Measurements

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1.general physics  1.1 length and time  -i-Physical Quantity Physical Quantities A physical quantity is a property of an object that can be measured with a measuring instrument. A physical quantity is usually expressed as “a magnitude ( numerical value)  and a  physical unit or more ” Example :- Normal body temperature of a healthy human being is  37  0 C -ii-Length Measuring Length. Length is “the distance between two points in a space”. According to SI units, the length quantity is expressed in meter. There are several kinds of measuring devices used to measure quantity of length, among others, are as follows. Tools used… a-   Measuring tape. b-   Ruler c-   Vernier callipers. d-    Micrometer Measurement of length Length is measured accurately through many meth...
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A.C Generator And transformer

A.C generator


In direct current (DC), the electric charge (current) only flows in one direction. Electric charge in alternating current (AC), on the other hand, changes direction periodically. The voltage in AC circuits also periodically reverses because the current changes direction.


• The coil is made of insulated copper wire and is rotated by turning the shaft. The slip rings are fixed to the coil and rotate with it. The brushes are 2 contacts which rub against the slip rings and keep the coil connected to the outside part of the circuit, usually made of carbon. When the coil is rotated, it cuts magnetic field lines, so an EMF is generated, which makes a current flow. Each side of the coil travels upwards then downwards then upwards etc. so the current flows backwards then forwards then backwards etc. so it is an alternating current. The current is maximum when the coil is horizontal since field lines are being cut at the fastest rate and 0 when the coil is vertical, since it is cutting NO field lines. The EMF can be increased by:
-increasing the number of turns on the coil
-increasing the area of the coil
-using a stronger magnet
-rotating the coil faster 

Transformer

AC currents (only, not DC) can be increased or decreased by using a transformer. A transformer is made of a primary/input coil, a secondary/output coil and an iron core. The iron core gets magnetised by the incoming current. This magnetism then creates a current in the leaving wire. The power is the same on both sides (since we assume 100% efficiency and that all the field lines pass through both coils). You can figure out the number of coils and the voltage with:
Output voltage / Input voltage = Turns on output coil / Turns on input coil V2 /V1 =n2 /n1
Input voltage × input current = output voltage × output current V1×I1 =V2 ×I2
Power1 = Power2 
A transformer works by mutual induction. As you saw before, an EMF (and current) can be induced by moving a magnetic field. A changing magnetic field can have the same effect. Turning an electromagnet next to a coil on or off will induce a very short-lasting EMF in the coil, but leaving the electromagnet on will not, since the magnetic field is not changing. Switching the electromagnet off will induce an EMF in the opposite direction of switching it on. The EMF can be increased if the core of the electromagnet goes right though the second coil or increasing the number of coils in the second coil. An alternating current in a transformer’s primary coil creates an alternating magnetic field in the core and therefore in the second coil. The alternating magnetic field creates an alternating voltage in the second coil. 
A step-up transformer increases the voltage and a step-down transformer decreases it.
• Transformers are used to make high voltage AC currents. Since power lost in a resistor = R × I2, having a lower current will decrease the power loss. Since transmission cables are many kilometres long they have a lot of resistance, so a transformer is used to increase the voltage and decrease the current to decease power lost.
•The advantages of high-voltage transmission:
-less power lost
-thinner, light, and cheaper cables can be used since current is reduced

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