phy question

2015-03-30 1:20 am

Q1.
What is the magnitude and direction of the electric field at a point midway between
a -8.0µC and a 7.0µC charge 8.0cm apart? Assume no other charges nearby.

Q2.
a) Draw approximately the electric field lines about two point charges, Q and -3Q
which are at a distance l apart.
b) Draw approximately the electric field lines emanating from a uniformly charged
straight wire whose length l is not great. The spacing between lines near the wire
should be somewhat less than l. [Hint: Also consider points very far from the
wire.]

Q3.
The work done by an external force to move a -8.1µC charge from point A to B is
9.00x10-4
J. Ig the charge was started from rest and had 2.10x10-4
J of kinetic energy
when it reached point B, what must be the potential difference between A and B?

Q4.
a) An electric field of 640V/m is desired between two parallel plates 11.0mm apart.
How large a voltage should be applied?
b) How strong is the electric field between two parallel plates 5.0mm apart if the
potential difference between them is 110V ?

回答 (1)

天同

2015-03-30 2:31 am

✔ 最佳答案

1. Electric field at mid-point
= [k(-8x10^-6)/0.04 + k(7x10^-6)/0.04] v/m
where k is the Coulomb constant (=9x10^9 N.m^2/C^2)
The direction of the field is towards the -ve charge.

2. These are standard electric field diagrams, which you could find in physics textbook or from the web.

3. Potential energy acquired by charge when it reaches B
= (9 x10^-4 - 2.1x10^-4) J = 6.9 x 10^-4 J
Hence, potential difference between A and B
= 6.9 x 10^-4/8.1x10^-6 v = 85 v

4. (a) Voltage = 640 x (11/1000) v = 7.04 v
(b) Electric field intensity
= 110/0.005 v/m = 22 000 v

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