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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D.C motor

D.C motor

When a current-carrying coil is in a magnetic field, it experiences a turning effect. A DC motor runs on a direct current. The coil is made of insulated copper wire. It is free to rotate between the poles of the magnet. The commutator, or split-ring, is fixed to the coil and rotates with it. When the coil overshoots the vertical, the commutator changes the direction of the current through it, so the forces change direction and keep the coil turning. The brushes are two contacts which rub against the commutator and keep the coil connected to the battery. They are usually made of carbon. The maximum turning effect if when the coil is horizontal. There is no force when the coil is vertical (but luckily it always overshoots this position).


The turning effect can be increased by:
-increasing the current
-using a stronger magnet
-increasing the number of coils (increases the length of coil) -increasing the area of the coil (increases the length of coil) •Reversing the rotation can be done by:
-reversing the battery
-reversing the poles
This equation isn’t needed but is useful for remembering the ways to increase the turning effect:
Force exerted on wire = magnetic field strength × current × length of wire

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