Category: polarity

A Few Words On DC And AC: What Exactly Are They?

A few disorganized concepts about direct and alternate current.

There are two main variations of the electrical current: the Direct Current, or DC, and the Alternate Current, or AC. But what does that mean?

The DC is the one you obtain when you power a device using batteries, for example. Batteries provide what is called Direct Voltage and that generates a Direct Current once applied to an electric circuit. If we draw a diagram of the voltage and, correspondingly, the current that flows in a circuit powered with DC, here is what we obtain:

This diagram basically tells us that the value of the voltage, and of the current, does not change over time. We usually define the current as flowing from the positive to the negative pole of the battery and that flow never changes over time.

The AC works like the DC, going from the positive to the negative voltage. The difference is that the voltage keeps switching: positive becomes negative and then becomes positive again, and so forth. And so the current keeps changing its direction accordingly.

Also, the AC does not change suddenly back and forth, but it does that progressively, following a shape called sine wave. All electrical energy distributed in our homes has this shape.

In USA, the AC current changes direction 120 times per second, which means that in one second there are 60 full periods of the sine wave. We say that the frequency of the current is 60 Hertz, abbreviated 60 Hz.

In Europe, 50 Hz is used instead. Other parts of the world either use one or the other.

The AC voltage is the one created in the power plants and provided, for example, at the wall outlets in your house by the energy service provider.

The voltage at the outlet is not constant as the one in the batteries. Instead, it changes continuously following the shape of a sine wave. Because of that, the polarity at each electrode of the outlet changes over time from positive to negative and vice versa, following the shape of the sine wave.

So, when we connect a device to the electric outlet, the current that will flow through that device will be an AC current as well.

The sinusoidal shape of the AC voltage depends on the way the electricity is generated. In the power plants, there are devices called alternators, a much bigger version of those that you can find inside your car to recharge its battery, or on a bike, to provide electricity to turn on the lights at night.

Depending on the power plant, a different kind of energy is used to put in motion the alternator. It could be fossil fuel or nuclear energy that heat a reservoir of water and create the steam that makes the alternator rotate. Or it could be the rotation of a propeller-like device that is put in motion by the wind.

Whatever is the source of the mechanical energy, the alternator converts that energy in electrical energy. But, since the rotation translates into a sine wave when described on a Cartesian reference system, the resulting electrical energy acquires that shape too.

But, why do we need both forms of voltage, DC and AC?

First of all, DC voltage is necessary to power up any electronic device, from your TV to your smartphone or radio or computer.

AC voltage, from the electrical engineering perspective, is used to transmit the electrical energy from the places where it is created to the places where it is used.

Back to the time where the first experiments of electricity transmission were conducted, there was a famous diatribe between Thomas Edison and Nikola Tesla.

Edison believed that the safest way to transmit electricity was to do that with cables powered with DC current.

Tesla argued that it was better to use AC current because it allowed much less waste of energy during the transportation, thanks to the fact that it is easier to convert the voltage from a low value to a higher one and vice versa, when using AC. And it is also very simple to convert the AC into DC when DC is needed, through a process called rectification.

As history tells us, Tesla won that battle, rightfully. And so, today, AC is used to bring the electrical energy to our homes from the power plants.

A Few Facts On Coulomb’s Law

A few words on the modern interpretation of Coulomb’s Law in terms of Fields Theory.

At the base of all the electromagnetism theory lays Coulomb’s Law, which describes why and how charges move in a medium, whether the medium is a conductor or the void.

Electrical and Electronics engineers are therefore supposed to be very familiar with this law. Let’s spend some time to provide a few important facts of the law. For that, we have to go back to the concept of ‘field’.

In modern physics, we usually tend to identify the effect of an entity over another entity with the name of field. Mathematically speaking, a field is a 3-dimensional matrix that assigns a specific value, usually a vector, to each point in the observed space.

Each time we put in that space an object capable of reacting with the field, we end up with a force applied to that object that is the product of the field in that point and the measurable value of that object.

So, in the case of charges, a single charge creates a field in the surrounding space. When we put in that space a second charge, the new charge will be subject to a force that is the product of the charge itself and the value of the field in that point in space.

Because charges can repel or attract, depending on their positive or negative sign, the field itself is directional, and therefore represented with vectors..

Here is a picture representing a field generated by a positive charge:

And here is a picture representing a field generated by a negative charge:

If the charge creating the field is Q, then the field can be represented by the following equation:

where E is the value of the field, actually the module of the field vector, and r is the distance from the charge of a point in space where the field is calculated. The constant ε0 is called “dielectric constant” and, in this formula, it assumes that the charge is located in the void or in thin air. An adjustment to the constant needs to be made when the medium is different.

We call this field an Electrostatic field, hence the E, because the charge that generates it does not move, it is static.

The force on a charge q put in the field can be calculated as:

And, since E is actually a vector, F is therefore also a vector.

If we want to represent this in actual vector format, we can write it as:

where

is the position vector of the charge q with respect to the charge Q. This is what is called Coulomb’s Law.

You can see that if the charges are both positive or both negative, the vector F is oriented such that the charge q moves away from the charge Q and, vice versa, when the charges have opposite sign, F is oriented toward Q.

That is exactly what we expected: charges of the same sign repel each other and charges of opposite sign attract each other.

As a further example, here is the graphical representation of an electrostatic field generated by two charges of opposite sign:

… and finally an electrostatic field generated by two charges of the same sign: