topic 3 - particles (flashcards)

states of matter, thermal energy transfer, specific heat capacity, specific latent capacity

properties of solids

properties of solids

• closely packed

• arranged neatly

• vibrate in place

• incompressible

• cannot be mixed

properties of liquids

• closely packed

• random

• some freedom

• incompressible

• can be mixed

properties of gas

• no contact

• random

• fills the space

• free to move

• compressible

• can mix

kinetic theory

• particles of solids vibrate slightly on the spot (unless at absolute 0)

• during heating, particles vibrate more; in liquids & gases, particles vibrate faster - gained kinetic energy

• when cooling, particles of a liquid/gas move slower; in solids, vibrations become less vigorous - particles lost kinetic energy

thermal energy transfers

• conduction (in solids)

• convection (in liquids)

• evaporation (from surface of liquids)

• radiation

conduction

When particles nearest the heat source gain kinetic energy, they vibrate more vigorously → will collide into their neighbours, causing them to vibrate; process repeats until all particles are vibrating more & object achieves thermal equilibrium with the heat source.

• metals are best conductors as they have free electrons

• small possibility of particles in liquids/gases colliding since they are further apart → conduction is unlikely

convection

Method of heat transfer in liquids & gases as particles can move and flow past each other.

• heated particles vibrate more vigorously + spread apart, causing the fluid to expand

• as particles are further apart, the fluid is less dense, so is able to rise upwards

convection currents

1. Heated particles can displace particles at the top of the container

2. cooler particles have less energy, therefore vibrate less; particles get closer, making the fluid more dense

3. denser fluid sinks to the bottom, where it can be heated

4. process repeats

evaporation

• the particles at the surface turn into vapour

• causes cooling - particles with the most kinetic energy escape the surface, so the average kinetic energy of particles that remain, decreases

• draught increases rate of evaporation

infrared radiation

• type of electromagnetic wave given off all objects

• the hotter the object, the more infrared radiation it gives off

• is absorbed by all objects, regardless of temp

• different types of surfaces can absorb & emit infrared radiation at different rates

emitters, absorbers, reflectors

• surfaces that are good absorbers will also be good emitters - infrared radiation transfers more efficiently both ways

• ‘Perfect black body’ will absorb all radiation directed at it + act as a good emitter; cannot reflect any radiation

thermal insulators

materials that are not good thermal conductors will be good thermal insulators.

useful:

• to prevent heat loss, thus reducing energy bills;

• to prevent injuries;

• to keep something hot or cold

how do insulators prevent heat loss?

• reducing the amount of conduction, convection or radiation

• air is an excellent insulator - trapping air prevents conduction

• if air cannot flow, convection cannot take place

• shiny surfaces can reflect radiant heat back to where it’s required

home insulation methods

• loft insulation - made of fibres trapping air

• cavity walls - double-layer walls with an air gap between, preventing conduction

• double glazed windows - panes of glass separated with a narrow air gap

how to calculate payback time

payback time = initial cost/annual saving

how to calculate profit

profit = (annual saving x time) - initial cost

specific heat capacity

the amount of energy needed to raise the temperature of 1kg of the substance by 1°C

• good thermal conductors have lower SHC values

energy = mass x SHC x temperature change

• SHC measured in J/kg/°C

specific latent heat

SLH of a substance is the energy needed to change the state of 1kg of the substance. each substance has 2 SLH values:

• one for melting/freezing

• one for boiling/condensing

SLH of fusion of a solid is the amount of energy to melt 1kg of substance

• greater the mass, the more energy needed

SLH of evaporation of a liquid is the amount of energy needed to boil 1kg of substance

energy = mass x SLH

SLH is measured in J/kg

how do you calculate density?

density = mass / volume

• can be measured in kg/m³ or g/cm³

• 1 g/cm³ = 1000kg/m³

how do you calculate pressure?

pressure = force (N) / area (m²)

• pressure is measured in N/m² or pascals, where 1 Pa = 1 N/cm²

• area can be measured in cm², so the unit for pressure would be N/cm²

describe a simple hydraulic system

• consists of two cylinders/pistons, connected by a tube

• INPUT - master piston

• OUTPUT - slave piston

• as the system is sealed, the pressure in the whole system is constant

why is the hydraulic system a force multiplier?

the output force is larger than the input force

how do you calculate hydrostatic pressure?

pressure = height x density x g

how does atmospheric pressure vary with altitude?

air is more dense close to earth’s surface, but this decreases as you ascend

→ there is a lower weight of air above you

what is Boyle’s law?

at a fixed temperature, the pressure of a gas is inversely proportional to its volume

pressure x volume = a constant (assuming that its temperature remains constant)

what is Charles’ law?

gases occupy a greater volume when they are at a higher temperature

at constant pressure, the volume of a gas is directly proportional to its thermodynamic temperature

what is Gay-Lussac’s law?

as temperature rises, particles in the gas gain more kinetic energy, therefore more collisions occur, increasing pressure of the gas

at constant volume, the pressure of a gas is directly proportional to its thermodynamic temperature