What do they have in common Worms Insects Loud speaker microphone earphone Hard drive Credit card Electric generator motor Bullet train Preinstalled software modern microwaves ovens watches calculators MRAM Antilock break Hybrid cars the list goes on and on Auto Motors with permanent magnets PM Motors per car avg 2000 25 2005 32 Door Gasket Defogger Motor Automatic Speakers Temperature Control TailGate Motor Sun Roof Motor Lumbar Support Head Rest Motor Tape Drive Motor Door Lock Motor Windshield Washer Pump Gauges Cruise Control Mirror Motors Liquid Level Indicators Anti skid Sensor and Motor Electric Power Steering and Sensor Economy and Pollution Control Four Wheel Steering Headlight Door Motor Ignition Systems Window Lift Motor Suspension System Fuel Pump Motor Chip Collector Coolant Fan Motor Seat Adjust Motors Seat Belt Motor Antenna Lift Motor Heat Air Condition Motor Traction Control Throttle and Crankshaft Position Sensors Windshield Wiper Motor Starter Motor Chapter 19 Magnetic Forces and Magnetic Fields 19 1 Magnets A magnet always has a north pole and a south pole at the same time Like poles repel and unlike poles attract S S N N S Break it up N Similar to the field of a positive and a negative charge electric dipole Similar to electric fields between parallel plates 19 2 Earth s Magnetic Field electron Nucleus Poor frogs and grass hoppers in the hands of nutty physicists 19 3 Force on a Charge v q v B v F q B v sin B F q v sin SI Unit Tesla 1 gauss 10 4Tesla Only true for a positive charge If it is a negative charge still use the RHR Then reverse the direction 19 4 The force on a current in a magnetic field Force on 1 moving charge F q v B sin Out of the page RHR v Force on many moving charges F q t vt B sin I L B sin Out of the page B v L vt I q t For HW you can be asked to find the magnetic force and combine it with friction or gravity Recall what total force you would need to have constant velocity Deal with one force at a time Example An electron moves between two plates of a capacitor where there is a magnetic field pointing into the page Which plate should be positively charged so that the electron can go straight without being deflected by the fields 19 5 What happens when it is a current loop Doesn t matter how ugly it is IABsin B z There is always at least one loop whose normal is not parallel to the B field This is what spins your CD player B y N loops x May you have more magnetic moments r is called magnetic moment IAN N number of turns Bsin For HW Be careful with the angle between the normal of the loop and the magnetic field Example A triangle as shown in the diagram experiences a torque of 3 0 Nm from a magnetic field that is perpendicular to the normal of the plane of the triangle 1 What is the current going through the loop 2 With this current what is the torque after the triangle rotates 90o 19 6 Moving charge in a magnetic field Force is perpendicular to B v Uniform B into page x x x x x x x x x x FR x x x x x x x F Like a satellite without the earth Solve for R F m x v F v v F B does no work W Fdcos x x v x Fx x x x F v Speed is constant x x x x x x x W K E 0 x x xv x x x x Circular motion x x x v2 R FM qvBsin qvB because 90 o v2 qvB m R Example Each chamber has its own magnetic field A positively charged particle enters chamber 1 with velocity 75m s and follows the dashed trajectory The magnetic field in region 1 is 1T the radius of the dashed line in region 1 is 0 2 m R mv qB 2 1 q 25 mC What are the magnetic field directions in regions 1 and 2 What is the mass of the particle If the radius of the dashed curve in region 2 is 0 4 m what is the magnetic field in region 2 x x x x 1 x x x x x x x x x x x x x x x x x x x x x x x x x x x x What are the signs of the charges 2 Mass spectrometer The radius gives the mass of the atom or molecule A magnet can experience a force from another magnet A current can experience a force from a magnet 19 7 Magnetic Field Produced by Currents Magnitude 0I 2 r 0 4 10 7 Tm A B Permeability of free space r distance from wire Right hand rule similar to that for magnetic moment with thumb and fingers exchanged between current and magnetic moment field Side view 19 8 magnetic force between two parallel wires B 0I 2 r Example Each of the two straight wires 1 m apart has a current 1 0 A going through it direction indicated What is the magnetic field mid point between the two wires If a proton moves at 2 5 x 108m s parallel to the wires at a distance of 1 0 m from the wire on the right What is the force magnitude and direction on the proton v I I p Let s bend a wire 19 9 Magnetic Fields of current loops and solenoids B In 19 5 we introduced the idea of magnetic moments which is another way of describing the same thing same hand rule Field inside a solenoid B 0nI n N L n Number of turns N per length L Electromagnet you can turn it on and off A real magnet can also be viewed as having currents going around except we can t easily turn them off Much more widely used than permanent magnets e g MRI