### Coulomb's Law

We provide the transcript below to those who for whatever reason would find the written words to be preferred over or in addition to the actual video.

You may view the video below or on YouTube.

Also available:
Coulomb's Law Video || Video Transcript || Video Notes ||  Full-Length Tutorial Video

#### Coulomb's Law

Video Transcript

Charged objects attract or repel with a force (Felect) that is directly proportional to the quantity of charge (Q1 and Q2) on either of the objects and inversely proportional to the square of the distance (d) between the object's centers. This relationship is known as Coulomb's Law.

To say the force (Felect) is directly proportional to the quantity of charge (Q) means that an increase in a Q value causes an increase in the F value. And a decrease in Q causes a decrease in F. But it also means that the factor by which a Q value is changed is equal to the factor by which the Felect is changed. So a doubling of Q1 causes a doubling of Felect. And a tripling of Q1 causes a tripling of Felect. A halving of Q1 causes a halving of Felect. If both Q1 and Q2 are changed, then you must change the F twice – once for each Q change.

In Example 1, the value of Q1 is tripled and the value of Q2 is halved. So to determine the new Felect, take the original Felect of 32 units and triple it (for the Q1 change) and halve it (for the Q2 change). The new force is 48 units.

To say that the force (Felect) is inversely proportional to the square of the distance (d) means that an increase in d causes a decrease in the Felect value. And a decrease in d causes an increase in Felect. That's the "inverse" part of the statement. But it also means that the factor by which the Felect is decreased is the square of the factor by which the d is increased. So a doubling of d causes F to decrease by a factor of 4 (22). A tripling of d causes F to decrease by a factor of 9 (32). And a halving of d causes the F to increase by a factor of 4.

In Example 2, the value of d is one-third the original value. Since d decreases by a factor of 3, Felect increases by a factor of 9. So to determine the new Felect, take the original Felect of 32 units and multiply it by 9.

In Example 3, Q1, Q2, and d are all changed. So the original force of 32 units must be changed three times … once for each variable. Since Q1 is quadrupled, F must be multiplied by 4. Since Q2 is tripled, F must be multiplied by 3. And since d was doubled, F must be divided by 4. Use your calculator to determine that the new force is 96 units.

These three examples illustrate how Coulomb's Law can be used to determine a new force. I’m Mr. H, letting you know that … You got this!

Visit: Concept Builder || Teacher Notes || Directions || Questions (For Teachers Only)