Electrostatic

Question 1

What is voltage?

Answer 1

• Is the energy transferred per unit charge passed
  1. When an electric charge goes through a change in voltage then energy is transferred
  2. Energy is supplied to the charge at the power source to ‘raise’ it through the voltage
  3. The charge gives up this energy when it ‘falls’ through any voltage drop in components elsewhere in the circuit
  4. The bigger the change in voltage, the more energy transferred for a given amount of charge passing through the circuit
  5. That means that a battery with a bugger voltage will supply more energy to the circuit for every coulomb of charge which flows round it, because the charge is raised up “higher” at the start, and as the diagram shows, more energy will be dissipated in the circuit too

Question 2

What is another unit than volts for voltage?

Answer 2

The volt is a joule per coulomb

Question 3

How is a positive electrostatic charge formed?

Answer 3

By there being a lack of negative electrons

Question 4

What are the forces between like charges?

Answer 4

• Forces of repulsion

- These forces get weaker the further apart they are

Question 5

What are the forces between unlike charges?

Answer 5

• Forces of attraction

- These forces get weaker the further apart they are

Question 6

What do objects that are uncharged contain?

Answer 6

They contain equal numbers of positive and negative charges

Question 7

How can insulating materials become charged by friction?

Answer 7

1. If an uncharged plastic rod is rubbed with an uncharged cloth, it is possible for both of them to become charged
2. This is sometimes called charged by friction
3. During the rubbing, electrons from the atoms of the rod may move onto the cloth
4. There is now an imbalance of charges in both objects
5. The rod is short of electrons and so is positively charged
6. The cloth has an excess of electrons and so is negatively charged

Question 8

What are some examples of electrical insulators?

Answer 8

Materials such as plastic, rubber, glass, wood

Question 9

Why does someone’s hair stand up on a Van de Graaff generator?

Answer 9

1. When it is turned on charges flow onto the large metal dome
2. Some of the charges flow over her hands and onto all parts of her body including her hair
3. Each strand of hair has the same type of charge as its neighbour, so there are repulsive forces between the strands.
4. These forces cause her hair to stand on end
   - For this demonstration to work, she must stand Ina n insulator to prevent any of the charges she is receiving from the generator from escaping into the floor. At the end of the demonstration the girl steps off the insulator, the charges can now escape and her hair falls (when a path is provided for charges to escape it is called earthing)

Question 10

What is a negative electrostatic charge and a positive electrostatic charge produced on materials by?

Answer 10

1. A negative electrostatic charge is produced by the gain of electrons
2. A positive electrostatic charge is produced by the loss of electrons

Question 11

Describe the Gold-Leaf Electroscope experiment

Answer 11

1. You can see whether a material is charged by using a gold-leaf electroscope
2. A gold-leaf electroscope has a metal disc connected to a metal rod, at the bottom of which are attached two thin pieces of gold leaf
3. When a rod with a known charge is brought near to the disc of the electroscope, electrons will either be attracted to, or repelled from, the metal disc, depending on the charge of the rod
4. This induces a charge in the metal disc, which in turn induces a charge in the gold leaves
5. Both gold leaves will have the same charge, so they will repel each other, causing them to rise
6. When the rod is taken away, the gold leaves will discharge and fall again

Question 12

Describe suspending a charged rod experiment

Answer 12

1. Another way of testing whether a rod of material us charged is to suspend a rod with a known charge on a thread and see if there is repulsion of attraction when the rod you are testing is brought close to it
2. If there is an attraction, then the test rod has the opposite charge to the suspended rod
3. If there is a repulsion, then the test rod has the same charge as the suspended rod

Question 13

What do positive charges never do?

Answer 13

move

Question 14

What happens as a charge builds up?

Answer 14

1. The greater the charge on an isolated object, the greater the voltage between it and the Earth. If the voltage gets big enough there is a spark which jumps across the gap

Question 15

What is a build up of Static caused by?

Answer 15

• Friction
  1. When two insulating materials are rubbed together electrons will be scraped off one and dumped on the other
  2. This will leave a positive electrostatic charge on one and a negative electrostatic charge on the other
  3. Which way the electrons are transferred depends on the tow materials involved
  4. Electrically charged objects attract small objects placed near them e.g. rub a ballon on a woolly pullover then put it near tiddly bits of paper and watch them jump
  5. The classic examples are polythene and acetate rods being rubbed with a cloth duster

Question 16

What happens when a polythene rod is rubbed?

Answer 16

Electrons move from the duster to the rod and the rod becomes negatively charged and the duster is left with an equal positive charge

Question 17

What happens when an acetate rod is rubbed?

Answer 17

Electrons move from the rod to the duster and the duster becomes negatively charged and the rod is left with an equal positive charge

Question 18

How can a charged conductor be discharged safely?

Answer 18

• By connecting it to earth with a metal strap
• The electrons flow down the strap to the ground if the charge is negative and flow up the strap from the ground if the charge is positive
• The rate of flow of electrical charge is called electric current

Question 19

What are charges produced by?

Answer 19

The movement of electrons

Question 20

What is a Van de Graaff generator made up of?

Answer 20

A rubber belt moving round plastic rollers underneath a metal dome and an electrostatic charge is built up in the metal dome as the belt goes round

Question 21

How is static electricity used in an inkjet printer?

Answer 21

1. Tiny droplets of ink are forced out of a fine nozzle, making them electrically charged
2. The tiny droplets are deflected as they pass between two metal plates. A voltage is applied to the plates, one is negative and the other is positive
3. The droplets are attracted to the plate of the opposite charge and repelled from the plate with the same charge
4. The size and direction of the voltage across each plate changes so each droplet id deflected to hit a different place on the paper
5. Lots of tiny dots make up your print out

Question 22

How is static electricity used in a photocopier?

Answer 22

1. The image plate is positively charged. An image of what you are copying is projected onto it
2. Whiter bits of what you are copying make light fall on the plate and the charge leaks away in those places
3. The charged bits attract negatively charged black power, which is transferred onto positively charged paper
4. The paper is heated so the powder sticks
5. Then you get a photocopy of your piece of paper

Question 23

When is static electricity annoying?

Answer 23

• Clothing crackles
• When synthetic clothing are dragged over each other e.g. in a tumble dryer or over your head, electrons get scraped off, leaving static charges on both parts, and that leads to the inevitable attraction (they stick together) and little sparks/shocks as the charges rearrange themselves

Question 24

How is static electricity a serious problem in lightning?

Answer 24

1. Rain drops and ice bump together inside storm clouds, knocking off electrons and leaving the top of the cloud positively charged and the bottom of the cloud negative
   - (Discharge to earth, air conducts, see lightning)
2. This creates a huge voltage and a big spark

Question 25

How is static electricity a potential danger when fuelling tankers?

Answer 25

1. As fuel flows out of a filler pipe static can build up
2. This can easily lead to a spark and in dusty or fumy places where petrol which is flammable is near? Leads to explosions
3. The solution is to make the nozzles out of metal so that the charge is conducted away, instead of building up
4. It is also good to have earthing straps between the fuel tanks and the fuel pipe

Question 26

What is the potential dangers of electrostatic charges when fuelling aircrafts?

Answer 26

1. As an aircraft flies through the air, they can become charged with static electricity
2. As the charge on an aircraft increases, so too does the potential difference between it and the earth
3. With high potential differences there is the possibility of charges escaping to the earth as a spark during refuelling, which could cause an explosion
4. The solution to this problem is to earth the plane with a conductor as soon as it lands and before refuelling commences.
5. Fuel tankers that transport fuel on roads must also be earthed before any fuel is transferred, to prevent sparks causing a fire or explosion