Category: 12 Electric Field

12.4 Electric Potential

Do you know what is elevation or altitude? E.g. Bukit Timah hill has an elevation of 164 m above sea level. The lowest point of the Dead Sea is 413 m below sea level. You can think of electric potential as the “elevation” of the “electric landscape”.

12.3.1 Electric Field of a Point Charge

Positive (source) charges set up outward fields, which blow away positive test charges but suck in negative test charges.

Negative (source) charges set up inward fields, which suck in positive test charges but blow away negative test charges.

Here is an worked example on how to calculate the resultant electric field strength.


Try setting up an electric field to guide a hockey puck to the goal

PhET Electric Hockey: Click HERE

12.3 Electric Field

Masses in gravitational field are subject to gravitational forces. Charges in electric fields are subject to electric forces. So if you know your g-field, e-field is just the same concept. Just watch out for negative charges.


This applet illustrates how a test charge can be used to detect and measure the strength of an electric field.

PhET Charges and Fields: Click HERE

12.1 Electric Forces

I don’t think there is an explanation for why like charges repel. They just do. The world would be totally different if they don’t.

Here is an worked example on the calculation of the resultant electric force.

Unfortunately, many electrostatic demonstrations are difficult to pull off in Singapore because of the humidity. But you can watch them in the following video.

The ultimate demonstration of “like charges repel and unlike charges attract”: combining the tinsels that came with the fun-fly sticks with the good old fashioned van der Graff generator. Notice also that both “charge by induction” and “charge by conduction” are demonstrated.

Detail at


This applet illustrates why a charged balloon sticks to walls.

PhET Balloon and Static Electricity: Click HERE

This quirky applet illustrates how we get static shocks.

PhET John Travolta: Click HERE

Supplementary Reading

It’s All Electric: Click HERE

C01 Electric Force

It is quite challenging to build up static charges in Singapore. Because of the humidity. The H2O molecules in the air keep removing the charges from any charged surface.

Even the Van der Graaff generator in my school lab cannot quite make the hair stand. All it can do is throw off some aluminium cups.

See detail at

One trick that electric charges can pull that masses cannot is this: a charged body can induce charges on other bodies. Even initially neutral bodies can start attracting or repelling each other after they have exchanged charges!

See how the Van der Graaff generator makes itself attractive by inducing (unlike) charges on aluminium foils, and then repulsive by conducting (like) charges onto the foils.

See detail at

C05 Field Lines

Using the field pattern of a point charge as the basic building block, we can sketch the field pattern for more complicated arrangement of more than 1 point charges.

For the one with four point charges, can you tell the polarity of the charges?

C04 Electric Potential

We imagine that an electric field causes a distortion to the electric landscape. Positive charges raise “electric mountains” and negative charges drop “electric pits”.

To any (test) charge that wanders into this electric landscape, it will have to use up its KE when it is climbing up mountains or out of pits. Likewise, it can gain KE when it is rolling down mountains or into pits.

You can think of the electric potential as an “electric altitude” . It marks the elevation at each point, so that we can easily calculate the changes in EPE as a charge travels around in an electric field.