Energy
The ability to do work, which is the ability to exert a force causing displacement of an object.
Work
Results from the application of force over distance. When work is
done, energy is transformed from one form to another.
Energy Is A Useful "Currency" For Describing Change
In physics, energy is a way to keep track of what can cause change. If an object can do work (for example, lift a load, speed something up, heat something up), then it has energy.
The idea of work connects forces and energy transfer. When a force moves an object through a distance, energy is transferred. For a constant force in the same direction as the motion:
$$W = Fd$$
Energy is measured in joules (J), the same unit as work.
Energy appears in everyday language ("I have no energy"), but in physics we use it precisely: it is a measurable quantity that helps explain and predict changes in motion, temperature, height, and more.
Energy Stores And Pathways Explain Where Energy "Is" And How It Moves
A good way to organize thinking is:
- Energy stores: where energy is held in a system.
- Energy transfers (pathways): how energy moves between stores.
Common Energy Stores At This Level
- Kinetic energy: energy of motion.
- Gravitational potential energy: energy due to position in a gravitational field (height).
- Thermal energy: energy associated with the random motion of particles (temperature).
- Elastic potential energy: energy stored when an object is stretched or compressed.
- Chemical potential energy: energy stored in chemical bonds (released or absorbed in reactions).
- Nuclear potential energy: energy stored in the nucleus due to nuclear forces (very large, harder to access).
Kinetic energy
Kinetic energy is the energy an object possesses due to its motion.
Gravitational potential energy
Gravitational potential energy is the energy stored due to the position of an object in a gravitational field.
Typical Transfer Pathways
- Mechanical work (forces doing work)
- Electrical work (charges moving through a potential difference)
- Heating (due to temperature difference)
- Radiation (energy carried by waves, including light and infrared)
Light and sound can carry energy away from a system, but in many everyday processes the energy transferred as light or sound is small compared with the total energy involved.
Conservation Of Energy Is A Fundamental Law
The law of conservation of energy states:
- Energy can be transferred between objects and transformed from one form to another.
- Energy cannot be created or destroyed.
- Therefore, the total energy of a closed system remains constant.
This is one of the most fundamental ideas in physics. You use it whenever you track energy before and after a process.
Conservation of energy
Energy cannot be created or destroyed. It can only be converted from one form to another.
"Energy is lost" usually means "energy is transferred to a store we are not interested in" (often thermal energy of the surroundings). The energy is not destroyed.
Gravitational Potential Energy Often Transforms Into Kinetic Energy
When an object falls, its gravitational potential energy decreases and its kinetic energy increases. If we ignore air resistance, the decrease in gravitational potential energy equals the increase in kinetic energy.
Gravitational potential energy near Earth's surface is:
$$E_p = mgh$$
where $m$ is mass (kg), $g \approx 9.8\,\text{m s}^{-2}$, and $h$ is vertical height (m).
Kinetic energy is:
$$E_k = \frac{1}{2}mv^2$$
What Conservation Predicts For Falling Objects
If no energy is transferred to thermal stores (no significant air resistance), then:
$$mgh = \frac{1}{2}mv^2$$
The mass cancels, so the speed after falling through height $h$ does not depend on mass:
$$v = \sqrt{2gh}$$
Two ball bearings (10 g and 100 g) are dropped from the same height (ignoring air resistance). The heavier one has more gravitational potential energy at the start, but it also needs more energy to reach a given speed because its mass is larger. Conservation of energy shows both reach the same speed at a given height, so they land at the same time.
In energy questions:
1. Choose a system (for example, "ball + Earth").
2. List initial and final energy stores.
3. Decide whether losses (friction, air resistance) are negligible.
4. Write an energy equation (initial total = final total).
5. Substitute formulas ($mgh$, $\tfrac12 mv^2$) only after the energy equation is set up.
Friction And Air Resistance Transfer Energy To Thermal Stores
In real situations, friction and air resistance do work that transfers energy out of the "useful" mechanical stores.
For example, a ball rolling down a slope:
- Gravitational potential energy decreases.
- Kinetic energy increases, but not as much as it would in an ideal case.
- Some energy is transferred to thermal energy of the ball, ramp, and air.
- A small amount may become sound.
This does not violate conservation of energy. It changes where the energy ends up.
When a process seems "inefficient," try adding a thermal store for the surroundings in your energy diagram. The missing energy usually appears there.
Elastic, Chemical, And Nuclear Energy Are All "Stored By Work Against Forces"
A key unifying idea is that many energy stores arise because work was done against a force:
Elastic Potential Energy
Stretching an elastic band or compressing a spring requires work against a tension (restoring) force. That work becomes elastic potential energy, which can later be transferred to other stores (often kinetic energy).
Chemical Potential Energy
At the atomic level, energy can be stored in chemical bonds. During some reactions, bonds break and form, and energy may be released, often as thermal energy. Explosives are an extreme example: stored chemical energy is rapidly transferred into kinetic energy of fragments, work done breaking materials, and thermal energy, with a small fraction as sound.
Nuclear Potential Energy
Some nuclei are unstable and can decay. Energy stored by short-range nuclear forces is nuclear potential energy. It is difficult to access, but the quantities involved can be enormous.
Do not confuse "potential energy" with "gravitational potential energy." Potential energy is a category that includes gravitational, elastic, chemical, and nuclear stores, it means stored energy that can be released.
Energy Transfer Chains Describe Devices And Living Systems
Many technologies can be described as an energy transfer chain (a sequence of transfers and transformations). Being able to identify these chains is a key skill.
Examples:
- Loudspeaker: electrical energy transferred to kinetic energy (vibration of the cone), then to sound energy in the air, with some thermal energy.
- Microphone: sound energy causes vibrations (kinetic), transformed into electrical energy.
- Light bulb: electrical energy transformed into light (radiation) and thermal energy (often most becomes thermal).
- Solar (photovoltaic) cell: radiation from the Sun transferred into electrical energy.
- Plant leaf (photosynthesis): light energy transferred into chemical potential energy stored in molecules.
Think of energy like money in different accounts. You can transfer money between accounts (transfer) and exchange currency (transform), but if you account for everything properly, the total amount is conserved.
Efficiency Explains Why Useful Output Is Less Than Input
In many systems, we care about how much of the input energy becomes the desired output.
Efficiency
Using scarce resources in the best possible way to avoid welfare loss.
Efficiency is less than 1 (or less than 100%) when energy is transferred to unwanted stores, usually thermal energy. Improvements in technology often aim to reduce these unwanted transfers, which is important for sustainability.
"Saving energy" in everyday life usually means reducing energy transfers into unwanted stores (for example, better insulation reduces energy transferred from a warm house to the colder outdoors by heating and convection).
Energy Diagrams Help You Communicate Conservation Clearly
A clear energy explanation usually includes a diagram showing:
- the starting store(s)
- arrows showing transfers
- ending store(s), including "wasted" thermal energy
1. A cyclist brakes to a stop. Which energy store decreases, and where does most of the energy end up?
2. A stretched elastic band launches a paper ball. Describe the main energy transfers.
3. Why is sound energy often described as "insignificant" in the overall energy budget of many processes?
Laws And Theories, Why Conservation Matters
A scientific law describes something that always holds under specified conditions. Conservation of energy is treated as fundamental: in any correct description, total energy must balance.
A scientific theory aims to explain why laws and observations occur. Theories can be revised if new evidence contradicts them, whereas a law is a robust relationship that must be obeyed within its domain.
When scientists say energy is "conserved," they are making a claim about what must remain invariant while other quantities change form. What counts as "the same total" depends on how the system boundary is chosen and how carefully we account for less obvious stores (for example, thermal energy of the surroundings).
