Transfer Of Thermal Energy

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Lesson notes

Demonstrating Conduction

Good thermal conductors (e.g., aluminium, copper) transfer heat easily; insulators (e.g., wool, cardboard) transfer heat poorly.
Tiles feel colder than a rug at the same temperature because tiles conduct heat away from the foot faster.
In the wood–metal rod experiment, paper over metal stays unburnt because metal conducts heat away; paper over wood chars because wood insulates.
Relative conductivity of metals can be compared by timing when wax melts and ball bearings drop from metal strips heated at a common centre.

Apparatus for demonstrating conduction with a conduction ring and Bunsen burner

Thermal Conduction (Extended)

Conduction in solids occurs via atomic vibrations and, in metals, via free electron collisions.
Metals are the best conductors because they have many free electrons that transfer energy rapidly.
Liquids and gases are poor conductors because particles are too far apart or slide past each other, hindering vibration transfer.
Thermal conductivity varies widely among materials; conductors are typically metals with delocalised electrons.

Conduction in a metal via atomic vibrations and free electron collisions

Convection

Convection is the main heat transfer method in fluids (liquids and gases); it cannot occur in solids.
Heating a fluid causes it to expand, become less dense, and rise; cooler fluid sinks, creating a convection current.
Cooling a fluid makes it contract, become denser, and sink, also driving a convection current.
A convection current can be demonstrated using potassium permanganate crystals in water: the purple dye reveals the rising and falling motion.

Convection current from a fire

Radiation

All objects emit thermal radiation (infrared); hotter objects emit more.
Thermal radiation can travel through a vacuum (e.g., from the Sun to Earth).
Black/dull surfaces are good absorbers and good emitters of radiation; white/shiny surfaces are poor absorbers and poor emitters.
At thermal equilibrium, an object absorbs and emits radiation at the same rate, so its temperature remains constant.

Hot object emitting thermal radiation vs. cooler object

The Greenhouse Effect

Without an atmosphere, Earth's average surface temperature would be about −18 °C.
Greenhouse gases (e.g., CO₂, water vapour, methane) absorb and re-emit longer-wavelength infrared radiation from Earth, trapping heat.
This process keeps Earth warm enough for life; increasing greenhouse gases raises the rate of radiation absorbed, leading to global warming.

The greenhouse effect

Investigating IR Radiation

Experiment: use identical flasks painted black, grey, white, and silver, filled with hot water at the same initial temperature.
Measure temperature at regular intervals (e.g., every 30 s for 10 min).
The black flask cools fastest (best emitter); the silver flask cools slowest (poorest emitter).
Plot temperature vs. time; the curves show that surface colour affects the rate of infrared emission.

Different coloured flasks for investigating infrared radiation

Consequences of Thermal Energy Transfer

Conduction applications: metal pans heat food quickly; plastic handles insulate; double-glazed windows use trapped air as an insulator.
Convection applications: room radiators heat air which rises and circulates; steam rises from hot drinks.
Complex transfers: a hot drink loses heat by conduction through the cup, convection from the surface, and radiation from the cup's sides.
A car radiator uses conduction from engine to liquid, then to radiator, and radiation from the dark, large-surface-area radiator to air.

Heat transfer processes from a hot mug of tea

Slides

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Practice questions

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1.Which of the following is the main method of thermal energy transfer in solids?
Easy
- AConduction
- BConvection
- CRadiation
- DEvaporation
2.Thermal radiation is part of which electromagnetic spectrum?
Easy
- AVisible light
- BInfrared
- CUltraviolet
- DMicrowave
3.In the experiment comparing conduction in wood and metal, a rod made of half wood and half metal is wrapped in paper and heated at the join. What is observed?
Easy
- AThe paper chars only where it touches the metal
- BThe paper chars only where it touches the wood
- CThe paper chars evenly across both materials
- DThe paper does not char at all
4.Which of the following is a greenhouse gas that contributes to the greenhouse effect?
Easy
- AOxygen
- BNitrogen
- CCarbon dioxide
- DHydrogen
5.In the investigation of infrared radiation using flasks painted different colours, which flask is expected to cool the fastest?
Easy
- ABlack flask
- BWhite flask
- CSilver flask
- DDull grey flask
6.Convection currents in fluids are caused by changes in:
Easy
- ATemperature and density
- BPressure and volume
- CColour and texture
- DMass and weight
7.Which of the following is a good thermal insulator?
Easy
- ACopper
- BAluminium
- CWool
- DIron
8.A student places one foot on a tile floor and the other on a rug in the same room. The tile feels colder because:
Medium
- AThe tile is at a lower temperature than the rug
- BThe tile conducts heat away from the foot faster than the rug
- CThe rug radiates heat towards the foot
- DThe tile has a higher specific heat capacity

More questions

9.Thermal radiation can travel through:
Easy
- ASolids only
- BLiquids only
- CGases only
- DA vacuum
10.Which surface is the best absorber of thermal radiation?
Easy
- AShiny white
- BDull black
- CShiny silver
- DDull grey
11.In a car radiator, thermal energy is transferred from the engine to the surrounding air mainly by:
Medium
- AConduction through the liquid, then radiation from the radiator
- BConvection only
- CRadiation only
- DConduction through the metal directly to the air
12.Which of the following is not a method of thermal energy transfer?
Easy
- AConduction
- BConvection
- CRadiation
- DEvaporation
13.In an experiment to compare cooling rates, a student records the temperature of hot water in a beaker every 30 seconds. The results are: t=0 s, θ=94°C; t=30 s, θ=93°C; t=60 s, θ=92°C; t=90 s, θ=91°C; t=120 s, θ=90°C; t=150 s, θ=89°C. What is the average cooling rate over the first 90 seconds?
Medium
- A0.033 °C/s
- B0.033 °C/min
- C0.1 °C/s
- D1 °C/s
14.
A student investigates cooling of hot water in a beaker. The table shows the temperature at 90 s intervals.
Time (s) Temperature (°C)
0 95.0
90 79.0
180 68.5
270 62.5
Which statement about the cooling rate is correct?
Medium
- AThe cooling rate decreases as the water cools.
- BThe cooling rate increases as the water cools.
- CThe cooling rate is constant throughout.
- DThe cooling rate is fastest in the last 90 s.
15.In a cooling experiment, a student uses a beaker with insulation and another with a lid. The temperature drop over 180 s is 14.5°C for the insulated beaker and 7.5°C for the beaker with lid. Which is more effective at reducing cooling rate?
Medium
- AThe lid is more effective.
- BThe insulation is more effective.
- CBoth are equally effective.
- DNeither reduces cooling.
16.A student calculates average cooling rate for beaker A over 180 s as x = (θ0 - θ180)/T. If θ0 = 83.0°C and θ180 = 68.5°C, what is x?
Medium
- A0.0806 °C/s
- B0.0806 °C/min
- C0.0833 °C/s
- D0.0833 °C/min
17.In the investigation of infrared radiation using flasks of different colours, which flask is expected to have the highest temperature after 10 minutes?
Easy
- ABlack flask
- BWhite flask
- CSilver flask
- DGrey flask
18.Which of the following is the best description of a convection current?
Easy
- AHot fluid rises because it is less dense, then cools and sinks.
- BHot fluid sinks because it is more dense, then warms and rises.
- CCold fluid rises because it is less dense, then warms and sinks.
- DHot fluid rises because it is more dense, then cools and sinks.
19.A car radiator is painted black and has a large surface area. Which combination of reasons explains why this helps cool the engine?
Medium
- ABlack surfaces are good emitters of infrared radiation; large area increases radiation rate.
- BBlack surfaces are good absorbers of infrared radiation; large area increases conduction.
- CBlack surfaces are poor emitters of infrared radiation; large area reduces convection.
- DBlack surfaces are poor absorbers of infrared radiation; large area reduces radiation.
20.In the wood and metal rod demonstration, paper wrapped around the rod is heated at the join. Why does the paper not burn where it touches the metal?
Medium
- AMetal conducts heat away from the paper, keeping it cool.
- BMetal reflects heat away from the paper.
- CMetal is a poor conductor, so heat stays in the paper.
- DThe paper is a better conductor than metal.
21.A student investigates the rate of cooling of hot water in a beaker. The temperature at t=0 is 95.0°C and at t=90s is 79.0°C. Using the equation x = (θ0 - θ90)/T where T=90s, what is the average cooling rate?
Easy
- A0.178 °C/s
- B0.178 °C/min
- C1.78 °C/s
- D0.178 K/s
22.In the investigation of infrared radiation using flasks of different colours, four identical flasks painted black, grey, white, and silver are filled with hot water at the same initial temperature. After 10 minutes, which flask is expected to have the lowest temperature?
Easy
- ABlack flask
- BGrey flask
- CWhite flask
- DSilver flask
23.A student wants to investigate whether the difference in cooling rate between a beaker and a metal can is due to surface colour or material. Which pair of changes to the apparatus would best isolate the effect of surface colour?
Medium
- AUse a metal can painted shiny silver and a metal can painted matt black.
- BUse a metal can and a glass beaker, both painted matt black.
- CUse a metal can painted black and a glass beaker painted white.
- DUse a metal can and a glass beaker, both unpainted.
24.A student plans an experiment to test how the thickness of a lid affects the cooling rate of hot water. Which of the following should be controlled variables?
Medium
- AVolume of water, initial temperature, room temperature, and material of the lid.
- BThickness of the lid, volume of water, and initial temperature.
- CRoom temperature, material of the lid, and thickness of the lid.
- DVolume of water, initial temperature, and thickness of the lid.
25.In an experiment to compare cooling rates, a beaker of hot water and a metal can (painted matt black) of hot water are used. The temperature drop over 150s is 5°C for the beaker and 6°C for the can. Which conclusion is correct?
Medium
- AThe water in the can has a greater rate of cooling than the water in the beaker.
- BThe water in the beaker has a greater rate of cooling than the water in the can.
- CThere is no significant difference between the rates of cooling.
- DThe can cools slower because it is metal.
26.A student plans to investigate whether the difference in cooling rate between a beaker and a metal can is due to surface colour or material. Which pair of changes to the apparatus would best distinguish between these two factors?
Hard
- AUse a beaker painted matt black and a metal can painted shiny silver.
- BUse a beaker painted shiny silver and a metal can painted matt black.
- CUse a beaker and a metal can both painted matt black, and also a beaker and a metal can both painted shiny silver.
- DUse a beaker and a metal can both made of the same material and painted the same colour.
27.Convection can occur in solids.
Easy
True or false?
28.Thermal radiation can travel through a vacuum.
Easy
True or false?
29.Match each method of thermal energy transfer with its correct description.
Medium
- Conduction
- Convection
- Radiation
- Transfer through particle vibrations and free electrons in solids
- Transfer by density changes in fluids
- Transfer by electromagnetic waves that can travel through a vacuum
30.Arrange the following steps in the correct order to describe how a convection current is set up when a liquid is heated from below.
Medium
- Hot liquid becomes less dense and rises
- Cooler liquid sinks to replace the rising liquid
- Liquid is heated from below, particles gain energy and move apart
- The rising liquid cools, becomes denser, and sinks
31.Which of the following are greenhouse gases? (Select all that apply)
Easy
- ACarbon dioxide
- BOxygen
- CMethane
- DWater vapour
- ENitrogen
32.Match each surface type to its ability to absorb and emit infrared radiation.
Medium
- Dull black
- Shiny white
- Dull white
- Shiny black
- Good absorber and good emitter
- Poor absorber and poor emitter
- Reasonable absorber and reasonable emitter
- Very poor absorber and very poor emitter
33.Arrange the following steps in the correct order to explain why a car radiator helps cool the engine.
Medium
- Liquid travels from the radiator to the engine
- The liquid absorbs thermal energy from the engine by conduction
- The liquid travels back to the radiator and transfers heat to it by conduction
- The radiator transfers thermal energy to the surrounding air by radiation
34.In the experiment to compare cooling rates of flasks painted different colours, which variables must be controlled? (Select all that apply)
Medium
- AStarting temperature of water
- BVolume of water in each flask
- CColour of the flask
- DTime interval between measurements
- EMaterial of the flask
35.Match each everyday object to the main method of thermal energy transfer it uses.
Easy
- Metal saucepan handle
- Radiator in a room
- Black car radiator
- Conduction
- Convection
- Radiation
36.Arrange the following steps in the correct order to describe how a wood fire heats a room.
Medium
- The fire transfers thermal energy to nearby objects by radiation
- Air surrounding the fire is heated and rises
- A convection current is set up, transferring thermal energy throughout the room
- The fire burns and becomes very hot

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Time (s)	Temperature (°C)
0	95.0
90	79.0
180	68.5
270	62.5