There are two types of current. Direct Current is current that always flows in the same direction. and Alternating Current is current which is reversed at regular intervals.
Resistance or ohms is usually the load we put on a circuit. This could be lights, it could be a motor or it could be a resistor or coil placed in the circuit.
Voltage Current And Resistance are measured by what we call Ohms Law E=IR, or I=E/R, or R=E/I. E=voltage, I=current, and R=resistance.
If you have 10 amps of current and 10 ohms of resistance you than will have 100 volts. E=IxR. If you have 100 volts and 10 amps you than will have 10 ohms. R=E/I. Watts is the power used by electricity and is measured by the formula P=IxE. If you have a light that is rated at 100 watts and the voltage is 100 volts the current will be 1 amp.
These are just basic formulas, There could be other factors that could slightly change these calculations. An electric circuit has voltage inducing a current to flow thru a resistance. Say you have 120 volts a.c. sending a current thru a light bulb. The bulb will stay on as long as there is voltage.
To shut the bulb off we need to break the circuit and stop the current flow. This can be done by placing a switch in the line. A switch is nothing more than a pair of contacts that open and close. Computers are basically on and off switches. Industrial motors turn off and on by switches. Just about everything electrical has some sort of switch loop.
There are two principal effects of electric current. The heating effect and magnetic effect.Whenever an electric current passes through a material there is a heating effect due to the current. A certain portion of electrical energy that is put into the circuit is transformed into heat energy due to the resistance of the material. This loss of energy causes a drop in voltage. Thus you have the term voltage drop.
Also whenever an electric current is passed thru a conductor the electric current tends to set up a magnetic field. You can check this out by holding a compass near the wire that is carrying the current. The compass needle will point towards the wire. If you were to put a piece of metal in an energized coil it would grab the metal.This is the process magnetic motor starters and relays use to pull in contacts for high voltage motors while using low voltage coils. You can also find this in solenoids to open and close many types of valves.
Perhaps the most widely used application for magnetism is in the use of transformers. A transformer tranfers electrical energy from one coil to another by means of an alternating magnetic field. It consist essentially of three parts: the primary coil which carries alternating current from the supply lines: the core whch produces an alternating magnetic flux, and the secondary coil which is produced an emf by the change of magnetism in the core for which it surrounds. Now in plain english. The primary coil is the winding of the tranformer which is connected to the source of power.The secondary coil is the winding which delivers power to the load. The core is a magnetic circuit upon which the windings are wound.
A step down transformer is one where the primary voltage is higher than the secondary voltage. For instance a bell or thermostat transformer would have a primary voltage of 120 volts and a secondary voltage of 12 or 24 volts. The advantage of this is you will less likely get a shock from 24 volts than 120 volts A step up transformer is one where the primary voltage is lower than the secondary voltage.
A voltage regulator is a transformer that regulates the secondary voltage. If you have a primary voltage which has a tendency to fluctuate this transformer will produce a constant secondary voltage. For instance say the primary voltage changes from 98 volts to 120 volts the secondary will stay constant at 120 volts. If you center tap the secondary of a 240 volt transformer you will get 120 volts. The less windings you use the lower the voltage. The more of the windings you use the higher the voltage. A multi tapped transformer is one where you can manually change the voltages. There are a series of taps to connect for each different voltage.
The wires going to the outside of your home from the transformers you see on the poles in the street are step down transformers.They change the high voltage coming from the utility company to the lower voltage you use in your home.
There are several types of motors the direct current motors that run on direct current that when applied to brushes excite the windings thus causing the motor to turn. The single phase alternating current motors which also need starting help which is usually accomplished by having a starting capacitor, a starting switch a starting winding or a split phase winding. These are the motors used mostly for mosty smaller appliances or machines.
Three phase alternating current motors which are used for both small and heavy duty applications. They have three windings and a rotor that run on alternating current. When the A.C. is applied it magnetizes the windings thus causing the rotor to turn. Protection for the smaller of these motors (fans, refrigerators etc.) is usually accomplished by using thermal overloads inside the motors or an overload switch with a heater. Most circuit breakers are not used to protect motors, only when they short to ground. Circuit breakers are used to protect wires feeding branch circuits and direct shorts to ground.
The larger three phase motors mostly used in industry are protected by motor starters or controllers. This is usually accomplished by overload heaters which when overloaded will shut down all three phases. These heaters usually have lead filled shafts that when heated will melt and trip overloads. The overloads in turn will open the voltage to a coil which is keeping contacts closed by magnetizing the coil. A lot of these starters are being replaced by smart controllers which do all this electronically.
The power that feeds your home and factorys and other structures which use electricity begins with large power generators which are run from mostly large water falls and partially from nuclear power plants. This power is run through main and sub stations. The wires that carry this power can be kept fairly small by using high voltage and low amperage. When they reach your home they are connected to transformers which step down the voltage and give you more amperage. In most states it is the utilitys resposibiity to bring these wires to the service loop above the meter or disconnect on your home. The voltage that is brought into most homes today is single phase 240 volts which is center tapped at the transformer to give a neutral wire for 120 volts. This wire is brought into the top of your main panel. Than electricity is distributed through circuit breakers and fuses into branch circuits to feed your electrical devices.