In practice, most of us do not have a fundamental and precise understanding of electric charge. Instead, we rely on simple statements that we have heard, such as ”like charges repel” or ”opposite charges attract”. In introducing a more subtle definition of charge, we will rely on this practical knowledge to explain a few fundamental experiments described below. We will then give a formal definition of electric charge and will list its properties.

Experiment 1: two repelling rods

A glass rod is rubbed with a silk cloth and then suspended by a thread as shown. A second identical glass rod is then rubbed with a silk cloth and brought close to the suspended rod without being allowed to touch it.

Observation: the two rods repel each other which is made obvious by the suspended rod rotating around the thread. This suggests that a repulsive force exists between the two rods.

Explanation: rubbing the original glass rod with the silk cloth caused charged to be transferred from the glass to the silk. Because the second rod is also made of glass and rubbed with a silk cloth, charge it transferred from the rod to the silk and the same type of charge remains on the second rod as does on the first. The observed repulsion between the two rods lets us conclude that ”like charges repel” (and we do not technically need to know if the glass is positively charged or negatively charged).

Experiment 2: two attracting rods

A glass rod is rubbed with a silk cloth and then suspended by a thread as shown. A plastic rod is then rubbed with a silk cloth and brought close to the suspended rod without being allowed to touch it.

Observation: the two rods attract each other which is made obvious by the suspended rod rotating around the thread. This suggests that an attractive force exists between the two rods.

Explanation: rubbing the original glass rod with the silk cloth caused charged to be transferred from the glass to the silk. The second rod being made of plastic, we do not necessarily know what kind of charge remains on it after being rubbed with the silk cloth but we can convince ourselves that it must be opposite that remaining on the glass rod since otherwise the rods would repel and not attract each other.

Direction of charge transfer:

It is important to be aware that determining the direction of the charge transfer and the type of charge left on each rod after being rubbed is learned through many different experiments in practice and is not obvious or intuitive. For example, a Styrofoam ball rubbed with fur leads to the ball and the fur being attracted to one another. However, it is not clear from that experiment alone which carries positive charge, and which one carries negative charge. We can document these findings by created the following table for a few experiments.

We observe the following:

everything in A repels everything else in A
everything in B repels everything else in B
everything in A attracts everything in B

We also note that any item that is not rubbed, has no effect on an item from category A or an item from category B.

Historically, items from category A were labeled as carrying positive charge and items from category B were labeled as carrying negative charge. The unrubbed items were said to be neutral and to carry zero net charge.

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