increases rate).

What is a common mistake students make about thermal expansion?

Thinking the molecules themselves expand, rather than the material as a whole due to increased separation between particles.

Name the three states of matter and describe their shape and volume properties.

A solid has a definite shape and a definite volume. A liquid has no definite shape but has a definite volume and will flow. A gas has no definite shape and no fixed volume; it will expand to fill its container completely.

What happens to mass and molecules during a change of state, and are these changes physical or chemical?

The number of molecules and the total mass remain unchanged during a change of state. Only the energy of the particles changes. These are physical changes, meaning they are reversible.

What are the names for the four main processes of changing state?

Melting: solid to liquid. Freezing: liquid to solid. Boiling/evaporation: liquid to gas. Condensing: gas to liquid.

Describe the arrangement and motion of particles in a solid.

Particles are very close together in a regular pattern; they vibrate around fixed positions.

Describe the arrangement and motion of particles in a liquid.

Particles are close together but arranged randomly; they move and slide past one another.

Describe the arrangement and motion of particles in a gas.

Particles are far apart and move randomly at high speed in straight lines until collisions occur.

How do intermolecular forces differ between solids, liquids, and gases?

Solids: strong forces. Liquids: partially overcome forces. Gases: forces fully overcome.

What is Brownian motion and what does it provide evidence for?

Random movement of tiny particles due to collisions with smaller invisible molecules; evidence for the particle model.

In terms of the particle model what causes gas pressure?

Collisions of gas particles with container walls.

How does increasing temperature affect gas particles and pressure?

Particles move faster, collide more often and with more force, increasing pressure (if volume is constant).

What is absolute zero and its value in Celsius?

-273°C; the temperature at which all particle motion stops.

How do you convert from Celsius to Kelvin?

Add 273. (T in K = θ in °C + 273)

What is Boyle's Law and what condition is required?

For fixed mass of gas at constant temperature: pressure is inversely proportional to volume (pV = constant).

Formula for specific heat capacity?

mass x specific heat capacity x change in temperature