Internal energy
Energy moves from hotter parts of a body to colder parts. Particles interact; in a solid they vibrate around their positions. In metals, free electrons also carry energy.
A flow of matter carrying energy is convection.
THERMAL CONDUCTION
/ 01
Heat you cansee.
Feel physics in motion. Start with conduction: energy moves from hot to cold without transferring matter.
THERMAL FIELDFEEL THE ENERGY
20 °C80 °C
Heat in motion. From hot to cold.
An artistic animation. The physics is explained below.
FIRST TOPIC · THERMAL CONDUCTION
Energy passes the baton.
Energy passes the baton.
Matter stays in place.
From spoons to insulation: three ideas that explain familiar things.
Thermal conductivity λ
A material’s thermal conductivity is described by λ (or k), measured in W/(m·K). A smaller λ means less heat flow for the same thickness, area and temperature difference.
λ is thermal conductivity; c is specific heat capacity. These are different properties.Temperatures become equal
Heat transfers spontaneously from hotter to colder. In a thermally isolated system this leads to thermal equilibrium. If a heat source keeps heating, a temperature difference can remain.
Touch senses heat flow, not an exact temperature.For curious minds: thickness matters too
For a uniform flat layer at steady state, the heat-transfer rate is P = λAΔT/d. A is area and d is thickness. Doubling the thickness halves the rate when everything else is equal. This model ignores edge effects; it is not a complete building calculation.
Conductivity also depends on temperature and material structure. Do not compare insulation using just one number if the thicknesses differ.
Rotation and waves on the first screen are an artistic image of heat, not a physical diagram of conduction.
PLAY AND DISCOVER
Every spin —
Every spin —
a new discovery.
8 worlds of heat. No timer. Spin, think and discover why. Or sign in to collect ranking points.
PRESS THE CENTRE · DISCOVER A TOPIC
0 / 0 discoveries
CURIOSITY LAB
What secrets does a cup of tea hold?
From particles to solar energy, every topic has a little story from everyday life.
- Temperature and particles
- Heat transfer
- Heat capacity
- Heat balance
- Changes of state
- Heat at home
- Weather and nature
- Energy and inventions
VIDEO → IDEA → YOUR DISCOVERY
A lesson you
A lesson you
can explore.
Pavlo Viktor’s explanations in our concise interpretations. Read, change conditions in a live model and return to the original video.
Two objects.
One temperature.
Why must a thermometer settle? Change the mass and predict how warm and cold objects reach agreement.
Explore equilibrium ↗Coins fall.
The reason remains.
Copper, steel or wood? Design a fair comparison and solve the “cold” metal trap.
Solve the thermal case ↗Boiling.
And not hot?
Lower pressure in the virtual vessel. Discover how water can boil at room temperature.
Explore cold boiling ↗Original videos by Pavlo Viktor. Educational interpretations and implementation by Yana Kostova. Each article links to its original video.
All published lessons · choose your route
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FOUR ADVENTURES. YOUR DISCOVERIES.
Now you are
Now you are
the lead researcher.
Побудуй станцію, розкрий загадки, відкалібруй термометр або стань тепловим інженером. Обирай, перевіряй, пробуй знову. Тут немає таймера — є час зрозуміти.
From a small guess to a big “aha!” Each route has 12 steps with animated clues.
Choose an adventure ↓Deliver heat. Save the station.
Pack your cargo, build protection and adjust the heating. Every choice has a physical reason.
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Cold cases. Hot evidence.
Do not trust first impressions. Open a clue, make a prediction and find an explanation without magic.
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Catch the degree. Outsmart the scale.
Read the scale, check equilibrium and uncover the secret of careful measurement. 12 steps without rushing.
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Find the bridge. Keep the heat.
Compare materials fairly, manage heat paths and support your conclusion. 12 engineering challenges.
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These are learning models, not instructions for heating things at home. Progress stays in this browser if storage is available. These games do not add to the overall ranking.
YOUR PHYSICS / YOUR DISCOVERIES
From curiosity to mastery.
Take your time. Every new question gives one chance to earn points.
The difficulty grows with your discoveries.
Save your progress.
Create a username and password and come back for new questions. Your chosen nickname appears in the public ranking only with your consent.
Your result appears in the public ranking if you agree. For playing with friends there are private team rooms with a shared mission and combined points, without chatting with strangers.
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Understanding earns points.
A correct answer earns 10, 20 or 30 points depending on the level. Every 10 correct answers raise your level, up to level three.
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Virtual laboratory
Boiling under control.
Predict → test → explain. Change one variable and see why the result changes.
1000 g0 kJ0 s
Ready to explore
Virtual experiment. The thermal colour is symbolic: pure water does not turn red when heated. Boiling bubbles contain water vapour.
🔬 Molecular magnifier
A schematic, not to scale. Each dot is an H₂O molecule: boiling does not break it apart. Distances and speeds are illustrative; measure temperature with a thermometer. The white “cloud” above the flask represents condensed droplets; gaseous water vapour is invisible.
Other initial conditions are identical for A/B. Graphs share the same scales.
⚗️ Next: pressure, losses and cooling
Only in free time-based mode. Pressure can change during the experiment: a pressure drop may cause flash boiling. At 0 W the water approaches the ambient temperature: warmer water cools and colder water warms. U is a set property of the model flask; its heat capacity is ignored.
Change conditions and begin.
A — solid line. B — dashed line.
🧂 Separate model: a pinch of salt
1 kg of water · 1 atm. An approximation for a dilute solution.
Pure water: 99.97 °C. Solution: 99.97 °C. ΔT = 0.00 °C.
i ≈ 2; Kb = 0.512 K·kg/mol. This card does not predict how long the solution takes to heat.
💎 Salt workshop: from solution to crystals
A separate equilibrium experiment at 25 °C, starting with 1 kg of water. Change the fraction evaporated and see how much salt still dissolves. This is not a model of evaporation rate or boiling brine.
NaCl solubility is about 360 g/kg of water at 25 °C. Dissolved salt + crystals = initial salt. Salt does not disappear with the water. Do not extend the “pinch of salt” formula to concentrated solutions.
What does the model include?
Basic mode: uniformly mixed water, constant absolute pressure, vapour escapes. It ignores heat losses, flask heat capacity, superheating and evaporation before boiling. At 0 W water does not cool in basic mode; extended mode adds heat exchange U(T − Tambient). This describes the model, not every real condition.
Q = ∫Pdt; c = 4180 J/(kg·K); Qheat = mc(Tboil − 20); mvapour = (Q − Qheat)/L(p). Tboil: IAPWS-IF97, region 4. L(p): IF97 table at 0.1 atm intervals, with linear interpolation between points.
Explore heating and pressure only virtually. Do not repeat pressure experiments at home.
My discoveries
Only in this browser on this device. Nothing is sent to the server.
01 / FOUNDATIONS
It all starts with energy.
Three ideas to help you
understand thermal processes.
Absorbing heat
The body receives energy. Its temperature rises during heating; during melting it can remain constant.
For example, ice melts in a glass.Releasing heat
The body gives energy to its surroundings. Its temperature falls during cooling; during freezing it can remain constant.
For example, hot tea cools down.Heat balance
In an isolated system, the heat released by hotter bodies equals the heat absorbed by colder bodies.
Q is measured in joules (J).How is heat transferred?
Conduction. Energy passes between particles and neighbouring parts of a body without transferring matter. This is how a metal spoon warms in tea.
Convection. Heat is carried by flowing liquids or gases. Heated water rises and colder water sinks.
Radiation. Energy travels in electromagnetic waves. This is how energy from the Sun reaches Earth through the vacuum of space.
02 / REFERENCE
Less memorising. More meaning.
All the formulas you need
in one place.
Q = c·m·(t₂ − t₁)
How much energy is needed?
More water or a larger temperature change means more heat is needed.
Explore the formula with an example
- Q
- Amount of heat, in joules (J).
- c
- Specific heat capacity: the joules needed to heat 1 kg of a substance by 1 °C. Unit: J/(kg·°C).
- m
- Mass of the substance, in kilograms (kg).
- t₁ → t₂
- Initial and final temperatures, in °C. Subtract first: Δt = t₂ − t₁.
Half a litre of water is about 0.5 kg. Heat from 20 to 30 °C: Δt = 10 °C. Q = 4200 × 0.5 × 10 = 21 000 J = 21 kJ.
This is the heat absorbed by the water. A real heater will use more energy if some warms the air and container.
Qreleased = Qabsorbed
Energy is not lost.
The heat released by warmer bodies equals the heat absorbed by colder ones.
Who released heat, and who absorbed it?
- Qreleased
- Heat released, taken as a positive amount, in J.
- Qabsorbed
- Heat absorbed, in J.
An imaginary mixture: equal masses of water at 60 and 20 °C reach 40 °C if there are no losses and the container absorbs no heat. One portion cools by 20 °C and the other warms by the same amount.
Different masses? The larger portion has more influence on the final temperature. Try it in the mixing model below.
4 200 J / (kg·°C)
Water stores a lot of energy.
Approximately this much is needed to heat 1 kg of water by 1 °C.
Why 4200 here, but 4180 in the model?
4200 J/(kg·°C) is a common school rounding. The lab uses 4180 J/(kg·K). These are close approximations: water’s heat capacity varies slightly with temperature. A change of 1 °C equals a change of 1 K.
The same example: 4180 × 0.5 × 10 = 20 900 J. With school rounding: 21 000 J, a difference of about 0.5%.
Specific heat capacity c tells us how much energy is needed for heating. Conductivity λ tells us how a material conducts heat. These are different properties.
03 / PRACTICE
Your little laboratory.
Change the conditions.
Observe the result.
EXPERIMENT 01
Hot + cold.
Hot + cold.
What happens next?
Choose the mass and temperature of the water.
We will calculate the result straight away.
Cold water
Hot water
THERMAL EQUILIBRIUM
and ignoring the container’s heat capacity.
Colour is a symbolic temperature scale. Particle motion is not boiling.
40.0°C
Final mixture temperature
(2 × 20 + 1 × 80) / (2 + 1)
Ideal model: no heat losses and ignoring the container’s heat capacity.
Can you reach the target temperature?
EXPERIMENT 02
See particles move.
The material determines how quickly a temperature change spreads.
SCHEMATIC PARTICLE MODEL
Choose conditions and start the experiment.
This is a qualitative illustration: speed and colour show a trend, not exact temperature values.
EXPERIMENT 03
Where does the energy go?
Decide whether the body absorbs or releases heat.
SITUATION 1 / 5
Heating water in an electric kettle
Choose an answer.
THREE MORE EXPERIMENTS · FROM IDEA TO CONCLUSION
Now heat is
Now heat is
under your control.
Change one condition. Watch the diagram and numbers. Explain what happened.
01 / HEATING → BOILING
Bubble, bubble. But the temperature stays?
1 kg of water started at 20 °C. It has now received 560 kJ: you can see it boiling! Change the energy to find where heating becomes boiling.
Water boils at 100 °C. Reduce the energy to return to heating.
100.0 °CTemperature
280 sHeating time in the model
0.901 kgRemaining liquid water
3. Why does more power not always mean hotter?
Before boiling, Q = cmΔt. Heating 1 kg of water from 20 to 100 °C needs 336 kJ. Further energy turns water into vapour: Q = rm, r ≈ 2260 kJ/kg. At constant pressure, temperature hardly changes during boiling. A more powerful heater supplies energy faster and makes boiling more vigorous.
The model ignores heat losses and container heat capacity; pressure is 1 atm. The energy control sets energy already absorbed by the water; time is calculated as Q/P.
1 ATM · THERMAL MODEL
Colour is a symbolic temperature scale; water does not turn orange. Bubbles appear during boiling. Gentle container shaking is an artistic effect.
Build a jacket for a house.
The same area, 1 m², and a temperature difference of 20 °C. Which layer lets less energy through each second?
16.0 W
Heat-transfer rate through the layer
3. Check the “double” rule
At steady state P = λAΔT/d. Double the thickness: the rate halves. Reduce λ: the energy flow also falls. Material values are approximate; a uniform flat-layer model ignores moisture, thermal bridges and edge effects.
One energy. Different temperatures.
The starting temperature is 20 °C. Compare how water and metals heat up with the same mass and absorbed energy.
The fill shows temperature on a symbolic scale, not expansion of the substance.
30.0 °C
4. Why does water warm more slowly?
Δt = Q/(cm). A higher specific heat capacity c means a smaller temperature change for the same Q and m. With 42 kJ, 1 kg of water warms by 10 °C, aluminium by about 46.7 °C and copper by 109.1 °C. The model ignores heat losses and changes of state; c is constant.
04 / AROUND US
Physics is closer than you think.
Familiar things.
Surprising explanations.
Snow is a blanket.
Air is trapped between snow crystals. It conducts heat poorly, so a layer of snow protects soil and plants from rapid cooling.
Is metal “colder”?
Metal and wood can have the same room temperature. But metal takes heat from your hand faster, so it feels colder.
Heat in a trap.
Porous materials such as aerogel limit heat transfer. Fluffy clothes use a similar idea: air between the fibres helps retain heat.
Heat across emptiness.
A vacuum has no matter for convection or conduction. Energy passes between separated bodies by radiation.
HEAT DETECTIVE · 12 PUZZLES
Obvious?
Obvious?
What if we test it?
Conduction only. Everyday traps, unusual comparisons and a little logic, without timers or school marks.
THERMAL TRAP
“Cold” metal.
“Cold” metal.
Hot suspicions.
Arrows show the direction of energy transfer. One cube melts faster. Both surfaces were in the same room. What did our detective miss?
Test it in a real experiment →Ready to solve a thermal mystery?
Make a guess. Choose an explanation. After answering, discover why one idea works and another fails.
0 / 0 solved
PREDICT · TEST · EXPLAIN
A small experiment.
A small experiment.
A big “wow!”
6 real experiments with simple things. It is interesting to make a wrong prediction here, and understand why.
Open the instructions, make a prediction and check off the steps in order. Compare your observations with your prediction and repeat. Diagrams explain the idea; they are not photos or promised results.
Motion in the diagrams is a repeated illustrative animation, not a measurement. Work through your own experiment step by step below.
01Insulation · 20–40 min
A jacket for an ice cube
Does an ice cube in a jacket melt faster or slower?
Test an idea
You need
Two identical plastic cups with lids, two similar ice cubes, dry fabric, a tray and a clock.
Choose a prediction. Then check off the completed steps in order.
Procedure
A fair comparison
Change only the covering. Keep the amount of ice, cups, lids and location the same.
The physics explained
The wrapped cube usually lasts longer: insulation reduces energy entering from the warm room. The “jacket” does not create cold. If there is no difference, check the fabric thickness and repeat.
Insulation works both ways. It slows cooling of warm things and warming of cold ones. How could this help bring ice cream home?
Stay safe: Keep the fabric dry. Catch water on the tray and keep it away from electrical appliances.
02Conduction · 5–15 min
Ice races
Will “cold” metal save the ice? Make a prediction!
Test an idea
You need
Metal and plastic surfaces without sharp edges, two similar cubes, a tray and a clock.
Choose a prediction. Then check off the completed steps in order.
Procedure
A fair comparison
Use equal ice sizes, locations and starting temperatures. Choose similarly sized surfaces; their thickness also matters.
The physics explained
Ice often melts faster on metal because metal transfers energy to it better. Colder to touch does not mean colder by thermometer. This is a qualitative comparison, not an exact measurement of conductivity.
One temperature — different sensations. Follow the energy from the room-temperature surface to the ice. Melting speed depends on both the material and the surface dimensions.
Stay safe: Do not chill the metal in a freezer or touch ice with your tongue. Clean up the water afterwards.
03Condensation · 10–20 min
The cup under investigation
The cup is “sweating”: a leak, or water from the air?
Test an idea
You need
Two clear plastic cups, room-temperature water, ice, lids and a cloth.
Choose a prediction. Then check off the completed steps in order.
Procedure
A fair comparison
Change water temperature; keep cups, lids and surrounding air the same.
The physics explained
Water vapour in the air condenses on a sufficiently cold surface. Outside droplets are not proof of a leak. In very dry air there may be no visible droplets. Repeat under different conditions rather than inventing a result.
Air contains water too. Water vapour can become droplets on a cold surface. Why are they easier to see in a humid room?
Stay safe: Use stable cups and a tray; do not drink the water after the experiment.
04Radiation · 15–30 min
The Sun chooses a T-shirt
Dark or light: which “T-shirt” catches more energy?
Test an idea
You need
Two identical cups, black and white paper, water, two safe thermometers and a clock.
Choose a prediction. Then check off the completed steps in order.
Procedure
A fair comparison
Only colour changes. Keep water volume, cups, exposure time and wind conditions similar.
The physics explained
A dark covering usually absorbs more solar radiation, so its water may warm more. Material, wind and clouds also affect real results. This simplified experiment is inspired by the NASA JPL solar heater.
Light can heat things. Compare the temperature change, not just the final number. Clouds and wind matter too: record the conditions.
Stay safe: No magnifying glasses, mirrors or concentrated rays. Do not look at the Sun. Try another day if it is cloudy.
05Work and energy · 1 min
A power station in your hands?
Heat without a battery: where did the energy come from?
Test an idea
You need
Your dry palms and a clock. You already have a laboratory!
Choose a prediction. Then check off the completed steps in order.
Procedure
A fair comparison
Compare stationary contact with sliding. Do not rub harder.
The physics explained
While sliding, you do work against friction. Some energy becomes internal energy in your palms, warming them. The source is your body’s energy. Touch provides an observation, not an exact temperature reading.
Mechanical work can cause heating. Find another example of mechanical energy becoming internal energy. Which part of the system warms up?
Stay safe: Move gently. Stop if uncomfortable; do not rub damaged skin.
06Evaporation · 15–30 min
The escape of invisible droplets
The water vanished without boiling. Where is it now?
Test an idea
You need
Two sealed bags, room-temperature water and warm water up to 40 °C, two identical paper-towel pieces, a dropper and a tray.
Choose a prediction. Then check off the completed steps in order.
Procedure
A fair comparison
Keep paper, drops, lighting and airflow the same. Only water temperature inside the bags changes.
The physics explained
Water evaporates without boiling. Warming the paper usually speeds evaporation: particles gain energy to enter the gas phase. Water vapour is invisible; arrows are symbolic. Humidity affects the rate.
Water becomes invisible vapour. During evaporation it enters the air. Predict how moving air will affect it, and change one condition at a time.
Stay safe: No boiling water or alcohol. An adult checks the warm water and bag seals; use a tray.
Water experiments need a tray and an adult’s help. Results depend on conditions: record what you see, even if your prediction was wrong.
SMALL LESSONS · BIG IDEAS
Physics lives
Physics lives
right beside you.
Start by asking “why?”. Then open the explanation and test the idea in the laboratory.
01
Why does metal feel colder than wood?
If both objects have spent a long time in the room, their temperatures are approximately equal. Metal carries energy away from a warm hand faster, so it feels colder. Touch does not replace a thermometer.
Think: why are saucepan handles made from poor heat conductors?
02
Does a blanket heat you or keep heat in?
Your body produces heat through metabolism. A blanket traps air and slows heat transfer. An ordinary blanket is not an energy source.
Experiment without heating: compare how long ice lasts in a bare cup and one wrapped in fabric.
03
How does heat arrive from the Sun?
Space between the Sun and Earth is mostly a vacuum, so air currents cannot circulate there. Electromagnetic radiation carries energy, some of which Earth’s surface absorbs.
Think: why is shade often cooler than direct sunlight?
04
Why does the sea cool slowly?
Water needs a lot of energy for a given temperature change: its specific heat capacity is about 4200 J/(kg·°C). A large mass of water heats and cools slowly.
Problem: 0.5 kg of water was heated by 10 °C. Q = 4200 × 0.5 × 10 = 21 000 J, ignoring losses.
05
Why does ice melt without much temperature rise?
When pure ice melts at normal pressure, energy changes its state. While ice and water coexist in equilibrium, the temperature stays near 0 °C. Once melting is complete, the water can warm up.
Distinguish heating, Q = c·m·(t₂ − t₁), from melting, Q = λ·m. Here λ is specific latent heat of fusion (J/kg), not the conductivity coefficient used in another topic.
06
Why does a wet T-shirt cool you?
Evaporating water needs energy, which can come from fabric and skin. Moving air carries humid air away and can speed evaporation.
Observe safely: how does airflow affect drying speed? Do not get too cold.
07
Why does a thermos keep things both cold and hot?
A thermos slows heat exchange in both directions. The vacuum gap reduces conduction and convection, reflective surfaces reduce radiation, and the lid limits exchange through the neck.
Test the idea: can insulation help a cold drink in a warm room?
08
How can we predict the result of mixing water?
For two portions of water without losses, mixture temperature depends on their masses: t = (m₁t₁ + m₂t₂)/(m₁ + m₂). Equal masses give the ordinary average. A real container may also absorb energy.
Example: 2 kg at 20 °C + 1 kg at 80 °C → 40 °C. Test other proportions in the lab.
09
Does water always boil at 100 °C?
100 °C is water’s boiling point at normal atmospheric pressure. In the mountains pressure is lower, so water can boil at a lower temperature. Unlike boiling, evaporation is possible even at 20 °C.
Do not experiment with boiling water on your own. Models and observations are enough for explanation.
10
How does friction become heat?
Rub your palms: mechanical work can increase their internal energy. Heat transfer and work are two ways to change internal energy. Energy does not appear from nothing.
Explain in words: where does the energy come from during friction?
11
How do you read a heating graph?
On a temperature–time graph, a sloping segment may show heating. A horizontal segment during a change of state means constant temperature even though energy enters. Always check axis labels and experiment conditions.
Ask: does a horizontal line necessarily mean the heater was switched off?
12
How can we save energy without magic?
Heat only as much water as you need. Reduce heat losses with insulation. Power tells us the rate of energy transfer or conversion: 1 W = 1 J/s.
Example: 500 W for 10 s → 5000 J. Not all the energy from a real appliance goes only into the water.
Materials prepared for grade 8. Project creator — Yana Kostova. Hot experiments require an adult; interactive labs can be explored independently.
05 / YOUR TURN
Now you know.
Now you know.
Shall we test it?
Three levels. Six questions each.
No marks — just new discoveries.
6 QUESTIONS · NEW SETBasic level
Physics made easy. Created together.
Yana Kostova
EASYPhysics creator and technical implementer. She makes explanations, interactive experiments and games that invite you to explore and understand physics.
Yana on Telegram ↗Dionis Kostov
Yana’s dad. Supports the server infrastructure and offers advice during development and testing.
Contact on Telegram ↗