Creating Magnetic Fields

1. Creating Magnetic Fields Text: Ch. 20 M. Blachly, AP Physics

2. Magnetic Field • Magnetic fields are created by moving charges. • For a long, straight wire, the magnetic field circulates around the wire.

3. Magnetic Field • Direction of B is given by right hand rule: •  Thumb in direction of current, fingers curl in the direction of B

5. Ampère’s Law Ampère’s law relates the magnetic field around a closed loop to the total current flowing through the loop.

6. Ampère’s Law Ampère’s law can be used to calculate the magnetic field in situations with a high degree of symmetry.

7. Magnetic Field of a Long Straight Wire The field is inversely proportional to the distance from the wire: The constant μ0 is called the permeability of free space, and has the value:

8. Summary • A current moving in a wire produces a magnetic field • A magnetic field produces a force on a wire that carries a current.

9. Two Wires • What will happen if there are two long, parallel wires that each carry a current?

10. Force between Two Parallel Wires The magnetic field produced at the position of wire 2 due to the current in wire 1 is: The force this field exerts on a length l2 of wire 2 is:

11. Force between Two Parallel Wires Parallel currents attract; antiparallel currents repel.

12. A loop of wire • What if we bend our wire into a loop?

13. Solenoid • What if we bend our wire into lots of loops?

14. The Right-hand Rule

15. Magnetic Materials • Electrons “orbit” the nucleus and also “spin”. This produces a magnetic field • Electrons generally pair up • most of the magnetic field cancels

16. Magnetic Materials • In some materials, the magnetic fields do not cancel • iron, cobalt and nickel • The atoms “align” in a small region and create a domain. • The domains persist when the external magnetic field is removed • Also called “hard” magnets or permanent

17. Paramagnetic material • The magnetic spins align in small regions forming a domain. • Domains can align with an external magnetic field • The domains do not persist when the external magnetic field is removed • Also called “soft” magnets

18. Nonmagnetic material • Atom has no net magnetic moment so there can be no domain • Exhibits no magnetic effects: magnets cannot “stick” to nonmagnetic metals • Example: copper, stainless steel

19. Cores • Adding a core to a solenoid can greatly increase the magnetic field strength.

20. Applications A galvanometer takes advantage of the torque on a current loop to measure current.

21. Applications An electric motor also takes advantage of the torque on a current loop, to change electrical energy to mechanical energy.

22. Applications Loudspeakers use the principle that a magnet exerts a force on a current-carrying wire to convert electrical signals into mechanical vibrations, producing sound.

23. Applications A mass spectrometer measures the masses of atoms. If a charged particle is moving through perpendicular electric and magnetic fields, there is a particular speed at which it will not be deflected:

24. Mass Spectrometer All the atoms reaching the second magnetic field will have the same speed; their radius of curvature will depend on their mass.

25. Links • Additional Links: • http://www.physics.sjsu.edu/becker/physics51/induction.htm • http://hyperphysics.phy-astr.gsu.edu/HBASE/magnetic/magcon.html#c1 • All about how audio speakers work: http://electronics.howstuffworks.com/speaker6.htm