Worksheet 2: Static Electricity - electric forces and charges¶

In this lab, we will be studying the concept of static electricity, which looks at charges and the forces between them. We will generate positive and negative charges and observe the force between like-sign (both positive or negative) and unlike-sign (positive and negative) charges.

There exist two types of charges: positive and negative. An atom is electrically neutral, but it is made of a positively charged nucleus with a number of electrons surrounding it. When one or more electrons are removed from an atom, it becomes positively charged. One can also add electrons to an atom to make it negatively charged. A charged atom is called an ion (you should be able to recall this from chemistry). Now for an object that is made of many atoms or molecules, when it has more positive charge than negative charge in it, we also say that this object is positively charged. The same is true vice versa. An object with equal amounts of positive and negative charge is said to be neutral.

Charges can be introduced to (or removed from) an object by friction (rubbing a plastic rod with a cloth for example). This is known as the triboelectric effect as this effect describes electric charge transfer due to contact between two objects. They can also be transported to an object by a vehicle or a belt (literally, like in the Van de Graff generator). Inside an object, charges (this time mostly the electrons) can be displaced by external forces. In the case of metals or conductors, this is called induction. In the case of a dielectric or insulator, this is called polarization.

There is a force between charges separated by some distance. The force between charges is governed by Coulomb’s Law ( $\mathbf{F} =\frac{1}{4 \pi \epsilon_0} \frac{q_1 q_2}{r^2} \hat{\mathbf{r}}$ ). Refer to your textbook to review this formula. From this law, we can read that like-signs repel, and unlike charges attract. To determine the force between two objects, each carrying many charges, we just need to add (integrate) all the forces between the many point charges in these objects two at a time. This is called superposition.

Static Field Detector¶

This is a static field detector. It must be connected to a plug and connected to the building ground with the green wire. Set the sensitivity to HI. Make sure that the detector probe is not near any objects, and press the RESET button to clear any LEDs that might be lit. Now the detector is ready to measure electric potential at its probe and indicate the polarity and relative strength of the source charge.

*Figure 2:* The green wire is what we use as a ground. It is connected to the buidling ground as shown.

Measurement 1¶

*Figure 3:* From left to right: microfiber cloth, wool cloth, PVC pipe, acrylic rod

To remove and residual charge on the PVC rod (and acrylic rod), grab a paper towel, wet it, and squeeze out the excess water (you want the paper towel damp not saturated with water). Grab a grounding wire that is connected to the building ground, hold it in the same hand as the damp paper towel (there should be contact between them), and run the paper towel over the PVC pipe. Use the static field detector to check that it is electrically neutral.

Measurement 2¶

To remove residual charge, run the end of the grounding wire on the aluminum can as well as the pith ball. Use the static field detector to check that they are electrically neutral.

Measurement 3¶

*Figure 5:* 15 cm piece of tape with approximately 1 cm folded under on the edge to make a "handle" for it.

Measurement 5¶

To remove residual charge, run the end of the grounding wire on the charge separation rods. Use the static field detector to check that they are electrically neutral. For this measurement, do not forget to remove residual charge from the PVC rod (like you did for Measurement 1) and then charge it using the same method as you did in Measurement 1.

Supplementary Material: