Electrical Engineering Fundamentals/Basics of Electricity

Basics

Review

• Electric current is caused by the flow of electric charge.
• Two types of materials in the world of electricity are:
  • Conductors - allow the flow of charge.
  • Insulators - stop the flow of charge.
• Power stations of various sorts provide electricity.
• Things that "use" electricity are called a "Load" on the circuit.
• The flow of current is always from positive to negative.
• Current flow can be either Direct (from positive to negative) or Alternating (Pos/Neg/Pos/Neg) usually 60 cycles per second.
• Electricity is measured in Volts, Amperes (Amps), and Watts.
• Electricity can be deadly but it is necessary in modern life.

Atomic Physics

Electrons & Holes

Electricity comes from the movement of charge. Most commonly, this is the movement of electrons from atom to atom. When an atom gains an electron, it becomes more negatively charged. The atom then pushes away other electrons, similar to the way like poles of a magnet repel one another. However, conductors don't hold onto that newly acquired electron very well. Think of a row of atoms. When one atom in the line becomes positively charged (because it lost its electron), then another electron is drawn to that atom. This same atom gains this other electron from one of the other atoms in the conductor, making that atom a prime candidate for another electron. The process continues like a row of dominoes.

When an electron moves, it leaves a "hole". This is not an actual physical entity but rather a space-holder that becomes useful in much more advanced electronics. In a piece of conductive material that does not have anything connected to it, the process of switching electrons is random and does not cause anything to happen. When there is a voltage difference applied across a chunk of material, electrons will move to the positively charged side of the material, filling holes but also leaving holes behind. This constant process of holes being created and filled causes more electrons to move and creates a current. The current is much like a stream and continues to flow until the voltage difference is zero.

As described, the electrons are traveling toward the positive end. We define "conventional current" as going the opposite way from the flow of electrons. The flow of electrons is negative, because they are negatively charged particles moving toward a positively charged area. We can think of the holes as flowing in the opposite direction, however this is just a mental visualization to help describe behaviors of materials in more advanced electronics. For now, think of electrons flowing from the negative terminal of a voltage source to the positive terminal while the holes that constitute what we think of as "conventional current" flow from the positive terminal of the voltage source to the negative terminal.

Insulators are simple. They are stable atoms of elements not prone to losing their electrons. This means they hardly ever share electrons, except in extreme conditions. Electrical equipment is always designed to operate within a particular temperature range, and this range varies upon the instrument's design. Semi-conductors and super-conductors also exist, but those have characteristics that we don't need to cover at the moment.

What You Need to Know

• Electricity flows along conductors.
  • Electrons flow from negative terminal to positive terminal.
  • Holes flow from positive terminal to negative terminal. (Conventional Flow)
• Current, or flow of charge, is stopped by insulators.
• There are such things as semi-conductors and super-conductors.

Water Analogy

The River

Imagine a nice mountain stream. What does it do? It flows downhill until it comes to an area of equilibrium. When the water is at the top of the stream, it has the potential to fall down. If we took away the mountain, the water would fall straight down. We know from physics that the water has a potential energy. The equation for potential energy (PE=mgh, m=mass, g=gravity acceleration, h=height) tells us that the potential energy of this water at the top of the mountain increases with greater mass of water. When thinking of electricity, these ideas of 'potential energy' and 'flow' are similar to what we've discussed about water, but instead, Electrical Engineers deal with a 'potential voltage' applied that causes electrons to 'flow'. In other words, higher amounts of potential energy in water are analogous to higher concentrations of stored charge (therefore higher potential voltage) in a material. The flow of electrons in a conductor can be thought of as the flow of water molecules through a pipe. The electromotive force, EMF, or simply Voltage Potential (V) in the field of Electrical Engineering is analogous to the potential energy which 'motivates' the water to flow downhill to a lower 'potential'.

When the water is flowing, it has a current, and so when electricity flows it also has a current. When the water flows faster, the current is higher, and the faster electrons flow by, the higher electrical current. The electrical term is Amperes which is often shortened to Amps, and notated as A.

In a mountain stream, if you cut a straight channel across a bend, the water will soon only follow the cut. When considering electricity, we call this a 'short circuit'. A short circuit can be very bad if you are part of this new shorter path to relieve the potential. So, always be careful when dealing with electricity! The dangerous part is the current, a static shock between a finger and doorknob has several hundred-thousand volts of potential. A wall socket has 120 V in the US, but the amperage (A) is more than enough to kill.

What You Need to Know

• Concentrations of charge (electrons or holes) results in a potential voltage (V).
• The movement of charge is known as current, measured in Amperes (A).
• Electricity passing through a body can be dangerous, even in small amounts.
• Be careful with visualizations, electricity can be modeled by several things, but a misconception of one may influence how you learn about electricity.

• It is important to move beyond these visualizations after some practice with electrical circuits, because electrons have much more strange behavior in special situations that cannot be described by simple analogies.

The Power of Electricity

In the physics classroom, a very common practice problem involves solving for how much power is needed/expended for/during a certain task. The same is true for electricity. Watts are a measure of power, the same as horsepower, but the question never arises as to how much horsepower your sub-woofer is. Usually, the only time horsepower is mentioned when dealing with electricity is when the electrical power is being converted by a motor into mechanical power. Don't worry about equations yet; just know that the power in an electrical system is dependent upon the voltage potential (hereafter simply voltage) and current (ie. flow of charge) , amps (A). ..... The power of a light bulb, for example, is given in Watts as an indication of how much current it will use at the rated voltage and how much light it will provide if it is switched on. Light bulbs also are rated in Volts, meaning that they are supposed to be used in a system with that approximate voltage. If the voltage is lower than rated then that light bulb will provide less light if is switched on, and if the voltage is higher then damage to the light bulb can be expected.

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Bgorges 20:50, 6 October 2008 (UTC)
William Leismer 22:05, 16 June 2015 (UTC)

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