David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.
What Is an Electric Circuit?
Circuits are everywhere in modern life: lights, TVs, computers, washing machines, air conditioning, you name it. Life would be totally different if we didn't have circuits. But what exactly is a circuit? Put simply, an electric circuit is a complete loop where electricity is flowing, to power components in that loop. And electricity is the continuous flow of electrons.
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Relationship of Energy and Electric Circuits
On a basic level, electric circuits transfer energy. If they didn't, none of our electrical devices would even work. How could a light bulb produce light energy if it wasn't given that energy from the electricity we supply to it? How could the motor in a tumble dryer turn with mechanical energy if that didn't come from somewhere? Electrical energy is a particular type of energy contained within an electric current, flowing around a circuit.
When you roll a ball down a hill, gravitational potential energy is changed into kinetic, or movement, energy. Circuits work in a similar way: The battery provides a difference in electric potential energy between the two terminals, the positive and negative sides of the battery. Then, if you connect those two sides of a battery in a circuit loop, charges have to flow, just like how the ball has to roll down the hill. This is why a circuit works in the first place.
Conservation of Energy
Another thing we understand about energy is that it's always conserved. Conservation of energy says that energy is neither created nor destroyed, it only moves from one type to another, or one place to another. But how does this work in terms of circuits?
Well, as we mentioned, there is an electrical potential energy difference between the two sides of the battery. Or, in other words, when the electrons in the circuit start to move, they initially have electric potential energy. That electric potential energy changes to regular electric energy as the electrons move around the circuit.
Then, that electric energy is transferred to the components in the circuit. If the circuit contains a bulb, it comes out as light energy and wasted heat energy. If the circuit contains a buzzer, it comes out as sound energy. If it contains a washing machine, it comes out as mechanical and heat energy. Whatever it is, the energy isn't lost; it all goes somewhere, even if it isn't all turned into useful work. Energy is always conserved.
Where does that energy actually come from in the first place, then? Well, the battery contains chemicals inside which allow it to maintain that electric potential energy difference between the two terminals. Chemical reactions happen inside the battery to achieve this, and those chemical reactions use up the chemical energy stored in the battery. That chemical energy was put inside there when the battery was created or charged: kind of like how our food contains chemical energy that we use to live, the battery contains chemicals which have energy inside them too.
For wall sockets, the electricity comes from power plants, where chemicals like coal, oil, and gas are burned to release their stored energy, or turbines are pushed by the kinetic energy of the wind. But whatever the source, energy always comes from somewhere and can be tracked all the way back to the start of the universe.
Let's review what we've learned. An electric circuit is a complete loop where electricity is flowing to power components in that loop. And electricity is the continuous flow of electrons. A circuit has a lot in common with rolling a ball down a hill. There is gravitational potential energy at the top, and this energy is released when the ball rolls. The same thing is true of a circuit, except that it has electric potential energy instead of gravitational.
Electric circuits also involve energy transfers. Chemical energy in the battery is changed into electrical potential energy on the two sides of the battery. This electrical potential energy is changed into electric energy in the electricity that flows around the circuit. Then, this electric energy comes out in the components of the circuit: light from a bulb, or sound from a buzzer.
Energy is always conserved. Conservation of energy says that energy is neither created nor destroyed, it only moves from one type to another, or one place to another. And this is just as true for circuits.
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Electrical Circuits: Energy Transfer & Conservation
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