Lecture 11 Magnetism of Matter Maxwell s Equations Chp

Lecture 11 Magnetism of Matter: Maxwell’s Equations Chp. 32 Wednesday Morning • • Cartoon -. Opening Demo Warm-up problem Physlet Topics – – – • Finish up Mutual inductance Ferromagnetism Maxwell equations Displacement current Exam Demos

What is Mutual Inductance? M When two circuits are near one another and both have currents changing, they can induce emfs in each other. I 1 1 2 I 2 On circuit boards you have to be careful you do not put circuits near each other that have large mutual inductance. They have to be oriented carefully and even shielded.

71. Two coils, connected as shown, separately have inductances L 1 and L 2. Their mutual inductance is M. (a) Show that this combination can be replaced by a single coil of equivalent inductance given by B We assume that the current is changing at (nonzero) rate di/dt and calculate the total emf across both coils. First consider coil 1. The magnetic field due to the current in that coil points to the left. The magnetic field due to current in coil 2 also points to the left. When the current increases, both fields increase and both changes in flux contribute emf’s in the same direction. B

B B e Thus, the induced emf’s are Therefore, the total emf across both coils is which is exactly the emf that would be produced if the coils were replaced by a single coil with inductance

(b) How could the coils in this figure be reconnected to yield an equivalent inductance of B We imagine reversing the leads of coil 2 so the current enter at the back of the coil rather than front (as pictured in the diagram). Then the field produced by coil 2 at the site of coil 1 is opposite to the field produced by coil 1 itself. B e The fluxes have opposite signs. An increasing current in coil 1 tends to increase the flux in that coil, but an increasing current in coil 2 tends to decrease it. B

B B B The emf across coil 1 is Similarly, the emf across coil 2 is The total emf across both coils is This is the same as the emf that would be produced by a single coil with inductance

75. A rectangular loop of N closely packed turns is positioned near a long, straight wire as shown in the figure. (a) What is the mutual inductance M for the loop-wire combination? (b) Evaluate M for N = 100, a = 1. 0 cm, b = 8. 0 cm, and l = 30 cm. (a) The flux over the loop cross section due to the current i in the wire is given by Thus,

(b) Evaluate M for N = 100, a = 1. 0 cm, b = 8. 0 cm, and l = 30 cm. (b) From the formula for M obtained,

Ferromagnetism Iron, cobalt, nickel, and rare earth alloys exhibit ferromagnetism. The so called exchange coupling causes electron magnetic moments of one atom to align with electrons of other atoms. This alignment produces magnetism. Whole groups of atoms align and form domains. (See Figure 32 -12 on page 756) A material becomes a magnet when the domains line up adding all the magnetic moments. You can actually hear the domains shifting by bringing up an magnet and hear the induced currents in the coil. Barkhausen Effect Two other types of magnetic behavior are paramagnetism or diamagnetism.

What is the atomic origin of magnetism? Electron spinning on its axis Electron orbiting around the nucleus

Spin Magnetic Dipole Moment of the Electron S is the angular momentum due to the electron’s spin. It has units kg. m 2/s. m has units of A m 2 - current times area Recall for a current loop, the magnetic dipole moment = current times area of loop . In the quantum field theory of the electron, S can not be measured. Only it’s component along the z axis can be measured. In quantum physics, there are only two values of the z component of the electron spin.

Therefore, only the z component of m can be measured. Its two possible values are: Corresponding to the two values of the electron spin quantum number +1/2 and -1/2 The above quantity is called the Bohr magneton and is equal to: The magnetic moment of the electron is the prime origin of ferromagnetism in materials.

22. The dipole moment associated with an atom of iron in an iron bar is 2. 1 x 10 -23 J/T. Assume that all the atoms in the bar, which is 5. 0 cm long and has a cross-sectional area of 1. 0 cm 2, have their dipole moments aligned. (a) What is the dipole moment of the bar? (b) What torque must be exerted to hold this magnet perpendicular to an external field of 1. 5 T? (The density of iron is 7. 9 g/cm 3) (a) The number of iron atoms in the iron bar is Thus, the dipole moment of the bar is

(C) Use the dipole formula to find the magnitude and direction of the magnetic field 1 cm from the end of the bar magnet on its central axis at P. 5 cm . A = 1 cm 2 m= 8. 9 A. m 2 P z

Big. Bite is a 50 ton electromagnet with a 25 cm by 100 cm gap B = 1 Tesla

Maxwells Equations: In 1873 he wrote down 4 equations which govern all classical electromagnetic phenomena. You already know two of them.

A magnetic field changing with time can produce an electric field: Faraday’s law Line integral of the electric field around the wire equals the change of Magnetic flux through the area Bounded by the loop Electric lines curl around changing magnetic field lines Example

Faraday’s Law B is increasing in magnitude Note that induced E field is in such a direction that the B field it produces opposes the original B field. Note there is no electric potential associated with the electric field induced by Faraday’s Law

Can a changing electric field with time produce an magnetic field: Yes it can and it is called Maxwell’s law of induction.

Maxwell’s law of induction Consider the charging of our circular plate capacitor B field also induced at point 2. When capacitor stops charging B field disappears.

Find the expression for the induced magnetic field B that circulates around the electric field lines of a charging circular parallel plate capacitor rR

Ampere-Maxwell’s Law This term has units of current Maxwell combined the above two equations to form one equation How do we interpret this equation?

What is the displacement current? This is called the displacement current id The term is really is a transfer of electric and magnetic energy from one plate to the other while the plates are being charged or discharged. When charging stops, this term goes to zero. Note it is time dependent.

Show that the displacement current in the gap of the two capacitor plates is equal to the real current outside the gap Can I detect the magnetic field associated with displacement current?

Calculation of id First find the real current i For the field inside a parallel plate capacitor Solving for q This is the real current i charging the capacitor. Next find the displacement current = real current. No charge actually moves across the gap.

Calculate Magnetic field due to displacement current Current is uniformly spread over the circular plates of the capacitor. Imagine it to be just a large wire of diameter R. Then use the formula for the magnetic field inside a wire. Inside the capacitor Outside the capacitor

Question 11: A circular capacitor of radius R is being charged through a wire of radius R 0. Which of the points a, b, c, and d correspond to points 1, 2, and 3 on the graph Where is the radius R 0 and R on the graph?

37. A parallel-plate capacitor has square plates 1. 0 m on a side as shown in the figure. A current of 2. 0 A charges the capacitor, producing a uniform electric field between the plates, with perpendicular to the plates. (a) What is the displacement current id through the region between the plates? At any instant the displacement current id in the gap between the plates equals the conduction current i in the wires. Thus, id = i = 3. 0 A. (b) What is d. E/dt in this region? The rate of change of the electric field is

(c) What is the displacement current through the square, dashed path between the plates? The displacement current through the indicated path is

(d) What is around this square, dashed path? The integral field around the indicated path is (e) What is the value of B on this path? ? ?

Summary of Maxwell Equations Integral form

Warm up set 10 Due 8: 00 am Tuesday 1. 2. 3. HRW 6 31. TB. 02. [120186] Suppose this page is perpendicular to a uniform magnetic field and the magnetic flux through it is 5 Wb. If the page is turned by 30° around an edge the flux through it will be: 4. 3 Wb 10 Wb 5. 8 Wb 2. 5 Wb 2. HRW 6 31. TB. 08. [120192] Faraday's law states that an induced emf is proportional to: the rate of change of the electric field the rate of change of the magnetic field zero the rate of change of the magnetic flux the rate of change of the electric flux 3. HRW 6 31. TB. 09. [120193] The emf that appears in Faraday's law is: around a conducting circuit perpendicular to the surface used to compute the magnetic flux throughout the surface used to compute the magnetic flux none of these around the boundary of the surface used to compute the magnetic flux