Thermal Physics Flashcards by xxx xxx

Q

Potential Energy

A

The energy the particles of a substance have due to stretching of the bonds between them.

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Q

Kinetic Energy

A

The energy the particles of a substance have due to their motion.

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Q

Thermal Energy

A

The sum of the potential and kinetic energy of the particles in a substance.

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Q

Temperature

A

Measure of average kinetic energy of the particles in a substance.

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Q

Absolute Zero

A

Coldest temperature that can theoretically exist. Particles have zero kinetic energy. 0 K or -273(.15) °C

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Q

Thermal Equilibrium

A

A state in which two objects are at the same temperature; the net flow of energy between them is zero.

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Q

Specific Heat Capacity

A

The energy change required to change the temperature of 1 kg of a substance by 1 K.

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Q

Water high specific heat capacity

A

Amongst solids and liquids water has a notably high specific heat capacity. This means that large changes in energy are needed for small changes in temperature.

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Q

Latent Heat

A

The energy change required to change 1 kg of a substance from one state to another. The energy absorbed/released by a substance as it changes states.

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Q

Conduction

A

Transfer of energy through matter by passing energy from particle to particle by collisions, dominant energy transfer method in solids. Very weak in gases, very strong in metals.

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Q

Convection

A

Transfer of energy through matter by particles carrying energy, dominant energy transfer method in fluids.

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Q

Natural Convection

A

Convection is where the particle movement occurs naturally due to density/pressure gradients in the fluid e.g. hot air rising

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Q

Forced Covection

A

Convection where the particle movement occurs due to an artificial driver e.g. fan-driven computer cooling

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Q

Radiation

A

Transfer of energy by emitted electromagnetic radiation, dominant energy transfer method in a vacuum.

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Q

Factors Promoting Radiation

A

Rough (high surface area), black surfaces are the best at absorbing and emitting radiation.

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Q

Factors Minimising Radiation

A

Smooth (low surface area), white/silver surfaces are the worst at absorbing and emitting radiation.

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Q

Minimising Conduction/Convection

A

Insulation typically traps pockets of air. As a gas, the air is a very poor conductor and since it is trapped it cannot convect. E.g. jumper, ceiling batts, fur.

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Q

Conservation of Energy

A

The total energy in a system cannot change, it can be transferred between objects and transformed into different forms.

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Q

Change of State (melting, boiling)

A

At a specific temperature, the substance does not gain kinetic energy, so temperature remains constant, it only gains potential energy as the bonds between particles are stretched and then broken as it changes states.

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Q

Change of state (freezing, condensing)

A

At a specific temperature, the substance does not lose kinetic energy, so temperature remains constant, it only loses potential energy as new bonds form between particles as it changes states.

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Q

Thermal Expansion

A

As matter gains potential energy the particles stretch further apart, causing the substance to expand.

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Q

Thermal Contraction

A

As matter loses potential energy the particles relax closer together, causing the substance to contract.

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Q

Latent Heat of Fusion

A

Solid and Liquid

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Q

Latent Heat of Vaporisation

A

Liquid and Gas

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Energy Cooling In Cars

A petrol vehicle only converts 12-30% of the chemical energy stored in its fuel into kinetic energy in the wheels. The rest is converted into other forms of energy – mostly heat. Excess heat must be removed to prevent damage to the engine.

Car Cooling System

A pump: this pushes coolant around the engine. A thermostat: this releases coolant into the radiator once it reaches a high temperature. A radiator: this is a set of tubes surrounded by thin pieces of metal called radiator fins. Heat from the coolant is transferred to the fins, which then transfer it to the air moving past them. A fan: this pulls air through the radiator as required to increase the rate of cooling.

Heat Pump (movement of energy from cold place to hot place)

1. A compressor pressurises a gas called refrigerant, driving up its temperature. 2. The refrigerant travels through a set of tubes called a condenser, where it releases heat to its surroundings and condenses into a liquid. 3. The refrigerant is depressurised by an expansion valve, which causes it to cool down. 4. The now-cool refrigerant travels through an evaporator, absorbing heat from its surroundings and turning into a gas as it does so. It then re-enters the compressor and the cycle repeats.

Refrigerator (heat pump)

the condenser is on the back, surrounded by radiator fins (similar to a car radiator) the evaporator is inside the refrigerator, where it absorbs heat from the refrigerator’s contents Refrigerators are usually equipped with adjustable thermostats, which turn off the compressor at a set temperature.

Air Conditioning

Refrigerate air conditioners also rely on heat pumps: the condenser is outside, where fins and a fan assist with heat transfer to the air (similar to a car radiator) the evaporator is inside, where it absorbs heat from the air The cooled air is blown into the room by another fan.

Evaporative Air Conditioning

Evaporative air conditioners cool a room by using the cooling effect of evaporating water. air is forced through a wet filter pad heat from the air is absorbed as the water in the pad evaporates the cooled air is forced into the room by a fan

Positioning of Rooms

Rooms used most during day in north end to use winter sun Bedrooms placed in south where they stay cool during summer

Insulation

Double Glazed Windows Double Brick Walls Fibreglass Insulation

Double Glazed Windows

Window made up of two glass panels with an air pocket in between - since air is a poor conductor of heat, this slows down the flow of heat into the house

Double Brick Walls

Made of two brick panels with air pocket in between

Fibreglass insulation

As an insulator, fiberglass works to trap air and slow the transfer of heat.

Shade

Trees and other sources of shade prevent sunlight into a house Deciduous trees lose leaves in colder months

Thermal Mass

A materials ability to absorb, store and release heat

High Thermal Mass

Materials with high thermal mass (brick, concrete, stone) are helpful in summer because they absorb lots of heat before slowly releasing it into the house In winter they absorb heat during day and release it at night, keeping house warm

Builders often insulate parts of homes with material that consists of a thin shiny sheet of reinforced aluminium foil. How does this material reduce heat transfer?

Aluminium foil reduces heat transfer because aluminium reflects 95% of the heat that hits its surface. This blocks heat transfer. It blocks the flow of radiation which makes it useful in preventing heat loss.

Swimmers often notice that when they start to swim in a still body of water after a hot, sunny day the surface water is warm but the deeper water is cold. Explain why this happens.

The water at the top of the pool is heated by the suns radiation and hotter water is less dense so it’ll stay at the surface, whereas colder water stays at the surface. This is convection because convection only occurs in liquids. There’s a convection current when warm fluids rise up and cool fluids fall.

Efficiency

Efficiency is the ratio between the useful work done by a machine and the total energy expended to do that work. Its expressed as a percentage according to the following formula:

When the temperature of a substance rises

The kinetic energy of the particles within that substance increases

Solids

Attractive forces hold particles in fixed positions, repulsive forces prevent lattice from collapsing Vibrate around equilibrium positions, given solids fixed volume and shape

Liquids

Particles have more KE, inter-particle forces weaker Particles move past one another, variable shape Fixed volume

Gases

Greater KE than liquid/solid Move rapid in random straight lines until they collide with other particles Collisions elastic due to high speed and low attractive forces Variable shape and volume

Zeroth Law of Thermodynamics

“If two systems are in thermal equilibrium with a third system, then they are in thermal equilibrium with each other”

When particles move further apart or change state,

, the energy used to overcome the inter-particle forces is converted into potential energy

If the heating causes an increase in temperature,

it gains kinetic energy and if it causes expansion or a change of state, it gains potential energy

When mechanical work is done on a system,

it increases the internal energy of the system and when mechanical work is done by a system, its internal energy decreases

First Law of Thermodynamics

“When energy passes into or out of a system as work or heat, the systems internal energy changes in accord with the law of conservation of energy”

A systems change in internal energy (U) is equal to the heat added to the system (Q) plus the work done on the system (W)

Change in Thermal energy = Heat + Work

If the mass of water in the pot is doubled, it will take twice as long to achieve a given temperature increase

Thermal energy proportional to mass

The longer the water is heated, the more its temperature will increase

Thermal energy proportional to change in temp

Liquid Water High Specific Heat Capacity

Liquid water has a particularly high heat capacity meaning that it takes a large amount of energy to change its temperature (useful in motor cooling)

Metal Low Heat Capacity

Metals with low heat capacities but high melting points like iron are useful for cookware as they are excellent conductors of heat and their low heat capacity minimises the heat that is “wasted” in heating up the cookware rather than the food inside

Temperature remains ___ when state changes

Constant (it doesnt change)

Whenever a substance goes from solid to liquid or liquid to gas it must gain ____

Potential Energy

When water is heated it gains ____ causing a rise in temp (temp to increase)

Thermal energy

Different substances have diff latent heats due to differences in the

arrangements of particles and the strength of their interparticle forces

Conduction

No net movement of particles Mostly in solids Occurs by collisions or movement of free electrons Transfers heat slowly due to low vibrational velocity of particles

Thermal Conductivity

Conductors are substances that transfer heat by collisions and free electrons Insulators have no free electrons Solids best conductors, particles closer together Gas good insulator, low density limit number of colissions

Rate of conduction increases with

Increasing thermal conductivity/ temp difference Decreasing thickness/surface area

Convection (movement of hot particles of fluid)

Mass movement of particles over a distance When heated, Particles gain KE so they move rapidly and further apart Decrease density of hotter portion of fluid Less dense, hotter fluid rise up and denser cool fluid fall down

Convection Current

The upward movement of hot fluid and downward movement of cold fluid is called a convection current

Natural Convection

When a fluid rises as its heated Land and sea breezes, air above land more vulnerable to temp changes than air above ocean Water high (C) keeps temp constant

Sea Breeze and Land During Day

Air above land heats up and rises This air replaced by cooler, denser air from above ocean (sea breeze) at night reverse process occurs

Sea Breeze and Land During Night

Warm air above ocean rises and is replaced by cool dense air from above land (land breeze)

Forced Convection

Occurs when fluid heated and blown by fan Example: fan forced oven, ducted heating

Radiation (electromagnetic radiation)

Electromagnetic radiation consists of electromagnetic waves that travel at the speed of light

Rate of Radiant Depends on

Surface area, increase with rate of radiant Surface colour and texture: rough matte black surfaces absorb more than white shiny smooth surfaces