What is “Resistors”

As its name suggests, a resistor is an electric part that has resistance electrically.

Following is the explanation in detail.
If current “I” flows when voltage “V” is applied to an object, relationship with
resistance “R” of the object can be represented by Ohm’s law (formula 1)

(formula 1)

According to Ohm’s law, R is a proportional constant of voltage “V” and current “I”. 
That is to say, R provides an index that shows the degree of ease of current’s flow. 
R is called electric resistance (unit is Ω: ohm) or just called resistance, and
the resistance that is made into an electric part is called resistor.

If resistance R is constant (ideal resistance), voltage and current are proportional.

Taking in another viewpoint, let’s transform Ohm’s law as the below formula 2.


(formula 2)

If the voltage V in the above formula is constant, the smaller resistance R becomes,
the bigger current I becomes. On the contrary, the bigger resistance R becomes,
the smaller current I becomes.  As you can see from this formula,resistance shows
the degree of ease (difficulty) of current’s flow.  Let’s calculate for real. 


For example, if current I = 1 (unit: A) flows when source voltage V = 1 (unit: V) in the
above circuit, resistance R is calculated (defined) by Ohm’s law as the formula below.


Besides, if the resistance becomes 10Ω while the voltage remains 1V, what will the
current be?  Let’s calculate using the formula 2.


The answer is as in the above formula.  Furthermore, in other words, “If current I flow
through resistance R, voltage V is generated (at both ends of resistance R).”  
This is so fundamental but important concept in understanding electric circuits.


Resistors in electric circuits play a role to lower and divide the voltage and make the
current constant as necessary. Although we hardly ever see resistors in daily life,
they are used in a wide range of areas such as home electronics,information equipments,
cars, electric trains, industrial equipments, and aerospace.  It is said that resistors are
one of three key factors in the circuits along with capacitors and coils.


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