Finish Chapter 8: E & M

Transcription

1 Finish Chapter 8: E & M Concept of FIELD Magnetism only a few examples Chapter 9: Waves, incl. e & m waves, light Waves generalities Light Electromagnetic waves/radiation

2 Important concept of a Field, represented by field lines : (let s not make it more magical than it is!) It s a representation of a (fundamental) force, incl. long-range, non-contact forces such as gravity, electric & magnetic forces. Think of it as a disturbance or a stress in space. Force by A on B occurs because A causes this stress, i.e. sets up this field, and B feels it. A field exists at a point P whenever an object would feel a force at that point P. Important: The above field exists at P even if there is no object there! (Of course, this field is due to some other object somewhere else.) Is there a gravitational field in this room? How do you know? Everywhere in this room? Outside the room?

3 A(ny) mass sets up a gravitational field, and feels forces due to the gravitational field of other masses. An(y) electric charge sets up an electric field, and feels.. A(ny) moving electric charge sets up a magnetic field, and feels Visualization of fields via field lines, electric case: Direction: taken as the force direction a + charge would feel. Guess: how might the strength of the force be represented pictorially? Concept checks 7 9!

4 C.E. 31: No, field not made of atoms. Same for gravitational field. Something to keep in mind (C.E. 32): there is energy associated with a field. Quiz # 54: How would a proton s motion differ from an electron s at the same point in the same electric field? Both are at rest initially. (a) They would both remain at rest. (b) They would both accelerate at the same rate and in the same direction. (c) They would both accelerate at the same rate, but in opposite directions. (d) Electron s acceleration would be larger, and they d move in opposite directions. (e) Proton s acceleration would be larger, and they d move in the same direction. Hint: don t forget that the magnitude of F electric on a particle is proportional to the magnitude of its electric charge.

5 (Electro)MAGNETISM: Fascinating from early childhood on and enormous societal impact (electrical power generation, modern electronic data storage, your credit card, etc ) Experiments clearly demonstrate that moving charges create an additional force/field, separate from electric, although tied together in that electric charges are required for both. In the end, different aspects of a single electromagnetic force the first great unification of fundamental forces in the history of physics. Three important points: 1) No magnetic monopoles! (in contrast to electric charges) 2) Intrinsic magnetism of magnetic materials, such as permanent magnets (coming from the atomic motions/ orbits of electrons and their intrinsic spin ), but also 3) magnetism from macroscopically moving charges (not within atoms), such as electrical currents.

6 In the language of fields: moving charges create magnetic fields, and feel a force due to the magnetic fields of other moving charges. Plenty more to electromagnetism, but for us one more phenomenon shall suffice: Faraday s Law, a.k.a. electromagnetic induction Whenever the magnetic field through a wire loop changes (say, by moving either), an electric current is induced in said loop, and for as long as the change continues. In other words: A changing magnetic field creates an electric field. And, it turns out, a changing electric field creates a magnetic field. ( formal unification of e & m via the famous Maxwell equations)

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8 One principle of electric power generation using induction (Faraday s Law) Reverse (i.e. E electric E mechanical )? Electric motor! Here you have one reason why we use AC (alternating currents) in our houses.

9 Quiz # 55: How does the electrostatic force between two hydrogen nuclei placed a certain distance apart compare with the force between two helium (atomic # 2) nuclei placed twice as far apart? (a) It s the same. (b) For hydrogen twice as large as for helium. (c) For helium twice as large as for hydrogen. (d) For helium four times as large as for hydrogen. (e) For hydrogen four times as large as for helium. Quiz # 56: The direction of the force on a negative charge placed at point C would be (a) upward (b) downward (c) leftward (d) rightward

10 WAVES: Familiar to everyone, but let s be careful about what is a true wave in physics. Example: a breaker at the beach is not a real wave! Best to start with demos Note the connection with oscillations/vibrations. What is (not) moving along? What is being transferred? Clearly different from particle motion! In Physics: A wave is a disturbance (or dislocation or shape or configuration) that travels through a medium, with no transfer of matter but with energy transfer.