الفيزياء العامة الجزء الثاني#موقع الفيزياء.كوم

Current & Resistance
Example 7.2
Calculate the resistance of a piece of aluminum that is 20cm long and
has a cross-sectional area of 10 -4 m 2 . What is the resistance of a piece of
glass with the same dimensions? ρ Al =2.82×10 -8 Ω.m, ρ glass =10 10 Ω.m.
Solution
The resistance of aluminum
R Al
l
= ρ = 2.82×
10
A
⎛ 0.1 ⎞
⎜ ⎟ = 2.82×
10
−4
⎝10
⎠
−8
−5
Ω
The resistance of glass
l 10 ⎛ 0.1 ⎞ 13
R glass
= ρ = 10 ⎜ ⎟ = 10 Ω
−4
A ⎝10
⎠
Notice that the resistance of aluminum is much smaller than glass.
Example 7.3
A 0.90V potential difference is maintained across a 1.5m length of
tungsten wire that has a cross-sectional area of 0.60mm 2 . What is the
current in the wire?
Solution
From Ohm’s law
V
I = where
R
therefore,
l
R = ρ
A
−7
VA (0.90)(6.0 × 10 )
I = =
= 6. 43A
−8
ρl (5.6 × 10 )(1.5)
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