physic lab report

where the constant G = 6.7 X10-11 Newton-meter2/kg2. The existence of both positive and negative charges means that the Coulomb force can be either attractive or repulsive (attractive for charges of opposite sign, repulsive for charges of the same sign), whereas the gravitational force is always attractive. Both forces act in a direction along the line connecting the two objects. The orders of magnitude difference in the force constant means that very small charges can exert very large forces.

Electrical Materials can be categorized in terms of how

easily the charges (usually electrons, sometimes ions) can move within them. In a conductor (e.g. metals), a few electrons per atom are “free” to move within the solid. If a positively charged object is brought near one end of a conductor, electrons will be attracted towards that point, resulting in a net negative charge near that point, and a net positive charge on the rest of the conductor. That net positive charge is due to the excess of positive protons (in nuclei) that are left behind. If a conductor becomes charged, then the free electrons will distribute themselves uniformly all around the outside of the conductor. Qualitatively, in a negatively charged conductor, the excess electrons distribute themselves to be as far apart as possible.

In an insulator (glass, ceramics, rubber, most plastics), all of the electrons are firmly attached to individual atoms or molecules, and they are not free to move within the solid. You will find in this lab that friction can transfer charges between insulators, leaving one locally charged positive, and the other locally charged negative. The relative tendency of insulators to become positive or negatively charged is referred to as the triboelectric series. Also, if a charged insulator is brought near enough to a conductor, some charge can be transferred either as a spark, or directly when the two objects touch.

Monitoring the Presence of Charge

To monitor the presence of charge you will use an electroscope . This simple device uses the deflection of a light metal foil as an indicator of the presence of charge (see figure). An excess of charge on the center conductor plate is shared between the conductor and the foil, and the two repel each other. The movable foil thus moves away from the center conductor, and the angle between the foil and the plate provides a measure of the excess charge.

• Can you tell the sign of the charge just by looking at the electroscope? Why or why not?

Procedure:

A. RODS and CLOTHS

Get a collection of rods and cloths from the supply table. These are insulators, and can become charged by vigorous rubbing.

HELPFUL HINT: When the rods are handled excessively, the oil from your skin gets on them and they become difficult to charge. Washing the rods in soapy water and drying them well should restore their original state.

• First rub the hard rubber rod with fur. Do you hear sparks? What is happening? Observe the interaction between the charged fur and rod, and of the fur with itself. Explain what you see in terms of the attractive and repulsive Coulomb interaction. The rod will be negatively charged. What is the charge on the fur? Could you tell just by looking at the fur itself, or do you need the rod?

• Rub the rod with the fur, and bring the rod slowly towards the electroscope ball. Does the electroscope respond? How? Listen carefully for whether or not a spark jumps between the rod and the electroscope. Bring the rod away from the electroscope before it touches or sparks. (If it sparks unintentionally, pull the rod away and touch the electroscope with your finger. This is called grounding the electroscope. Then bring the rod in again without sparking.) What happens to the electroscope? Explain what is happening. Can you get a deflection of the electroscope foil when the net charge on the electroscope is zero? Why or why not?

• Bring the charged rod in until it sparks or touches. What happens? Why? While the charged rod is far away and the foil is deflected, touch the electroscope with your finger to ground the electroscope. What happens? Explain what is happening.

Draw a sketch of the charge distribution on the electroscope for the cases of (a) hard rubber rod, near, but not touching the electroscope and (b) after touching or having a spark jumps between the rod and the electroscope. What is the net charge on the electroscope in each cases?

• Charge the electroscope by touching the top plate/ball with the charged rod. Remove the rod, and bring it back again, this time not touching the electroscope. What happens? Why? Now, bring the rod close to the main body of the electroscope (near the foil). What happens? Why?

• Using a single pith ball (lightweight ball with a conducting surface) suspended from a support, observe the behavior of a neutral (uncharged), free-swinging pith ball as a charged rod is brought near it. Bring the rod slowly closer to the pith ball until they touch. Explain the behavior of the pith ball in terms of electric charge. Sketch the charge distribution on the rod and ball before and after they touch.

Next, take other combinations of rods and cloths.

• Rub the Lucite rod with Saran wrap and repeat the above experiments (ground the electroscope before you start). For results which look the same as with your first rod, you only need to state that fact in your lab write-up. If there are differences, describe them. Can you tell from these experiments whether or not the Lucite and rubber rods have the same charge?

• Design an experiment to determine the relative sign of any two charged rods using the equipment available in this lab. Use this experiment to determine the sign of the charge on the rods when rubbed by each of the cloths (i.e., make a chart (triboelectric series) for three or four rods and three or four cloths, and fill in the charge on the rod for each case). Describe what experiment you did, and why it told you what charge each rod had. What charge do you think the cloths have in each case?

• Can you tell from these experiments whether there are exactly two kinds of charge? (Zero, or NO charge is not a kind of charge) If so, how? If not, how many kinds of charge do you find?

B. charging by induction

In this experiment, you will investigate the phenomenon of charging by induction. This takes advantage of the polarization of charges within conductors which you observed in the previous experiment.

• Charge a rod with fur. Bring the rod close to the electroscope, but do not touch it or allow a spark to jump to it. When the foil is deflected and the rod is nearby, touch the electroscope ball with your finger. Remove first your finger, and then the charged rod. The foil should now move, indicating an excess of charge. Use your knowledge of induced polarization and the idea that your finger is a conductor to predict the sign of the charge. Determine the sign of the charge on the electroscope. (describe how you did this). Does your measurement agree with your prediction? Discuss this with your lab partners (and the instructor) until you understand why you got the result you did. Write this explanation (include a sketch of the charge distribution) in your lab write-up.

• A similar experiment can be performed using the pith ball on an insulated stand. Bring a charged rod near the sphere, then touch the sphere with your finger. Remove your finger, and then the rod. What is the sign of the charge on the sphere? Does it matter where you touch the sphere (i.e. near or far from the rod)? Why or why not?

C. Quantitative experiment [OPtional, weather (humidity) dependent]

In the qualitative experiments, you saw that two objects charged with the same sign repelled each other. In this part of the lab you will measure the strength of this force relative to the force of gravity on the same object. Assuming Coulomb’s law, you will calculate the charge on these objects.

You will need to perform these measurements very quickly, especially if it is particularly humid.

On the lab bench, you should find two pith balls (light weight, conductive surface) hanging on insulating strings.

1. Calculate the force of gravity (in Newtons) acting on each pith ball. Adjust the tops of the strings so that the balls are at the same height above the table and are just touching when they are uncharged. Measure the length “L” of the strings.

2. Charge up the two pith balls as much as you can using your choice of rods. Be careful not to ground the pith balls once they are charged by touching them directly with your hands.

3. Touching just the strings, force the pith balls to touch each other to try and get the charge on the two balls to be equal. Release the balls.

4. The two balls should now look like the upper figure on the right. Using a plastic ruler or caliper, being careful not to touch them, measure the separation R between the two balls. (NOTE: R is the distance between the centers of the balls.) If the separation is less than about 1 diameter of the balls, try to put more charge on. If your pith balls can’t hold enough charge to separate more than 0.5 cm no matter how hard you try, share with another lab group that has better luck.

5. Draw a free body diagram for one of the charged pith balls as at the right, where F = electrostatic (Coulomb) force, G = mg = gravitational force and T = string tension. Show that if the Coulomb force is horizontal, then the ratio of the Coulomb force to the gravitational force on one of the balls is given by:

= tanΘ