Magnetic Field

Physics 102Professor Lee

CarknerLecture 16

PAL # 16 RC Circuits

From left loop: I2 = (15 – 11I1)/7.5 = 2 –1.5I1 From big loop: I3 = (15 –11I1)/18.2 = 0.82 – 0.6I1 Put in junction rule: I1 = 2 – 1.5I1 + 0.82 – 0.6I1 3.1I1 = 2.82 or I1 = 0.91 A Put in loop rules: I2 = 0.64 A, I3 = 0.27 A

Junction rule: I1 = I2 + I3 Loop rule (left loop): 15 – 7.5I2 –11I1 = 0 Loop rule (big loop): 15 – 6.2I3 – 12 I3 –11I1 =

0 15 – 18.2I3 –11I1 = 0

I1

I2 I3

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

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

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?

Right Hand Rule

If v is your straight fingers, and B is your curled fingers, F is your thumb

F

v

B

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 ()

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

Charged Particle in Field

q

vB

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 ~

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!

Earth as a Magnet

Jupiter’s Magnetosphere

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

Next Time

Read 20.2-20.6, 20.9 Homework: Ch 20, P 1, 9, 15, 27

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/3B) QC) 3QD) 6QE) 9Q

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 increasesB) It decreasesC) It depends on CD) It stays the sameE) None of the above