Basic Electricity Concepts and Terms

Last update: March 8, 2007

The terms you are required to know for the Electricity Merit Badge are: Volt, ampere (amp), watt, ohm, resistance, potential difference, rectifier, rheostat, conductor, ground, circuit, and short circuit. The others in here are good to know.

Bohr model for Aluminum. 
  13 electrons: two in the inner shell, eight (4 pairs) in the middle
  shell, and three in the outer shell, all orbiting a nucleus. Electrons are tiny negatively charged particles that orbit the positively charged nucleus of atoms. Electrons are so small that they only account for a very small portion of the overall mass of the atom.

To get a better idea of what's going on, see the Bohr model for Aluminum to the right. The number of protons in an atom (in this case, 13) indicate the atomic number of an atom.

The nucleus of an atom is made up of protons, and neutrons. Protons have a positive charge, while neutrons have no (neutral) charge. Hydrogen is the only atom that doesn't have to have neutrons in its nucleus, but it can have as many as two neutrons. An atom can have any number of neutrons, but if it has too many, or too few, it will break down in what we call radioactive decay. Radioactive decay means that an atom will eject part of it's self in order to become more stable.

Atoms of the same type, but with different neutrons are called isotopes. The number of protons in an atom give it it's chemical properties, not the neutrons (neutrons have no charge, so they can't interact very well with electrons). The number of protons in an atom is the same as it's atomic number. This means that a hydrogen atom with zero neutrons, and one with one neutron will react chemically the same way to other hydrogen atoms; but, the mass of these atoms are not the same (a hydrogen atom with one neutron is twice as heavy as one with zero neutrons).

The interaction of electrons between atoms allow the formation of chemical bonds. Current flow through a circuit are these electrons jumping from atom to atom at about the speed of light.

Electrons in an atom exist in specific orbits, or energy levels. When an electron gets more energy, it can be bumped up an orbit, or even out of the atom. When an electron looses energy, it will fall down one orbit.

The energy that is "lost" from an electron is released as a photon. A photon is a little packet of energy that travels through space at the speed of light. The light we see is from photons in a narrow energy band which we simply call "visible light". Lower energy photons make up radio waves, while higher energy photons are ultra violet, gamma, and other harmful radiation that can damage our cells. All the light we see is produced by electrons switching energy levels and giving off photons.

When an atom looses or gains an electron, the atom no longer has a neutral charge becomes ionized. Here is an example:

Bohr model for
    Aluminum, plus one electron in the outer shell Bohr model for
    Aluminum, minus one electron in the outer shell
A negative Al ion Al-, has an extra electron A positive Al ion Al+, missing an electron

Now, if ion looses an electron, it is still an ion! All an ion is, is an atom that does not have a neutral charge (the number of electrons and neutrons are not equal)-- that's it. A similar term, "Isotope" refers to atoms of the same type (same number of protons), but with a different number of neutrons. The chemical properties of isotopes (hydrogen with zero neutrons compared to one with two neutrons) are the same, with the exception that their atomic weights are different.

An electron can have any amount of extra energy behind it. Special chemical reactions can cause electrons to be attracted strongly to other atoms, strong enough that they will flow through a wire to do it. Some other things that we do with magnetic fields like in a generator, gives electrons more energy and makes them want to move. This extra energy creates a potential difference between two points which is the driving force behind current flow.

One thing to remember about the Bohr model, and all other models of the atom you will see: We don't really know what an atom looks like, there is no way we can take a look at it's actual structure and see those electrons zipping around. Atomic models are used to help us predict how atoms will react to other atoms, and that's it. We can make educated guesses on what an atom looks like based on experiments, but that is the most we can do.

Potential Difference
The difference in electrical potential, or voltage, from one point in a circuit to another. The voltage rating on a battery describes the potential difference between it's terminals.

Regardless of the potential in circuit, there can be no current flow until the terminals are connected, or there is enough energy to overcome a barrier (like air) and electrons are allowed to flow from one terminal to another. So, if you have a 10,000 volt battery, and it's not connected to anything, there will be no current flow-- but as soon as you touch both terminals you'll get a nasty surprise.

Volt (V)
Voltage a unit of measure is the driving force, or potential difference behind electron flow, and hence the force behind the flow of current in a electrical circuit. Energy always flows from an area of high energy to low energy.

Higher voltages mean that electrons are able to overcome greater levels of resistance, including jumping through the air. Stun Guns, produce very small amounts of current, but have a high level of voltage that allows sparks to jump from one terminal to the other. Tesla Coils, and Lightning are other examples of the effect of having a large potential difference from one point to another.

If you think about water flowing through a pipe, you can think of the pressure behind the water as voltage, and the amount of water flowing through the pipe as current. The difference between water and electrons are, that when water has more pressure behind it, it travels faster, electrons will always travel at the same speed through a wire regardless of energy behind them.

The resistance of a circuit to the flow of electrons, or current flow. Everything has some resistance associated with it, even metal wires. Some of these resistances are so small that we will ignore them in most cases.

Heat changes the resistance of all materials. Some materials will increase their resistance as they get hotter, others will decrease their resistance. This property of materials can be used to make electronic circuits that can measure heat.

Another method to measure heat is to wrap two wires together made of different materials like copper and iron. When these wires are exposed to high levels of heat, a small voltage can be detected between the two wires. These devices are called thermocouples.

When electrons flow through a circuit, some of their energy (measured in volts) lost and becomes heat energy due to the resistance in the circuit. This energy lost can me measured with a volt meter across a resistor in a circuit.

The more current traveling through a conductor, the more heat is produced. So much heat can be produced that it will cause wires to glow, or even melt.

Ohm (Ω)
The unit of measure for a circuits resistance to current flow.
Resistors have resistance, and their primary function is to add resistance to a circuit. The electrical symbol for a resistor is __/\/\/\__.

Below is a picture of what some resistors look like. The large white one is a power resistor. Power resistors can handle a lot of current flow without burning up. The color bands indicate the resistance, the color of the body indicates the type of resistor (carbon, wire, and so on). The tan resistors are carbon resistors.

A five different
  resistors with quarters for size comparison (one is the California

The first two bands (or first three if there are five color bands) are the significant digits. The second to last band is the multiplier. The last band (which may not be present) indicates the resistors tolerance, or allowed error associated with it's manufacture (a ±5% tolerance for a 100Ω resistor means it has a resistance range of 95Ω to 105Ω).

Here is the color codes:

Black 01
Brown 110
Red 2100
Green 5100,000
Blue 61,000,000
Grey 81x108
White 91x109
Gold 0.1
Silver 0.01
Tolerance color codes:
None ±20%
Silver ±10%
Gold ±5%
Red ±2%

The red color band for tolerance is not very common

So, if you saw a resistor with the following color bands: Brown, Red, Red, Gold, it would be a 1200Ω resistor (or 1.2KΩ) with a tolerance of ±5%. This tolerance means the actual resistance of the resistor could be between 1140Ω and 1260Ω.

A Prefix of 'K' (Kilo) means 1000 Ohms; and a prefix of M (Mega) means 1000 Kilohm, or 1,000,000 Ohms.

There is a mnemonic people use to help remember this color sequence, it goes like this: "Bad Boys Run Our Young Girls Behind Victory Garden Walls, Get Started Now." The last word "Now" is supposed to remind you of the "none" color code for the resistor tolerance. Remember that you start counting from zero, not one!

The flow of electrons in a circuit, measured in Amps.

Following the water analogy, you can think of current as the amount of water flowing through a system at any given point in time. We measure water flow in some unit of volume per some unit of time, like 1 gallon per minute (GPM), or 1 liter per minute (l/sec).

The more voltage you have in a circuit, the more current flow you have, and the higher resistance in a circuit, the less current flow you have. This is Ohm's Law.

It is important to remember that volts don't kill you, it's current. You can be zapped by 100,000 volts and not die. When you shock someone with static electricity, that's many thousands of volts, but it's a low current flow.

You can normally handle circuits under 13 volts without getting zapped because your skin's resistance. However, if you have a break in your skin, or get it wet, that resistance is dramatically reduced and you can get zapped (put a 9 volt batter on your hand, then try it on your wet toung).

As little as 1/1000th (1 milliamp) you can feel, 1/100th (10 milliamp) can cause your muscles to contract, and 1/10th of an amp (100 milliamp) across the heart can kill you.

Ohm's Law
"Current is directly proportional to the voltage in a circuit, and inversely proportional to the resistance of the circuit." We express this as an equation:

           V      V:Voltage
        I= -      I:Current
           R      R:Resistance

This equation is a very good one to remember. You will use it often when working with electrical circutis.

Ampere, Amp (A)
The unit of measure of current flow in a circuit.
Direct Current (DC)
Direct current sources never have current flowing in the opposite direction, that is, the positive and negative sides never reverse. A battery only produces direct current when connected to a circuit (the circuit can change the DC into AC).
Alternating Current (AC)
Alternating current means the current flow goes one way and then reverses it's direction at regular time interval. This happens because one terminal is negative and the other is positive for a while, then they switch from being negative to positive and from positive to negative.

In the US, household AC is 60Hz (60 Hertz) that means current switches back and forth 60 times in one second. If we were to graph the change in voltage of household AC over one time period (in this case, 1/60th of a second), we would get the graph below. As you can see, it is a sine wave.

The graph of a single period of a sine wave

The human body has bio-electrical reactions that control such things as muscles, and the heart beating, and they operate at at a frequency very close to 60 Hertz. This makes AC even more dangerous than DC because one shock could throw off your biorhythms and could trigger a heart attack within hours. A person that has been shocked for by AC should be watched for a few hours to make sure he doesn't have a heart attack.

CPR alone may be enough to help someone who has had a heart failure due to electrical shock. Even if CPR brings someone back, if it was caused by a shock or not, they must go to a hospital.

CPR will help keep blood flowing through their system, but it may not be able to restart their heart because the electrical signals have been thrown off-- that's where a defibrillator comes in. Defibrillators resequence the heart's electrical impulses. You are able to get auto-defibrillators that allow people with only basic training to save a persons life (a computer, along with special sensors, determine the proper sequence of electrical pulses to start a heart).

TV programs and movies show people using defibrillators instead of CPR. This is not correct. Defibrillators are used with CPR, they don't replace CPR.

NEVER STOP CPR ONCE YOU'VE STARTED! If you stop CPR, and try starting again, further chest compressions may not push blood throughout the body, instead just circulate it inside heart like a balloon filled with water. Naturally, if they start yelling and try getting up, you should stop ;-).

Batteries convert chemical energy into electrical energy. Multiple battery cells can be connected in series to increase the voltage of the circuit. All batteries have some internal resistance, which means some batteries are better at high current flows than others.

(elect. symbol 
  for battery) is the electrical symbol for a battery cell. The plus sign isn't always there. The longer end indicates the positive terminal, and the smaller one is the negative terminal. Some electrical drawings have several of these battery cell symbols staked on each other. This means it's a battery pack, or just a battery.

Warning: you should only connect batteries of the same type in series, otherwise really bad things can happen: Ka-Boom! Owee, I have acid in my eyes! Okay, it's a remote chance, but it can, and does, happen-- especially with rechargeable batteries.

All batteries contain nasty chemicals that you don't want to get on you or to ingest/swallow. So opening up a battery is a really bad idea. Old batteries leak out acid, which you can see by some crusty stuff on the battery terminals or the battery can. Throw these batteries away and quickly wash your hands before your hands start burning, or you get the acid in your eyes.

Wikipedia's Battery (electricity) page has a great deal of information on batteries that you may be interested in. It also goes into the chemical reactions in different battery technologies.

Current Flow and Electron Flow
Both terms essentially mean the same thing when talking about an electrical circuit: electrons moving from one place in a circuit to another. The direction of current flow was determined by Benjamin Franklin, who said that current flows from the positive to negative. Well, he had a 50% chance of getting that right and just missed it.

Electrons really flows from the negative terminal to the positive terminal. When we talk about Current Flow we assume that current flows from positive to negative. When we talk about Electron Flow, we assume that current flows from negative to positive (which is really the case).

When working with circuits, it makes little difference if you use electron or current flow. Since most books use current flow, rather than electron flow (which is a newer term), that is what we will be using. When we start talking about what happens inside of semiconductors (transistors, diodes, etc), we have to talk about electron flow within that component to understand what is going on inside.

Here is an illustration of an electrical circuit to better help you understand this:

Diagram showing
  current and electron flow

The thing to remember is that Current flow is some imaginary thing we came up with, but still use because it works, and it's the traditional way of performing circuit analysis. But Electron Flow is what's really happening in a circuit on a subatomic level. When we talk about AC, the direction of current changes constantly, so we just pick a direction and sick with it when performing circuit analysis.

A material in which current is able to flow through easily. Most metals make good conductors, some better than others, silver being the best in normal conditions. Super-conductors have nearly no resistance to current flow, but they often have to be super-cooled to work and are very expensive.
These materials can either be good conductors or good insulators, or somewhere in between, based on certain conditions. They are used in diodes, transistors, and other electrical components. They are very useful and are the reason why we have the computers we have today.

The most common base material used in semiconductors is Silicon (Si), the second is Germanium (Ge). Germanium semiconductor components take away less of the electron's energy than Silicon, but are more expensive. Silicon is refined from beach sand.

A material that does not conduct current very well. It is the exact opposite of a conductor. The rubber around wires is an insulator. Glass is also used as an insulator.
Watt (W)
The measurement of power in a circuit, as determined by multiplying volts times amps:

P=V x I

A Kilowatt is 1,000 watts. A Watt-Hour is the number of watts used over an hour. You can think of Watt as measuring the speed you're using power, like MPH (miles per hour), and a Watt-hour as the distance you've actually traveled over that time.

Using Ohms Law, by substituting V for I x R we get P=R x I2. We will use this equation when talking about why the power companies step up line voltage to insane values in transmission lines, and later step it back down to household values.

Short Circuit
Current wants to take the path of least resistance. If there is a path of very low resistance, current will want to flow that direction more than any other. When there is a path of very low resistance back to the power source (which isn't supposed to be there), we call this a short circuit. A short circuit can cause a fire.
A fuse is a device that, if a current level is exceeded, the wire, or substance inside of the fuse, will heat up and then melt, breaking the electrical connection.

There are fast and slow blow fuses. Fast blow fuses are used to protect circuits from short circuit conditions. Slow blow fuses don't blow right away when they have reached their current rating. Slow blow fuses are used with motors and other components that have high starting currents.

The idea behind a fuse is that fuse will blow (melt) before something else in your circuit melts, catches fire, or blows up (sometimes they blow because something in a circuit failed in some really bad way).

Fuses are electrical safety devices and should never be jumped out with wire, foil, or any other metal. They are used to protect other electrical components in a circuit and to prevent electrical fires. Only replace a fuse with a fuse of the same type and rating.

Circuit Breaker
A Circuit Breaker provides overload protection. They are slower acting than a fuse. Circuit breakers are electro-mechanical devices (which means they are part electrical and part mechanical) which will break a circuit if too much current is flowing through a circuit for too long. Unlike fuses, they can be reset by flipping a switch-- you don't need to replace them unless they go bad.

All houses built today use circuit breakers rather than fuses. If your house still uses fuses, the wiring is old and and you could be at risk for an electrical fire.

Some smaller circuit breakers have a pot of soft solder that melts at a specific temperature. If the pot gets too hot due to excessive current flow, the solder will soften enough to allow a plunger to pop out. To reset it, push the plunger back in. The end of the plunger you push looks like a button.

When household circuit breaker trips, the circuit breaker will be in a position that's not quite in the off or on position. To reset, you need to take the breaker switch first to off, and then back to on. Taking it to off resets the trip mechanism and allows the breaker to be closed.

Rheostat/Potentiometer elect. symbol for rheostat
A Rheostat is similar to a resistor, except that the resistance of the component can be changed by turning a knob or moving a slider (slide-type rheostats are used on sound mixing boards). That is why the electrical symbol looks like a resistor with an arrow through it. That line means that it can be adjusted. You'll see something similar on other devices that can be adjusted by a knob, such as capacitors and inductors.

There are two major types of rheostats out there: Linear, and Logarithmic/Audio. Logarithmic rheostats, also called Audio rheostats or potentiometers, are used for sound applications because the human ear is logarithmic (in other words, something that sounds twice as loud to you really is a sound wave that is 10 times as strong, and something three times louder to you is a sound wave that is 100 times stronger). Linear rheostats are used for adjusting current flow to things such as lights.

A device that generates electron flow through moving a conductor through a magnetic field.

The amount of voltage induced in the generator coil is directly proportional to the strength of the magnetic field, and the speed at which the magnetic filed is changing or moving through the conductor.

Transformers are able to step up, or step down AC voltages through magnetic field interactions. They are also able to isolate an AC circuit form other AC circuits.

AC current causes the magnetic field to continually grow, decrease, and then increase over and over again. This produces a moving magnetic field-- just like what's needed to make a generator work.

electrical symbol for transformer transformer diagram To the right is a diagram of a transformer. To the left is the electrical symbol for a transformer. The the square on the diagram illustrates the iron core of a transformer.

The metal core in a transformer transfers the magnetic field from one set of windings to another, and thus induces a voltage in the second set of windings that changes at the same frequency as the original windings.

By making the number of windings on on the secondary side (load side) half those of the primary side (supply side), we are able to induce a voltage in the secondary windings that are one half that of the primary side.

A device that converts AC into DC. Diodes are used to rectify a AC into DC because they allow current to flow only in one direction. In order to produce a clean DC output, filters have to be used. Usually capacitors are used in these filters.

Another method that is used to convert AC to DC is the use of a motor-generator set. AC motor is used to spin a DC generator. It's not as efficient as using a rectifier, but they are cheaper for large current applications and are used on some older elevator systems.

Ground is considered at zero potential. There are two types of grounds: a circuit or chase ground, and an earth ground. An earth ground means that the ground is somehow physically connected to the ground your standing on. A circuit or chase ground just means that there is a wire or connection that is connected to the case that the circuit is housed in.

The third prong on your electrical outlet (US/Canada) is an earth ground (if it isn't, somebody wired your house wrong). The earth ground is often connected to the metal case of the machine. Electricity takes the path of least resistance, so, if there is a good earth ground to a metal case, electricity would rather flow through the grounding wire instead of you. The grounding wire in a house is either green or bare copper.

If you don't have three prong outlets in your house, you need to have a licensed electrician come out and rewire your house. He'll also put in a device called a GFCI, or Ground Fault Interrupter. GFCI's will break the circuit if there is a ground fault-- like what would happen if you drop something electrical in a tub.

Inductor elect symbol for inductor
An inductor is a coil of wire, any coil of wire is actually an inductor. Inductors, by themselves, tend to resist a change in current flow. They store energy in the magnetic field produced by current moving through a conductor. When current flow begins to slow, the magnetic field begins to collapse, and thus induces a voltage in the coil that increases current flow.

There are a few applications for inductors, such as used in filters.

Motors are also inductors, and, next to transformers, are the largest inductors in an electrical circuit. Cutting off power to these circuits can cause a large voltage spike which is called an "inductive kick." Capacitors are used to suppress this inductive kick by counteracting the effect of the inductor.

Capacitor elect symbol
  for capacitor
A device that resists changes in voltage. It stores electrons between metal plates and an insulator. They are used in filters, and power supplies to maintain voltage during current surges. They also cancel out the effect of inductors in AC circuits and suppress the "inductive kick" in both AC and DC circuits.

Capacitors come in many shapes and sizes. Here are a few:

A bunch of capacitors

Diode elect symbol for
Diodes were the first semiconductor component developed. They only allow current to flow in one direction. They find uses in many places, and are the main components of rectifiers. A minimum voltage is required in order for a diode to pass current through it. Here is what they look like:

five different

Lamp/Light Bulb elect symbol for lamp
Basic incandescent light. It produces light by heating up a special wire, called a filament, which then glows. The element that is commonly used in light bulbs is Tungsten, because it begins to glow well before it reaches it's melting point. The mantels in gas lanterns now use tungsten for the same reason (they used to use Thorium, which is radioactive, but stopped in the 1980s).
LED (Light Emitting Diode) elect symbol for LED.
LEDs are special diodes that, when current passes through them, they will light up. They use far less power than a lamp, and last far longer. The little arrows are supposed to show photons, or light, being emitted.

Resistors are often needed with LEDs to lower the current flow through the LED to a safe value (dependent on the LED, but around 10-30 milliamps is usually good).

Different color LEDs require different minimum voltages to turn on.

Wikipedia's LED page has more information on LEDs if you would like to learn more.