- #1
erisedk
- 374
- 7
Homework Statement
Charge Q is uniformly distributed on the rim of a thin insulating disc of mass m, radius R, which is initially at rest and placed on a smooth horizontal surface. What will be the angular velocity of the disk if a magnetic field B, perpendicular to the plane of the disk, is switched on?
Homework Equations
∫Eind.dl = -dφ/dt
The Attempt at a Solution
2πR. Eind = (B.πR2)/Δt
E=BR/2Δt
QE/m = QBR/2Δtm
QE/m is acceleration.
a = Rα (α is the angular acceleration)
α = QB/2Δtm
ω = ω0 + αΔt
ω = QB/2Δtm × Δt
= QB/2m
However, the answer is QB/m.
So, I tried another approach.
2πR. Eind = (B.πR2)/Δt
E=BR/2Δt
F=QE= QBR/2Δt
τ=Iα
QBR2/2Δt = (mR2)/2 × α
From here,
α = QB/Δtm
and
ω = QB/Δtm × Δt
= QB/m
And this is the correct answer.
What is the problem with the first answer?