Question

1)What potential difference is needed to stop an electron that has an initial velocity v=7.0×105m/sv=7.0×105m/s?

2)How strong is the electric field between the plates of a 0.80-μFμF air-gap capacitor if they are 3.0 mmmm apart and each has a charge of 72 μCμC ?

3)A 3700-pFpF air-gap capacitor is connected to a 30-V battery.If a piece of mica (K= 7) is placed between the plates, how much charge will flow from the battery?

Answer 1

1) Using work - energy theorem

W=q = K₂-K₁

q=0-mv²/2

Here m =9.1*10^-31 kg , v=7*10⁵ m/s q=1.6*10^-19 C

1.6*10^-19*=-9.1*10^-31*(7*10⁵)²/2

=-222.95‬*10^-21/1.6*10^-19

=-139.34*10^-2

=-1.3934 V

In magnitude and in three significant figure =1.39 V answer

2) We know that Q=CV So V=Q/C =72uC/0.8uF =90 V

So electric field between the plates E=V/d =90/(3*10^-3) =30*10³ V/m Answer

3) We know that Q=CV =KCV

Q=3700*10^-12*30

Q=111*10^-9 C

Q=111nC  

When a piece of mica is placed between the plates

Then Store Charge Q'=C'V =KCV

Q'=7*3700*10^-12*30

Q'=777nC

So the charge flow from the battery Q=777-111=666nC Answer