Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Electrostatics wikipedia , lookup
Field (physics) wikipedia , lookup
Maxwell's equations wikipedia , lookup
History of electromagnetic theory wikipedia , lookup
Condensed matter physics wikipedia , lookup
Neutron magnetic moment wikipedia , lookup
Time in physics wikipedia , lookup
Magnetic field wikipedia , lookup
Magnetic monopole wikipedia , lookup
Superconductivity wikipedia , lookup
Lorentz force wikipedia , lookup
Electromagnetism wikipedia , lookup
Summary Lecture 18 The principle of galvanometers, electric motors, and loudspeakers is based on the interaction between (i) a magnetic field due to permanent magnet and (ii) a magnetic field due to electric current in the loop. Galvanometer: transforms a magnetic field due to current into the angle of the pointer. Electric motor: transforms magnetic field due to current into rotation. Loudspeaker: transforms variable magnetic field in motion and sound. Electricity and Magnetism Electromagnetic Induction Faraday’s Law Lenz’s Law A galvanometer takes advantage of the torque on a current loop to measure current: φ= NIAB sin θ k There are three main types of materials: (1) diamagnetics, (2) paramagnetics, and (3) ferromegnetics. The source of a magnetic field at the atomic level is a motion of electrons around the nuclei (orbital magnetic moment) and around own axis (spin magnetic moment). Ferromagnetics contains tiny regions called domains; the magnetic field in each domain is in a single direction; the external magnetic field aligns the direction of the magnetic field in all domains. Physics 112, Spring 2010, Feb 24, Lecture 18 2 Physics 112, Spring 2010, Feb 24, Lecture 18 Electricity and Magnetism: Electromagnetism Electric Charge and Magnetic Field How electricity and magnetism are related each other? + The interaction between the electric and magnetic fields is what we refer to as electromagnetism. v=0 + B B Charge at rest: no effect of magnetic field So, electromagnetism explains how magnetic field affects the electrical current or how the electrical current affects the magnetic field. v Fm = q v⊥ B Moving charge: magnetic field affects the charge Magnetic field affects only moving charge! Only moving electric charge produces a magnetic field! Three important electromagnetic phenomena: 1. Moving charges (for example, electric current) produce a magnetic field. v - + + 2. Magnetic field affects only moving charge and a wire with electric current. 3. Electromagnetic induction: changing magnetic field induces the electric field. Physics 112, Spring 2010, Feb 24, Lecture 18 B I 3 Moving charge = electric current Right-hand rule #1 Physics 112, Spring 2010, Feb 24, Lecture 18 4 1 Energy Produced by Wind Turbines Electromagnetic Induction Wind turbines can be placed at any location. Wind Energy Center in Hancock County, Iowa. Iowa is the second largest producer of wind energy in the USA: Texas (9,410 MW) Iowa (3,670 MW) California (2,794 MW) Washington (1,980 MW) Minnesota (1,809 MW) TPI Composites, Newton, Iowa, is a leading provider of blades to the wind energy movement. 5 MW (5 Megawatts) wind turbine. Physics 112, Spring 2010, Feb 24, Lecture 18 Coil Renewable energy obtained by wind generators. Amazing physics phenomenon used in wind turbines: electromagnetic induction. 5 Permanent magnet 6 Physics 112, Spring 2010, Feb 24, Lecture 18 Induced Electromotive Force (EMF) Electromagnetic Induction If magnetic field changes in some space, it results in appearance of the electric field in this space. Faraday’s experiment to induce an EMF. Wire made of copper v N Effect from permanent magnet moving inside the coil. S v N S V Electric field lines A moving magnet produces an electric field in the space around it. A moving magnet produces an electric voltage in the coil. Electromagnetic induction: the electric voltage induced by changing magnetic field. Physics 112, Spring 2010, Feb 24, Lecture 18 7 Physics 112, Spring 2010, Feb 24, Lecture 18 8 2 Magnetic Flux Induction Magnetic flux through the area: r B r B Φ B = BA In addition to changing B, the magnetic flux will also change if the area of the loop changes. ΦB = 0 A=0 A Magnetic flux = (magnetic field) × (area) × cosθ Units: Similarly, flux will change if the angle between the loop and the field changes. ΦB = B⊥ A = BAcosθ T ⋅ m 2 = weber = Wb 9 Physics 112, Spring 2010, Feb 24, Lecture 18 Faraday’s Law of Induction For a coil containing N loops of wire: ε = −N ΔΦB Δt ΦB t Minus sign in Faraday’s law is there to remind us in which direction the induced EMF acts. ε = −N ΔΦB Δt The current produced by an induced EMF moves in a direction such that its magnetic field opposes the original change in flux. Note: The induced current tries to keep the flux constant. EMF induced in coil Units: 10 Physics 112, Spring 2010, Feb 24, Lecture 18 Lenz’s Law The induced EMF in a closed loop is proportional to the rate of change of magnetic flux, through the loop. ε Induced current Magnetic field increases into the plane of the paper. In which direction does the induced current flow in the loop? Induced field must point out of the paper (to keep the total flux inside the loop constant) rate of change of magnetic flux through coil - volts - webers - seconds IIND BIND points out of plane of paper B increasing into plane of paper Hence, IIND flows counterclockwise around the loop. Physics 112, Spring 2010, Feb 24, Lecture 18 11 Physics 112, Spring 2010, Feb 24, Lecture 18 12 3 Induction Electromagnetic Induction A square loop of wire 20 cm on a side contains 20 turns and rotates in a magnetic field of 3 T making 1 revolution per 1 second. Find: An iron loop and copper loop of equal area are placed in a changing magnetic field. Across which loop is an induced EMF (voltage) generated? 1. Maximum magnetic flux that can pass through the loop. ΦB = B⊥ A = BAcosθ = A cos00 = (3T )(0.2 m× 0.2 m)(1) = 0.12 Wb iron Changing B 2. Minimum magnetic flux that can pass through the loop. ε = −N ΦB = BAcosθ = Acos900 = (3T )(0.2 m × 0.2 m)(0) = 0 1) in iron loop 2) in copper loop 3. Electromotive force induced in the coli. ε = −N ΔΦB Δt 0.12Wb ΔΦB = (20) = 9.6 V 0.25 s Δt 3) in both loops copper 13 Physics 112, Spring 2010, Feb 24, Lecture 18 Physics 112, Spring 2010, Feb 24, Lecture 18 Electromagnetic Induction 14 Units The units for magnetic flux: A plastic loop and copper loop of equal area are placed in a changing magnetic field. 1) farad Across which loop is an induced EMF (voltage) generated? 2) coulomb 3) weber plastic 4) tesla Changing B ε = −N 1) in plastic loop ΔΦB Δt Units The units for magnetic field: 1) farad 2) in copper loop 2) coulomb 3) in both loops copper 3) weber 4) tesla Physics 112, Spring 2010, Feb 24, Lecture 18 15 Physics 112, Spring 2010, Feb 24, Lecture 18 16 4