- #1
Pushoam
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Homework Statement
Homework Equations
Flux rule: ξ= -dΦ/dt'
F= ma
The Attempt at a Solution
Let's assume that the loop is going down with speed 'v <0'.
Using Flux rule,
ξ = - Bva
Current will move in clockwise direction to increase Φ.
The effect of magnetic force on AC gets canceled by the effect of magnetic force on BD.
Net effect of magnetic force on the loop = Net effect of magnetic force on AB
In CD, the velocity of charged particle is
V = vx##\hat x## + v ##\hat y## : v<0
= ##\frac{- Bva}{Rλ}\hat x ## + v ##\hat y## :
where λ is charge per unit length
R is resistance across the loop = resistance across AB
Magnetic force on AB = λa(##\frac{- Bva}{Rλ}####\hat x ## + v##\hat y##)× B(- ##\hat
z##)
= λa(##\frac{- B^2 va}{Rλ}\hat y - vB(\hat x##))
##\frac{dv }{dt} \hat y = [\frac{-B^2a^2 v}{Rm} - g]\hat y##
Solving the equation gives,
v = ## \frac{-gRm}{B^2a^2} [1-e^{\frac{B^2a^2 t}{Rm}}##]
vterminal = ##- \frac{gRm}{B^2a^2 }##
For reaching 90% of vterminal,
0.9 vterminal = vterminal##[1-e^{\frac{B^2a^2 t}{Rm}}]##
solving it,
t = ##\frac {-v_{terminal} ~ln(1.9)} g##
t , here, depends on the dimensions while in question , it is said that it should be independent of dimension.
So, what is wrong here?