Electrical Technology

Brought to you by the students of P. A. Wiedorn at Wilde Lake High School


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Quick Start Version

Electromotive Force (EMF or E)

The push in the circuit. Measured in Volts (V). Often called "voltage".

Current (I)

The flow of electrons. Measured in Amps (A, a) or often milliamps (ma).

Resistance (R)

Limits current flow. Measured in Ohms (Ω).

Ohms Law

The Voltage, Current, and Resistance in any circuit are governed by Ohm's Law where

V = I R

OR: I = V / R

OR: R = V / I

Watt's Law

The power in any circuit is the current times the voltage. Using E for Electromotive Force:

P = I E

Kirchhoff's Laws

Current (point) Law

The sum of the currents in and out of any point is zero.

Voltage (path) Law

The sum of the voltage drops around any closed path is zero.

Charge (Q)

Charge is the number of electrons. 1 coulomb is the amount of electrical charge in 6.24x1018 electrons. Current is the rate of change of charge where 1 amps is equal to 1 Coulomb per second.

Capacitance (C)

A capacitor consists of two plates separated by an insulating layer called an electrolyte. Charge can be stored on these plates.

Capacitance resists a change in voltage. It is measured in farads (f). Since a 1 farad capacitor would be a very large capacitor, you often encounter capacitors measured in micro-farads (μf)

Inductance (L)

A coil of wire resists change in current flow. This property is called inductance. The symbol for inductance is "L".

Inductance is measured in Henries (H). Since one Henry is a rather large amount of inductance, you will often encounter milli-Henries (mH).

Sources Of Electricity

There are six major sources of electricity.

Circuits

A circuit is a complete path for current flow, usually to and from the same source of voltage

A closed circuit is a complete circuit in operation.

An open circuit has a break, or open, in it for some reason and will not operate.

The simplest way to open and close a circuit is with a switch.

A short circuit has bypassed the load. Perhaps two wires are touching that shouldn't be, or something has fallen across a circuit. Without a load there is nothing to limit current flow. Excess current causes overheating and could even cause a fire. In your house circuit breakers (or in years past fuses) trip to protect your home from fire in the event of a short circuit in the wiring.

Conductors, Insulators, Semi-Conductors

A conductor is a material with free electrons that is used to carry current. Wires are made out of copper or aluminum. Silver and now gold is used as a plating on connecter for printed circuit boards in computers.

An electical insulator is a material whose electrons are tightly bound. It is used to stop the flow of current for personal protection and to avoid short circuits. Wires are wrapped in paper, plastic, and rubber. Power lines are carried on wooden poles and held away from the pole with ceramic insulators.

Semi-conductors are materials that sometimes conduct and sometimes insulate depending on conditions. Semi-conductor components are usually made from Silicon (Si14) but some are made from the rare-earth metal Germanium (Ge32). Silicon is a one of the components of ceramics and is therefore an insulator, but a controlled amount of impurities can be added to make it a semi-conductor. This process is called doping. Electrical components made from semi-conductors include: diodes, light emitting diodes (LED), transistors, SCR, micro-chips, operational amplifiers (op-amps), micro-processors, and the central processing unit (CPU) of your computer.


Once More, From the Top

Basic Physics

Electro Motive Force E Volts V
Current I Amps A
Resistance R Ohms Ω

Current

When a Voltage is applied to a conductor, the electrons stop randomly drifting and all head off in the same direction. The rate of electron flow through a conductor is called current flow. The symbol for current flow is "I".

When the current flow is equal to 1 Coulomb (6.28 x 1018 electrons) per second, 1 ampere of current is flowing. Ampere is abbreviated amp.

1 ampere = 1 Coulomb / second

Kirchhoff's Law of Current (point Law) says that the sum of the currents in and out of any point is zero.

Voltage

Voltage is a name for the electrical pressure or force that pushes electrical current through a conductor.

Another name for voltage is "Electromotive Force" (EMF). EMF is measured in volts.

A difference in potential exists when the EMF at two different points in a circuit is different. Current will flow from the point of higher EMF to the point of lower EMF. When there is no difference in potential between two points in a circuit, no voltage difference exists, and there will be no current.

The voltage drop is the difference in voltage between two points in a circuit resulting from the loss of electrical "pressure" (voltage) as current flows through the circuit.

Circuit symbol for battery (a source of voltage) the longer bar is the positive terminal.

Kirchhoff's Law of voltage says that the sum of the voltage drops around a closed loop must be zero.

Resistance

Resistance is the opposition which a device or material offers to the flow of current. All materials oppose current, but some materials oppose current more than others.

A resistor is a circuit component specifically designed to provide a specific amount of opposition to current flow. Resistors are included in an electrical circuit to provide just the amount of resistance needed.

The unit of measure of resistance is the Ohm. The symbol for Ohm is the capitol Greek letter omega (Ω). One Ohm is the value of resistance present when a difference in potential of one volt causes a current of one ampere.

R = V / I

An Ohmmeter is an instrument used for measuring resistance.

The wattage rating of a resistor is an indication of how much heat a resistor can handle. In general, resistors with larger wattage ratings are physically larger.

The prefix Kilo means one thousand (1,000 or 103). A kilohm is one thousand ohms.

The prefix Mega means one million (1,000,000 or 106). A megohm is one million ohms.

Symbols and abbreviations

Circuit symbol for a resistor

Ω Ohm

K Kilo, as in 10 KΩ

M Mega, as in 5 MΩ

R Resistor, in a circuit with several resistors they will be labeled R1, R2, etc.

A Potentiometer is a variable resistor with three external leads.

A Rheostat is a variable resistor with two external leads.

Small resistors are marked with a color code that gives the value of resistance in Ohms and its "tolerance" in percentage.

an Interactive Graphical Resistor Calculator

Power

Electrical power is the rate of doing work by moving electrons across a difference in potential. The symbol for power is P.

Electrical power is measured in watts. One watt of power is expended when one ampere of current is flowing across one volt of electrical potential (Watt's Law). The abbreviation for watt is W. One thousand watts is one Kilowatt, abbreviated KW.

From the definition, the equation for power (Watt's Law) is

P = I V or P = I E

by using the relationship V = IR, other equations can be generated.

P = I2R

P = E2/R

The instrument that measures electrical power is a wattmeter.

Circuits

Series circuits

In a series circuit the same current flows through each load in turn.

Itotal = I1 = I2 = I3 = ...

The total resistance is the sum of the individual resistances

Rtotal = R1 + R2 + R3 + ...

And by Ohm's Law

Itotal = V / Rtotal

Parallel circuits.

In parallel circuits each load sees the same voltage.

Vtotal = V1 = V2 = V3 = ...

The current across each load is equal to the voltage divided by the resistance of the load.

I1 = V / R1

I2 = V / R2

etc.

The current through each load combines to return to the source, so the total current is equal to the sum of the currents in each branch.

Itotal = I1 + I2 + I3 + ...

The equivalent (sometimes called total) resistance is the Voltage divided by the total current.

Requivalent = V / Itotal

Rearranging

Itotal = V / Requivalent

Let's take another look at the current equation

Itotal = I1 + I2 + I3 + ...

Let's replace each current with the associated V/R.

V / Requivalent = V / R1 + V / R2 + V / R3 + ...

Dividing out the V on both sides...

1 / Requivalent = 1 / R1 + 1 / R2 + 1 / R3 + ...

Which is the formula for finding equivalent resistance in a parallel circuit. Use the 1/x button on your calculator to make the calculation simple. Try it with two 100Ω resistors in parallel.

1 / Requivalent = 1 / 100 + 1 / 100

Did you get 50Ω? If not try again.

Note: The equivalent resistance in any parallel circuit will be smaller than the smallest resistor.

Series-Parallel Circuits

Redraw a set of equivalent circuits, replacing paralled resistors with their equivalent resistance and series resistors with their total resistance. Find the current in each branch and use that to find the voltage across each resistor. Use Ohm's law.

 

Electrical Safety Rules
  • Keep your work area neat, clean and well organized.
  • Never work with electrical equipment if the floor is wet or you are wet.
  • Don't work alone, make sure someone is near by in case of emergency.
  • Never remove the ground wire of a three prong plug.
  • Never touch electrical circuitry using both hands.
  • Avoid unnecessary talking. Pay close attention to your work.
  • Work deliberately, patiently and cautiously. Don't take chances.
  • You could receive a painful burn if you touch a hot soldering iron or its associated accessories. Be careful not to touch hot solder.
  • Never work on energized circuits of more than 30 volts.
  • Never totally trust fuses, switches etc. They can be defective.
  • Some components will remain electrically charged for years, even if the plug is removed. A serious shock is possible.
  • Sometimes electrical components get very hot and you could get burned.
  • Watch out for sharp metal edges on electrical equipment.
  • Wear safety glasses at all times that project work is being performed.
  • Always check electrical cords before plugging them in. This is especially important when using soldering items. The cords sometimes get burned and must be taped to prevent electrical shock.
  • If you don't know how to do something, ask the teacher.
  • Avoid practical jokes and fooling around.
  • If possible turn equipment off when testing. Use extra care when equipment must remain on.

Electric Motor

By: Sarah S-K

So you want to know how an electric motor works? Let's start with some background information. If you already know this, then just bear with me here. An electric motor is a motor that uses electricity by converting it to mechanical work. The first electric motor was built in 1821 by Michael Faraday. It was simply spinning wire suspended over a bowl of liquid mercury. Of course, much has improved since then. The basic parts of an electric motor, nowadays, is a much more complicated list. The main parts of an electric motor are the following:

Check out the diagrams


What you're looking at is a simple form of an electric motor. Now that you see what it looks like, you should know how it works. As you can see, there are two magnets: The "North" and the "South". The motor uses these magnets to create the motion. The flow of opposition between the magnets creates a rotational motion. The armature is an electromagnet, but instead of being wound around a nail like you did in middle-school, it is thin wire coiled around two or more metal poles, called the stator (not labeled above). The axle is part of the armature. The commutator can be described as a pair of plate attached to the axle, providing connection for the electromagnet. The commutator and brushes work together as a team to let the current flow to the rotating armature. They flip the direction of the flowing electrons at the right moments. Thus, the commutator is attached to the axle of the electromagnet and it spins (this is also called the rotor). The field magnet is a permanent magnet, unlike the electromagnet.

Of course, the electric motor is only one type of motor out of the many interesting inventions. There are different ways of creating mechanical action, such as the series-wound motor. In this motor, the field is also an electro-magnet. The current travels through the field winding and then through the armature winding via the commutator and brushes. If you reverse the direction of the current through the field, the current is also reversed in the armature and the motor continues to spin in the same direction. Because of this the series-wound motor is described as the universal motor because it will operate on either AC or DC power.

An AC is an alternating current that switches between positive and negative. The DC is different, it's a direct current that stays either positive or negative. Most small electric motors require a steady DC supply.

In conclusion, an electric motor creates energy off of magnets, and is simply one of the many ways motors can work. There are still scientists working today, discovering more and more about this fascinating field and leading us farther into the path of scientific engineering.

Links

AC, DC, and Electrical Signals
Electric Motor
How Electric Motors Work
Animation
Another Animation


Wind Power

For a magazine about wind energy visit www.wpm.co.nz/.

For a good overview visit solstice.crest.org/renewables/re-kiosk/wind/index.shtml

Want to know what the government is doing? Visit www.nrel.gov/wind/ For other news visit www.worldenergynews.com

Tired of paying your electric bill? Visit www.homepower.com/ or to check out some models to buy visit www.windgenerators.com

To see what other states are doing visit www.cogreenpower.org/

For a complete tour of wind energy visit www.windpower.dk/tour/

Last updated on 2 June 2012 by P. A. Wiedorn

 

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