An electromagnet is wound with 800 turns. When the coil is excited by 230V, the current flowing through it is 2.5A and the magnetic flux linking the coil is 0.09Wb. Calculate the self- inductance of the coil
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Answer:
Measurement is the act, or the result, of a quantitative comparison between a given
quantity and a quantity of the same kind chosen as a unit. The result of the measurement is
expressed by a pointer deflection over a predefined scale or a number representing the
ratio between the unknown quantity and the standard. A standard is defined as the physical
personification of the unit of measurement or its submultiple or multiple values. The
device or instrument used for comparing the unknown quantity with the unit of
measurement or a standard quantity is called a measuring instrument. The value of the
unknown quantity can be measured by direct or indirect methods. In direct measurement
methods, the unknown quantity is measured directly instead of comparing it with a
standard. Examples of direct measurement are current by ammeter, voltage by voltmeter,
resistance by ohmmeter, power by wattmeter, etc. In indirect measurement methods, the
value of the unknown quantity is determined by measuring the functionally related
quantity and calculating the desired quantity rather than measuring it directly. Suppose the
resistance as (R) of a conductor can be measured by measuring the voltage drop across the
conductor and dividing the voltage (V) by the current (I) through the conductors, by
Ohm’s
1.2 FUNDAMENTAL AND DERIVED UNITS
At the time of measuring a physical quantity, we must express the magnitude of that
quantity in terms of a unit and a numerical multiplier, i.e.,
Magnitude of a physical quantity = (Numerical ratio) × (Unit)
The numerical ratio is the number of times the unit occurs in any given amount of the
same quantity and, therefore, is called the number of measures. The numerical ratio may
be called numerical multiplier. However, in measurements, we are concerned with a large
number of quantities which are related to each other, through established physical
equations, and therefore the choice of size of units of these quantities cannot be done
arbitrarily and independently. In this way, we can avoid the use of awkward numerical
constants when we express a quantity of one kind which has been derived from
measurement of another quantity.
In science and engineering, two kinds of units are used:
• Fundamental units