Answer:
The
electrical resistivity of a material is defined as the resistance of a
conductor made of that material of unit length and unit cross-sectional area. Its
Sl unit is Ohm metre
Consider an electric circuit consisting of a cell, an ammeter, a nichrome
wire of length L(marked 1 to 4) and a plug key as shown below
At point plug the key and note the current ammeter. Replace the nichrome
wire by another nichrome wire of same thickness but twice the length that is 2l
at point 2. Again note the reading. Now, replace the wire by a thicker nichrome
wire of same length (marked 3).
A thicker wire has a larger cross-sectional area.
Again note down the current through the circuit. Replace
nichrome wire with copper wire of same length and same area of cross-section at
point 4, Note the value of current. Notice the difference in the current in all
cases. When the length of the wire is doubled, then the ammeter reading
decreases to half its previous value i.e., current through the wire is halved.
Since, resistance of the wire , then R is doubled
which implies R x L.
When the nichrome wire is replaced by a thicker one of
same material and length, the current in the wire increases which means that
the resistance of the thicker wire (3) is less than that of the thinner wire
(1). This implies .
When the nichrome wire is replaced by a copper wire (4) of
the same length and same cross-sectional area, then the current recorded by the
ammeter is more. This means that the resistance of copper wire is less than
that of the nichrome wire of the same dimensions i.e. ,the resistance of the
wire depends on the nature of its material.
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