Section 4
Part 2

Technical and Safety Information about Electricity

Technical information:

A magnetic field is the charge produced by electrons. The magnetic field is measured as flux. The unit of measure is webers or maxwells.

An electric field is the charge difference between two magnetic fields.

An insulator has a very high resistance.
A conductor has a very low resistance.

The basic formulas that relates voltage, current, resistance and power are:

I = V / R
V = I * R
R = V / I

P = V * I
P = V * V / R
P = I * I * R

I is the current in amps ( a ).
V is the voltage in volts ( v ).
R is the resistance in ohms ( ).
P is the power in watts ( w ).

A ground reference is defined as the reference point that all voltage measurements are made ( a 0 volt reference ). Normally the cathode of a battery is defined to be the ground reference making the battery's voltage rating positive. However, the anode can just as easily be defined as the ground reference, thus making the battery voltage rating negative.

The cathode is the source of electrons and thus has a negative polarity relative to a ground reference.

The anode generates a positive voltage ( from positive ions ) and thus has a positive polarity relative to a ground reference.

With a direct current source ( DC source ) the polarity of the power source's terminals does not change.

A battery becomes a constant current source if the circuit resistance is to low. This is because the battery can only deliver so many electrons. If the circuit resistance becomes to low , then the battery can not supply enough electrons, so the voltage will drop. Basically, the internal resistance of the battery becomes the largest circuit resistance.

The power rating of a battery divided by it's rated voltage will tell you the maximum current the battery can deliver.

The voltage rating divided by the maximum current tells you the smallest resistance the battery can handle.

Most batteries have a power dissipation rate. From this rate you can determine how long a battery can deliver a given amount of power before it over heats.

Most batteries have a short circuit rating. This rating determines how long the cathode can be connected directly to the anode with a conductor before it overheats. Most simple batteries have a slow chemical processes so they have a longer short circuit rating ( maybe a tenth of a second ). High power batteries have very fast chemical processes and will explode if shorted for even a very short period of time. High power batteries like car batteries usually have a fuse on one or both terminals.

In an open electrical circuit energy is transferred somewhere in the electrical circuit though an electric field.

Parallel circuits reduce resistance, because a power source must supply current to more circuits. From the formula, R = V / I, if the voltage is constant and the current increases, then the resistance must decrease.

Series circuits increases resistance, because the same current must pass through more components. Each component offers additional resistance to the current.

Inductors:

Refer to figure - Fields, for the following discussion.

When current flows through a circuit it generates an electric field across the conductor and a magnetic field around the conductor.

The magnetic field is produced by the number of electrons at any given point on the conductor. The larger the number of electrons, the larger the magnetic field. The electrons producing the magnetic field are valency electrons, who's energy level has been rasied to the point that they can move through the conductor.

Electric fields are produced by the differences in magnetic fields along the conductor.

An electric field is weaker than a magnetic field.

The electric field can be picked up by another conductor running parallel to it and will produce a current in the second circuit. This affect is called induction, the second current was induced by the first current.

The amount of current induced is dependent on several factors, the distance between the conductors, the length of the coupling and the material separating the two conductors. To get maximum coupling you need to get the two conductors as close as possible, parallel to each other for as long as possible and make sure the separating materials have minimum affect on the fields.

The insulating properties around the conductors determines how close the conductors can get before electrons try to cross over to the other circuit. A good insulator will not react to a field.

If a coil is wrapped around a magnetic rod and the coil's two ends are connected to an amp meter instead of a battery you might think a current would flow. This doesn't happen because the magnet is supplying a single magnetic field, thus no electric field is being generated. To get a current to flow in the coil, you must move the magnet through the coil. This moving magnetic field produces an electric field across the coil, thus causing a current to flow in the coil. If you pull the magnet back though the coil in the opposite direction you will get a current flow also in the opposite direction.

A moving magnetic field through coils is the principle of primary power generation.

When a voltage is applied to an inductor, the electrons don't instantly flow, because the electrons in one coil opposes the electrons in the adjacent coils. As the electrons begin to flow, the resistance of the inductor drops ( increased current means reduced resistance ). The inductor is just a wire and has little resistance, so a resistor is used to limit the amount of current that will flow. As the current flow increases the voltage across the inductor drops ( as electrons are pulled from the inductor, the opposing charge is reduced ). So when voltage is first applied to an inductor there is no current. When maximum current is flowing through the inductor there is no voltage across the inductor. This means the peak current occurs after the peak voltage or current lags the voltage in an inductor.

Transformers:

To couple as much electric field as possible between two conductors, a conductor is wound around a rod. The second conductor is then wound around the same rod on top of the first conductor . In a linear inch, the conductors are actually much longer, which produces a longer period of time the first conductor's electric field is in contact with the second conductor, thus transferring more energy to the second circuit. Generally, the rod is removed, thus producing an air core transformer. The rod can be magnetic, in which case the transformer is said to be an iron core transformer.

The inductor that induces an electric field is called the primary coil. The inductor that receives the induced electric field is called the secondary coil.

A transformer transfers power from the primary to the secondary, making it a constant power source.

A transformer comes in three basic types; step-up, step-down and isolation.

A step-up transformer, increases the voltage from the primary to the secondary. The secondary voltage is higher so the current will be lower ( I = P / V ).

A step-down transformer, decreases the voltage from the primary to the secondary. The secondary voltage is lower so the current will be higher.

An isolation transformer is used to electrically isolate two circuits. It makes no changes to the voltage between the primary and secondary.

Most transformers in use are compound transformers containing several transformers in one. A compound transformer has more than two coils, either two or more primary coils or two or more secondary coils or both. A compound transformer can be composed of any or all of the basic transformer types.

Capacitors:

A capacitor is very similar to a battery except it has no chemical to supply electrons. It stores electrical energy by using very large metal foils separated by an insulator. When a voltage is applied to a capacitor, electrons move from the battery and collect on the foil. When a voltage is first applied to a capacitor, there is no charge on the capacitor ( no electrons ), so the metal foil is at 0 volts. Because all the voltage is across the connecting wire from the battery to the capacitor, and the wire has little resistance, the current flow will be at maximum ( whatever the battery can deliver ). As electrons collect on the foil, the voltage on the foil increases. As the voltage on the foil increase the current flow decrease ( less voltage across the wire, constant resistance means less current ). When the voltage on the capacitor equals the battery voltage, the current flow will be 0 amps. This means the peak current occurs before the peak voltage or current leads the voltage in a capacitor. The quality of the insulator will determine how long the capacitor will hold it's charge when removed from the voltage source. This is why it's so dangerous to poke around high voltage circuits that contain high capacity ( large ) capacitors.

A capacitor's capacity is measured in farads.

Inductors and capacitors have opposite current/voltage behaviors. Designed correctly an inductor/capacitor combination can be made to oscillate ( an AC generator ).

Technical and Safety Notes:

Voltage ( unit of force ) is 1 joule ( unit of energy ) per coulomb ( unit of charge ). Ampere is 1 coulomb per second. Coulomb is 6.28 * 10 ^18 ( 6,280 million billion ) electrons.

A voltage is the measure of force between two charges and the equation looks identical to the equation that measures the gravitational force between two masses ( like the earth and sun ). Interesting to note that an electric force is 10^39 ( that's 1 followed by 39 zeros ) times stronger than the gravitational force.

Technically everything under a strong enough electrical force will conduct electricity.

The elements; copper ( Cu ), silver ( Ag ) and gold ( Au ), all have 1 electron in the s-orbital of their outer shell. This means that under force they can give up an electron or add an electron to fill the s-orbital. They also sit in the middle ( group 11 ) of the periodic table ( chemically more stable than other metals ).

Alkali batteries, like the standard D cell, use a positive electrode of carbon ( C ) and a negative electrode of zinc ( Zn ) which is also used for the battery casing.

Batteries using ionic chemicals which are acids and alkalis is very good reasons for not opening them. Some chemicals used in batteries are very toxic as well, like lithium ( Li ). Larger sources of power require that batteries be composed of many and larger cells, like with a car battery ( lead-acid ).

Since a battery uses a chemical process to generate electricity, heat and gas are generated inside the battery during the conversion process. If this process takes place to quickly, then the gas pressure can become high enough to cause the battery to swell even to the point of a breach in the casing. The breach can produce a cloud or spray of heated acid or alkali. A very corrosive sludge around one or both terminals indicates a breach in the batteries casing.

Last updated: Mar 4, 2011

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