# Physics E&M (2)

An insulated object A is charged by induction and earthing so that it gets a net charge of -3x 10^-6 C. Another identical conduction object B is charged by sharing so that it gets a net charge of +2.0 x 10^-6 C.

a) State the difference between the two charging methods.

b) If the 2 charged objects are brought into contact, how many electrons are transferred between them?

ans.

b) 1.5625 x 10^13

### 2 個解答

• 最愛解答

a)

The first method is that use induction to induce some charges, for example positive charges, on one side of the object A and, since it is earthing, only negative charges are free to move( positive charges are induced on one side which is fixed by induction), the electron on the surface of object A will then leave object A to the ground. Therefore, since there is more positive charges than negative charges on object A, A is positively charged.

The second method is only by sharing, which means object B is in contact with a charged object and net charges are transferred to object B directly.

The main difference is that object A is charged without any direct contact with any charged object, it is charged by leaving electron to the ground(if it is positively charged) or by gaining electron from the ground(if it is negatively charged)

b)

Since two charged objects are identical, when they are in contact with each other, charges will be balanced.

The final charge on each of the object

= (-3+2)/2 x 10^-6

= -0.5 x 10^-6C

Therefore, the change in charge on each of the object

= -3x10-6 - (-0.5x10^-6)

=-2.5x10^-6C

Therefore, the number of electrons transferred

= -2.5x10^-6 / -1.602x10^-19 (where -1.602x10^-19 is the charge of one electron)

=1.56x10^13

The answer is slight different from yours because your ans take the charge of electron with more accurate number.

資料來源： Myself
• (a) The charges on the object using the "induction and earthing method" are of opposite polarity (i.e. opposite sign) to that of the charging body.

On the other hand, the polarity (sign) of charges using the "sharing" method is the same as that of the charging body.

(b) There is an excess of 3x10^-6 C of electrons in object A, but lacking of 2x10^-6 C of electrons in object B. Hence, there is an overall excess of (3x10^-6) - (2x10^-6) = 1x10^-6 C of electrons in the system.Because the two objects are identical,when they are brought into contact, the number of excess electrons in each object must be the same.

Hence, the excess of 1x10^-6 C of electrons would be shared equally by the two objects, each of which would possess 0.5x10^-6 C of excess electrons.

Thus, object A has lost (3x10^-6 - 0.5x10^-6) C = 2.5x10^-6 C of electrons, whereas object B has gained (2x10^-6 + 0.5x10^-6) C = 2.5 x 10^-6 C of electrons.

Then the number of electrons transferred from objects A to B

= 2.5x10^-6/1.6x10^-19 = 1.56x10^13