# General Physics I - Department of Physics and Astronomy

Magnetism Physics 114 02/04/20 Lecture VII 1 Concepts Magnetic field Magnetic force 02/04/20 Lecture VII

2 Skills Determine the direction of magnetic field created by electric current Determine the direction of magnetic force on electric current Two right hand rules 02/04/20 Lecture VII 3 Magnets Magnets have magnetic

poles north and south Like poles repel Unlike poles attract Similarity to electric interaction 02/04/20 Lecture VII 4 Magnetic field Introduce magnetic field: Field lines go from north pole to south Mnemonic rule birds fly from north to south

02/04/20 Lecture VII 5 Magnetic field of the Earth Earth has a magnetic field B~5x10-5 T Compass - a small magnet in a form of an arrow - is used to determine the direction of the magnetic field South magnetic pole is located close to the north geographic pole, that is why north end of

the compass is pole is pointing there (unlike poles attract) 02/04/20 Lecture VII 6 Magnetic field Magnetic field is labeled by B It is measured in Tesla and Gauss 1T=10 kG 1T=1N/A m 02/04/20 Lecture VII

7 Magnets North and south poles do not exist separately!!! two halves of a broken magnet still have south and north pole each Different from electric charges positive and negative charges can exist separately 02/04/20

Lecture VII 8 Magnetic field created by currents Electric currents moving electric charges - create magnetic field Stationary electric charges do not create magnetic field First right hand rule Thumb along the current Wrap your fingers around the wire fingers show the direction of the magnetic field 02/04/20 Lecture VII 9

Magnetic field created by current Magnetic field B created by current I at a distance r from the conductor is 0 I B 2 r Tm/Amagneticpermeability of free space 02/04/20 Lecture VII 10

Magnetic field of loop current Magnetic field is in ~ same direction inside a current loop Several loops create stronger magnetic field solenoid 02/04/20 Lecture VII 11 Magnetic field of a solenoid Magnetic field inside solenoid Is parallel to its axis Depends on the current I

Depends on the number of loops per unit length n=N/l: B=0nI Does not depend on diameter Does not depend on the total length 02/04/20 Lecture VII 12 Magnetic force on electric currents Magnetic field exert a magnetic force on electric currents moving electric charges F=IlBsin

angle between B and I F is max when B is perpendicular I and zero when B is parallel to I 02/04/20 Lecture VII 13 Direction of the magnetic force Second right hand rule: Fingers along the current

Bend to show the direction of the magnetic field Thumb shows the direction of the force 02/04/20 Lecture VII 14 Magnetic force on moving charge Magnetic force F is perpendicular to the velocity v of a particle with charge q F=qvB sin Charged particles move in circles in

magnetic fields 02/04/20 Lecture VII 15 Magnetic fields are used to separate matter from antimatter And measure particle velocity Bubble chamber picture 02/04/20 Lecture VII

16 Torque r F Fr sin rF rF sin 02/04/20 Lecture VII 17 Current loop in magnetic field F1 IaB

b 1 F1 2 b 2 F2 2 1 2 b b IaB IaB 2 2 IabB IAB A=ab area of the loop Magnetic field exerts a torque on a loop parallel to the

magnetic field: IAB F2 IaB 02/04/20 Lecture VII 18 Current loop in magnetic field b 1 F1 2 b

2 F2 2 b 1 F1 sin 2 b 2 F2 sin 2 IAB IAB sin 02/04/20 Lecture VII

angle between magnetic field and a perpendicular to the loop !!! Magnetic field orients a loop current perpendicular to B. N loops: NIAB sin 19 Force between two currents Parallel currents attract Anti-parallel repel

0 I1 B1 2 L F21 I 2lB1 l 0 I1 I 2 l F21 2 L 02/04/20 Lecture VII 20

Ferromagnetism Electrons inside atoms little loop currents When placed inside magnetic field microscopic currents orient to enhance the external field This effect happens only in some materials ferromagnetic, e.g. iron. 02/04/20 domains Lecture VII

21 Magnetic materials Ferromagnetic Km>>1 (~1000) Iron, cobalt, nickel, some aloys residual magnetism can be destroyed (orientation of the domains randomized) by mechanical shock or high T Paramagnetic Km>1 just a bit (1+10-4,-5) No domains, but molecules have magnetic moment Aluminum, magnesium, oxygen, platinum Diamagnetic Km<1 a really tiny bit (1-10-5,-6) Molecules do not have a magnetic moment Copper, diamond, gold, silicon, nitrogen 02/04/20

Lecture VII 22

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