2/8/2010 Do Now: 12/16/2013 (on last week’s paper) What makes a magnet a magnet? What makes a magnet a magnet? Why are some magnets stronger than others? Why are some magnets stronger than others? What else do you know about magnets? What else do you know about magnets?

2/8/2010 Objectives Discuss characteristics of magnets. Discuss characteristics of magnets. Describe magnetic field lines Describe magnetic field lines Quantify magnetic fields. Quantify magnetic fields. Calculate force on wires and charges. Calculate force on wires and charges. Calculate force on wires and charges Calculate force on wires and charges Describe origins of induced emf. Describe origins of induced emf. Apply Faraday’s Law of Induction. Apply Faraday’s Law of Induction. Apply Lenz’s Law. Apply Lenz’s Law.

2/8/2010 Magnets (some basics) 2 poles…always 2 poles…always Can’t isolate a single magnetic charge. Can’t isolate a single magnetic charge. Opposites attract. Opposites attract. Ferromagnetic materials include iron, cobalt, nickel, and gadolinium. Ferromagnetic materials include iron, cobalt, nickel, and gadolinium. Paramagnetic materials – everything else. Paramagnetic materials – everything else.

2/8/2010 Magnetic Fields Described with field lines. Described with field lines. Direction of field is tangent to line at any point. Direction of field is tangent to line at any point. Number of lines per unit area proportional to the magnitude of the field. Number of lines per unit area proportional to the magnitude of the field. Lines go from N to S. Lines go from N to S. N S

2/8/2010 Quantifying Magnetic Field Direction based on compass needle Direction based on compass needle Magnitude of B defined as torque exerted on compass needle Magnitude of B defined as torque exerted on compass needle Magnetic Field is a vector with symbol B. Magnetic Field is a vector with symbol B. N Pole is really the south magnetic pole. N Pole is really the south magnetic pole.

2/8/2010 Units for B Tesla (use this for calculations!) Tesla (use this for calculations!) 1T = 1 N/A-m 1T = 1 N/A-m Gauss Gauss 1G = 1 x T 1G = 1 x T

2/8/2010 Electric Currents & Magnetism 1820—Oersted found deflection of compass needle near electric wire. 1820—Oersted found deflection of compass needle near electric wire. An electric current produces a magnetic field. An electric current produces a magnetic field. Direction of field around current carrying wire described with the “Right Hand Rule” (See page 591). Direction of field around current carrying wire described with the “Right Hand Rule” (See page 591). Use conventional current flow. Use conventional current flow.

2/8/2010 Force on I in B Current exerts force in B, B exerts force on I. Current exerts force in B, B exerts force on I. Force is perpendicular to current in wire and magnetic field ( B ) Force is perpendicular to current in wire and magnetic field ( B ) Right hand rule applies Right hand rule applies F = IlBsin(  ) F = IlBsin(  )

2/8/2010 Force on I in B F = IlBsin(  ) F = IlBsin(  ) Assume uniform magnetic field Assume uniform magnetic field Current parallel to field B, Force 0. Current parallel to field B, Force 0. Current perpendicular to field B, Force max. Current perpendicular to field B, Force max.

Example: A proton moves at 8x106 m/s along the x- axis. It enters a region in which there is a magnetic field 2.5 T, directed at an angle of 60 with the x-axis and lying along the x- y plane. Calculate the initial force and acceleration of the proton. A proton moves at 8x106 m/s along the x- axis. It enters a region in which there is a magnetic field 2.5 T, directed at an angle of 60 with the x-axis and lying along the x- y plane. Calculate the initial force and acceleration of the proton. 2/8/2010

Electric Charge in B Force on Moving charge in B – Lorentz Force. Force on Moving charge in B – Lorentz Force. From F=Il B sin(  ) From F=Il B sin(  ) We have N particles of charge q passing a reference point in time t, so I=Nq/t We have N particles of charge q passing a reference point in time t, so I=Nq/t Since t is time to travel distance l and v=d/t, then we can let l = vt and so F=(Nq/t)(vt) B sin  so… Since t is time to travel distance l and v=d/t, then we can let l = vt and so F=(Nq/t)(vt) B sin  so…

2/8/2010 Force on a single charge F = qvBsin(  ) v is a vector, B is a vector so we have to take the cross product and use right hand rule. pointer finger points in direction of v pointer finger points in direction of v middle finger points in direction of B middle finger points in direction of B Thumb points in direction of F Thumb points in direction of F Rule for positive charge only! Rule for positive charge only!

Examples (on handout) (on handout) 2/8/2010

Long Straight Wires B = (  0 /2  )(I/r) Field strength is proportional to current and inversely proportional to distance from wire. Field strength is proportional to current and inversely proportional to distance from wire. Constant of proportionality is Constant of proportionality is  0 /2  Where mu is permeability of free space = 4  x10 -7 T-m/A Review example Review example 20-7.

2/8/ Long Wires 2 current carrying wires will exert forces on each other. 2 current carrying wires will exert forces on each other. Right hand rules to determine field direction and force direction on wire. Right hand rules to determine field direction and force direction on wire. Currents same direction—attractive. Currents same direction—attractive. Currents opposite directions—repulsive. Currents opposite directions—repulsive.

2/8/ Long Wires F = I 2 lB 1 and B 1 = (  0 /2  )(I 1 /r) F = I 2 lB 1 and B 1 = (  0 /2  )(I 1 /r)So F = I 2 l (  0 /2  )(I 1 /r) F = I 2 l (  0 /2  )(I 1 /r)So F/l = (  0 /2  )(I 1 I 2 /r) F/l = (  0 /2  )(I 1 I 2 /r) Review example 20-8 and Review example 20-8 and 20-9.

Practice: 1-4 MC in Chapter MC in Chapter 19 2/8/2010

Do Now (12/18/13): *Pass in your HW, please! 1. A wire carries a current of 22 A from east to west. Assume that at this location the magnetic field is 0.5 G and points from North to South. Find the magnetic force on a 36 m length of wire. 2. How does this change if the current runs west to east? 3. If the current is directed north to south, what is the magnetic force on the wire? 2/8/2010

Torque on a Current Loop Torque Torque 2/8/2010

Example: A circular loop of radius 50 cm is oriented at an angle of 30 to a magnetic field of 0.5 T. The current in the loop is 2 A. Find the magnitude of torque at this instant. A circular loop of radius 50 cm is oriented at an angle of 30 to a magnetic field of 0.5 T. The current in the loop is 2 A. Find the magnitude of torque at this instant. 2/8/2010

Galvanometer Galvanometer is the historical name given to a moving coil electric current detector. When a current is passed through a coil in a magnetic field, the coil experiences a torque proportional to the current. If the coil's movement is opposed by a coil spring, then the amount of deflection of a needle attached to the coil may be proportional to the current passing through the coil. Such "meter movements" were at the heart of the moving coil meters such as voltmeters and ammetersuntil they were largely replaced with solid state meters Galvanometer is the historical name given to a moving coil electric current detector. When a current is passed through a coil in a magnetic field, the coil experiences a torque proportional to the current. If the coil's movement is opposed by a coil spring, then the amount of deflection of a needle attached to the coil may be proportional to the current passing through the coil. Such "meter movements" were at the heart of the moving coil meters such as voltmeters and ammetersuntil they were largely replaced with solid state meterselectric currenttorquemoving coil metersvoltmetersammeterselectric currenttorquemoving coil metersvoltmetersammeters 2/8/2010

galvanometer galvanometer galvanometer 2/8/2010

Galvanometer A galvanometer is the basis of an ammeter and a voltmeter A galvanometer is the basis of an ammeter and a voltmeter 2/8/2010

Motion of a Charged Particle in a B-field motion motion 2/8/2010

Example A proton is moving in a in a circular orbit of radius 14 cm in a uniform magnetic field of magnitude 0.35 T, directed perpendicularly to the velocity of the proton. Find the orbital speed of the proton. A proton is moving in a in a circular orbit of radius 14 cm in a uniform magnetic field of magnitude 0.35 T, directed perpendicularly to the velocity of the proton. Find the orbital speed of the proton. 2/8/2010

Mass spectrometer The mass spectrometer is an instrument which can measure the masses and relative concentrations of atoms and molecules. It makes use of the basic magnetic force on a moving charged particle. The mass spectrometer is an instrument which can measure the masses and relative concentrations of atoms and molecules. It makes use of the basic magnetic force on a moving charged particle. magnetic force magnetic force 2/8/2010

Mass Spectrometer How it works How it works 2/8/2010

Example: Mass Spectrometer Two singly ionized atoms move out of a slit at a point S (shown on board) and into a magnetic field of 0.1 T. Each has a speed of 1 x10 6 m/s. The nucleus of the first atom contains one proton and the second contains a proton and a neutron. Atoms with the same chemical properties but different masses are called isotopes. The two isotopes here are hydrogen and deuterium. Find their distance of separation when they strike a photographic plate at P. 2/8/2010

Practice: Complete MC 1-4 (5-6 are bonus) in Chapter 19 Complete MC 1-4 (5-6 are bonus) in Chapter 19 Problem 30 Problem 30 Will be collected Will be collected 2/8/2010