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NCEA Physics

Electricity and Magnetism

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Charging by friction

Aims:

•To be able to describe common materials which are electrical conductors or insulators including metals and plastics.

•To know that insulating materials can be charged by friction.

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Charging by friction

Aims:

•To be able to explain that positive and negative electrostatic charges are produced on materials by the loss and gain of electrons.

•To know that there are forces of attraction between unlike charges and repulsion between like charges.

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Static electricity

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Nature’s sparks

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Lightning not lightening

This is the cool bit. A rare photo of an upward spark attracted to the lightning.

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Electro-static

•In this part of the course we will be studying the effects and properties of electrostatic events.

•Electro – electric charge. Both positive (from a proton) and negative (from an electron), forget about neutrons for a while.

•Static – not moving, like where the buses and trains stop. Electric charge cannot move on some materials.

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Inside the atom

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Electrostatic forces

•Electric charge can be positive and negative.

•The forces that act between two charged objects can be both attractive and repulsive

•Opposites charges attract, similar charges repel.

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Gravity and electricity

• The big difference between the effects of gravity and electricity is repulsion.

• All objects are attracted towards one another because of gravity regardless of their electric charge. Gravity cannot repel.

• Electricity is stronger than gravity on a small scale.

• Gravity wins when we look at large objects, such as the Earth.

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Protons do not move

•A key idea when studying static electricity is that protons do not move.

•An object can become positively charged when electrons leave it, but the protons do not move.

•An object can become negatively charged when electrons are added to it, but the protons do not move.

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Protons do not move

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Electrons on = Negative charge

Electrons off = Positive charge

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Protons do not move

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Conductors and insulators

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Conductors conduct a lot

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•Any sort of charge, in this case negative electrons, can flow along the surface of a conductor.

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Insulators only a little

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Insulators are materials that do not have any ‘free electrons’. Charge cannot flow along the surface of an insulator. However if the voltage is big enough electric charge can flow through any material. We know that humans, water and the air can conduct electricity in certain extreme conditions.

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Charge distribution

•When electrons leave an object it becomes positively charged. On a conductor the remaining electrons spread themselves out, leaving a constant positive charge.

•When electrons leave an insulator it becomes positively charged. The remaining electrons cannot move so the charge is concentrated in one place.

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Give it a rub

We have all done it at some time or another, and we all love it.

Rub a balloon on a sweater and it will become charged enough to stick to a wall.

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Why does it stick ?•Electrons from your jumper jump onto the balloon, it becomes negative.

•When it goes near a wall the balloon repels the electrons in the wall away from their atoms.

•The electrons on the balloon are attracted to the atoms that are now positive and stick to the wall.

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Carpets and friction

•If you are wearing certain shoes and walking on a certain type of carpet you will become charged. In this example electrons move from the shoe to the carpet.

•When we touch a conducting object that is Earthed electrons flow onto our bodies to neutralise the charge. Although the voltage is large the current is small so we are not harmed.

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Rods and cloths

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Learning about static

•We can learn a lot of information about static charges by charging plastic rods by friction.

•Charge a rod and attempt to move a thin stream of water or pick up a tiny piece of paper.

•If you can suspend your rod try charging another and see if it attracts or repels.

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Rub that rubber

•Rub a rod of rubber with a piece of fur.

•Electrons from the fur will move to the rubber.

•The rubber will have a negative static charge.

•Only the electrons have moved.

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Electrons on

Leaves the rod negative

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Electrons off

Leaves the rod positive

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Protons do not move

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Other examples

• Acetate can be rubbed to gain a positive charge,• Vinyl can be rubbed to gain a negative charge,• If acetate has a positive charge it must have an

extra amount of :• Protons, because the electrons have left.• If vinyl has a negative charge it must have an extra

amount of :• Electrons, because they have been added.

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Rubber repulsion

•Both pieces of rubber have gained electrons.

•They both have a negative charge.

•The pieces of rubber repel one another.

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Glass loves rubber

•The rubber has gained electrons and has a negative charge.

•The glass has lost electrons and has a positive charge.

•The rubber and glass are attracted towards one another.

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Opposite charges attract

Two different rods will attract each other if they have different charge.

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Like charges repel

Two different rods will repel each other if they have the same charge.

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Charging a conductorIf a conductor is insulated from the rest of the world, such as this metal sphere, it can become charged.

•We charge the rod and then touch the sphere.

•Some electrons will pass from the rod to the sphere until the forces of repulsion are equal.

•The charge then spreads itself out over the surface until it is equal in all places.

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Harder thinking

•The negative rod does not affect the neutral ball.

•When the rod touches the ball some electrons move onto the ball, charging it negatively.

•The rod is moved away from the ball. The two now repel each other as they have the same charge.

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Harder thinking

•When a negatively charged rod is brought near a neutral metal sphere it will repel some of the electrons on its surface.

•The rod does not touch the metal.

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Harder thinking part two

•We now make a connection with the ground.

•The electrons that were repelled by the rod will escape to the ground.

•The ground connection can then be disconnected.

•The sphere now has less electrons.

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Harder thinking part three

•With less electrons the sphere has a positive charge.

•The rod can now be moved away from the sphere.

•Electrons on its surface move around to spread out the charge .

•This is called charging by induction.

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Some questions require a higher level of skill in the use of calculators. So test yourself with this.Remember:-

Students succeed better in Physics when answers that require calculations:-

1. show all working i.e., equation, substitution and answer. (C2),

2. have correct significant figures (C1)

3. show the correct unit (C1)

Particles become charged by the addition of electrons or the

removal of electrons.

An oil droplet has 3.0 x 106 electrons added to it. Show that

the oil droplet has a total charge of –4.8 x 10-13 C.

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Some questions require a higher level of skill in the use of calculators. So test yourself with this.

Particles become charged by the addition of electrons or the

removal of electrons.

An oil droplet has 3.0 x 106 electrons added to it. Show that

the oil droplet has a total charge of –4.8 x 10-13 C.

= 3.0 x 106 x (-1.60 x 10-19)

= - 4.8 x 10-13.

Total charge = Number of electrons x charge of each electron

AchievedNOT Achieved

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Summary – charging by friction

•When a rod is rubbed by a cloth electrons may move from one insulator to another.

•Losing electrons results in a positive charge.

•Gaining electrons results in a negative charge.

•Charged insulating materials can attract and repel each other.