Conservation law facts for kids

A conservation law in physics is a rule that says the total amount of something stays the same over time. It doesn't change, even if things move around or transform. This "something" could be simple, like mass or electric charge. Or it could be something you have to calculate, like energy or angular momentum.

For example, the "law of conservation of mass" means that the total amount of mass in a system always stays the same. Even if it changes form, like a log burning into ash and smoke, the total mass is still there. If you could measure all the mass in the universe today, it would be the same tomorrow.

A Look Back at Conservation Laws

For a long time, scientists thought that mass and energy were always conserved separately. This meant the total mass in the universe never changed, and the total energy never changed.

But then, Albert Einstein came along. He showed that mass and energy are actually connected. He discovered that mass can turn into energy, and energy can turn into mass! If this happened, it would seem like the old conservation laws were broken. The total amount of mass would go down, and the total amount of energy would go up.

Einstein explained that conservation laws still work if you combine mass and energy. He said that even if mass changes or energy changes, their total sum when added together always stays the same. So now, there is one big conservation law for mass and energy combined.

How Mass and Energy Connect

Mass is measured in kilograms, and energy is measured in joules. You can't just add them directly. But Einstein found a way to combine them using his famous equation: .

This equation means that to add mass to energy, you first need to multiply the mass by the speed of light (c) and then multiply it by the speed of light again (c²). This converts mass into an energy equivalent, allowing them to be added together.

Why Conservation Laws Are Useful

Conservation laws are super helpful for scientists and engineers when they solve problems in physics. If they know that a certain quantity is conserved, it gives them important mathematical information about the problem they are working on.

Some of the things that are thought to be conserved include:

• momentum
• Mass and energy combined
  • Conservation of energy
  • Conservation of mass
• angular momentum
• Electric charge

Symmetries and Conservation

A brilliant mathematician named Emmy Noether showed a deep connection between conservation laws and symmetries in the laws of physics. Her discovery, called Noether's theorem, gives physicists a very powerful tool to understand the universe.

Basically, if the laws of physics stay the same even when you change something, that's a symmetry. And each symmetry leads to a conservation law!

For example:

• Because there's no absolute position in space (only relative position), the total momentum of a closed system is conserved. This means if you move a system to a different spot, the physics still works the same.
• Because there's no absolute time (only relative time), the total energy of a closed system is conserved. This means the laws of physics don't change over time.
• Because there's no special direction in space (only relative orientation), the total angular momentum of a closed system is conserved. This means if you rotate a system, the physics still works the same.
• More complex symmetries, like local gauge invariance, lead to the conservation of electric charge.

Types of Conservation Laws

Conservation laws can be described in two main ways: global or local.

Global Conservation

A global conservation law simply states that the total amount of something in the entire universe does not change over time. It's like saying the total number of marbles in a giant, sealed box never changes, no matter how they roll around inside.

Local Conservation

A local conservation law tells us a bit more. It says that if the amount of something changes in one specific place, it's because that "something" moved into or out of that place. And we can actually measure that movement. It's like saying if the number of marbles in one corner of the box changes, it's because marbles rolled in or out of that corner, and you can see them moving.

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