# 13 Electric Fields

## Electric Potential between Two Point Charges

In preparing for blended learning lessons for my JC2 students, I tweaked the Gravitational Potential applet made last year for a similar display of the electric potential between two point charges. This is a testament to the similarities between the two concepts as well as the ease of adapting a GeoGebra applet for education.

We can scaffold students’ learning using this interactive applet by asking questions such as:

1. By observing the electric potential graph, are you able to find a point when the net field / force acting on a test charge is zero? What are the necessary conditions?
2. The slope of the sum of the electric potentials is analogous to that of a physical slope where a ball will roll downhill in the same way that a positive test charge will accelerate based on the potential gradient. However, this analogy will work differently for a test charge that is negative. Why?
3. Given that $E = -\dfrac{dV}{dx}$, where x is the distance from the point charge, is the direction of the E-field vector consistent with the negative of slope?

To paste this applet into SLS, use the following embed code. In SLS, create a new component within an Activity within a Lesson using the “+” button. Choose Text/Media and select the button that shows “</>” or reads Embed Website/App”. Copy and paste the following codes to the box.

<iframe scrolling="no" title="Electric Potential of Two Point Charges" src="https://www.geogebra.org/material/iframe/id/z8cr66wb/width/640/height/480/border/888888/sfsb/true/smb/false/stb/false/stbh/false/ai/false/asb/false/sri/false/rc/false/ld/false/sdz/true/ctl/false" width="640px" height="480px" style="border:0px;"> </iframe>

By embedding the gravitational potential distance graph for two masses, a comparison can be made between the two. This will help students draw connections between the two concepts based on the fact that the forces both follow an inverse-square law.

This is the embed code for the applet on gravitational potential.
<iframe scrolling="no" title="Gravitational Potential between Two Planets" src="https://www.geogebra.org/material/iframe/id/ff55x6vr/width/638/height/478/border/888888/sfsb/true/smb/false/stb/false/stbh/false/ai/false/asb/false/sri/true/rc/false/ld/false/sdz/false/ctl/false" width="638px" height="478px" style="border:0px;"> </iframe>

## Using a mobile phone in a petrol station.

So it’s safe to use a mobile phone in a petrol station. But not wearing nylon.

For more on this, read the Straits Times article : http://www.straitstimes.com/asia/se-asia/use-of-mobile-phones-at-petrol-stations-do-not-cause-fires-experts

## Explanation for Water Bending with Static Electricity

It’s interesting to note the differing views regarding the explanation for how a thin stream of water can get bent when a charged object is placed near it. It started with these two videos from Veritasium:

This video then sets out to disprove Veritasium’s model of ions being removed from the water stream.

## Ionisation of Air to Remove Static Electric Charges

Materials

1. Wool
2. PVC pipe or plastic comb
3. String
4. Lighter

Procedure

1. Hang the string from an elevated position. Leave the bottom end free.
2. Rub the PVC pipe with wool. This deposits negative charges, or electrons, onto the surface of the PVC pipe.
3. Place the side of the pipe that is rubbed near the string. You should notice the string being attracted  towards the PVC pipe.
4. Holding the PVC pipe still while attracting the string, light a flame using the lighter and place it in between the string and pipe. You should observe the string falling back to its original position.

Science Explained

When air is ionised with the help of a flame, it serves as a conducting medium through which static electric charges can escape from a surface.

## Water Bender

A thin stream of water can be easily bent using a plastic comb or ruler which was previously rubbed with wool. This demonstrates the attractive forces between unlike charges.

Materials

1. Plastic ruler
2. Wool
3. Water from a tap

Procedure

1. Turn on the faucet for the thinnest stream of water with a consistent flow.
2. Rub the plastic ruler with the wool.
3. Place the part of the ruler which was rubbed near the stream of water without touching.

Science Explained

Water molecules are polar in nature, which means that one side (where the oxygen atoms are) is more negative while another side (where the hydrogen atom is) is more positive. When wool is rubbed with plastic, it deposits electrons on the ruler.

The electrons will remain on the plastic as it is a poor conductor of electricity. When placed near the stream of water, the water molecules reorientate themselves such that the positive pole of each molecule is now nearer to the ruler than the negative pole.

The resulting attractive forces are stronger than the repulsive forces as the forces between charges decrease when the distance apart increases.

## Leyden Jar

A Leyden jar is a device used to store static electric charge. It can be used to conduct many experiments with electricity such as creating a spark across a gap.