1. CH22: Magnetism
The magnificent spectacle of the Aurora Borealis, or northern lights, glows in the northern sky above Bear Lake near Eielson Air Force Base, Alaska. Shaped by the Earth’s magnetic field, this light is produced by radiation spewed from solar storms. (credit: Senior Airman Joshua Strang, via Flickr)

2. Outline Magnets, Magnetic fields, and Magnetic field of the Earth
Force on a moving charge in a magnetic field
Motion of a charge particle in a magnetic field
The mass spectrometer
Force on a current in a magnetic field
Magnetic fields produced by currents
Magnetic Resonance Imaging (MRI)
Cathode Ray Tube (CRT)
Magnetic storage

3. Magnetic Compass
The needle of a compass is a permanent magnet that has a north magnetic pole (N) at one end and a south magnetic pole (S) at the other.
Navigation in animals:
Vertebrates (ex: migratory birds) and invertebrates (ex: Spiny lobsters) use the earth's magnetic field to navigate and determine their geographic position.
Cow’s magnet and feeding.

4. Magnetic Poles
Like poles repel each other, and unlike poles attract each other.

5. North and south poles always occur in pairs
North and south poles always occur in pairs. Attempts to separate them result in more pairs of poles. If we continue to split the magnet, we will eventually get down to an iron atom with a north pole and a south pole—these, too, cannot be separated.

6. Magnetic Field of Magnets

7. Magnetic Field of the Earth
The earth behaves magnetically almost as if a bar magnet were located near its center. The axis of this fictitious bar magnet does not coincide with the earth's rotational axis; the two axes are currently about 11.5° apart.

8. Angle of Declination
The south magnetic pole does not coincide with the north geographic pole but, its position is not fixed but moves over the years.
The angular difference between the magnetic north and the geographical north is called the angle of declination.

9. Angle of Dip
Earth's magnetic field lines are not parallel to the surface at all points.
For instance, near the north magnetic pole the field lines are almost perpendicular to the surface of the earth.
The angle that the magnetic field makes with respect to the surface at any point is known as the angle of dip.

10. The Force That a Magnetic Field Exerts on a Moving Charge
The following two conditions must be met for a charge to experience a magnetic force when placed in a magnetic field:
1.The charge must be moving. No magnetic force acts on a stationary charge.
2.The velocity of the moving charge must have a component that is perpendicular to the direction of the magnetic field.

11. Magnetic Force
Consider a charge q0, moving with velocity v, in a magnetic field. Magnetic field will be denoted by the symbol B.
The magnetic force is given by,
where θ is the angle between the magnetic field and velocity.
10. An electron moving at 4.00×103 m/s in a 1.25-T magnetic field experiences a magnetic force of 1.40×10−16 N . What angle does the velocity of the electron make with the magnetic field?

12. Right-hand Rule No. 1
When the right hand is oriented so the fingers point along the magnetic field B and the thumb points along the velocity v of a positively charged particle, the palm faces in the direction of the magnetic force F applied to the particle.

13. Application of RHR
1. What is the direction of the magnetic force on a positive charge that moves as shown in each of the six cases shown below?

14. Definition of the Magnetic Field
Magnetic field is the force on a unit charge moving with a unit velocity perpendicular to the magnetic field.
The SI unit of the magnetic field is the tesla, T named after the Croatian-born American engineer Nikola Tesla ( ).

15. Units of Magnetic Field
The cgs unit for the magnetic field is the gauss, G.
1 T = 104 G.
The Earth’s magnetic field near its surface is about
0.5 G or 5  10-5 T.
Laboratory electromagnets can produce magnetic fields as high as 3 T.
Superconducting magnets can produce magnetic fields up to 30 T.

16. The Motion of a Charged Particle in an Electric or Magnetic Field

17. Velocity Selector
17. A velocity selector in a mass spectrometer uses a T magnetic field. (a) What electric field strength is needed to select a speed of 4.00×106 m/s ? (b) What is the voltage between the plates if they are separated by 1.00 cm?

18. The Circular Motion

19. Mass Spectrometer
Physicists use mass spectrometers for determining the relative masses and abundances of isotopes.
Chemists use these instruments to help identify unknown molecules produced in chemical reactions.
Mass spectrometers are also used during surgery, where they give the anesthesiologist information on the gases, including the anesthetic, in the patient's lungs.

20. Problem 20
20. A mass spectrometer is being used to separate common oxygen-16 from the much rarer oxygen-18, taken from a sample of old glacial ice. (The relative abundance of these oxygen isotopes is related to climatic temperature at the time the ice was deposited.) The ratio of the masses of these two ions is 16 to 18, the mass of oxygen-16 is 2.66×10−26 kg, and they are singly charged and travel at 5.00×106 m/s in a 1.20-T magnetic field. What is the separation between their paths when they hit a target after traversing a semicircle?

21. The Force on a Current in a Magnetic Field

22. Loudspeaker

23. The Torque on a Current-Carrying Coil

24. Electric Motor