“PHLYZICS”

Static Electricity and CHARGING objects The basics of CHARGE:

• There are only two types of CHARGE à Positive (+) and Negative (-) • Objects that have TOO MANY electrons are negative. An object does not need to

have ALL electrons to be negatively charged. • Objects that have TOO FEW electrons are positive. An object does not need to

have only protons to be positively charged. • Objects that have the same number of protons and electrons are NEUTRAL. • UNLIKE CHARGES ATTRACT (for example, negative attracts to positive). • LIKE CHARGES REPEL (for example, negative repels negative).

Why do objects become charged?

• In nature, atoms prefer to be neutrally charged. • The atom:

- The nucleus contains protons (+) and neutrons (neutral). - Electrons orbit the nucleus in clouds. They occupy energy

levels. When they are energized (for example, by heating the atom), they jump UP energy levels. When they jump back down, they release their energy.

- Electrons are held in orbit by a CENTRIPETAL FORCE, which is supplied by the attraction between the + nucleus and the – electron.

- Some atoms hold their electrons VERY tightly (with a very strong attractive force).

- Some atoms hold their electrons less tightly. These atoms tend to lose their electrons when different atoms come into contact with each other.

• Objects become charged when electrons are transferred from

one atom to another. • PROTONS NEVER GET TRANSFERRED!!!! They are

locked in the nucleus and cannot move from atom to atom. Only “free electrons” get transferred.

• So, how do we know which atoms GAIN electrons and which atoms LOSE electrons when two atoms rub together?

…. Use the ELECTROSTATIC SERIES à à à

• When two materials are rubbed together, the material closer to the top of the table loses electrons, while the other material gains them.

How can you charge an object? There are four ways to charge an object: 1) By FRICTION

2) By CONTACT 3) By INDUCTION 4) By POLARIZATION

Charging by CONTACT • First of all, to charge by contact, you need to start with an object that is already

charged. • To charge a neutral object, you simply touch the charged object to the neutral object. • The neutral object will gain electrons (if the charged object was negatively charged)

and it will lose electrons (if the charged object was positively charged). • The result of charging by contact: … A newly charged object with the SAME charge as the originally charged object.

+ + + + +

- - -

-

Charging by FRICTION - Rub two unlike materials together.

- One will gain electrons (becoming - )

and one will lose electrons (becoming + ) - Use to ELECTROSTATIC SERIES to

figure out which one becomes + or -

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Conductors vs. Insulators Before we can learn the 3rd method of charging an object, we need to understand the difference between CONDUCTORS and INSULATORS. CONDUCTORS

o Examples include metals o Allow electrons to flow freely through them (they “conduct” electricity)

INSULATORS

o examples include wood, glass, ceramics o DO NOT allow electrons to flow freely through them o They CAN be charged, but the charge will NOT move through them. It will

stay at the location on the object that was charged.

Charge Separation When a charged object is brought near (but not touching) a neutral conductor, the charged object causes the charge on the conductor to separate.

Charging by INDUCTION Induction is a charging method that charges a neutral object WITHOUT actually allowing the charged object to contact the neutral object. To use this method, it is important to follow a few steps VERY carefully, and in order:

Step 1) Bring a charged object close to (BUT NOT TOUCHING) a neutral conductor. Step 2) The electrons will move (causing a charge separation). Step 3) Make the separation permanent (while keeping the charged object close, but not

touching). Step 4) Remove the charged object, leaving a newly charged object with THE OPPOSITE CHARGE.

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NOTE:

THE Newly charged object has the OPPOSITE charge as the originally charged object when charged by INDUCTION. This is the opposite of the “Charging by Contact” case (where both objects had the same charge at the end.

Charging by POLARIZATION

Another example of Charging by

Polarization is a balloon bending water (aka “The Great Water-

Bender”

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+ + + + + + +

+ + + + + + +

Top view

1. If two charged rods repel each other, what can be said about the sign of the charge on each rod?

2. Suppose you have four charged objects, A, B, C, and D. You find that A repels B, A attracts C, and C repels D. If you know that D is positively charged, what kind of charge does B have?

3. A small metal rod is positively charged. You touch this metal rod with another metal rod that you are holding

in your hand. Fully explain what will happen. 4. Explain why objects are often “grounded” by touching them to a metal pipe that is driven into (or staked into)

the earth. (Remember: grounding is a way of neutralizing an object) 5. You have two metal spheres of the same size mounted on insulating stands, a strip of plastic, and a piece of

paper. Describe what you would do to give the two spheres equal electrical charges a. of the same sign. b. of opposite sign.

6. A neutral pith ball is hanging from a string. A positively charged rod is brought into contact with it, thus

causing the ball to repel away from the rod. You notice that the pith ball slowly returns to the rod, and then immediately “jumps” away from it again. Explain what you think happened to cause this.

7. In charging an object, you can add or remove negative charges or you can add or remove positive charges.

Which procedure would charge the object negatively? (There is a common misconception in this problem. Can you see why the first sentence is not entirely true?)

8. Must all negative charges be removed in order for a rod to become positively charged?

9. Three metal blocks in contact are resting on an insulating tablecloth,

as shown at the right. You place two objects with strong positive charges, one at each end of the blocks, close but not touching the blocks. You then poke the blocks apart with an insulating, uncharged rod while the objects with strong positive charges are nearby. Finally, you remove the two positively charged objects. What is the charge on each block?

10. Repeat the previous problem, this time with a positively charged sphere and a negatively charged sphere on

each end (instead of the two positively charged spheres). Assume that both spheres have equal amounts of their respective charge and that all three blocks are exactly the same size.

11. Two metal rods are mounted on insulating blocks and aligned along a common axis, with a space between

them. One is positively charged, the other negatively charged. A light sphere with a conducting coating suspended by a long insulating thread is introduced into the space between the rods and touched to the positive rod. What do you predict will happen? Explain your reasoning.

12. A Van de Graff generator (as shown at the right … like the one we electrocuted ourselves in class with J) becomes charged because a rubber belt and a silicon roller rub against each other inside the generator. If the belt supplies the charge to the metal sphere on top of the generator, will this sphere be positively or negatively charged? Explain.

13. If you put 5 metal pans (in a nice, neat stack) on top of a working Van de Graff generator, the

pans will fly off, one by one. Explain why.

Practice Problems (write mini-essay answers to these questions)

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