Download Honors Physics Unit 8 Review Guide Magnetism Test Format 15

Honors Physics
Unit 8 Review Guide
Magnetism
Test Format
 15 multiple choice question (2 pts each)
o All concepts and directions, right hand rules, etc.
 5 short problems (6 pts each)
o Short math problems
 1 drawing problem (10 pts)
o Drawing fields, forces and directions
 1 explanation/discussion (10 pts)
o Conceptual understanding/explanation
 1 medium problem (20 pts)
o Multi-step problem with drawing and math
Topics Covered

Magnetic Fields
o Bar magnets
 2 poles
 Opposites attract
o Compasses
o Magnetic field on Earth
o Magnetic field lines
 Come out of N poles
 Go into S pole
 Never end
o Created by moving charges
 Electrons in certain materials (bar magnets – ferromagnets)
 Currents – charges moving in wires
o Unit is Tesla (T) and symbol is B
o Right-hand rule #1
 Thumb in direction of current and fingers curl in direction of magnetic field
o Ampere’s Law – source of magnetic field from a current

o
𝐵=
𝜇0 𝐼
2𝜋𝑟
Solenoids
 𝐵 = 𝜇0 𝑛𝐼
 Acts like a magnet with 2 poles
o
o

Forces in Magnetic Fields
 F = qvB – Lorentz force on a moving charged particle
 F = BIL – force on a wire
 Direction given by right-hand rule #2 (gun position with fingers)
 Thumb = force
 Index = velocity/current
 Middle = magnetic field
 Negative charges go the other way
 Force is perpendicular to velocity and magnetic field  charge move in circles
Velocity selector
 Has both a E and B field
 Forces can balance for a particular velocity
 Useful in a mass spectrometer
Electromagnetism
o Faraday’s Law: changing magnetic fields create electricity
o Magnetic Flux
o Lenz’s Law: gives direction of induced current  opposes original change in field
o Alternating current
o Generators
o Power Plants
Practice Problems
Multiple Choice practice: go over old homework questions
Possible conceptual topics:
Units of things
Directions of fields and forces (Right-hand rules)
How fields, forces depends on things
Induction directions
Power Plant information
Short Problems
1 . What is magnetic field a distance of 10 cm away from a wire carrying 4 Amps of current.
2. I can make a solenoid that has 1000 turns and carries 50 Amps of current. How long does it
need to be to make a magnetic field in the middle that is equal to 1 Tesla?
3. The Earth’s magnetic field is 0.5 Gauss running from N to S. If I placed a proton in it with a
speed of 1000 m/s running East, what would happen to it. Ignore any effects of gravity
4. Two wires have equal currents running in opposite direction. They are each 1 meter long and
have a mass of 0.1 kg. How much current would have to pass through the wires in order for one to
levitate on top of the other at a distance of 20 cm?
Drawing Problems
5. Draw the magnetic field for a wire with a current coming out of the page toward you
6. Draw what happens to a proton that is initially moving up the page in a magnetic field
directed into the page.
7. I have a setup with a constant magnetic field directed out of the page. I put a loop of wire in
it and slowly increase the area of the loop. Draw the setup and indicate what direction the induced
current flows?
Explanation Problems
 How magnets work
 How magnets can be used to levitate things
 How a velocity selector/mass spectrometer works
 How generators work
 Discussion of different power plants and how they work
Medium Problem
8. A wire carries a current of 5 Amps across the page to the right.
a. Draw the magnetic field surrounding that wire
b. What is the value of the magnetic field 4 cm below the wire?
c. A proton is placed at that point 4 cm below the wire and moves to the left with a velocity of
100 m/s. What is the magnitude and direction of the force on that proton?
d. Suppose that instead of a proton, I placed another wire with a length of 25 cm and a current
of 10 Amps going to the left at that same point 4 cm below the original wire. What is the magnitude and
direction of the force on that wire?
Practice Problems Key
Short Problems:
1. B = 8 x 10-6 T
2. L = 6.3 cm
3. Force would be “down” which is towards the ground in this case. It would act like gravity.
4. Both currents would need to be about 1000 Amps.
Drawing Problem:
5. Concentric circles going counter-clockwise
6. Force is to the left
7. Induced current is clockwise
Medium Problem:
8. a. Field coming out above the wire and going in below the wire
b. B = 2.5 x 10-5 T
c. F = 4 x 10-22 N down the page
d. F = 6.25 x 10-5 N down the page