1. Magnetism
Chapter 27 opener. Magnets produce magnetic fields, but so do electric currents. An electric current flowing in this straight wire produces a magnetic field which causes the tiny pieces of iron (iron “filings”) to align in the field. We shall see in this Chapter how magnetic field is defined, and that the magnetic field direction is along the iron filings. The magnetic field lines due to the electric current in this long wire are in the shape of circles around the wire. We also discuss how magnetic fields exert forces on electric currents and on charged particles, as well as useful applications of the interaction between magnetic fields and electric currents and moving electric charges.

2. Magnets and Magnetic Fields
Electric Currents Produce Magnetic Fields
Force on an Electric Current in a Magnetic Field; Definition of B
Force on an Electric Charge Moving in a Magnetic Field
Torque on a Current Loop; Magnetic Dipole Moment

3. Applications: Motors, Loudspeakers, Galvanometers
Discovery and Properties of the Electron
The Hall Effect
Mass Spectrometer

4. Magnets and Magnetic Fields
Magnets have two ends – poles – called north and south.
Like poles repel; unlike poles attract.
Figure Like poles of a magnet repel; unlike poles attract. Red arrows indicate force direction.

5. Magnets and Magnetic Fields
However, if you cut a magnet in half, you don’t get a north pole and a south pole – you get two smaller magnets.
Figure If you split a magnet, you won’t get isolated north and south poles; instead, two new magnets are produced, each with a north and a south pole.

6. Magnets and Magnetic Fields
Magnetic fields can be visualized using magnetic field lines, which are always closed loops.
Figure (a) Visualizing magnetic field lines around a bar magnet, using iron filings and compass needles. The red end of the bar magnet is its north pole. The N pole of a nearby compass needle points away from the north pole of the magnet. (b) Magnetic field lines for a bar magnet.

7. Magnets and Magnetic Fields
Magnetic field lines:
Start from the north pole and end at the south pole.
Do not intersect
Tangent to the lines
Density of the lines

8. Magnets and Magnetic Fields
The Earth’s magnetic field is similar to that of a bar magnet.
Note that the Earth’s “North Pole” is really a south magnetic pole, as the north ends of magnets are attracted to it.
Figure The Earth acts like a huge magnet; but its magnetic poles are not at the geographic poles, which are on the Earth’s rotation axis.

9. Magnets and Magnetic Fields
A uniform magnetic field is constant in magnitude and direction.
The field between these two wide poles is nearly uniform.
Figure Magnetic field between two wide poles of a magnet is nearly uniform, except near the edges.

10. Electric Currents Produce Magnetic Fields
Experiment shows that an electric current produces a magnetic field. The direction of the field is given by a right-hand rule.
Figure (a) Deflection of compass needles near a current-carrying wire, showing the presence and direction of the magnetic field. (b) Magnetic field lines around an electric current in a straight wire. (c) Right-hand rule for remembering the direction of the magnetic field: when the thumb points in the direction of the conventional current, the fingers wrapped around the wire point in the direction of the magnetic field. See also the Chapter-Opening photo.

11. Electric Currents Produce Magnetic Fields
Here we see the field due to a current loop; the direction is again given by a right-hand rule.
Figure Magnetic field lines due to a circular loop of wire.
Figure Right-hand rule for determining the direction of the magnetic field relative to the current.

12. Definition of the Magnetic Field
A magnet exerts a force on a current-carrying wire. The direction of the force is given by a right-hand rule.
Figure (a) Force on a current-carrying wire placed in a magnetic field B; (b) same, but current reversed; (c) right-hand rule for setup in (b).

13. Definition of the Magnetic Field
The force on the wire depends on the current, the length of the wire, the magnetic field, and its orientation:
This equation defines the magnetic field B.
In vector notation:

14. Definition of the Magnetic Field
Unit of B: the tesla, T:
1 T = 1 N/A·m.
Another unit sometimes used: the gauss (G):
1 G = 10-4 T.

15. Magnetic Force on an Electric Current
Magnetic Force on a current-carrying wire.
A wire carrying a 30-A current has a length l = 12 cm between the pole faces of a magnet at an angle θ = 60°, as shown. The magnetic field is approximately uniform at T. We ignore the field beyond the pole pieces. What is the magnitude of the force on the wire?
Solution: F = IlBsin θ = 2.8 N.

16. Magnetic Force on an Electric Current
Measuring a magnetic field.
A rectangular loop of wire hangs vertically as shown. A magnetic field B is directed horizontally, perpendicular to the wire, and points out of the page at all points. The magnetic field is very nearly uniform along the horizontal portion of wire ab (length l = 10.0 cm) which is near the center of the gap of a large magnet producing the field. The top portion of the wire loop is free of the field. The loop hangs from a balance which measures a downward magnetic force (in addition to the gravitational force) of F = 3.48 x 10-2 N when the wire carries a current I = A. What is the magnitude of the magnetic field B?
Solution: The wire is perpendicular to the field (the vertical wires feel no force), so B = F/Il = 1.42 T.

17. Magnetic Force on an Electric Current
Magnetic Force on a semicircular wire.
A rigid wire, carrying a current I, consists of a semicircle of radius R and two straight portions as shown. The wire lies in a plane perpendicular to a uniform magnetic field B0. Note choice of x and y axis. The straight portions each have length l within the field. Determine the net force on the wire due to the magnetic field B0.
Solution: The forces on the straight sections are equal and opposite, and cancel. The force dF on a segment dl of the wire is IB0R dφ and is perpendicular to the plane of the diagram. Therefore, F = ∫dF = IB0R∫sin φ dφ = 2IB0R.

18. Torque on a Current Loop
The forces on opposite sides of a current loop will be equal and opposite (if the field is uniform and the loop is symmetric), but there may be a torque.
The magnitude of the torque is given by
Figure Calculating the torque on a current loop in a magnetic field B. (a) Loop face parallel to B field lines; (b) top view; (c) loop makes an angle to B, reducing the torque since the lever arm is reduced.

19. Torque on a Current Loop

20. Torque on a Current Loop

21. Torque on a Current Loop

22. Torque on a Current Loop

23. Torque on a Current Loop
For N loops, define the magnetic dipole moment, μ:
The torque on and the potential energy of the loop depends on its orientation in the field:

24. Torque on a Current Loop
Torque on a coil.
A circular coil of wire has a diameter of 20.0 cm and contains 10 loops. The current in each loop is 3.00 A, and the coil is placed in a 2.00-T external magnetic field. Determine the maximum and minimum torque exerted on the coil by the field.
Solution: The minimum torque is zero; the maximum is NIAB sin  = 1.88 N·m.

25. Torque on a Current Loop
Magnetic moment of a hydrogen atom.
Determine the magnetic dipole moment of the electron orbiting the proton of a hydrogen atom at a given instant, assuming (in the Bohr model) it is in its ground state with a circular orbit of radius r = x m. [This is a very rough picture of atomic structure, but nonetheless gives an accurate result.]
Solution: The centripetal force is provided by the Coulomb force between the electron and proton: mv2/r = e2/4πε0r2. This can be solved to find the velocity of the electron: v = 2.19 x 106 m/s. The time required for one orbit is T = 2πr/v; the current is I = e/T. Finally, the dipole moment is μ = IA = 9.27 x J/T.

26. Homework
Ch. 29
Exer. 27
Exer. 28, 29, 30, 31, 32,
Exer. 35, 37, 36, 38, 39,
Exer. 47
Prob. 2

27. Force on an Electric Charge Moving in a Magnetic Field
The force on a moving charge is related to the force on a current:
Once again, the direction is given by a right-hand rule.
Figure Force on charged particles due to a magnetic field is perpendicular to the magnetic field direction.

28. Force on an Electric Charge Moving in a Magnetic Field
Negative charge near a magnet.
A negative charge -Q is placed at rest near a magnet. Will the charge begin to move? Will it feel a force? What if the charge were positive, +Q?
Solution: There is no force on a motionless charge, be it positive or negative, so in neither case will it begin to move.

29. Force on an Electric Charge Moving in a Magnetic Field
Magnetic force on a proton.
A magnetic field exerts a force of 8.0 x N toward the west on a proton moving vertically upward at a speed of 5.0 x 106 m/s (a). When moving horizontally in a northerly direction, the force on the proton is zero (b). Determine the magnitude and direction of the magnetic field in this region. (The charge on a proton is q = +e = 1.6 x C.)
Solution: Since the force is zero when the proton is moving north, the field must point in the north-south direction. In order for the force to be to the west when the proton is moving up, the field must point north. B = F/qv = 0.10 T.

30. Force on an Electric Charge Moving in a Magnetic Field
Magnetic force on ions during a nerve pulse.
Estimate the magnetic force due to the Earth’s magnetic field on ions crossing a cell membrane during an action potential. Assume the speed of the ions is 10-2 m/s.
Solution: F = qvB; assume B = 10-4 T, and use the electron charge for q. F = N.

31. Force on an Electric Charge Moving in a Magnetic Field
If a charged particle is moving perpendicular to a uniform magnetic field, its path will be a circle.
Figure Force exerted by a uniform magnetic field on a moving charged particle (in this case, an electron) produces a circular path.

32. Force on an Electric Charge Moving in a Magnetic Field
Electron’s path in a uniform magnetic field.
An electron travels at 2.0 x 107 m/s in a plane perpendicular to a uniform T magnetic field. Describe its path quantitatively.
Solution: The magnetic force keeps the particle moving in a circle, so mv2/r = qvB. Solving for r gives r = mv/qB = 1.1 cm.

33. Force on an Electric Charge Moving in a Magnetic Field
Stopping charged particles.
Can a magnetic field be used to stop a single charged particle, as an electric field can?
Solution: No, because the force is always perpendicular to the velocity. In fact, a uniform magnetic field cannot change the speed of a charged particle, only its direction.

34. Force on an Electric Charge Moving in a Magnetic Field

35. Force on an Electric Charge Moving in a Magnetic Field
A helical path.
What is the path of a charged particle in a uniform magnetic field if its velocity is not perpendicular to the magnetic field?
Solution: The path is a helix – the component of velocity parallel to the magnetic field does not change, and the velocity in the plane perpendicular to the field describes a circle.

36. Force on an Electric Charge Moving in a Magnetic Field
The aurora borealis (northern lights) is caused by charged particles from the solar wind spiraling along the Earth’s magnetic field, and colliding with air molecules.
Figure (a) Diagram showing a negatively charged particle that approaches the Earth and is “captured” by the magnetic field of the Earth. Such particles follow the field lines toward the poles as shown. (b) Photo of aurora borealis (here, in Kansas, a rare sight).

37. Force on an Electric Charge Moving in a Magnetic Field
Velocity selector, or filter: crossed E and B fields.
Some electronic devices and experiments need a beam of charged particles all moving at nearly the same velocity. This can be achieved using both a uniform electric field and a uniform magnetic field, arranged so they are at right angles to each other. Particles of charge q pass through slit S1 and enter the region where E points into the page and B points down from the positive plate toward the negative plate. If the particles enter with different velocities, show how this device “selects” a particular velocity, and determine what this velocity is.
Figure 27-21: A velocity selector: if v = E/B, the particles passing through S1 make it through S2.
Solution: Only the particles whose velocities are such that the magnetic and electric forces exactly cancel will pass through both slits. We want qE = qvB, so v = E/B.

38. Motors
An electric motor uses the torque on a current loop in a magnetic field to turn magnetic energy into kinetic energy.
Figure Diagram of a simple dc motor.
Figure The commutator-brush arrangement in a dc motor ensures alternation of the current in the armature to keep rotation continuous. The commutators are attached to the motor shaft and turn with it, whereas the brushes remain stationary.

39. Galvanometers
A galvanometer takes advantage of the torque on a current loop to measure current; the spring constant is calibrated so the scale reads in amperes.
Figure Galvanometer.

40. Electron
Electrons were first observed in cathode ray tubes. These tubes had a very small amount of gas inside, and when a high voltage was applied to the cathode, some “cathode rays” appeared to travel from the cathode to the anode.
Figure Discharge tube. In some models, one of the screens is the anode (positive plate).

41. Electron
The value of e/m for the cathode rays was measured in 1897 using the apparatus below; it was then that the rays began to be called electrons.
Figure goes here.
Figure Cathode rays deflected by electric and magnetic fields.

42. Electron

43. Electron
Millikan measured the electron charge directly shortly thereafter, using the oil-drop apparatus diagrammed below, and showed that the electron was a constituent of the atom (and not an atom itself, as its mass is far too small).
The currently accepted values of the electron mass and charge are
m = 9.1 x kg
e = 1.6 x C
Figure Millikan’s oil-drop experiment.

44. The Hall Effect
When a current-carrying wire is placed in a magnetic field, there is a sideways force on the electrons in the wire. This tends to push them to one side and results in a potential difference from one side of the wire to the other; this is called the Hall effect. The emf differs in sign depending on the sign of the charge carriers; this is how it was first determined that the charge carriers in ordinary conductors are negatively charged.
Figure The Hall effect. (a) Negative charges moving to the right as the current. (b) Positive charges moving to the left as the current.

45. The Hall Effect Drift velocity using the Hall effect.
A long copper strip 1.8 cm wide and 1.0 mm thick is placed in a 1.2-T magnetic field. When a steady current of 15 A passes through it, the Hall emf is measured to be 1.02 μV. Determine the drift velocity of the electrons and the density of free (conducting) electrons (number per unit volume) in the copper.
Solution: The drift velocity is EH/Bd = 4.7 x 10-5 m/s. Since I = nevdA, n = I/evdA = 11 x 1028 m-3.

46. Mass Spectrometer
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, which then allows the measurement of its mass:

47. Mass Spectrometer
All the atoms reaching the second magnetic field will have the same speed; their radius of curvature will depend on their mass.
Figure Bainbridge-type mass spectrometer. The magnetic fields B and B’ point out of the paper (indicated by the dots), for positive ions.

48. Mass Spectrometer

49. Mass Spectrometer Mass spectrometry.
Carbon atoms of atomic mass 12.0 u are found to be mixed with another, unknown, element. In a mass spectrometer with fixed B′, the carbon traverses a path of radius 22.4 cm and the unknown’s path has a 26.2-cm radius. What is the unknown element? Assume the ions of both elements have the same charge.
Solution: The ratio of masses is equal to the ratio of the radii, or Therefore the unknown element has a mass of 14.0 u, and is probably nitrogen.

50. Summary Magnets have north and south poles.
Like poles repel, unlike attract.
Unit of magnetic field: tesla.
Electric currents produce magnetic fields.
A magnetic field exerts a force on an electric current:

51. Summary A magnetic field exerts a force on a moving charge:
Torque on a current loop:
Magnetic dipole moment: