Physics Lab #13

 

Coulomb’s Law

Theory

Coulomb’s Law does for the force of electrical attraction and repulsion what Newton’s Law of Gravitation does for the force of gravitational attraction.  The similarities between the laws are apparent (see below) but there are important differences:

1. The electrical force only acts on particles that carry a charge.
2. Charges (unlike mass) can be positive or negative.
3. The electrical force can be attractive (between opposite charges) or repulsive (between like charges).

 

In form the law is just like Newton’s:

 

                                   F_c = kq₁q₂/r²

 

Where F_c is the size of the electrical force (coulombic force) in Newtons, q₁ and q₂ are the charges involved in units called coulombs ( C ), r is the distance between the charges in m and k is the coulombic constant (analogous to G) equal to

8.99 x 10⁹Nm²/C².  It is surprising that Coulomb derived this almost 100 years after Newton given how similar it is.

 

Atoms are in general electrically neutral but, for example, when you rub a balloon against your hair electrons get knocked off some of the atoms and get transferred between the balloon and your hair.  This gives the balloon a net charge.

 

 

 

 

Procedure

1. Take a pair of similar balloons and mass them on the electronic balance to confirm that they have about the same mass (to two sig figs).  Blow them up pretty full and about the same amount.  Weigh them again and note the mass.
2. Take two to three meters of thread and loop it through a ceiling tie so equal lengths hang down.   Tie a balloon on each end.  Measure the length from the top of the thread to the middle of each balloon (ensure it is about the same for each balloon) and note the measurement.
3. Take a balloon in each hand and rub against your hair in a symmetrical fashion.  The same charge (about) should build up on each balloon.
4.  The balloons will repel each other and you will measure the distance between them.
5. Draw the picture of what you see and label it with the lengths and distances measured.
6. Draw a free body diagram for the scenario and use similar triangles to get the magnitude of the coulombic force between the balloons.
7. Use Coulomb’s Law, the force calculated and the distance between balloons to get the charge on each balloon.
8. Given that the charge on an electron is 1.6 x 10^-16 C calculate how many electrons were transferred in the rubbing of the balloon.
9. From the mass of the balloon you can get an idea of how many electrons there are in the balloon in total before rubbing by diving the mass of the balloon by the mass of each electron-proton pair which is 1.66 x 10^-27 kg.
10. Calculate the fraction of the electrons that were transferred.