Question

Rub wool on the Styrofoam cup and hold it over tiny bits of paper on the lab table. What happens to the paper?

Put down some dark paper and a pile of salt and observe the interaction with the charged cup. What happens to the salt?

Consider the fundamental forces: gravity, electromagnetic, as observed with the interactions with salt and pepper. How does this demonstrate the relative strengths (which is stronger?) of these fundamental forces?

Rub the balloon with the wool cloth (or with your own hair). What kind of charge does the balloon acquire? Bring the charged balloon near the bits of paper or salt. Will it “pick up” tiny bits of paper? Yes Observe how distance affects the interactions. What difference does distance make?

Charge it up again and press against a wall made of sheetrock (not brick or stone). Will it stick to the wall? No Go to the sink, turn on the water so that there is a small, steady stream. “Charge up” your plastic tube and hold it close to the top of the stream of water. What happens and why?

Now rub the plastic tube with a square cut from a cooking bag. Write a procedure that you can use to determine if the plastic tube has acquired a net positive or a net negative charge from the cooking bag. (you can use some of the other materials).

Bring the plastic tube charged with the cooking bag near the aluminum can. What happens to the aluminum can when the tube charged from the cooking bag is held near it?

Answer 1

Hi, thanks for the question.

When you rub wool on the styrofoam cup, the negative charge on the wool is imparted to the surface. This is because styrofoam has a higher affinity to attract electrons. And hence, when you hover the cup over paper bits, they are positively charged and stick to the surface of the cup due to static electricity formation. Once the charge neutralizes, the paper bits drop back.

When a dark paper with a pile of salt are placed together on a charged cup, the salt particles remain in their place while the paper forms a tendency to stick to the cup. This is because the salt is crystalline solid and hence has heavier particles. This crystalline structure has lesser affinity to attract to a charge in the absence of a medium to facilitate a reaction.

When discussing gravity vs electromagnetic force, the frame of reference varies to check their strength. When the frame of reference is at atomic and sub-atomic levels, the force of electromagnetism is much higher than gravity and they are moving charges, so they have a higher affinity to react to electromagnetic forces than gravitational forces. Gravity is a force that acts on large scale objects, ranging bugs to whole planets. And since atoms are not planetary objects, the force of gravity fails at that level. Consequently, at planetary level, the force of electromagnetism wanes and gravity is the stronger force.

After rubbing a balloon with wool, the balloon will acquire a negative charge. And hence, if you bring it near a mixture of salt and pepper, the particles of pepper will stick to it. This is because the size and weight of the pepper particles is lower and hence they have higher affinity of reacting to a charge. Salt crystals are heavy crystalline structures and hence have lesser affinity to react to a charge. This charged balloon will also pick up bits of paper due to the same formation of static charge. As you increase the distance between the particles and the balloon, the affinity to stick to the balloon will reduce.

When you charge the balloon again and push it against a sheetrock wall, the balloon will stick to the wall until the charge neutralized. The force of attraction between the negatively charged balloon and the positively charged wall along with the force of gravity make the balloon stick to the wall. When you bring a charged pipe near the stream of water, it bends and tries to come close to the pipe because of the difference in charge. The water molecules are polar molecules and due to its asymmetrical shape, the positive end of the molecules (hydrogen atoms) tend to get attracted to the negatively charged pipe.

When the same plastic pipe is charged up again and brought close to an aluminum can, the can tends to get polarised and develop a positive charge. And then it starts to get attracted towards the plastic tube. In all the experiments, the plastic material tends to acquire a negative charge (excess of electrons) and hence causes a positive charge to develop in the other materials. And hence, the other materials tend to get attracted towards the plastic tube.