# My son's questions: Energy and Light

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My son is 10 years old and he has questions which many times I can't answer. I don't and honestly can't answer his questions in a simple way that he can understand. Or do I underestimate him? Not so sure about it.

His questions are:

1. What is energy and what is its form: is it liquid, solid or gas?

2. What is light?

Can somebody help me, please? Thanks.

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Energy isn't a substance, so it's neither a solid, liquid nor gas. It's a property that we noticed that objects have that seems to be related to the ability to do things (move objects, change the temperature of things) and is especially convenient because the amount in a system never seems to change (unless you deliberately add more or take it away). That's good because now we can make predictions about what will happen or what we will need — for example, you can figure out how much coal you might need to boil a certain amount of water.

Light is electromagnetic radiation — electric and magnetic fields oscillating in a particular way.

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1) Energy is the ability to do "work". It is neither solid, liquid, nor gas -- however, all those states of matter may have stored energy (think coiled spring, hydroelectric power, compressed air. In physics, "work" is applying a certain force over a certain distance, and the work is the product of the force and the distance over which it is applied, W = F*d. Applying a force over no distance does not count as work, although for a human it may actually be physically tiring.

There are many forms of energy, in two classes: potential energy and kinetic energy. The potential energy is due in some way to postitioning and can somehow be released. The kinetic energy is energy of motion. There are many forms: thermal energy, kinetic energy, light, sound, water waves, electric energy, gravitational potential energy, compression or tension potential energy, ...

If you understand energy you pretty much understand all of physics.

2) Light is an electromagnetic wave. It is a solution to Maxwell's Equations assuming zero charge (these equations are difficult to understand without crazy mathematical knowledge). The result is a changing electric field which causes a changing magnetic field which again causes a changing electric field, etc, in a self-sustaining manner. The speed of light is exactly 299,792,458 meters per second, and it is exactly that because we have defined the meter in terms of it.

Visible light is only a tiny fraction of the electromagnetic spectrum. Gamma rays, x-rays, ultraviolet rays, visible light, infrared, microwave, radio wave, are all light. Each individual "particle" of light is called a photon. The higher the frequency (or equivalently, the shorter the wavelength), the higher the energy of the photon. Light at ultraviolet and above is highly energetic and can damage your body.

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Thanks. It's not easy to explain to him but I'll do the best I can.

We are discussing about temperature and heat now, but I can see from his eyes that it is a heavy subject for him.

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Those are good answers but they might be a little heavy for your average ten-year-old. Prof. Louis a. Bloomfield, of the University of Virginia, has a good website that helps kids learn science. http://www.howeverythingworks.org/

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It's worth emphasising the difference between an adjective and a noun. Say you have any given object: it could be hot or cold, shiny or dull, red or blue, spinning or stationary. None of those properties can really be separated from the object itself, and in fact two of them are forms of energy: thermal and kinetic.

Thermal energy (heat) can be described as the amount that molecules are shaking and moving around within an object, the more movement - the hotter an object is. When molecules move around too much then a substance can't hold itself together which is why things melt and then evaporate.

Also, when molecules are moving fast enough they can hit each other hard enough to break apart and reform, which is basically what a chemical reaction is, and that's why you tend to involve heat when you are cooking things.

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However — and this is a point that is routinely fumbled even by people with physics under their belts — heat is thermal energy flowing to or from an object. A body does not contain heat.

Which is really confusing, because we say an object with a lot of thermal energy is hot, and we call the amount of energy it can contain, as related to a temperature change, the heat capacity. So while a lot of people get this wrong, it's not really surprising why.

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• 3 months later...
1) Energy is the ability to do "work". It is neither solid, liquid, nor gas -- however, all those states of matter may have stored energy (think coiled spring, hydroelectric power, compressed air. In physics, "work" is applying a certain force over a certain distance, and the work is the product of the force and the distance over which it is applied, W = F*d. Applying a force over no distance does not count as work, although for a human it may actually be physically tiring.

There are many forms of energy, in two classes: potential energy and kinetic energy. The potential energy is due in some way to postitioning and can somehow be released. The kinetic energy is energy of motion. There are many forms: thermal energy, kinetic energy, light, sound, water waves, electric energy, gravitational potential energy, compression or tension potential energy, ...

If you understand energy you pretty much understand all of physics.

2) Light is an electromagnetic wave. It is a solution to Maxwell's Equations assuming zero charge (these equations are difficult to understand without crazy mathematical knowledge). The result is a changing electric field which causes a changing magnetic field which again causes a changing electric field, etc, in a self-sustaining manner. The speed of light is exactly 299,792,458 meters per second, and it is exactly that because we have defined the meter in terms of it.

Visible light is only a tiny fraction of the electromagnetic spectrum. Gamma rays, x-rays, ultraviolet rays, visible light, infrared, microwave, radio wave, are all light. Each individual "particle" of light is called a photon. The higher the frequency (or equivalently, the shorter the wavelength), the higher the energy of the photon. Light at ultraviolet and above is highly energetic and can damage your body.

This sounds reasonable!

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• 2 months later...

if thermal energy is formed when molecules are shaking and moving around within an object, can i conclude that energy = movement of molecules?

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energy = movement of molecules

Edited by vordhosbn
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You can't say energy = movement, since mass at rest contains a whopping load of energy. Kinetic energy is due to movement.

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You can't say energy = movement, since mass at rest contains a whopping load of energy. Kinetic energy is due to movement.

What about energy = movement and/or the potential thereof?

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What about energy = movement and/or the potential thereof?

That's closer, and is already incorporated in the definition: the capacity to do work.

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How old is your son by the way?

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How old is your son by the way?

My son is 10 years old

My money's on 10 (as of the end of February, at least)

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That's closer, and is already incorporated in the definition: the capacity to do work.

That's true, but then you have to explain the physics meaning of "work" in contrast to the possible lay meaning, which could involve frying french fries or tearing movie tickets in half.

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