For example, we have chemical energy in the form of fuels and can convert that chemical energy into electricity. How does that process work? How can we turn kinetic energy of wind mills to electricity? How does sunlight turn into heat energy? How come we can measure energy in Joules when there are so many forms of it?
In: Chemistry
One thing to think about is that we can’t really measure energy. We can only measure things related to it, such as electric current. Or the mass of a moving body, and it’s velocity.
Actually, we can’t *really* measure velocity either – but we can measure displacement and time, and we know how they connect mathematically to form the useful concept of velocity.
You mention heat energy. One way to measure heat energy is to measure the temperature increase in eg. a volume of gas the energy flows into. But on a microscopic level, temperature is related to the kinetic energy of the gas molecules, which brings us to mass, time and displacement again.
The energy in the form of radiation from the sun is related to the frequency of the electromagnetic wave, which is a function of time again.
What makes energy such an important concept in physics is that it’s conserved, it cannot be created or destroyed. And conservation laws are very powerful tools in doing physics. So maybe try to think of energy as a powerful concept rather than anything ”changing form”. I’ll admit however that Einstein’s famous mass-energy equivalence stretches this intuition quite a bit.
Chemical energy can be directly converted into electricity by exploiting chemical reactions which make electrons move from one side to another (electricity is the controlled flow of electrons).
Kinetic energy is converted into electricity often by using rotating devices. This works because kinetic energy can make something rotate and a rotating magnetic field (for example produced by a rotating magnet) produces AC voltage because of the interconnection between electric and magnetic fields.
There are also more indirect ways to convert chemical energy into electrical energy, by going through extra steps, including:
– burning the fuel and using the steam to move something, and in turn convert that motion into electricity (steam engines)
– spraying fuel into air and generating a high temperature gas that is used to move something which is in turn used to produce electricity (combustion engines)
>How come we can measure energy in Joules when there are so many forms of it?
We can measure energy with the same unit because energy is always energy. Energy can be used to produce a force. The simplest example is some kind of energy pushing, say, an object by 2 feet. If the thing pushing is parallel to the path taken by the object, the energy is simply the product E=F*s, where F (in the simplest case) is the weight. You can also measure the energy produced by burning something (e.g. the food energy in calories) by measuring the increase in temperature of something else.
How can we measure energy? Most measurements measure the relevant energy and not the “total” energy content. Energy cannot be measured directly but it is possible to calculate the energy that was converted based on the relevant effects. For example, it is possible to measure the rise in temperature of a fixed amount of water to determine how much energy was used to heat it up. The precision of this measure depends on the nature of the experiment and devices used. Experimental physics isn’t about “absolutely true and 100% accurate” measures, it is about what is “good enough” for that application. For example, if measuring heat output, calculating the bonding energy between neutrons and protons is irrelevant if the amount that this energy contributes to the experiment is negligible.
Conversion of energy is about releasing or storing energy in certain states. For example a water pump moving water from the ground to an overhead tank converts electrical energy to motion of the pump which is then converted to kinetic energy of water movement and then to gravitational potential energy of that mass of water suspended above the ground. A battery stores energy by converting electrical energy to the energy of chemical bonds and when the battery is discharged, the chemical bond energy is converted to electrical energy.
A windmill to a wind turbine converts the motion of wind to the rotational motion, this rotation is applied to the coils within the generator. These coils are suspended in magnetic fields. A moving coil in a magnetic field induces electrical potential and in a suitable circuit, this causes electrons to flow. This flow of electrons is the electrical current which can be transmitted through wires to do stuff which is where the energy is released or used.
Most of it works with expansion of gasses, which in turn drive a turbine generator. Heating up water (by means of focussed sunlight, heat from radioactive decay or burning any fuel) turns it to steam which expands. This expanding gas pushes through a system of pipes until it reaches a turbine generator that exchanges the rotational force of the turbine into electricity.
Similarly, an internal combustion engine (like in a car), burns fuel inside the cylinders, the expanding gasses from the explosion push the pistons which then turn the crankshaft and creates the rotational force that drives the wheels.
>For example, we have chemical energy in the form of fuels and can convert that chemical energy into electricity.
Chemicals store energy in the form of chemical bonds between atoms. There are other chemicals and configurations that these atoms would prefer to bond with, but are unable to spontaneously. For example, lets take a gas made up of Hydrogen (H2) and Oxygen (O2). The ideal configuration for these atoms would be to form water, but the hydrogen and oxygen are stable enough that they aren’t just going to break their bonds for no reason.
Lets introduce a source of heat to the gas. This can be in the form of a flame, a spark, or just a really hot piece of metal. The heat adds energy to the molecules, giving them the strength to break their chemical bonds and reconfigure in their ideal form. The result is water + energy in the form of heat since the new molecule is in a more ideal state than just pure H2 and O2. This heat is enough to trigger the reaction in more molecules, and now you have a self sustaining reaction as long as you keep adding more H2 and O2 to replenish what is consumed. Your exhaust gas (H2O) is also now very hot compared to the initial temperature.
Hot things want to expand. We can exploit this by turning the heat energy into kinetic energy. In a car, this expanding gas pushes a piston which drives a crankshaft. In a power plant, the gas pushes through a turbine. As the gas expands, it loses its kinetic energy and cools down again.
If we want to extract electricity from this rotational energy, we need to use magnets. When a changing magnetic field passes over a conductor of electricity (copper wire), a current is conducted in that wire. We wrap wires around a hollow chamber, and then stick a magnet attached to our rotating crankshaft or turbine shaft into that chamber. As the magnet rotates, the orientation of its magnetic field changes. Because the wire experiences this changing magnetic field, a current is created inside of it. This is how an electrical generator works. Side note: Permanent magnets are not strong enough to generate a very strong electrical current, so we’ll actually siphon off a small portion of the electricity we generate to run a powerful electromagnet attached to our rotating shaft instead. Because the magnetic field is stronger, we generate even more electricity and are easily able to offset the loss required to run the electromagnet.
Keep in mind that this only follows a simple form of energy generation from chemical bonds. There are other more complex forms such as fuel cells which would turn our H2 and O2 into H2O with the generation of electricity more directly, but these are rather inefficient and we don’t use them as much. I used hydrogen and oxygen because they’re simple and only have one reaction product, but you can just as easily substitute the hydrogen for something like methane (CH4).
Nuclear reactors use the heat from their fuel decaying to heat water into steam and run turbines, and wind farms or hydroelectric plants get their kinetic energy from the force of the wind or water flow.
Adding to the other answers, a more philosophical take:
“Forms of energy” are a simplification – a convenient picture. In theoretical physics, there are no ‘forms’ of energy, there is only a single concept of ‘energy’, which can usually be broken up into ‘potential’ and ‘kinetic’ parts – referring to the part of energy that is dependent on the system’s state, and the part of energy that is dependent on the system’s rate of change, respectively.
But basically, when we say something like “energy is converted from X to Y” we are not really being rigorous. What it means is “when you want to analyze the dynamics of this problem, you should include both terms for X and terms for Y – terms for other things are negligible and can ignored”.
A full picture of physics would be needlessly difficult if not outright impossible to apply to practical problems, so in practice we oversimplify things until what’s left are the relevant parts of the whole. Simplifications like these are why we speak of ‘forms’ of energy so often, even though it’s fundamentally a caricature of what’s really happening.
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