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Download Honors Physics Unit 8 Review Guide Magnetism Test Format 15
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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