Conductors and Insulators.

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 through a conductor such as a wire from high to low . In direct current, the flow always in the same direction, which distinguishes it from (AC). DC is commonly found in many low- applications, especially where these are powered by , which can only produce DC. Most automotive applications use DC although the generator is an AC device which uses a to produce DC. Most circuits require a DC power supply. Direct current installations usually have different types of sockets, switches, and fixtures, mostly due to the very low voltages used, from those suitable for alternating current. It is usually extremely important with a direct current appliance to not reverse polarity unless the device has a bridge to correct for this. (Most battery-powered devices don't.)

Types of direct current

Measurement of current

Current in a circuit is measured by an ammeter. An ideal ammeter has zero resistance so that the circuit in which it has been placed is not disturbed. An ideal ammeter is a short circuit. When we are measuring current we use units to tell us what we are measuring. The units used are OHMS (Ω)

Potential difference

In the sciences, potential difference is the difference of a between two points. It can be described as the , where is the . The product of the flux and the potential difference is the , which is the rate of change of the conserved quantity, e.g . A potential difference is generated between the ends of an that moves perpendicular to a . There are many types of potential difference:

In electrical engineering the potential difference is the voltage.
In fluid systems the potential difference is the
in thermal systems ...

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Potential difference

In electrical engineering the potential difference is the voltage.
In fluid systems the potential difference is the
in thermal systems the potential difference is the .

Voltmeter is a device used for measurement of potential difference between two points. Voltmeter is basically a high resistance meter connected in series and is connected across two points between which p.d. are to be measured. The deflection produced in voltmeter is the measure of p.d. between two points.

Resistance

Resistance restricts the flow of charge, so a resistance makes the current smaller. Connection of wires have a very low resistance; the wires used in lamp filaments have a higher resistance. Bt they are all conductors. Insulators like the plastic around the wires in a cable have a much higher resistance – an infinate resistance.Resistance is measured in Ohms the equation is

P.D. across a = Current through x A Conductor Conductor Constant

& V = I X R 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 of 60 (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 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.C need batteries.

MAGNETISM.

The space around a magnet where it produces a magnetic force is called a magnetic field. The force around a bar magnet can be detected and shown to vary in direction. Lines of force are related to magnetism. A line of force is how we draw which way the lines of magnetism are.

Diagram

The laws of magnetic poles are: If the N pole of a magnet is brought near the N pole of another magnet repulsion occurs. Two S poles also repel. By contrast, N and S poles always attract. The laws of magnetic poles summarises these facts and states:Like poles repel, unlike poles attract.

The basic idea behind an electromagnet is extremely simple: By running electric current through a wire, you can create a magnetic field. By using this simple principle, you can create all sorts of things, including , solenoids, read/write heads for and , , and so on.

An electromagnet starts with a (or some other source of power) and a wire. What a battery produces is electrons. The figure below shows the shape of the magnetic field around the wire. In this figure, imagine that you have cut the wire and are looking at it end-on. The green circle in the figure is the cross-section of the wire itself. A circular magnetic field develops around the wire, as shown by the circular lines in the illustration below. The field weakens as you move away from the wire (so the lines are farther apart as they get farther from the wire). You can see that the field is perpendicular to the wire and that the field's direction depends on which direction the current is flowing in the wire. The compass needle aligns itself with this field (perpendicular to the wire). These fields can be made stronger by increasing the amount of electricity flowing through the wires, also by increasing the amount of coils you have.

Electromagnetic radiation

Above is a picture of a x- ray machine being used on a patient.

X-rays were first discovered in 1895 when, during experiments with electric currents passed through a vacuum tube, Wilhelm Conrad Röntgen noted that a nearby fluorescent screen glowed when the current was being passed. When the current was switched off the screen stopped glowing. He recognized this effect to previously unknown rays which, X being the symbol for an unknown quantity, he called X-rays .A special type of photographic film is used to record X-ray pictures. The X-rays are converted into light and the more energy that has reached the recording system, the darker that area of the film will be. This is why the bones on an X-ray image appear whiter (less energy passes through) than the lungs (more energy passes through). Within two months of their discovery X-rays were being used both in Europe and North America, not just to take pictures of the internal organs of living people but also to treat a wide variety of diseases. The energy that does not pass through the body is deposited within it and it is this energy that causes the effects of radiation. Simple X-ray image can be extremely informative. For example it can Show whether or not a bone is broken or whether or not there is a shadow on the lung.
Special X-ray techniques can also be used to investigate other problems with the soft tissues of the body. By injecting special dye into arteries and/or veins the blood vessels can be made visible. By swallowing special dye the gullet and stomach can be examined. is a further development of the use of X-rays. By using a sophisticated scanner connected to a computer, it is possible to construct a series of pictures that look at the living body in cross-section.

Other diagnostic and therapeutic techniques

There are many situations in which ultrasound are performed. Perhaps you are pregnant, and your obstetrician wants you to have an ultrasound to check on the developing baby or determine the due date. Maybe you are having problems with circulation in a limb or your , 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 . In ultrasound, the following events happen:

The ultrasound machine transmits high-frequency (1 to 5 megahertz) sound pulses into your body using a probe.
The sound waves travel into your body and hit a boundary between tissues (e.g. between fluid and soft tissue, soft tissue and bone).
Some of the sound waves get back to the probe, while some travel on further until they reach another boundary and get reflected.
The reflected waves are picked up by the probe and relayed to the machine.
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).
The machine displays the distances and intensities of the echoes on the screen.