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# Conductors and Insulators.

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Introduction

### Conductors and Insulators

Not all substances are good conductors of electricity. As a general rule, metals are good conductors whereas non metals are poor conductors. The poorest of conductors are commonly called "insulators," or "non conductors." Aluminium, copper, gold, iron, mercury, nickel, platinum, and silver are examples of good conductors. Amber, glass, mica, paper, porcelain, rubber, silk, and sulphur are all non conductors. The difference between a conductor and an insulator is that in a conductor, there are free electrons, whereas in an insulator, all of the electrons are tightly bound to their respective atoms. In an uncharged body, there are an equal number of positive and negative charges. In metals, a few of the electrons are free to move from atom to atom, so that when a negatively charged rod is brought to the end of the conductor, it repels nearby free electrons in the conductor, causing them to move. They in turn repel free electrons in front of them, giving rise to a flow of electrons all along the conductor.

Direct current

Direct current (DC) is the continuous flow of electricity through a conductor such as a wire from high to low potential. In direct current, the electric charges flow always in the same direction, which distinguishes it from alternating current (AC). DC is commonly found in many low-voltage applications, especially where these are powered by batteries, which can only produce DC.

Middle

The constant R is the resistane of the conductor and the equation is knowen a OHMS law. We can also say that the P.D. across the conductor is directly proporional to the current flowing through it at constant temperature. The equation can be rearranged as:

R = V                                                                                        I                                                      Where V is the P.D. across the conductor and I is the current through it . From this equation it is clar that:                1Ω = 1V                                                                                          1A

Alternating current

Electrical AC (alternating current) occurs when charge carriers in a conductor or semiconductor periodically reverse their direction of movement.  Household utility current in most countries is AC with a frequency of 60 Hz (60 complete cycles per second), although in some countries it is 50 Hz. Some AC waveforms are irregular or complicated. An Irregular AC wave is produced by audio amplifiers that deal with analogue voice signals and/or music. The voltage of an AC power source can be easily changed by means of a power transformer. This allows the voltage to be stepped up (increased) for transmission and distribution.  High-voltage transmission is more efficient than low-voltage transmission over long distances, because the loss caused by conductor resistance decreases as the voltage increases. D.C current is different to A.C because it does not reverse direction of movement it continues one way, D.C is now not used as widely as A.C and most appliances that use D.

Conclusion

blood circulation in a limb or your heart, and your doctor has requested a Doppler ultrasound to look at the blood flow. Ultrasound has been a popular medical imaging technique for many years.Ultrasound or ultrasonography is a medical imaging technique that uses high frequency sound waves and their echoes. The technique is similar to the technique used by bats, whales and dolphins, as well as SONAR used by submarines. In ultrasound, the following events happen:
1. The ultrasound machine transmits high-frequency (1 to 5 megahertz) sound pulses into your body using a probe.
2. The sound waves travel into your body and hit a boundary between tissues (e.g. between fluid and soft tissue, soft tissue and bone).
3. Some of the sound waves get reflected back to the probe, while some travel on further until they reach another boundary and get reflected.
4. The reflected waves are picked up by the probe and relayed to the machine.
5. The machine calculates the distance from the probe to the tissue or organ using the speed of sound in tissue and the time of the each echo's return (usually on the order of millionths of a second).
6. The machine displays the distances and intensities of the echoes on the screen.

This is a picture of the order of how a ultrasound works.

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