Remember Ohms law from last time. Here are two circuits below. If you know Ohms law you should be able to make the light come on in one only one try as the amperage, I, is now specified.
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 .
These ideas were first suggested by Michael Faraday and Farady's law states that the voltage is proportional to the rate of change of the magnetic flux. In the diagrams below, this magnetic flux is represented by blue vectors going from North to South.
Diagram of a simple electric generator:
In this position the magnetic flux is essentially zero but its rate of change is large. Hence, the voltage is at a maximum here and current flow is also at a maximum.
In this position the Voltage is now zero and their is no current flow. The magnetic flux 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 at 1200 Volts we have only 69.4 watts of energy loss and a 99% enery efficient delivery system.
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.
Internet Resource Alert
How Electricity Works
Previous Lecture Next Lecture Course Page