1. You can verify this by moving test puff towards and away from the stationary puff several times. At each distance, you should get the same displacement of the test puff if the charges are same when you are moving towards and away from the test puff.
2. To plot the graph, you would need more data. The smallest value of r is 1 cm. That is the distance where you would get maximum value of x. But it seems that you have missed out measuring the other values of x (at r = 4cm, r = 3 cm) as well along with the value of x at r =1 cm.
| Sr No | r | 1/r2 | x |
| 1 | 5 cm | 0.04 | 0.5 cm |
| 2 | 4 cm | 0.0625 | ?? |
| 3 | 3 cm | 0.111 | ?? |
| 4 | 2 cm | 0.25 | 0.7 cm |
| 5 | 1 cm | 1 | ?? |
You can complete the above table and plot the graph of x vs 1/r2 in excel. This graph should pass through the origin and you can find the slope of this line.
3. We have,
Since the values of k, Q1, Q2, L, m and g are constant (we can call this term as m), on the right hand side, we have variable as (1/r2).
Comparing this equation to the equation for a straight line, we can write
4. For the straight line equation, y = mx, m is the slope of the line. From the question (3), we can write the value of m, which is LkQ1Q2/mg. So we can verify that the equation for the slope m is LkQ1Q2/mg
5. As briefed in question (2), we can find the slope (m) from the graph and use the value for the calculation as below:
Assuming that the charges are same on both puffs, we can write.
We know the value of L; value of k is 8.99*109 N.m2/C2; value of m would be addition mass of puff and mass of needle; also, value of L is measured by you (please convert it in SI unit i.e. from cm to m); finally, value of g is 9.81 m/s2.
6. To find the charge on each puff, you will have to put the values in the equation we got in question (5) and solve for Q. You will get the value of Q in C.
7. To find the number of electrons, you will have to divide the charge Q you found in question (6) by the charge on a single electron. Charge on each electron is 1.6*10-19 C.
8. Does this number seem surprising large or small to you? Why or why not?
Since electron is very small, its mass is 9.1*10-31 kg, the number you would find will be very large as very large number of electrons would contribute to this charge Q.