# Electric fields

1. Mar 4, 2008

### Lalitha

Hello Sir ,
i am new to the site and so is not familar of that but i have question and i found u online so wish u to help me with that.

Electric forces are used to accelerate small particles (like ink particles in a printers) but consider electrostatic action on macroscopic objects .Suppose that a 4 kg chicken were to be supported by a vertically upwardly directed electric feild .Since the amximum E strength in air is about 3*10^6.The net charge on each chicken would be unreasonably large since it must be least.

a) 1.3*10^-6

b)1.3*10^-5

c)-1.3*10^-5

d)7.7*10^4

none

Last edited by a moderator: Mar 4, 2008
2. Mar 4, 2008

### Staff: Mentor

In a static situation, the upward electric force must be equal and opposite the weight of the mass being suspended.

3. Mar 4, 2008

### Lalitha

thanks a lot sir i got that.I am very Thankful for your help .

4. Mar 4, 2008

### Lalitha

as we are said that the electric force is equal to mass but in oppostie direction then the force that makes the body stantionary in an electric feild is F=-mg=-4*10 (g=9.8m/sec^2=10 app),

And as E=3*10^6 and using the realtion that F=qE

we get q=F/E=-4*10/(3*10^6)=-1.3*10^-5

5. Mar 4, 2008

### Staff: Mentor

Be careful here.

The force of gravity is down so the force of the electric field must be up, so Fe = -Fg, so if Fg = -40N, the Fe = 40 N. What does that indicate about the polarity of the charge?

6. Mar 4, 2008

### Kurdt

Staff Emeritus
EDIT: beaten by Astro

7. Mar 4, 2008

### Lalitha

Sir here as we consider the force of gravity so it need to be +40N NA SIR as we have given that m=4kg,
and f=mg =4*10=40,
and Fe=-Fg=-40.

Is that not right Sir ?

8. Mar 4, 2008

### Lalitha

Hello Sir Plz let me know am i correct or going wrong there mean as fg=mg=4*10=40 and Fe=-Fg=-40N

9. Mar 4, 2008

### Staff: Mentor

Fe=-Fg= 40N

The gravitational force is downward, and by convention, that is negative.

Acceleration is a vector as is force. By convention a downward vector is negative.

The acceleration of gravity is $$\vec{a}\,=\,-9.81 \hat{j}\,m/s^2$$, where $$\hat{j}$$ is the unit vector in the vertical orientation and is taken as positive.

10. Mar 4, 2008

### Lalitha

Ok thanks a lot for that sir .I got it now .

11. Mar 7, 2008

### Lalitha

The plates are connected to a battery and then the connection is removed so the two plates are left charged and isolated. If a dielectric material such as a sheet of glass is inserted between the plates:

a. no work is done and the energy stored stays constant.
b. work is done, increasing the amount of stored energy in the capacitor.
c. work is done, decreasing the amount of energy stored in the capacitor.

12. Mar 7, 2008

### Lalitha

Plz can anybody help me with this one