EK 1001 Physics #804, 805

[MediCynical]

Both of these questions deal with a point charge q1 a distance d from a hollow sphere with a radius r made from a conducting material.

For 804, the problem asks "Which of the following most accurately represents the force on q1 due to the charged sphere?" The answer is kq1q2/[(d+r)^2], because a hollow sphere behaves as if it were a point charge at its center.

However, problem 805 asks "When q1 is removed, the force on the sphere changes. Which of the following represents the time between the moment q1 is removed and the moment the sphere experiences any change in force?" The answer for this is d/(3E8) but I put (d+r)/(3E8) for the same reasoning as above in 804.

Can someone explain to me why the force on q1 would be in relation to the center of the sphere, but the change in force when q1 is removed is felt at the edge of the sphere and not the center?

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[sangria1986]

Both of these questions deal with a point charge q1 a distance d from a hollow sphere with a radius r made from a conducting material.

For 804, the problem asks "Which of the following most accurately represents the force on q1 due to the charged sphere?" The answer is kq1q2/[(d+r)^2], because a hollow sphere behaves as if it were a point charge at its center.

However, problem 805 asks "When q1 is removed, the force on the sphere changes. Which of the following represents the time between the moment q1 is removed and the moment the sphere experiences any change in force?" The answer for this is d/(3E8) but I put (d+r)/(3E8) for the same reasoning as above in 804.

Can someone explain to me why the force on q1 would be in relation to the center of the sphere, but the change in force when q1 is removed is felt at the edge of the sphere and not the center?

Can u please explain first how you got 3e8? Is that 3*10^8? And how did you arrive at this

 

[collegestud2013]

3 x 10^8 m/sec is the speed of light, and also the speed at which gravity and electrostatic forces propagate through space.

The force btwn q1 and the sphere considers the entire charge of the sphere as it is the net effect of all the infinitely small charge elements the sphere is composed off, and because of the symmetry the force on sphere overall would act at its center. For the sphere to experience any change in force as a result q1 being instantaneously removed the force only needs to propagate to the edge of sphere since we're only talking about any element of the sphere that will experience a change in force, not necessarily the whole sphere.

 

[MediCynical]

3 x 10^8 m/sec is the speed of light, and also the speed at which gravity and electrostatic forces propagate through space.

The force btwn q1 and the sphere considers the entire charge of the sphere as it is the net effect of all the infinitely small charge elements the sphere is composed off, and because of the symmetry the force on sphere overall would act at its center. For the sphere to experience any change in force as a result q1 being instantaneously removed the force only needs to propagate to the edge of sphere since we're only talking about any element of the sphere that will experience a change in force, not necessarily the whole sphere.

thanks

 

[Salient]

Can u please explain first how you got 3e8? Is that 3*10^8? And how did you arrive at this

3E8 is just shorthand for 3x10^8. A lot of calculators have an "EE" button that allows you to enter numbers in like this.

 

[ThirdEye]

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