Questions 4 - Electrostatic

Questions (with answers)
1. A plastic rod is rubbed with a cloth. When placed near pieces of paper, it will:
a) attract them
b) repel them
c) neither (do nothing)
Answer: a - it will attract them because the rod is charged

2. When rubbed with the same cloth, describe what would happen to a copper rod.
Answer: It would not become charged, because copper is a conductor.

3. When an insulator becomes positively charged it:
a) gains electrons
b) loses electrons
c) gains protons
Answer: b - this is because electrons have a negative charge, so if it has less electrons, it will be positive. It cannot be b, because that would mean the negative charge increases, so it would be negative. It definitely cannot be c because protons cannot move. 

4. You wash and dry your hair, then comb it with a plastic comb.  As you move the comb away from your head some hairs are attracted to the comb.

(a) What has happened to the comb to make it attract the hairs? (1 mark)

Answer: It has been charged by friction because you used it to comb your hair - it is an insulator and so is your hair.

(b) If the comb is now held above some small pieces of dry tissue paper what is likely to happen? (1 mark)

Answer: Comb will attract the paper

(c) If you rub your hands all over the comb it will no longer attract your hair.
Explain why. (2 marks)

Answer: You are standing on the ground, so the electrons from the comb will travel through you down to the Earth so the comb is no longer charged, therefore the electrons and the charge is gone.

Do-them-yourself questions (not answered)

1. One method of painting a car uses electrostatics. A paint spray produces paint droplets, all of which are given a positive charge. The car body is given a negative charge.

(a) Explain why it is important to give all of the paint droplets a positive charge. (2 marks)

(b) Explain why it is important to give the car body a negative charge. (2 marks)

2. An aircraft is being refuelled after a long flight. Explain why it is important that, before refuelling starts, the aircraft is first connected to the fuel pump by a metal wire. (4 marks)

More practice questions

Go to this website for more practice - the answers are provided so you can mark your work. Cheating is pointless..the only person that's going to benefit from doing these questions honestly is you.

Revision top tip - make a list of everything you did wrong and then work on that. Then search for more questions and re-do and mark.

Disclaimer: The questions were not created by me, with the exception of question 2 on the answered questions. All rights and credit goes to the person that wrote them, the author, who I was not able to find. Questions 1 and 3 come from BBC Bitesize and the rest were found on a website with no author. I mean no harm. Seriously.

You have the approval of the Dark Side.

2.25 Explain some uses of electrostatic charges, eg in photocopiers and inkjet printers.

Photocopier

BBC Bitesize has a very clear diagram (above) that explains how this works. Steps are as follows:

• Original document placed on photocopier
• Light goes through
• Image of it is projected onto a positively charged plate/surface
• Where the light touches the plate, the electrical charge is gone (poof.) so that only the writing/image/whatever is left
• Negatively charged toner (ink) particles are sprayed or rolled onto the thingie
• The ink sticks to the positive areas
• Paper is placed over this and the ink is transferred to the paper
• It is heated so the ink sticks
• Photocopy is taken off the photocopier and turned over - 'TIS DONE!

This page is particularly useful - it is a link to BBC Bitesize and it has various uses for electrostatic charge. (If the hyperlink doesn't work, copy and paste the link below into your browser)

http://www.bbc.co.uk/schools/gcsebitesize/science/add_gateway_pre_2011/radiation/electrostaticsusesrev1.shtml

2.24 Explain the potential dangers of electrostatic charges, eg when fuelling aircraft and tankers

Electrostatic charges can sometimes be dangerous. For example, when fuelling a car.

The problem
The tube that the fuel runs through is rubber. This is an insulator. The fuel rubs against the rubber, creating friction so the fuel is charged. The static build-up can cause a spark - the car is made of metal, and the electrons have nowhere to go, so they try to 'jump' onto the metal, which creates a spark. When fuelling, you'll often notice that a strong smell and a tiny bit of gas comes out. This is highly flammable. When there is a spark, this gas will catch fire and could harm the person(s) near it. In a more extreme case, it might even cause the car to explode :( 

The solution
Earthing the excess charge - this means having a metal rod that carries the excess electrons down to the earth to the charge is no longer a hazard :) Note that this must be done before fuelling starts. 

Everyone needs a bit of Star Wars in their life <3

2.23 Explain electrostatic phenomena in terms of the movement of electrons

Electrostatic phenomena is when static has a specific effect. For example, when you get a static shock (you know, like if your friend is somehow charged and then you touch them and like ow it feels like someone just pinched you...it's that).

Electrons can move from one material to the other, and their ultimate goal is to reach the earth. When you touch your friend (let's say it's a she and her name is Agoriel), the electrons jump from whatever is charged (eg a wooly sweater) to you and then down to the earth.

A better example: your friend Agoriel is touching a Van der Graaff generator (look it up if you don't know what it is) - her hair is beginning to stand on end and she is negatively charged. She is standing on an insulator. If you stick out a finger to touch her finger, there will be a spark, you'll probably go "owch!" and the electrons jump to you and then travel down to the earth.

2.21 Explain that positive and negative electrostatic charges are produced on materials by the loss and gain of electrons

When two insulators rub against each other, the electrons move from one to the other. The one that has gained electrons is now negatively charged, and the one that has lost electrons is now positively charged.

Note that only electrons move - protons are still (hence the name)

This is charge by friction.

Materials can also be charged by induction. 

This is when a electrons are pushed away. Let's say that (for some reason) you are negatively charged. You place your hand in front of a can and it rolls towards you. How does this work? Because your hand is negatively charged, and like charges repel, it repels the electrons at the front of the can. This leaves the protons, which are positively charged. Therefore, the can is now charged positively by induction. Because opposite charges attract, the can rolls towards your hand.

To demonstrate this visually (sorry about the bad drawing skills, that was me making the diagrams)

Key - purple dots are electrons and red dots are protons (note that on both diagrams, the hand is supposed to have electrons on it but I didn't quite get the same shade of purple the second time through)

Figure 1 - Step 1

Figure 2 - Step 2

2.19 Identify common materials which are electrical conductors or insulators, including metals and plastics

Examples of conductors include:

• copper
• iron
• nickel
• gold
• steel (that's a metal alloy)
• aluminum/aluminium
• pretty much any metal

Examples of insulators (non conducting materials):

• plastic
• Styrofoam
• rubber
• cloth
• pretty much any non metal

Explanations :)

Conductors are materials that allow an electrical current to be passed through them. They need to have 'free' electrons because current is a flow of electrons - this is why metals are conductors. Some materials will have more free electrons. The more of them they have, the less resistance, and therefore they are a better conductor.

Non conducting materials do not allow an electrical current to be passed through them. They do not have free electrons which means they cannot move. No flow of electrons therefore no current.