Unit 9: Electromagnetism Lesson 1: Magnetic Fields

Magnetism A force in nature that attracts certain metals, such as iron, nickel & cobalt. Magnets are surrounded by magnetic fields (regions where magnetic forces act). Magnetic field lines (show direction of force): 10 min - Point the way a compass would (from magnetic North to South) Density = field strength (strongest at poles in a bar magnet)

What causes magnetism? - In normal materials, the spins are randomly oriented. Electrons spin: - In magnetic materials (can be affected by magnetic forces) there is some interaction between spins. 10 min - In magnetic materials, the spins tend to line up in small areas called “domains”: - In a magnet (produces magnetic forces), the domains are lined up.

The Magnetic Field Around a Wire - All wires carrying current produce a magnetic field, B - Direction: determined by the right hand rule (RHR) 10 min Example: I (I out of page) B

Brain Break!

Magnetic Field in a Solenoid To determine direction: consider one loop of wire B Solenoid: a long straight coil of wire used to generate a magnetic field. I 10 min Whole coil: (similar to a bar magnet) Note: B direction inside coil new RHR thumb = magnetic field fingers = current

Example 2: Which end of the battery is positive? Which end is North? N Example 2: Which end of the battery is positive? 10 min Left side of battery

Homework: Pg. 206 #1 and 2

Unit 9: Electromagnetism Lesson 2: Magnetic Fields and Forces

Magnetic Field in a Solenoid (magnitude) Example: Find the magnetic field in the center of a solenoid of diameter 25 cm and length 32 cm with 500 turns and a current of 250 mA. 10 min

Quantitative Measure of Magnetic Forces Magnetic fields cause forces on moving charged particles. → B = magnetic field → v = velocity of particle q = charge 10 min → F = force on the charge θ is the angle between the velocity and the magnetic field Note: when the angle is 90°, sinθ = 1 F |F|= qvBsinθ B = qvsinθ Direction – use right hand rule Newton B is measured in Tesla = Coulomb•m/s

Right Hand Rule F Method: Thumb = force Pointer finger = velocity Positive charge 5 min Method: Thumb = force Pointer finger = velocity Middle finger = magnetic field Note: negative charge - reverse F

E.g. Positive charge (coming out of the page): E.g. What direction is the force? B B (out of page) v v - 10 min What’s the direction of the magnetic force? Force = up (negative charge) Force: left

Brain Break!

Application: Velocity Selector + + + + + E.g. Protons enter a region of uniform electric field 500 N/C as shown. v - - - - - - - 10 min Find the magnitude and direction of a magnetic field that would allow protons with velocity 2.0 x 106 m/s to pass through in a straight line. Direction : into page FB = FE } FB qvB = qE balanced E 500 B = = = 2.5 x 10-4 T v 2x106 FE

Homework: Pg. 216 #1-3 Pg. 217 #1-4 10 min

Unit 9: Electromagnetism Lesson 3: Moving Charges in Magnetic Fields

Force on a Current in a Magnetic Field B wire θ θ = 90o F = IlB q I F = IlBsinθ l Example: A wire carrying 20 A runs East to West along the equator where B = 5.0x10-5 T. Find the force per meter on the wire (include direction). 10 min N B F = IlB E W I = (20)(1)(5x10-5) S = 1.0 x 10-3 N (toward the ground)

Motion of Charged Particles in Magnetic Fields What happens when a proton enters the Earth’s magnetic field (5.0x10-5 T), in a direction perpendicular to the field lines, at a velocity of 3.0 x 106 m/s? x x x x x F is always perpendicular to v x x x x x F Work = F||•d = 0! x x x x x F Therefore, |v| stays constant. x x x x x v 15 min Uniform circular motion! x x x x x Find the radius of the circle: F = ma qvB = mv2/R mv (1.67x10-27)(3x106) R = = = 630 m qB (1.6x10-19)(5x10-5)

Brain Break!

Find the velocity of the helium nuclei. Try This: Helium nuclei enter a region of uniform magnetic field of 3.0 x 10-3 T (perpendicular to field) and make a circular arc of radius 3.7 m. Find the velocity of the helium nuclei. m = 4(1.67x10-27 kg) q = 2e = 3.2x10-19 C F = ma mv2 qvB = 15 min R qRB (3.2x10-19)(3.7)(3x10-3) v = = m 6.67x10-27 v = 5.3 x 105 m/s

Cathode Ray Tube (CRT) Example: Electrons accelerated through 50 000 volts in a CRT enter a region of uniform perpendicular magnetic field and follow a curved path of radius 37 cm. Find the magnetic field strength. KE gained = PE lost 15 min Homework: Pg. 223 #1a, 3-5 = 0.0020 T