Ece Tutorial 1 - Voltage

Ece Tutorial 1 - Voltage - What you always wanted to know...

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Unformatted text preview: What you always wanted to know about VOLTAGE but were afraid to ask Take an object in your hand, raise it to a certain position, then drop it. Your object will drop and - bang! - it will hit the floor. By doing that, your object performs work. You can see that by placing a fragile object on the floor: Your falling object will break it and that certainly requires work. If you measure the work done by the falling object, you'll see that the heavier your object is and the higher you've raised it, the more work it is capable of doing when it falls down. The capability of doing work is called energy. As your object is capable of doing work because of its position (being above the floor), we say that it has some potential energy. The word potential means 'possible but not yet realized,' or 'the inherent capacity for doing something.' Indeed, by raising your object to a certain position, you gave that object the capability to do work but the work is only being done when you release the object. Now, drop the object to a table instead of the floor. You will see that the object falling from its previous position will do less work. The reason is that its position relative to the table is not as high than relative to the floor. Therefore, we must note that the potential energy is always determined with respect to a given reference position. The reference position must be defined before we can determine the potential energy whose value is equal to the amount of work we have to apply to the object when we bring it from the reference position to the given position. What happens to the potential energy if you eliminate the object? Can a non-existing object have any energy? Clearly not. There is no potential energy if there is no object. But if you retrieve the object from its hiding place and put it back to the same position where it was before, it will have the same potential energy as before. We conclude that there is something at that position that creates a potential energy when you put an object at that position. That 'something' is called the potential at that particular position. Do not forget that the potential is also dependent on the choice of the reference position. The potential at a certain height will be smaller with reference to a table than with respect to the floor. It is convenient to characterize the potential as the potential energy of an object of unit mass. Its value is equal to the amount of work we have to apply to the object of unit mass when we bring it from the reference position to the given position. Then we can easily express the potential energy (W) of any object at that particular position by multiplying the potential at that position (P) with the mass (m) of the object: W = P x m. Imagine now that a charge (Q) is our object. It is resting comfortably at a certain reference position. Let us bring that charge to a new position in space. We have to apply work to do that because of the forces acting on our charge from other charges around it. These are electric forces rather than forces of gravity. The charge will have a certain amount of potential energy at its new position because it can do work when it is released from that position and goes back to the reference position. The reference position must be defined before we can determine the potential energy whose value is equal to the amount of work we have to apply to the charge when we bring it from the reference position to the new position. There is no potential energy if there is no charge. But if you put the charge back to the same position, it will have the same potential energy as before. We conclude again that there is a potential at that particular position. Do not forget that the potential is dependent on the reference position. Like the altitude of a geographical location is defined with reference to some common altitude (e. g., sea level), potential is also not an absolute but a relative quantity. We will characterize the potential as the potential energy of a unit positive charge of 1 coulomb. Its value is equal to the amount of work we have to apply to the unit positive charge when we bring it from the reference position to the given position. Then we can easily express the potential energy (W) of any charge at that particular position by simply multiplying the electric potential at that position (u) by the charge: W = u x Q. If you are still with me, then it is very easy for us to define voltage. Voltage is simply the potential difference between two points in space, i.e., the amount of work we have to apply to the unit positive charge when we bring it from the first point to the second one. If the potential of the first point is u(1) and the potential of the second point is u(2) and they are both defined with respect to the reference point whose potential is u(0), then the voltage is equal to V = [u(1) - u(0)] - [u(2) - u(0)] = u(1) - u(2). We conclude that the voltage does not depend on the choice of the reference point. Therefore, it is convenient to choose the potential of the reference point as u(0) = 0. Please note that all this is true only in case of so-called conservative fields where the potential energy (and thus the potential) is independent of the path along which we bring the object (charge) from the reference position to the given position. This is the case for simple electric circuits but we will see later (Faraday's law) that in the general case of fields changing in time voltage has a more sophisticated meaning. The unit of voltage is 1 joule/coulomb which is called 1 volt (V).Can you explain what 1 volt is in simple physical terms?. If you can, then you understand what voltage is. Congratulations! Please send me feedback about this tutorial. What do you think about it? Any questions? . ...
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