1. CH 18: Charged Particles and Electric Forces of Charged Particles on Other Charged Particles

2. Charged Particles
All ordinary matter is made of protons, neutrons, and electrons.
A proton has a charge of 1.6 x C.
An electron has a charge of 1.6 x C.
A neutron has no charge.

3. The Atom Classical Model (Planetary Model)
lithium (Li)
3 protons, 3 neutrons, and 3 electrons
What is the total charge of a lithium atom?

4. An ion An atom can gain an extra electron.
Or an atom can lose one of its electrons.

5. - - - - - - - - - - - - - - - + + + + + + + + + + + + + + +
Macroscopic Objects
Objects are generally neutral.
Equal numbers of + and – charges.
Net charge is zero. + + + + + - - - - - + + + + + - - - - - + + + + + - - - - -
Net Charge = 0

6. - - - - - - - - - + + + + + + + + + + + + + + + Positive Net Charge
What if we remove some negative charges? - - - + + + + + - - - + + + + + - - -
Net Charge = 0

7. - - - - - - - - + + + + + + + + + + + + + + + Positive Net Charge
What if we remove some negative charges?
Object has an overall positive charge.
How could we get a net negative charge? + + + + + - - + + + + + - - - + + + + + - - -
Net Charge = +

8. - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Negative Net Charge - - - + + + - + +
Add negative charges.
Object has an overall negative charge.
But how do we add or remove charges? - - - - - - - - - - - - + + + - - - + + - - - - - - - - - - - + + + + + - - - - -
Net Charge = -

9. Calculating Net Charge (Example)
Given an object that is composed of 2.0×1011 protons
and 4.0×1011 electrons, what is the object’s net charge?
Charge due to protons = (#protons)(charge per proton)
= (2.0×1011 protons)(+1.6×10-19 C/proton)
= +3.2×10-8 C
Charge due to electrons = (#electrons)(charge per electron)
= (4.0×1011 electrons)(-1.6×10-19 C/electron)
= -6.4×10-8 C
Net charge = Sum of all charges
= (+3.2×10-8 C) + (-6.4×10-8 C)
= -3.2×10-8 C

10. Poll
If you rub a balloon on your head, some electrons in the atoms of your hair move to the atoms of the latex balloon.
Is the charge of the balloon, positive, negative, or zero?
positive
negative
zero

11. (6.0×104 electrons)(-1.6×10-19 C/electron)
Example
Rubbing a plastic rod against a piece of wool
transfers 6.0×104 electrons from the wool to
the rod. What is the net charge of the rod?
(6.0×104 electrons)(-1.6×10-19 C/electron)
-9.6×10-15 C
What is the net charge of the piece of wool?
+9.6×10-15 C

12. - + “Magic” Tape Friction is a good way.
Take two neutral pieces of tape.
Stick the sticky side of one piece to the non-sticky side of other.
Rip apart quickly. - +
Net positive
charge
Net negative
Rip apart

13. Conservation of Charge
Negative charges moved from one neutral piece of tape to the other.
The total charge on both pieces together is still zero.
The net amount of charge produced in any process is always zero. - +
Equal amounts of positive
and negative charge

14. Types of Materials Conductors (metals) Insulators
Some electrons are loosely bound to nuclei and can move freely in the material.
Insulators
All electrons are tightly bound to nuclei.

15. Model of a neutral atom

16. Interaction of a charged particle and a neutral atom+q  +
neutral atom +q  +
Proton stays at the same place but electron cloud shifts toward the other positively charged particle.

17. Atom is polarized due to the other charged particle
The shifting of the electron cloud in a neutral atom due to an electric force of another charged particle on that electron is called polarization.
neutral atom +q  +

18. Representation of a polarized neutral atom

19. Poll
Will a charged particle and neutral atom attract, repel, or not interact?
attract
repel
not interact

20. Poll

21. Poll

22. Representation of a polarized insulator+ -

23. Neutral Conductor and Charged Insulator- + - + - + - - + + - + - + - + - + - + - + - + + - - +

24. Interaction of a charged insulator and a neutral conductor
More positively
charged area - +
Free electrons are repelled from rod.
Right side + charged & left side - charged.
Net attractive force between rod and conductor. -
More negatively
charged area - + + - - - + + - + - + + - - - + + - + + + - - +

25. Interaction of a neutral insulator and a charged insulator+ - - + + - + - + - + - + - + - + - + - + - + - + - + - + - + - + - + - + -

26. Induced Charge in an Insulator+ -
Negative charges repelled by rod.
Electrons are not free but polarization occurs.
Face of insulator becomes + charged.
Net attractive force between rod and insulator. - + + - + - + - + - + - + -