Electricity is an apparent force in nature that exists whenever there is a net electrical charge between any two objects.
Basics of Electrostatics:
Properties of Electricity:
VOLTAGE: Potential difference between a negatively charged object and a positively charged one (like two terminals on a battery). Potential difference is measured in units of Volts ( V )which represents the work done per unit charge to move electrons between the positive and negative terminals. If a potential difference exists, then energy can be extracted.
Imagine that you have two opposite charges that you want to separate. It takes work to separate the charge and thus the separated charges store energy. The amount of stored energy is given by:
E = qV where V is the voltage or electric potential of some system.
The units of voltage or Volts: 1 Volt = 1 Joule/Coulomb
If the separated charges get back together, work/energy can be extracted from the system. If there is some pathway for the charges to flow then we get a current. Current is denoted by I and is in units of amperes or amps 1 Ampere = 1 coulomb/second
RESISTANCE: Property of material that helps prevent the flow of electrons in it. Metals are good conductors due to low resistance. Wood is a poor conductor due to high resistance. Resistance, R , is measured in ohms and depends upon both the type of material and its size. Long wires have more resistance than short wires; thin wires have more resistance than thick wires. R is also temperature dependent.
Is there a relation between I, V, and R ? Let's do an experiment:
Experimental results then lead to Ohms law:
This is a linear relation. If you double the voltage (V) then for the same value of R you get twice the current. If you want to keep the current the same value after doubling V, you would have to double the resistance (R).
Your electricity bill essentially measures the amount of current that you use but you use this current as Power .
So the toaster has a power of 120x8 = 960 Watts.
Energy = Power * Time (and its energy --> kilowatt hours that you pay for - a 100 watt light bulb left on 10 hours = 1 kilowatt hour. )
If you leave your toaster on for one hour, than that would also be approximately 1 KWH (960 watt-hours if you want to nit pick).
1000 Watt-hours = 1 Kilowatt hour (KWH); A KWH will be our basic unit of energy in this class. You purchase KWHs from the electric utility whenever you use power in your home.
The Discovery of Electricy and Magnetism and the Generation of Electricity.
In the early 19th century the following similarity between two charged particles and two magnets was observed:
In 1820 Oersted did this experiment:
and discovered that an electric current creates a magnetic field
Similarly, a coil of wire with a current passing through it generates a magnetic field. This is known as an electromagnet or solenoid .
So now we know that a current can create a magnetic field. If a magnetic field can create a current then we have a means of generating electricity. Experiments showed that a magnetic just sitting next to a wire produced no current flow through that wire. However, if the magnet is moving a current is induced in the wire. The faster the magnet moves, the greater the induced current.
This is the principal behind simple electric generators in which a wire loop is rotated between to stationary magnetics. This produces a continuously varying voltage which in turn produces an alternating current .
Diagram of a simple electric generator:
In this position there is no current flow but their is a large potential difference (a large voltage)
In this position the Voltage is now zero and the current flow is at a maximum
Popular Methods of Turning the Crank:
Why do transmission lines carry such high voltages?
Consider the following:
Current = Power/Voltage; If we increase V by a factor of 10, then I lowers by a factor of 10 (at constant power) and the power dissipated as heat lowers by a factor of 102.
Hence if we increase 120 Volts to 1200 Volts we have only 69.4 watts of energy loss and a 99% energy efficient delivery system This is why high voltage (typically 760 thousand Volts or 760 kiloVolts) transmission lines are required to delivery electricity from central generating sources (e.g. a hydroelectric dam) to consumers/grids hundreds of miles away.
How to change the voltage: Use a Transformer
A transformer uses alternating current in one coil to induce alternating current in another. The induced voltage is given by: Vout = Vin x N2/N1 where N1 = Number of coils in the Primary and N2= Number of coils in the secondary. When N2 is less than N1, we reduce Vout. This is why there are transformers on power lines to step the voltage down to 120 Volts by the time it reaches your house.
Energy conservation tells us that Power In = Power Out
And that's the way the world works.