5/10/10 1 Physics 122 Theme Music: Matisyahu I Will be Light* Cartoon: Pat Brady Rose is Rose May 9, 2011 Physics 122 Prof. E. F. Redish *Thanks to Dan Makover For bringing this song to my attentionOutline Quiz 11: Capacitors (using clickers) What is light? – Unified theory of electromagnetism – Maxwell’s rainbow – Hertz’s invisible light – But… 5/10/10 2 Physics 122Model of magnetism (alpha): Magnetic poles Magnetism is a third “non-touching” force like gravity and electricity. Magnets are made up of magnetic charges analogous to electric charges but different. We will call them magnetic poles – North and South to distinguish them from Positive and Negative. Magnetic materials are made up of N and S just like they have + and - charges. When they are magnetized, the Ns and Ss separate. 5/10/10 3 Physics 1225/10/10 4 Physics 122 Is there magnetic charge ? If you break a magnet in half, both parts are still full (N-S) magnets. You can do this all the way down to a single electron and it still holds true (and you cant break a single electron). Our little dipoles turn out to be electrons.Model of magnetism (beta): Magnetic dipoles No separate magnetic poles have ever been found. All matter contains small magnetic dipoles – little bar magnets that cannot be broken. When magnetic materials are magnetized, the little dipoles are aligned and the forces they exert on each other hold each other in place. When they are heated, the little magnets lose their alignment and go random. 5/10/10 5 Physics 1225/10/10 6 Physics 122 Basic Particles The fundamental particles making up matter (electrons and protons and neutrons) can be classified by how they behave in response to (and as sources of) gravity, electricity, and magnetism. Mass Charge Magnetic Moment proton 1.67 x 10-27 kg 1.6 x 10-19 C 0.0015 Bohr magnetons neutron 1.67 x 10-27 kg 0 0.0010 Bohr magnetons electron 9.1 x 10-31 kg -1.6 x 10-19 C 9.28 Bohr magnetonsMoving electrons are deflected by a magnet. Is it because they are little magnets? 5/10/10 7 Physics 1225/10/10 8 Physics 122 Fields Field Source Response gravity mass mass electricity charge charge magnetism moving charge / magnetic dipoles moving charge / magnetic dipolesFoothold ideas: Magnetism 1.0 Magnetic fields are produced by magnets. Magnetic fields are felt by magnets and by moving charges. Magnetic force law: Although there are magnetic dipoles (e-, p+) there are no separate magnetic poles. 5/10/10 9 Physics 122 FB= qv ×B ΔF = IΔL ×B5/10/10 10 Physics 122 Fields Field Source Response gravity mass mass electricity charge charge magnetism magnetic dipoles moving charge / magnetic dipoles Note the asymmetry: So far we have only noted that moving charges feel a force from a magnetic field. Can they create them as well?Moving charges create as well as feel magnetic fields 5/10/10 11 Physics 122 This suggests that currents should attract and repel.Foothold ideas: Magnetism 2.0 Magnetic fields are produced by magnets and by moving charges (currents). Magnetic fields are felt by magnets and by moving charges (currents). Magnetic force law: Magnetic field law: Although there are magnetic dipoles (e-, p+) there are no separate magnetic poles. 5/10/10 12 Physics 122 FB= qv ×B ΔF = IΔL ×B ΔB = kAIΔL ׈rr2Some surprising results relating E and B Again we find an asymmetry: If a moving charge feels a magnetic field, does a moving magnet feel an electric field? Yes! Furthermore…never mind charges: – Changing magnetic fields create electric fields. – Changing electric fields create magnetic fields. 5/10/10 13 Physics 122Faradays Law: A changing magnetic field creates an electric field 5/10/10 14 Physics 122 And one which, by the way, does NOT satisfy the Kirchoff loop rule! Including magnetism in our treatment of electric currents – generators, transformers, etc. – requires additional foothold principles.Faradays law is immensely enabling This allows us to create immensely useful devices – Generators (moving magnets to create currents) – Transformers (carrying electrical energy across gaps) – Lots more 5/10/10 15 Physics 1225/10/10 16 Physics 122 Fields Field Source Response gravity mass mass electricity charge changing electric fields charge moving magnets magnetism magnetic dipoles changing magnetic fields moving charge / magnetic dipoles5/10/10 17 Physics 122 Maxwells Synthesis So our electric and magnetic fields seem deeply intertwined. In 1865, a Scottish physicist named James Clerk Maxwell created a unified theory of electric and magnetic fields.5/10/10 18 Physics 122 Maxwells Principles Maxwell 1: Point charges serve as sources to create electric fields. (Coulombs Law) Maxwell 2: There are no point poles that serve as sources to create magnetic fields. Maxwell 3: Moving electric charges and changing electric fields create magnetic fields. Maxwell 4: Changing magnetic fields create electric fields.5/10/10 19 Physics 122 A Unified Field Theory Maxwells equations provide the first Unified Field Theory – a set of equations that describe two fields (electric and magnetic) and specify the relationship between them. These equations turn out to be highly accurate until one gets to distances that are small compared to atomic sizes / energies that are large compared to atomic energies. These equations turn out to conflict with Newtons laws with respect how things look to moving observers – and Maxwells equations turn out to be right and Newtons wrong. This requires us to modify Newtons laws when speeds get near to c. This led directly to Einsteins special theory of relativity.5/10/10 20 Physics 122 A Remarkable Result Since in Maxwells equations electric and magnetic fields can bootstrap each other through empty space – with the changing electric field producing a changing magnetic field which produces… we can get fields that change continually and propagate through space. The speed of this wave is a combination of the constants obtained through studies of electric and magnetic
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