Electricity and Magnetism

For hundreds of years, the static electricity we met in the previous quiz was the basis for parlor tricks and games. Surprising friends and acquaintances with a mild electric shock passed the hours in social settings and was not considered awkward.

Electricity was captivating but had no practical use.

Benjamin Franklin Benjamin Franklin

Benjamin Franklin, an 18th-18^\text{th}\text{-}century American polymath, was among the first to realize that these amusing properties of certain materials might be used in new electric inventions.

In this quiz, we introduce Franklin's idea of net charge, an important leap forward in understanding electrical conduction, and the basis for the electrification of modern living.

Net Charge

                 

In the last quiz, you saw how a piece of plastic tape holds onto extra electrons when it's removed from a countertop. We can tell the tape is charged because the electric force, normally confined within atoms, comes into plain view.

What we didn't consider is what happens to the countertop. The surface has lost electrons that are now on the tape — does this have any effect? More specifically, can we do a test to deduce whether an object is missing electrons?

In this quiz, we'll answer these questions. Again, we provide instructions for another demo using adhesive tape, so that you can play with these concepts on your own.

This video shows you the setup:

Net Charge

                 

In the setup video on the previous page, you were directed to drag your fingers over the pair of strips from the surface before you pulled them apart.

What does dragging your fingers over the pair of strips do to the tape?

Net Charge

                 

After separating the pair of strips, use a neutral object (like a piece of foil) to test for charge on the two strips. Is one of the strips electrically charged, both strips, or neither?

If you get stuck, check out this video:

Net Charge

                 

Both tape strips behave as if they are electrically charged.

To make sense of this observation, let's think back to our sand grain analogy. In this imaginary scenario, the orange sand piles are analogous to uncharged objects — they're an even mixture of red and yellow grains.

If we move the yellow grains (analogous to electrons) from pile A\text{A} to pile B,\text{B}, then pile B\text{B} gets more yellow. As yellow grains are removed from pile A,\text{A}, the red grains (analogous to protons) will make up more of pile A.\text{A}.

Given this analogy, if we could inspect the number of protons and electrons on both tape strips, what would you expect to find?

Net Charge

                 

After pulling the strips apart, one gained electrons and the other donated them. Therefore, on both strips, the balance between the two types of electric charge is upset, and both strips will be able to attract a neutral piece of foil with an electric force.

This is an example of a law known as conservation of charge, which says that electric charge is neither created nor destroyed in any physical process.

In other words, electrons don't come from nowhere!

Many times in this course, we'll say that an object with an imbalance of protons and electrons has a net charge and that an object with a near-perfect balance of protons and electrons has zero net charge.

The reason for these terms is to remind ourselves that real materials are made up of huge numbers of charged particles and that they become electrically charged when there's a significant imbalance between the two types of charge.

Net Charge

                 

You may still be wondering how we can be sure that both strips don't have excess electrons? Is there any way to tell the difference between an object that's missing electrons and one that has excess electrons?

The only way to tell is with... a third charged object.

Try charging the PVC pipe with a nylon sock, and then hold it up to both pieces of charged tape.

What do you find about the electric force between the pipe and each of the strips?

Net Charge

                 

We just observed a repulsive electric force between the PVC and the left-hand tape strip. In a repulsive interaction, both objects carry a net charge of the same kind.

If we look back at the triboelectric series, PVC gains electrons from wool to become negatively charged, which means the left-hand tape strip is also negatively charged. The right-hand tape strip is positively charged.

Let's do another test and see if we can figure out what kind of net charge a single tape strip picks up when it is ripped off the surface. Stick a single tape strip on your clean surface, charge it, and observe the electric force when you hold this test strip up to each of the strips already hanging.

Does the single strip have positive or negative net charge?

Net Charge

                 

17th-17^\text{th}\text{-} and 18th-18^\text{th}\text{-}century scientists called the "substance" transferred during charging or discharging "electrical virtue." We now know that charging is due to the gain or loss of electrons (or protons, in specialized cases): an object gains a net charge when the balance between electrons and protons is disturbed:

  • Electric charge due to an excess of electrons is called net negative charge.
  • Electric charge due to an absence of electrons is called net positive charge.

Because the charge carried by electrons and protons "cancels out" on a neutral object, this naming scheme is natural.

We also discovered the basic rule that characterizes electric forces:

Like charges repel, and opposite charges attract.

Next, we'll build a mathematical model of the interaction between charges, and we'll see more reason to call the two kinds of electric charge positive and negative.

These basic ideas are all you need to understand the modern theory of electrical conduction, which is the subject of the next chapter.

Net Charge

                 
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