* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Electromagnetic Induction
Survey
Document related concepts
Fundamental interaction wikipedia , lookup
Field (physics) wikipedia , lookup
Electrostatics wikipedia , lookup
Maxwell's equations wikipedia , lookup
Neutron magnetic moment wikipedia , lookup
Time in physics wikipedia , lookup
Magnetic monopole wikipedia , lookup
Magnetic field wikipedia , lookup
Electromagnetic mass wikipedia , lookup
Superconductivity wikipedia , lookup
Electromagnet wikipedia , lookup
Lorentz force wikipedia , lookup
Aharonov–Bohm effect wikipedia , lookup
Transcript
Electricity and Magnetism Chapter 28 28 Electromagnetic Induction Electromagnetic Induction Electricity and Magnetism 28.1 Induced e.m.f. and induced current • An e.m.f. is induced whenever – the conductor cuts through magnetic field lines due to the relative motion between the conductor and the magnet. Induction by the relative motion between a coil and a magnet Experiment 28.1 28 Electromagnetic Induction Electricity and Magnetism • An e.m.f. is induced whenever – the conductor cuts through magnetic field lines due to the relative motion between the conductor and the magnet. Induction by moving a wire across magnetic field lines 28 Electromagnetic Induction Experiment 28.2 Electricity and Magnetism • An e.m.f. is induced whenever – the conductor cuts through magnetic field lines due to the relative motion between the conductor and the magnet. – the magnetic field through a coil changes. Induction caused by a changing magnetic field 28 Electromagnetic Induction Experiment 28.3 Electricity and Magnetism • This phenomenon is called electromagnetic induction. • Such an e.m.f. is called induced e.m.f. high potential induced e.m.f. low potential 28 Electromagnetic Induction Electricity and Magnetism • This phenomenon is called electromagnetic induction. • Such an e.m.f. is called induced e.m.f. • The current produced is called an induced current. wire frame induced current induced current 28 Electromagnetic Induction if a closed loop is formed Electricity and Magnetism Faraday’s law of electromagnetic induction The magnitude of the induced e.m.f. is directly proportional to the rate at which the conductor cuts through the magnetic field lines, or the field through the coil changes. smaller larger 28 Electromagnetic Induction Electricity and Magnetism Faraday’s law of electromagnetic induction The magnitude of the induced e.m.f. is directly proportional to the rate at which the conductor cuts through the magnetic field lines, or the field through the coil changes. smaller larger Checkpoint (p.358) O 28 Electromagnetic Induction Electricity and Magnetism Lenz’s law An induced current always flows in a direction so as to oppose the change producing it. N opposing the change magnet approaching Ring that cannot get through 28 Electromagnetic Induction S Electricity and Magnetism Lenz’s law An induced current always flows in a direction so as to oppose the change producing it. S opposing the change magnet withdrawing Ring that cannot get through 28 Electromagnetic Induction N Electricity and Magnetism • Fleming’s right hand rule can also be used to determine the direction of an induced current. Experiment 28.5 Example 28.2 Checkpoint (p.365) O Conducting loop moving across a magnetic field 28 Electromagnetic Induction Electricity and Magnetism 28.2 Faraday’s law and motional e.m.f. Magnetic flux • Magnetic flux F is a measure of the number of magnetic field lines through a surface. 28 Electromagnetic Induction Electricity and Magnetism • The magnetic flux F through a planar surface in a magnetic field B is F = BA cos q • Magnetic flux is a scalar quantity with the unit weber (Wb). • The magnetic field is also called the magnetic flux density. Example 28.3 28 Electromagnetic Induction Checkpoint (p.370) O Electricity and Magnetism Mathematical form of Faraday’s law • Faraday’s law can be expressed in mathematical form: The negative sign is another way to state Lenz’s law. F t • For an N-turn coil, if the magnetic flux F through each turn is the same, the total flux through the coil, called the magnetic flux linkage, is NF. Example 28.4 Example 28.6 28 Electromagnetic Induction Example 28.5 Checkpoint (p.374) O Electricity and Magnetism Motional e.m.f. • A motional e.m.f. is the induced e.m.f. arises from the motion of a conductor in a magnetic field. ++ ++ + magnetic force v 28 –– –– – uniform magnetic field B Electromagnetic Induction motional e.m.f. Electricity and Magnetism • A motional e.m.f. is the induced e.m.f. arises from the motion of a conductor in a magnetic field. At equilibrium, ++ ++ + FB = FE magnetic force FB = qvB = Blv l v motional e.m.f. electric force FE = qE = qV / l 28 Electromagnetic Induction –– –– – uniform magnetic field B Electricity and Magnetism • If the velocity v makes an angle f with the magnetic field B, the motional e.m.f. is = Blv sin f straight conductor rod of length l v f uniform magnetic field B Checkpoint (p.377) O 28 Electromagnetic Induction Example 28.7 Electricity and Magnetism 28.4 Applications of electromagnetic induction and generators • Electromagnetic induction is used in – moving-coil microphones 28 Electromagnetic Induction Electricity and Magnetism • Electromagnetic induction is used in – moving-coil microphones – magnetic storage Writing data 28 Reading data Electromagnetic Induction Electricity and Magnetism • Electromagnetic induction is used in – moving-coil microphones – magnetic storage – electric guitars 28 Electromagnetic Induction Electricity and Magnetism • Electromagnetic induction is used in – moving-coil microphones – magnetic storage – electric guitars – electrical generators A bicycle alternator 28 Electromagnetic Induction Generators in a power station Electricity and Magnetism Electrical generator • A generator is a device that converts mechanical energy into electrical energy. • The ones generating alternating currents are called a.c. generators (or alternators). • Those generating direct currents are called d.c. generators (or dynamos). A simple a.c. generator Simple a.c. generator 28 Electromagnetic Induction Electricity and Magnetism • The slip rings in an a.c. generator are used to prevent the twisting of wires during the rotation of the coil. slip rings 28 Electromagnetic Induction Electricity and Magnetism • The slip rings in an a.c. generator are used to prevent the twisting of wires during the rotation of the coil. 28 Electromagnetic Induction Electricity and Magnetism • The slip rings in an a.c. generator are used to prevent the twisting of wires during the rotation of the coil. 28 Electromagnetic Induction Electricity and Magnetism • The slip rings in an a.c. generator are used to prevent the twisting of wires during the rotation of the coil. 28 Electromagnetic Induction Electricity and Magnetism • The slip rings in an a.c. generator are used to prevent the twisting of wires during the rotation of the coil. 28 Electromagnetic Induction Electricity and Magnetism Variation of the induced e.m.f. in a simple a.c. generator 28 Electromagnetic Induction Electricity and Magnetism • The commutator of a d.c. generator is used to reverse the connection to the external circuit when the direction of the induced e.m.f. in the coil reverses. A simple d.c. generator Simple d.c. generator 28 Electromagnetic Induction Electricity and Magnetism • The commutator of a d.c. generator is used to reverse the connection to the external circuit when the direction of the induced e.m.f. in the coil reverses. Variation of the induced e.m.f. in a simple d.c. generator 28 Electromagnetic Induction Electricity and Magnetism • The induced e.m.f. of a generator can be increased by – increasing the rotational speed of the coil, – using stronger magnets, – winding more turns of wire on the coil, – increasing the area of the coil within the field, and – winding the coil on a soft iron core. Experiment 28.6 28 Electromagnetic Induction Example 28.9 Electricity and Magnetism • In some generators, it is the magnets that rotate but not the coil. A bicycle alternator 28 Electromagnetic Induction Electricity and Magnetism • For an a.c. generator in a power station, there is an electromagnet, called rotor, rotating in a set of fixed coils, called the stator. Thus, e.m.f. is induced in the stator coils. Checkpoint (p.390) O 28 Electromagnetic Induction Electricity and Magnetism 28.5 Eddy currents • Eddy currents are induced currents circulating through a conducting plate when the plate cuts through magnetic field lines or is placed in a changing magnetic field. eddy currents Conducting plate leaving the field uniform magnetic field Falling magnet 28 Electromagnetic Induction Coin detector Electricity and Magnetism • Eddy currents are induced currents circulating through a conducting plate when the plate cuts through magnetic field lines or is placed in a changing magnetic field. eddy currents Conducting plate entering the field uniform magnetic field Falling magnet 28 Electromagnetic Induction Coin detector Electricity and Magnetism Braking effect • In general, magnetic braking effect arises whenever eddy currents are induced due to the relative motion between a block of conductor and a magnetic field. external force magnetic force decelerates uniform magnetic field Experiment 28.7 28 Electromagnetic Induction Electricity and Magnetism Induction heating • Eddy currents can heat up the conductor itself as a result of the heating effect of current. • This phenomenon is called induction heating. A floating aluminium foil What makes the brass gong rotate 28 Electromagnetic Induction Checkpoint (p.397) O