Electrostatics Notes (614) (teacher)

Transcription

1 Electrostatics Notes (614) (teacher) Charge!!! Have you ever walked across the carpet and gotten shocked when you touched the doorknob?

2 ! What about static cling? Have you ever gotten to school only to be embarrassed when someone points out the sock sticking to your back? What s going on in these cases? Why did they occur?

3 Atomic Structure 3 Basic Particles make up Atoms: 1. Protons 2. Neutrons 3. Electrons Charge! Protons & Electrons have a property called electric charge Protons: positive electric charge (+) Electrons: negative electric charge (-) The strength of the positive charge on a proton is the same as the strength of the negative charge on the electron The unit of charge is the Coulomb (C)

4 Charge cont.! Neutrons do not have charge Neutrons: neutral Neutrons must be from Switzerland Can you tell the difference?! Can you tell the difference between an electron in an oxygen atom and an electron in a sodium atom?! The electrons of ALL atoms are identical. Each has the same quantity of charge and the same mass.

5 Particle Charge Summary:! Particles may be charged (positive or negative) or neutral (not charged) Particle Charge Protons positive (+) Neutrons neutral Electrons negative (-) Basically! Oppositely charged particles attract each other Ex: Protons (+) and Electrons (-) attract! Particles with the same charge repel each other Ex: 2 Electrons (-) would repel each other Ex: 2 Protons (+) would repel each other

6 Continued..! Particles with neutral charge do not interact Neutrons do not attract or repel each other Neutrons do not attract or repel electrons or protons In the real world, we rarely deal with individual charged particles! Everyday objects are made of lots of atoms! Most atoms have an equal number of protons and electrons! Therefore, most atoms are neutral Remember, even though atoms are neutral, they are still made of charges

7 What does it mean to say that an object is neutral?! A neutral object has no net (overall) charge! A neutral object has equal amounts of positive and negative charge What does it mean to say that an object is positively charged?! Positively charged object: more positive charges than negative charges

8 What does it mean to say that an object is negatively charged?! Negatively charged object: more negative charges than positive charges What does it mean for an object to be polarized?! When an object is polarized, its charges have shifted so that one side of the object has a net negative charge and the other side of the object has a net positive charge.! A polarized object might be neutral (no net charge) or have a net charge.

9 Electrons move, Protons don t!! Protons don t move! Protons are very massive. They have too much inertia. They are in the center of the atom.! Electrons are outside the nucleus. It is easier to move particles on the perimeter. What do you have to do to make an object positively charged?! You need to remove electrons from the object

10 What do you have to do to make an object negatively charged?! You need to add electrons to the object What is an ion?! When an atom gains an extra electron, it is negatively charged. It is then a negative ion.! When an atom loses an electron, it is positively charged. It is then a positive ion.

11 So what is happening when you rub a balloon on your head and it becomes charged? Is friction creating charge?! No! Charge cannot be created or destroyed. Conservation of Charge! Charges are being exchanged Conductors vs. Insulators! Conductors: loosely bound electrons Electrons flow easily Examples: metals! Insulators: tightly bound electrons Electrons flow slowly Examples: rubber, plastic, and styrofoam

12 Charge can exchange when conductors touch (Conduction)! When objects touch (or are close enough for a spark), electrons may be exchanged! Works best from conductor to conductor Charging by Conduction Suppose you have two identical metal spheres, imaginatively named A and B. Each sphere is on an insulating stand. Originally, A has 10 units of charge, and B is neutral. If the two spheres are briefly touched together, then separated, what is the charge on each? A. A has 10 units, B has zero. B. B has 10 units, A has zero. C. A has 10 units, and B also has 10 units. D. A and B each have 5 units. E. A and B each have zero charge.

13 Induction! A charged object charges a neutral conductor without contact! The conductor does make contact with a neutral object (often the ground)! The conductor ends up with a charge opposite that of the charged object brought near Charging by Induction Notice that we are charging by induction, grounding the side of the sphere next to the positive object. What will the final charge on the sphere be? A. Positive B. Negative C. Neutral D. It depends

14 Charge can exchange when insulators are scraped together! When objects are scraped together, friction is involved! Charging with friction works best with insulators! CHARGE IS NOT CREATED! Electrons are simply being. transferred The Triboelectric Series " Electron Donors (objects that give electrons become positive) (objects that take electrons become negative) " Electron Grabbers

15 Triboelectric Series A Triboelectric Sequence" ELECTRON GRABBERS " " " "ELECTRON DONORS" Rubber Amber Cotton Silk Cat fur Wool Glass Rabbit fur" If you rub cotton with amber, which becomes positive? A. Amber B. Neither C. Cotton Which of the following can make glass negative? A. Amber B. Cat fur C. Rabbit fur Charging with Friction You rub a balloon against your hair, and the hair becomes positively charged. This means that A. Electrons moved from the balloon to your hair. B. Protons moved from the balloon to your hair. C. Protons moved from your hair to the balloon. D. Electrons moved from your hair to the balloon. E. The rubbing destroyed electrons in your hair, leaving it positive.

16 When a plastic rod is scraped with wool! Both are insulators! The rod takes electrons and becomes negative! The wool lost electrons and becomes positive If two negative rods are held near each other! Predict: Will they ATTRACT or REPEL?! Watch your teacher do the demo. What happened?

17 If is a glass rod is rubbed with silk! The silk takes electrons and becomes negative.! What kind of charge will the glass rod have? When the charged plastic and glass rods are held near each other! Predict: Will they ATTRACT or REPEL? plastic! Watch your teacher do the demo. What happened? glass

18 Sometime neutral objects are attracted to charged objects! The negative balloon sticks to the neutral wall The wall is an insulator, so charges cannot move far Charges can rearrange in the neutral object s molecules Grounding Objects! When a charged conductor comes in contact with the earth, charges are exchanged until the object is neutralized.