B1 Thermal Energy Transfers

Solids (shape&volume, arrangement, movement and energy, IMF)

1. Fixed shape, fixed volume & do not flow

2. Regular arrangement

3. Particles only vibrate around fixed positions but cannot translate because they have relatively little kinetic energy

4. Strong intermolecular forces of attraction

Liquids (shape&volume, arrangement, movement and energy, IMF)

1. No fixed shape(takes the shape of the container), but a fixed volume at a particular temperature, do flow

2. Irregular arrangement

3. Particles have more kinetic energy than a solid, so they vibrate and they can move one over another

4. Weaker IMF than a solid of the same substance

Gases (shape&volume, arrangement, movement and energy, IMF)

1. No fixed shape or volume（depends on temperature and pressure）, flow and completely fill the container

2. Irregular arrangement & particles are far apart

3. Particles have relatively high kinetic energy and move quickly and randomly in all directions

4. Very weak IMF: high intermolecular potential energy

Thermal expansion

Thermal energy increases the kinetic energy of the particles, causing them to vibrate/move more. Due to the intermolecular forces, the increased vibration results in an increase in the average separation between particles, so the substance expands. This weakens their IMF and increases their intermolecular potential energy.

Phase changes

Internal Energy

The internal energy of a system is the the sum of the total intermolecular potential energy of the particles (due to the intermolecular forces) and the total kinetic energy of the particles (due to their random motion).

Explains in terms of particles what happens to the internal energy of a system when it's heated.

When thermal energy is transferred to the particles:

1）particles move faster, increasing their kinetic energy

2) particles move further apart, increasing their intermolecular potential energy

Therefore the internal energy of the system increases.

formula of density

Kelvin scale and degrees Celsius temperature scale

The Kelvin scale defines the temperature at which the average kinetic energy of the particles is 0 as 0K. = -273 °C

Kelvin to degrees Celsius : -273

Degrees Celsius to Kelvin : + 273

What is a change of temperature of 2 degrees Celsius converted Kelvin?

→ 2K

The change in temperature of a system is the same when expressed in Kelvin and in degrees Celsius (since an increment od the Kelvin scale is equal to an increment of the degrees Celsius scale).

Temperature and average kinetic energy of the particles + hence the formula for KE of particles

The average kinetic energy of the particles is directly proportional to the temperature in Kelvin.

What other equation can be used to work out the KE of particles?

KE = ½ mv²

Used when mass and velocity are given.

Specific heat capacity

The amount of heat/thermal energy required to increase 1kg of a substance by 1°C or 1K. (or The amount of thermal energy released by a substance when its temperature decreases by 1 °C or 1K )

Energy formula linked to specific heat capacity

Q=mc△T

Q=mcAT (meow)

Explain what happens to the energy of the particles and the temperature when a substance is heated and remains in the same phase.

The heat energy supplied is transferred to the kinetic energy of the particles, thus increasing the temperature.

Explain what happens to the energy of the particles and the temperature when a substance is heated and undergoes a phase change.

All the heat energy supplied is transferred to the intermolecular potential energy of the particles to provide them with more energy to overcome the intermolecular forces of attraction between them.

This allows them to move further apart and the volume expands.

The substance changes phase. The average kinetic energy of the particles stays the same.

As a result, the temperature remains constant

Specific latent heat (L)

The amount of energy required for 1kg of a substance to change phase.

Specific latent heat of fusion =….from solid to liquid

Specifc latent heat of vaporisation =…from liquid to gas

Formula linking energy to specific latent heat

Q=mL

Explain what happens, in terms of thermal energy transfer and temperature, when ice cubes are put into a drink at room temperature.

1） Heat energy is transferred from the drink to the ice cubes.

2）The temperature of the ice cubes increases to its melting point.

3）The ice cubes change phase at constant temperature.

4）The temperature of the liquid ice cubes continues to increase and the temperature of the water decreases until they reach a thermal equilibrium.

280K(2 s.f.)

Use the fact that energy gained by ice cubes = energy lost by water

Conduction

The transfer of thermal energy by the collision of particles in direct contact.

• Occurs best in solids as particles are close to each other (in direct contact).

When a solid is heated:

• Thermal energy is transferred to the kinetic energy of the particles closest to the heat source, so they vibrate more,

• hitting and causing adjacent particles to vibrate more too,

• transferring heat energy kinetic energy from hotter parts to cooler parts

• Until the thermal equilibrium is reached, where the net energy transfer is 0

Conductors and Insulators

• Conductors are materials that are good at transferring heat energy by conduction. Metals are good conductors.

• Insulators are the opposite. Wood and air are good insulators.

Why are metals good conductors

They contain delocalised electrons that can move freely through the structure and transfer energy through collisions.

Why is air a good insulator

Air particles are far apart, so there are fewer collisions between particles, reducing heat transfer by conduction. Small pockets of trapped air prevent the air from moving and circulating, reducing heat transfer by convection.

On what three factors does the rate of thermal energy transfer by conduction depend on? Hence give the formula for rate of thermal energy transfer by conduction.

! Thermal conductivity k is in W m₋⁻ ¹K⁻¹

Convection

The transfer of thermal energy due to the mass movement of particles caused by differences in density

Occurs in fluids only

When a fluid is heated:

• Thermal energy is transferred to the kinetic energy of the particles,

• causing them to move faster and collide harder and more frequently with one another,

• so they move further apart, decreasing the density.

• Therefore the hot fluid rises, and colder, denser fluid sinks to take its place

• This process continues and a convection current forms

Radiation

The transfer of thermal energy by electromagnetic waves.

• All objects above 0K emit IR.

• Does not involve particles so can occur in a vacuum.

• Black matt surfaces are the best absorbers ans emitters, but bad reflectors.

• Light/Silver shiny surfaces are the worst absorbers and emitters, but the best reflectors.

• Rate of energy transfer can be calculated using P(rate)= σAT⁴, the same fornula used to calculate Luminosity, which is defined as the amount of energy a star emits per second(unit time)=rate of radiation

Intensity

The amount of power on one square meter of the surface of the object.

Black bodies

A black body is an ideal object that absorbs and (at equilibrium) emits all the energy at all wavelengths of the electromagnetic spectrum.

No real object is a black body, but some stars are close approximations of it.

Sketch the graph of intensity against wavelength showing the emission spectrum of a black body at 6000K

What would happen to the graph of a black body with lower temperature?

1. Lower peak intensity.

2. Peak intensity occurs at longer wavelength.

3. Lower overall intensity.

4. Area under the graph decreases so total power emitted is also lower.

Wien's law

Luminosity（definition+formula）

Apparent brightness+formula

The amount of power per square meter received by an observer.（=how bright a star appears to an observer）

Unit= Wm⁻²

What do you need to watch out when using formulae involving temperature?

Convert temperature to Kelvin!!

(except if it's temperature change=the same in degrees Celsius and Kelvin )