1. Magnetic Field Physics 102 Professor Lee Carkner Lecture 16

2. PAL # 16 RC Circuits  From left loop: I 2 = (15 – 11I 1 )/7.5 = 2 –1.5I 1  From big loop: I 3 = (15 –11I 1 )/18.2 = 0.82 – 0.6I 1  Put in junction rule: I 1 = 2 – 1.5I 1 + 0.82 – 0.6I 1  3.1I 1 = 2.82 or I 1 = 0.91 A  Put in loop rules: I 2 = 0.64 A, I 3 = 0.27 A  Junction rule:  I 1 = I 2 + I 3  Loop rule (left loop):  15 – 7.5I 2 –11I 1 = 0  Loop rule (big loop):  15 – 6.2I 3 – 12 I 3 –11I 1 = 0  15 – 18.2I 3 –11I 1 = 0 I1I1 I2I2 I3I3

3. Electricity and Magnetism  Magnets exert a force on two types of objects:    Both of these forces are due to the same fact:  Magnetic fields produce a force on moving charges   Moving charges produce a magnetic field  Both electricity and magnetism are related to charge

4. Magnetic Domains  Magnetic fields are always dipolar   Magnetic fields are always due to moving charges   Regions where the electron spins reinforce each other  If you apply an external magnetic field to certain metals you will force the domains to line up and create a magnet

5. Vectors  A magnet produces a magnetic field (B)   The moving particle has a velocity (v)   B, F and v have a direction and a magnitude   i.e., if the B field points one way and the charge is moving another way, what is the direction of the force?

6. Right Hand Rule   If v is your straight fingers, and B is your curled fingers, F is your thumb F v B

7. Vector Conventions  The force on a negative particle is opposite that of a positive one   Vectors going into the page are represented with a cross (X), vectors going out of a page are represented with a dot (  )

8. Magnetic Force Magnitude  The magnitude of the magnetic force depends on 4 things:   The magnitude of the charge (q)   The angle between the v and B vectors (  )  The force can be written as: F = qvB sin 

9. Charged Particle in Field q v B 

10. Magnetic Field  We can use the expression for the force to write an expression for the magnetic field: B = F/qv sin    We will often use a smaller unit, the gauss (G)   Typical bar magnet ~  Earth’s magnetic field ~

11. The Earth’s Magnetic Field  The Earth has a magnetic field produced by the dynamo effect   We can find the direction of the Earth’s magnetic field with a small bar magnet called a compass   The Earth’s “north magnetic pole” is really a the south pole of a magnet!

12. Earth as a Magnet

13. Jupiter’s Magnetosphere

14. Planets and Magnetism  A planet will have a magnetic field if:    The planet’s magnetic field interacts with charged particles streaming off of the Sun known as the solar wind  This interaction region is known as the magnetosphere 

15. Next Time  Read 20.2-20.6, 20.9  Homework: Ch 20, P 1, 9, 15, 27

16. Three identical capacitors are connected in parallel. If a total charge Q flows from the battery, what is the charge on each capacitor? A)Q/3 B)Q C)3Q D)6Q E)9Q

17. A resistor R and capacitor C are connected to a battery. If the resistor is replaced with a resistor of 2R, what happens to the time needed to charge the capacitor? A)It increases B)It decreases C)It depends on C D)It stays the same E)None of the above