# 13 Electric Fields

## 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.

## Electroscope

An electroscope is a device that can be used to detect or measure the amount of charge in its vicinity. One of the earliest electroscopes is the gold-leaf electroscope which was invented by a British clergyman Abraham Bennet. This is a cheaper model of the leaf electroscope made using aluminum foil.

Materials

1. Paper clip
2. Aluminum foil
3. Modelling clay
4. Glass bottle with a narrow neck
5. Steel or brass sinker

Procedure

1. Cut two strips of aluminum foil measuring 2 cm by 0.5 cm.
2. Straighten the paper clip before bending both ends to make two hooks. Hang the paper clip using one hook from the sinker.
3. Pierce each aluminum strip at one end through the other hook of the paper clip, leaving it to hang from the hook.
4. Place the paper clip and aluminum strips inside the bottle. If the sinker is smaller than the neck of the bottle, use some modeling clay to keep it in place.
5. Now you can test the electroscope by rubbing a comb with some wool and placing it near the paper clip.

Science Explained

Negative charges (electrons) are deposited on the comb by rubbing with wool. When the comb is placed near the sinker without touching, the negative charges in the sinker are repelled. As glass is an electric insulator, the only way for them to go is downwards onto the aluminum strips. Both strips are now negatively charged and will repel each other. The extent of their repulsion is dependent on the amount of charge on the comb and its distance from the electroscope.