Electric charge

We begin today with the study of electric charge and the forces related to it.

First observation of the day, we have an aluminum sphere which is given a charge. This charge repels the piece of paper stuck to it because of the forces caused by the charge. These forces come from the charge of atoms. The forces between particles that are not moving or that are moving (slowly) are known as electrostatic forces.

Our first experiment:

Electrostatic Forces

Exploring the Nature of Electrical Interactions

You can investigate some properties of electrical interactions with the following equipment.  Each student should have:

• 4 Scotch tapes, approx. 10 cm long

• 2 small rod stands

• 2 metal rods

• 2 right angle clamps

The nature of electrical interactions is not obvious without careful experimentation and reasoning.  We will first state two hypotheses about electrical interactions.  We will then observe some electrical interactions and determine whether our observations are consistent with these hypotheses.

Hypothesis One:  The interaction between objects that have been rubbed is due to a property of matter that we will call charge.   There are two types of electrical charge that we will call, for the sake of convenience, positive charge and negative charge.

Hypothesis Two:  Charge moves readily on certain materials, known as conductors, and not on others, known as insulators.  In general, metals are good conductors, while glass, rubber, and plastic tend to be insulators.

a. You and your partner should each place a 10 cm or so strip of Scotch tape on the lab table with the sticky side down.  The end of each tape should be curled over to make a non-stick handle.  Peel your tape off the table and bring the non-sticky side of the tape toward your partner’s strip.  What happens?  How does the distance between the tapes affect the interaction between them?

b. Place two strips of tape on the table sticky side down and label them “B” for bottom.  Press another strip of tape on top of each of the B pieces;  label these strips “T” for top.  Pull each pair of strips off the table.  Then pull the top and bottom strips apart.

1. Describe the interaction between two top strips when they are brought toward one another

2. Describe the interaction between two bottom strips.

3. Describe the interaction between a top and a bottom strip.

c. Are your observations of the tape strip interactions consistent with the hypothesis that there are two types of charge?  Please explain your answer carefully, in complete sentences, and cite the outcomes of all your observations.

Answer for A was that they were attracted towards my lab partners hands. This is because her hands have the opposite charge of the tape.

answer for b1 is that they repel each other, this is because they have the same charge. 

answers for b2 and b3: b2: repelling; b3:slightly attract.

summary of our experiment. answer for C is yes, attract and repel.

This is our experiment in which we review a little of physics 4A. A ball hangs vertically by a string of length L. Initially ball is located at the origin of your coordinate system. The ball is pushed horizontally so that now it is in a new position with x-coordinate x2. Write an expression for the angle θ makes with the vertical.

The magnitude of the gravitational force between two masses m1 and m2 

Electric Force Law Video Analysis Activity 

Here we used logger pro to measure the electric force between to metal balls, one hanging and one on a stick. We got the one on the stick closer and closer to the hanging one. The hanging ball started to move away as because of this. From there we were then able to find the separation distance caused by the electric force.

b)Determine the percent difference between your experimental value for the exponent in this force law and the theoretical value.

Assuming that the charges on both balls are the same, use one of your data points and Coulomb’s Law to determine what the charge is on each ball.

Assuming that the charge on the hanging ball is half the charge of the other ball, use one of your data points and Coulomb’s Law to determine what the charge is on each ball.

Here we calculated the the magnitude of the force between the two charges.

Here we calculated the angle in which the string moved



Here a propeller was placed on top of the charged sphere once placed on top the it started to propel and the pieces of paper on the sides became neutral.

Here we calculated the the ratio of the electric force to the gravitational force between two particles. We were given a pair of electrons with a charge of 1.602x10^-19 C and a mass of  9.1*10^-31 kg  separated by a distance of 1 * 10^-10 m.