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## Review on Magnetism

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**Review on Magnetism**Chapter 28**Magnetism**• Refrigerators are attracted to magnets!**Where is Magnetism Used??**• Motors • Navigation – Compass • Magnetic Tapes • Music, Data • Television • Beam deflection Coil • Magnetic Resonance Imaging (MRI) • High Energy Physics Research**Cathode**Anode (28 – 8)**N**S Consider a Permanent Magnet The magnetic Field B goes from North to South.**q**• If the charge is moving, there • is a force on the charge, • perpendicularto both v and B. • F = q vxB q A Look at the Physics There is NO force on a charge placed into a magnetic field if the charge is NOT moving. There is no force if the charge moves parallel to the field.**The Lorentz Force**This can be summarized as: F or: v q m B q is the angle between B and V**The Wire in More Detail**Assume all electrons are moving with the same velocity vd. L B out of plane of the paper**i**. (28 – 12)**Current Loop**What is force on the ends?? Loop will tend to rotate due to the torque the field applies to the loop.**Side view**Top view C C (28 – 13)**Dipole Moment Definition**• Define the magnetic • dipole moment of • the coil m as: • =NiA t=m x B We can convert this to a vector with A as defined as being normal to the area as in the previous slide.**R**L L R L R (28 – 15)**v**B + + + + + + + + + + + + + + + + + + + + F Trajectory of Charged Particlesin a Magnetic Field (B field points into plane of paper.) B + + + + + + + + + + + + + + + + + + + + v F**Trajectory of Charged Particlesin a Magnetic Field**(B field pointsinto plane of paper.) v B B + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + v F F Magnetic Force is a centripetal force** = s / r s = r ds/dt = d/dt r v**= r s = angle, = angular speed, = angular acceleration r at = r tangential acceleration ar = v2 / rradial acceleration The radial acceleration changes the direction of motion, while the tangential acceleration changes the speed. at ar Uniform Circular Motion = constant v and ar constant but direction changes ar KE = ½ mv2 = ½ mw2r2 ar = v2/r = 2 r v F = mar = mv2/r = m2r Review of Rotational Motion**YES !**You have to remember these stuff.**Radius of a Charged ParticleOrbit in a Magnetic Field**Centripetal Magnetic Force Force = v B + + + + + + + + + + + + + + + + + + + + F r**v**B + + + + + + + + + + + + + + + + + + + + F r Cyclotron Frequency The time taken to complete one orbit is: V cancels !**Smaller Mass**Mass Spectrometer**An Example**A beam of electrons whose kinetic energy is K emerges from a thin-foil “window” at the end of an accelerator tube. There is a metal plate a distance d from this window and perpendicular to the direction of the emerging beam. Show that we can prevent the beam from hitting the plate if we apply a uniform magnetic field B such that**r**Problem Continued**#14 Chapter 28**A metal strip 6.50 cm long, 0.850 cm wide, and 0.760 mm thick moves with constant velocity through a uniform magnetic field B= 1.20mTdirected perpendicular to the strip, as shown in the Figure. A potential difference of 3.90 ηV is measured between points x and y across the strip. Calculate the speed v.**21. (a) Find the frequency of revolution of an electron**with an energy of 100 eV in a uniform magnetic field of magnitude 35.0 µT . (b) Calculate the radius of the path of this electron if its velocity is perpendicular to the magnetic field.**39. A 13.0 g wire of length L = 62.0 cm is suspended by a**pair of flexible leads in a uniform magnetic field of magnitude 0.440 T. What are the (a) magnitude and (b) direction (left or right) of the current required to remove the tension in the supporting leads?