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Friday, July 31, 2009

Voltage, Current, Power and Energy

Let's begin with the very basic concept of Electrical Engineering. We should do this because as we move along, we will use these basic concepts on more complicated applications. The following topics were already taken from our Physics course but for those who already forget it, let's take a short review.

The Voltage

We all know that Potential Difference between two points is the work done per unit charge when a charge is moved from one point to the other. The defining equation for this is,

V = W/Q
where :

V = the voltage in volts.
W= energy in joules.
Q = is the charge in coulombs, is always understood to be a positive charge in a conventional way, thus making definite the algebraic sign of the potential difference in any particular case.

One volt is defined as the potential difference between points in an electric circuit such that one joule of work must be done to move a charge of one coulomb between the points considered. Since the difference of potential between two points is expressed in volts, the potential difference is often referred to as voltage between these points. Thus, if an electric power line has a voltage of 220V, it follows that 220J of work have to be expended for each coulomb of electricity which is transferred through any apparatus connected between the two wires. The statement above could be shown in a simple expression as:

1 volt = 1 joule/coulomb, or 1 V = 1 J/C
Actually, voltage is being measured using the instrument called the voltmeter. There, you can see the actual voltage reading between two terminals of the actual circuit for example of a television sets, radio and other electronic /electrical instruments. If you have your Physics Laboratory before it was supposed to be tackled.

I hope you get my point here. I will not give much details about this topic above as well as the topics to be discuss below because this is just a review of your Physics. We only need to remember important concepts now just to be ready on the more complicated one. You have to trust me on this for the meantime.


The Current

Let's take an example of a light bulb connected to the battery source. A voltage source such as battery, that forces electric charge to move through the rest of the circuit from one terminal of the voltage source back to the other and an unbroken path through the rest of the circuit along which the charge can move. The charge that moves through this circuit is called an electric current. This is also defined in the other way around as the rate of charge flow of one coulomb per second.

Mathematically, this can be expressed as:

I = Q/t
where:
I = the current in amperes.
Q= the charge in coulombs.
t = the time in seconds.

Here in electrical engineering, we will be dealing with two types of current: the direct current and an alternating current. This is a huge topic to be discuss separately on my succeeding post because the idea is completely separate. Just to give you a little background about this. Take for example of a light bulb connected to the battery source. The current produced by the battery is one typical example of a direct current. When a direct current flows, charge always move in the same direction - electrons move away from the negative terminal of the emf source toward the positive terminal. Those other electronics devices that are connected to the terminals of the wall socket oscillate back and forth at 60Hz. The voltage across the wall socket in a home changes polarity at a frequency of 60Hz ( 60 times per second). This is a typical example of an alternating current.

Common instrument used to measure an electric current in an electric circuits is an ammeter.


Power and Energy

The property of the body or system of bodies by virtue of which work can be performed is called energy. Commonly defined as "ability to do work". Energy can exist in many forms and can be tranformed from one form to another. One of these energy that we will considered here in Electrical Engineering is the heat.

Heat is defined as the energy transferred to or from an object because of a difference in its temperature and that of some other object in contacts in its environment.

Mathematically, this is expressed as:

Q = mc delta T
where:
Q = heat in kcal
m = mass
C = specific heat
delta T = change in temperature

Always take note that, 1 kcal is the amount of heat required to raise the temperature of 1-kg of water by 1 degree celcius.

Update August 6, 2008: Don't forget to review the value of specific heat for every substance for you will used it in problem solving in the last part of this post.


Power is restricted to mean the time rate of doing work. The average power is the worked performed divided by the time required for the performance. Mathematically, this is expressed as:

P = W/t = VI

where:
P = the power in watts
W= the energy in joules
t = the time in seconds

Please take note that all units above are expressed in MKS. This is the commonly unit being used nowadays especially in board examinations.


A Teaser

Here in Electrical Engineering, there is no challenge if I will not leave any problem solving to my readers. For you to fully digest the topic presented here, please try to solve the problem below. Answers will be given before posting my next post and will be found on the separate page of this site. The problem below was given during Registered Electrical Engineering board exams dated October, 1997.

Problem No.1 :
A process equipment contains 100 gal of water at 25 degree celcius. It is required to bring it to boiling in 10 minutes. The heat loss is estimated to be 5%. What is the KW rating of the heater?


Please stay with me on my next post here in Learn Electrical Engineering for Beginners.
I will reveal the solution on my next post. Goodluck!

Cheers!

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