# CAN SUMBODY HELP EM WID DIS PROBLEMss ZZZZZZz life or death situation!

1. Jan 10, 2005

### hype_chicky

1. The electrostatic force between two point charges is 1.2 x 10^-2 N. If the distance between them is doubled, the charge of one of the point’s doubles and the charge of the other point triples. What will be the force between them?
Kq1q2/r2

3. Two charges one charge +1.5 x 10^-2 C and the other charge -2.7 x 10^-5 C, are 20.0 cm apart. The positive charge is to the left of the negative charge.
a) Draw a diagram showing the point charges and label a point Y that is 5.0 cm away from the positive charge, on the line connect the charge (field lines need not be drawn.)
b) b.) Calculate the electric field at point Y

4. Find the final speed of an electron, starting form rest, passing between two parallel plates with a potential difference of 4.5 x 10^3 V. An electron mass of 9.1 x 10^-31 kg and a charge of 1.6 x 10^-19 C. Is this answer valid? Explain.

(yes it is valid because …)

5. Two parallel plates labeled W and X are separated by 5.2 cm. The electric potential between the plates is 150V. An electron starts from rest at time tW and reaches plate X at time tX. The electron continues through the opening and reaches point P at time tp (remember e = - 1.6 X 10^-19 C and the mass of an electron is 9.1 X 10^-31 kg)

a.) Sketch the speed-time graph on the axes below
b.) Determine the kinetic energy of the electron as it arrives at plate X

2. Jan 10, 2005

3. Jan 10, 2005

### hype_chicky

what sticky?

4. Jan 10, 2005

### vincentchan

which part do you NOT understand? it seems like you have the formulas for problem 1 already.... and the rest seems like a graphing problem, just follow the question, draw the graph, and apply the formulas in your text books, that's it, do you not understand how to graph or what? Let me know what you do not understand so that i can help you.....I don't wanna just hand out the answer

5. Jan 10, 2005

### vincentchan

6. Jan 10, 2005

### hype_chicky

For question 1:
K doesn't change, q1 is doubled so it's 2, q2 is tripled, so it's now 3 and r doubled, so the new force is 2*3*k/4 which is 3/2 of the old one. So just multiply the 1.2 x 10^-2 N by 3/2. And the answer wud be 1.8x10^-2N

FOR QUESTION 3
For 3. I have no idea how to do the diagram I am like really lost L, for the electric field at point Y I have E on point y = kq1/r^2 = (9.0 x 10^9 Nm^2/c^2) (1.5 X10^-2)/ 0.05^2 and I got 5.4 X10^10N but I feel like something is missing.

7. Jan 10, 2005

### hype_chicky

sorry guys i am new..and i am not trying to get somebody to do my homework i wanna understand it myself i am just a little slow i would really appreciate it if you guys could help me, i will post my answers thus far for 4 and 5 next

8. Jan 10, 2005

### hype_chicky

FOR QUESTION 4

Q deltaV /r = ½ kmv^2

(1.6 X 10^ -19 C ) (4.5 X 10^3V) / 3,2 X10 ^-13 = ½ x 9.1 x 10^-31 X v^2
v^2 = 4.94 X 10^24
v = &.03 X 10^13

yes this is possible…but why?

9. Jan 10, 2005

### hype_chicky

FOR QUESTION 5

FE = Ek
Q delta V/r = 1/3 mv^2

1.6 x 10^-19 C (150V) / 0.05a = ½ (9.1X10^-31 kg) (v^2)
V= 2.9 X 10^-23 m/s

10. Jan 10, 2005

### vincentchan

i am not going to check your arithmatic...coz i don't have a calculater... sorry about that.. the first problem you did is totally correct, the second one is wrong, how? the positive charge and negative charge are separate 20cm, point Y is located between two charges and 5cm away from the negative one, how far away is it from the positive one? the E field @ Y is just the sum of the E field of two charges.... the last one, assume your v is right(i am not checking your arithimatic) , how is it compare with speed of light c= 3*10^8 m/s, notice nothing can travel faster than light

11. Jan 10, 2005

### vincentchan

Q delta V/r = 1/3 mv^2
should be one half @right hand side instead of one 3rd, but i think it is a typo...

1.6 x 10^-19 C (150V) / 0.05a = ½ (9.1X10^-31 kg) (v^2)
this is correct...if you change the a to 2......

V= 2.9 X 10^-23 m/s
WRONG, obviously the v is too small.... some arithmatic mistake, do it carefully
and, isn't that the problem asking you the kinetics energy... why are you doing speed here?

12. Jan 10, 2005

### hype_chicky

oh thanx buddy ur awesome!

13. Jan 10, 2005

### vincentchan

oh, sorry, one more mistake you have made..
because i didn't check your number, i missed this one.....

Q deltaV /r = ½ kmv^2===>>wrong
the right one should be
qV=1/2mv^2
where did you get 3.2*10^-13 in
(1.6 X 10^ -19 C ) (4.5 X 10^3V) / 3,2 X10 ^-13 = ½ x 9.1 x 10^-31 X v^2

14. Jan 10, 2005

### hype_chicky

4. Find the final speed of an electron, starting form rest, passing between two parallel plates with a potential difference of 4.5 x 10^3 V. An electron mass of 9.1 x 10^-31 kg and a charge of 1.6 x 10^-19 C. Is this answer valid? Explain.

wut is the value fo r in this question?

15. Jan 10, 2005

### vincentchan

you dont even need r
energy is qV, no matter what r is, the energy will be the same

16. Jan 10, 2005

### hype_chicky

i dunt understand

17. Jan 10, 2005

### hype_chicky

but for question 3 i got - 5.39903 x 10^14 n/c
does dat seem right?

18. Jan 10, 2005

### hype_chicky

oh man am so confused

19. Jan 10, 2005