February 05, 2011

Simple Basic Electronics (part 3)

(Elektronika Dasar)

Electrical Current

The conductor - insulator demonstration illustrates two other important points:

  1. An immobile static charge flows through a conductor as an electrical current. It then resumes its static state on the Electroscope foil leaves.
  2. Electrical current flows from a region of high charge or potential to a region of low potential.

The Magnetic Connection
A current flowing through a wire creates a magnetic field around the wire. You cannot see the field, but you can observe its effect. Orient a compass so it needle points to the north (N) mark. Place a copper wire over and parallel to the needle. Then connect a flashlight cell across the wire and the needle will move away from its north-south orientation. (Leave the wire connected for only an instant to prevent the cell from overheating!)

Measuring Current Electricity
The physical (or mechanical) motion of a magnetic compass needle in a magnetic field provides a convenient way to measure the quantity of current flowing in a wire. This is the basic of the moving coil current meter used in the analog multimeter. To provide high sensitivity, the wire is warped as a coil.

Direct Current Electricity
An electrical current can flow in either of two directions through a conductor. If it flow in only one direction, whether steadily  or in pulses, it's called direct current (DC). It's important to be able to specify the quantity and power of a direct current. Here are the key terms:

Current (I)
Current is the quantity of electrons passing a given point. The unit of current is the Ampere. One ampere is 6,280,000,000,000,000,000 (6.28 x 10^18) electrons passing a point in one second.

Voltage (V or E)
Voltage is electrical pressure or force. Voltage is sometimes referred to as potential. Voltage Drop is the difference in voltage between the two ends of a conductor through which current is flowing. If we compare current to water flowing through a pipe, then voltage is the water pressure.

Power (P)
The work performed by an electrical current is called power. The unit of power is the Watt. The power of a direct current is its voltage timesits current.

Resistance (R)
Conductor are not perfect. They resist to some degree the flow of current. The unit of resistance is the Ohm. A potential difference of one volt will force a current of one ampere through a resistance of one ohm. The resistance of a conductor is its voltage drop divided by the current flowing through the conductor.

Mr. Ohm's Law
Given any two of the above, you can find the other two using these formulas known as Ohm's Law:
V = I x R
I = V/R
R = V/I
P = V x I  or  I^2 x R

Summing Up: This is the "water analogy":

Alternating Current Electricity
A curent flowing through a conductor establishes a magnetic field around the conductor. This effect works both ways so that a current will flow in a field. You can easily demonstrate electromagnetic current generation with a coil of wire and a small magnet. Connect the leads of the coil to a meter designed to sense microampere. Insert a steel nail through the coil and stroke the magnet back and forth across the coil. The meter will indicate a few microampere each stroke.

When the magnet is stroked in one direction along the coil, electrons in the wire are moved in one direction and the direct current is produced. On away from the coil, the direction of current flow is reversed. Therefore, if the magnet is stroked back and forth along the coil, a current which alternates in direction or polarity is produced. It's called an alternating current (AC). Alternating current is ussually produced by rotating a coil in a magnetic field.

Sine Wave Measurement
AC voltage is usually specified at a value equal to the DC voltage capable of doing the same work. For a sine wave this value is 0.707 times the peak voltage. It's called the RMS (root-mean-square) voltage. The peak voltage (or current) is 1.41 times the RMS value. Household line voltage is specified according to its RMS value. Therefore, a household voltage of 120-volts corresponds to a peak voltage of 120 x 1.41 or 169.2-volts.

Why AC is used? AC is better suited than DC for transmission through long distance power lines. A wire carrying AC will induce a current in a nearby wire. This is the principle behind the Transformer.

to be continued..  Simple Basic Electronics (part 4)

Simple Basic Electronics (part 1)   Simple Basic Electronics (part 2)

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