Conservation of energy
In this topic you will:
- learn that energy cannot be created or destroyed
- understand that because energy is conserved, there is no increase or decrease in energy
- understand that thermal energy is dissipated from hotter places to colder places.
- discover that thermal energy always transfers from hotter places to colder places
- understand what is meant by heat dissipation.
Key words
- conserved
- created
- destroyed
- system
- colder
- hotter
Energy is conserved
People often talk about conserving energy when they mean using less energy. For example, you switch off a lamp when you leave the room.
In physics, saying that energy is conserved means something different from saying less energy is being used.
When we say that energy is conserved, we mean that the total quantity of energy stays the same. This happens when energy is stored, changed, transferred or even dissipated.
Look at the energy diagram for an electric lamp (extension material).
The energy diagram shows that every time 100 J of electrical energy is supplied to the lamp, 10 J of this is changed to light energy and 90 J is changed to thermal energy. The light energy is useful and the thermal energy is wasted.
The electrical energy is called the energy input and the light and thermal energy together are called the energy output.
The total energy input of 100 J is the same as the total energy output of 10 J + 90 J = 100 J.
This shows how energy is conserved in an electric lamp.
The energy diagram for the engine in a car has more energy outputs (extension material).
In the energy diagram for a car engine, the energy input and outputs are shown as percentages. Notice that the energy input is 100% and the total energy output adds up to 100%. Once again, energy is conserved.
Important Concept: System
In physics, a system is a part of the universe that is chosen for study. It could be a single object or a group of interacting objects, and we analyze the energy transfers in and out of this system.
In these examples, the electric lamp and the car engine are types of system. In physics, a system is something that has been chosen to be studied, especially in terms of energy changes.
The total energy output can never be greater than the total energy input in any type of system. This is because energy cannot be created. Created means to make something, or to bring something into existence.
Common Mistake
Students often think energy is “lost” in a system. In reality, energy is not lost—it is usually transferred to the surroundings, often as thermal energy, and becomes less useful but still conserved.
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200 J (input) – 60 J (useful output) = 140 J of energy is dissipated, usually as heat to the surroundings.
Wasted energy, such as thermal energy, is dissipated. Dissipated means this energy spreads out into the surroundings and becomes less useful.
Important Concept: Dissipated Energy
Dissipated energy is energy that has spread out and is no longer useful for doing work. It often becomes thermal energy transferred to the surroundings.
Although this energy is dissipated, it is not destroyed. Destroyed is the opposite of created. Destroyed means to damage something or to end its existence.
Common Mistake
Students often think that energy is “gone” when it is dissipated. In reality, the energy still exists—it has simply spread out and can no longer do useful work. It is not destroyed.
The law of conservation of energy states that energy cannot be created or destroyed, only changed or transferred.
Questions
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Energy cannot be created or destroyed, only transferred or changed from one form to another.
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False. The total energy input equals the total energy output; some of the output may be wasted or dissipated, but not lost.
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False. According to the law of conservation of energy, output cannot exceed input.
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True. The total input energy is equal to the total output, even if some is wasted or dissipated.
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Input = 1000 J. Kinetic = 500 J → X = 500 J thermal.
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Input = 100 J. Light/sound = 80 J → X = 20 J thermal.
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700 J + 1300 J = 2000 J chemical input → X = 2000 J.
Calculate the percentage of the chemical energy that is changed to light and waste chemicals.
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100% − 70% = 30% of the chemical energy is changed to light and waste chemicals.
Calculate the percentage of the chemical energy changed to electrical energy.
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100% − 55% − 10% = 35% changed to electrical energy.
Explain whether Sofia is correct.
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Yes, Sofia is correct. 18 J total – 7 J thermal = 11 J remaining, but she says 12 J becomes light. Since 12 J > 11 J, her answer is incorrect. She overestimated the useful energy.
Bridging Concept: From Energy Conservation to Heat Transfer
So far, you've learned that energy is always conserved—it cannot be created or destroyed, only transferred or changed. You've also explored how energy can be dissipated into the surroundings, often as thermal energy (heat).
But when energy is transferred as heat, where does it go? Why does a hot object cool down over time? Why does your hand feel warm near a fire?
To answer these questions, we now turn to how thermal energy moves—from one place to another. In particular, you'll explore how energy always flows from hot to cold until balance is reached.
Let’s take a closer look at what happens when objects of different temperatures interact…
Feeling heat
When you put your hands around a hot drink, you can feel heat in your hands. This will only happen if the drink is hotter than your hands. Hotter means at a higher temperature.
Thermal energy moves from the drink, through the container and into your hands.
Thermal energy always moves from hotter places to colder places. Colder means at a lower temperature.
We make use of this in many ways:
When thermal energy is removed from a hot object, we say that the thermal energy has dissipated.
Feeling cold
Imagine you are holding ice. The ice feels cold.
It is easy to think that the cold from the ice moves into your hand, but that does not happen. Cold is not an energy store and cold cannot move. Cold means there is less thermal energy.
When you hold the ice, thermal energy transfers away from your hand and into the ice. You feel cold because thermal energy has been transferred away from your hands. You can damage your skin by holding ice for too long, as your skin needs the correct quantity of thermal energy to function.
When it is colder outside and you open a window, the inside cools down. This is because thermal energy from the hotter inside moves to the colder outside. Cold air may enter the room, but thermal energy from the warmer air is still transferred to the colder air.
Dissipation
You met the word dissipation in Stage 7. Dissipation is used to describe energy that spreads out and becomes less useful.
When thermal energy moves from a hotter place to a colder place, we say that thermal energy has dissipated from the hotter place.
The rate, or speed, of thermal energy transfer increases when the temperature difference between the hot place and the cold place increases.
Remember that energy is always conserved, so the thermal energy has not disappeared or been destroyed, it has just spread out and moved to a colder place.
Some scientists think that, billions of years in the future, all the thermal energy in the universe will have dissipated. At that time, everything in the universe will be at the same low temperature and no more energy changes or transfers will be possible.
Questions
Thermal energy moves from ............... places to ............... places.
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Thermal energy moves from hotter places to colder places.
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The temperature of the hot food will decrease as it loses heat to the cooler surroundings.
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Thermal energy is transferred from the hot food to the cooler air around it, so the food gets cooler.
Explain what will happen to the temperature of the ice cream.
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Thermal energy will transfer from the warmer air to the colder ice cream, causing its temperature to rise and the ice cream to melt over time.
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Zara is correct. Cold does not move into the body. Gloves reduce heat loss by stopping thermal energy from escaping your hands to the colder air.
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The water absorbs thermal energy from the engine parts and carries it away, helping prevent the engine from overheating.
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The water transfers thermal energy to the air outside the radiator. The water cools as it flows through the radiator.
Think Like a Scientist
You will measure the temperature change in water caused by heat dissipation from a lamp. This is now an individual investigation.
• 2 V or 6 V lamp
• Lamp holder
• Cells
• Wires and connectors
• 50 cm³ (or smaller) beaker
• Water
• Thermometer
• Timer
• This experiment must be carried out in a school laboratory.
• Ensure that only the glass part of the lamp is in the water.
• Never put a lamp in water at home.
Step 2. Use as small a volume of water as possible.
Step 3. Ensure that only the glass part of the lamp is in the water.
Step 4. Record the temperature of the water.
Step 5. Switch on the lamp.
Step 6. Record the water temperature every minute.
Step 7. Continue recording until the temperature stops changing.
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Create a results table showing time in minutes and temperature in °C.
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Plot time on the x-axis and temperature on the y-axis to show how the water heated over time.
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The lamp emits thermal energy, which is transferred to the water, causing its temperature to rise quickly at first.
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The system reaches thermal equilibrium where the energy input equals energy loss to surroundings.
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LEDs convert more energy to light and less to heat. The water would heat more slowly or not at all.
Reflect on the following:
- Did I take measurements at the correct time?
- Did I record results clearly in a table?
- Did my graph show the temperature trend accurately?