# Homework Help: Electric Potaential of two spheres

1. Feb 9, 2012

### crybllrd

1. The problem statement, all variables and given/known data

Electric charge can accumulate on an airplane in flight. You may have observed needle-shaped metal extensions on the wing tips and tail of an airplane. Their purpose is to allow charge to leak off before much of it accumulates. The electric field around the needle is much larger than the field around the body of the airplane and, can become large enough to produce dielectric breakdown of the air, discharging the airplane. To model this process, assume that two charged spherical conductors are connected by a long conducting wire and a charge of 9.00 µC is placed on the combination. One sphere, representing the body of the airplane, has a radius of 6.00 cm, and the other, representing the tip of the needle, has a radius of 2.00 cm.
(a) What is the electric potential (V) of each sphere?
r = 6.00 cm
r = 2.00cm

(b) What is the electric field (V/m, direction) at the surface of each sphere?
r = 6.00 cm
r = 2.00cm

2. Relevant equations

r1 = 6e-2
r2 = 2e-2

3. The attempt at a solution
q1/q2=r1/r2
q1=3q2

q1+q2=9e-6

3q2+q2=9e-6
4q2=9e-6
q2=2.25e-6

q1+2.25e-6=9e-6
q1=6.75e-6

v1=kq1/r1

v1=101

When I submit it, it tells me that "Your response differs significantly from the correct answer. Rework your solution from the beginning and check each step carefully."

Any help?

2. Feb 9, 2012

### ehild

The potential is not a number, it has some unit. In what units is the potential 101? Check the magnitude of the data and give the potential in volts.

ehild

3. Feb 9, 2012

### crybllrd

v1=kq1/r1

v1=8.99e9 Nm2/C2*1.95e-5C/6e-2m

v1=101 Nm/C

Nm/C is a V

4. Feb 9, 2012

### ehild

q1=6.75 e-6C, is not it?

Check the magnitudes: You multiply 109 with 10-6; it is 103, and divide by 10-2: you get 105.

ehild

5. Feb 9, 2012

### crybllrd

101e5 tells me that I am off by a multiple of ten.

I am still confused though- in my calculator I put in
(K*6.75*10e-6)/6e-2
(I have K stored as the ke constant)
and get 101. It sounds like you're telling me that my calculator cannot handle scientific notation, and I need to tag on the " e5 " to my calculation of 101. Why is it not giving me the right answer? Or am I misunderstanding this?

6. Feb 9, 2012

### crybllrd

Never mind, I figured it out.
Thanks a lot!