Electricity part 2

Making Charges Flow

We now have all the tools to make charges flow. Electrons in atoms can act as our charge carrier, because every electron carries a negative charge. If we can free an electron from an atom and force it to move, we can create electricity.

Consider the atomic model of a copper atom, one of the preferred elemental sources for charge flow. In its balanced state, copper has 29 protons in its nucleus and an equal number of electrons orbiting around it. Electrons orbit at varying distances from the nucleus of the atom. Electrons closer to the nucleus feel a much stronger attraction to the center than those in distant orbits. The outermost electrons of an atom are called the valence electrons, these require the least amount of force to be freed from an atom.
Using enough electrostatic force on the valence electron–either pushing it with another negative charge or attracting it with a positive charge–we can eject the electron from orbit around the atom creating a free electron.

Now consider a copper wire: matter filled with countless copper atoms. As our free electron is floating in a space between atoms, it’s pulled and prodded by surrounding charges in that space. In this chaos the free electron eventually finds a new atom to latch on to; in doing so, the negative charge of that electron ejects another valence electron from the atom. Now a new electron is drifting through free space looking to do the same thing. This chain effect can continue on and on to create a flow of electrons called electric current.

Conductivity

Some elemental types of atoms are better than others at releasing their electrons. To get the best possible electron flow we want to use atoms which don’t hold very tightly to their valence electrons. An element’s conductivity measures how tightly bound an electron is to an atom.
Elements with high conductivity, which have very mobile electrons, are called conductors. These are the types of materials we want to use to make wires and other components which aid in electron flow. Metals like copper, silver, and gold are usually our top choices for good conductors.

Elements with low conductivity are called insulators. Insulators serve a very important purpose: they prevent the flow of electrons. Popular insulators include glass, rubber, plastic, and air.

Static or Current Electricity

Before we get much further, let’s discuss the two forms electricity can take: static or current. In working with electronics, current electricity will be much more common, but static electricity is important to understand as well.

Static Electricity

Static electricity exists when there is a build-up of opposite charges on objects separated by an insulator. Static (as in “at rest”) electricity exists until the two groups of opposite charges can find a path between each other to balance the system out.

When the charges do find a means of equalizing, a static discharge occurs. The attraction of the charges becomes so great that they can flow through even the best of insulators (air, glass, plastic, rubber, etc.). Static discharges can be harmful depending on what medium the charges travel through and to what surfaces the charges are transferring. Charges equalizing through an air gap can result in a visible shock as the traveling electrons collide with electrons in the air, which become excited and release energy in the form of light.

One of the most dramatic examples of static discharge is lightning. When a cloud system gathers enough charge relative to either another group of clouds or the earth’s ground, the charges will try to equalize. As the cloud discharges, massive quantities of positive (or sometimes negative) charges run through the air from ground to cloud causing the visible effect we’re all familiar with.
Static electricity also familiarly exists when we rub balloons on our head to make our hair stand up, or when we shuffle on the floor with fuzzy slippers and shock the family cat (accidentally, of course). In each case, friction from rubbing different types of materials transfers electrons. The object losing electrons becomes positively charged, while the object gaining electrons becomes negatively charged. The two objects become attracted to each other until they can find a way to equalize.
Working with electronics, we generally don’t have to deal with static electricity. When we do, we’re usually trying to protect our sensitive electronic components from being subjected to a static discharge. Preventative measures against static electricity include wearing ESD (electrostatic discharge) wrist straps, or adding special components in circuits to protect against very high spikes of charge.

Current Electricity

Current electricity is the form of electricity which makes all of our electronic gizmos possible. This form of electricity exists when charges are able to constantly flow. As opposed to static electricity where charges gather and remain at rest, current electricity is dynamic, charges are always on the move. We’ll be focusing on this form of electricity throughout the rest of the tutorial.

Circuits

In order to flow, current electricity requires a circuit: a closed, never-ending loop of conductive material. A circuit could be as simple as a conductive wire connected end-to-end, but useful circuits usually contain a mix of wire and other components which control the flow of electricity. The only rule when it comes to making circuits is they can’t have any insulating gaps in them.
If you have a wire full of copper atoms and want to induce a flow of electrons through it, all free electrons need somewhere to flow in the same general direction. Copper is a great conductor, perfect for making charges flow. If a circuit of copper wire is broken, the charges can’t flow through the air, which will also prevent any of the charges toward the middle from going anywhere.
On the other hand, if the wire were connected end-to-end, the electrons all have a neighboring atom and can all flow in the same general direction.

Electric Fields

We have a handle on how electrons flow through matter to create electricity. That’s all there is to electricity. Well, almost all. Now we need a source to induce the flow of electrons. Most often that source of electron flow will come from an electric field.

What’s a Field?

A field is a tool we use to model physical interactions which don’t involve any observable contact. Fields can’t be seen as they don’t have a physical appearance, but the effect they have is very real.

We’re all subconsciously familiar with one field in particular: Earth’s gravitational field, the effect of a massive body attracting other bodies. Earth’s gravitational field can be modeled with a set of vectors all pointing into the center of the planet; regardless of where you are on the surface, you’ll feel the force pushing you towards it.


The strength or intensity of fields isn’t uniform at all points in the field. The further you are from the source of the field the less effect the field has. The magnitude of Earth’s gravitational field decreases as you get further away from the center of the planet.

As we go on to explore

electric fields in particular remember how Earth’s gravitational field works, both fields share many similarities. Gravitational fields exert a force on objects of mass, and electric fields exert a force on objects of charge

Electric Fields

Electric fields (e-fields) are an important tool in understanding how electricity begins and continues to flow. Electric fields describe the pulling or pushing force in a space between charges. Compared to Earth’s gravitational field, electric fields have one major difference: while Earth’s field generally only attracts other objects of mass (since everything is so significantly less massive), electric fields push charges away just as often as the attract them.
The direction of electric fields is always defined as the direction a positive test charge would move if it was dropped in the field. The test charge has to be infinitely small, to keep its charge from influencing the field.

We can begin by constructing electric fields for solitary positive and negative charges. If you dropped a positive test charge near a negative charge, the test charge would be attracted towards the negative charge. So, for a single, negative charge we draw our electric field arrows pointing inward at all directions. That same test charge dropped near another positive charge would result in an outward repulsion, which means we draw arrows going out of the positive charge.

Electric Potential (Energy)

When we harness electricity to power our circuits, gizmos, and gadgets, we’re really transforming energy. Electronic circuits must be able to store energy and transfer it to other forms like heat, light, or motion. The stored energy of a circuit is called electric potential energy.

Energy? Potential Energy?

To understand potential energy we need to understand energy in general. Energy is defined as the ability of an object to do work on another object, which means moving that object some distance. Energy comes in many forms, some we can see (like mechanical) and others we can’t (like chemical or electrical). Regardless of what form it’s in, energy exists in one of two states: kinetic or potential.

An object has kinetic energy when it’s in motion. The amount of kinetic energy an object has depends on its mass and speed. Potential energy, on the other hand, is a stored energy when an object is at rest. It describes how much work the object could do if set into motion. It’s an energy we can generally control. When an object is set into motion, its potential energy transforms into kinetic energy

Let’s go back to using gravity as an example. A bowling ball sitting motionless at the top of Khalifa tower has a lot of potential (stored) energy. Once dropped, the ball–pulled by the gravitational field–accelerates towards the ground. As the ball accelerates, potential energy is converted into kinetic energy (the energy from motion). Eventually all of the ball’s energy is converted from potential to kinetic, and then passed on to whatever it hits. When the ball is on the ground, it has a very low potential energy.

Electric Potential Energy

Just like mass in a gravitational field has gravitational potential energy, charges in an electric field have an electric potential energy. A charge’s electric potential energy describes how much stored energy it has, when set into motion by an electrostatic force, that energy can become kinetic, and the charge can do work.

Like a bowling ball sitting at the top of a tower, a positive charge in close proximity to another positive charge has a high potential energy; left free to move, the charge would be repelled away from the like charge. A positive test charge placed near a negative charge would have low potential energy, analogous to the bowling ball on the ground

To instill anything with potential energy, we have to do work by moving it over a distance. In the case of the bowling ball, the work comes from carrying it up 163 floors, against the field of gravity. Similarly, work must be done to push a positive charge against the arrows of an electric field (either towards another positive charge, or away from a negative charge). The further up the field the charge goes, the more work you have to do. Likewise, if you try to pull a negative charge away from a positive charge–against an electric field–you have to do work.

For any charge located in an electric field its electric potential energy depends on the type (positive or negative), amount of charge, and its position in the field. Electric potential energy is measured in units of joules (J).

Electric Potential

Electric potential builds upon electric potential energy to help define how much energy is stored in electric fields. It’s another concept which helps us model the behavior of electric fields. Electric potential is not the same thing as electric potential energy!
At any point in an electric field the electric potential is the amount of electric potential energy divided by the amount of charge at that point. It takes the charge quantity out of the equation and leaves us with an idea of how much potential energy specific areas of the electric field may provide. Electric potential comes in units of joules per coulomb (J/C), which we define as a volt (V).
In any electric field there are two points of electric potential that are of significant interest to us. There’s a point of high potential, where a positive charge would have the highest possible potential energy, and there’s a point of low potential, where a charge would have the lowest possible potential energy.
One of the most common terms we discuss in evaluating electricity is voltage. A voltage is the difference in potential between two points in an electric field. Voltage gives us an idea of just how much pushing force an electric field has.

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With potential and potential energy under our belt we have all of the ingredients necessary to make current electricity.