Induced EMF Generators Transformers Physics 114 2192021 Lecture

Induced EMF Generators , Transformers Physics 114 2/19/2021 Lecture VIII 1

Concepts • Electric generator • Magnetic flux • Induced EMF 2/19/2021 Lecture VIII 2

Electric motor electric generator • Electric motor: – electric energy mechanical. • Electric generator: – Current loop rotates in – mechanical electric energy magnetic field – conductor loop rotating in magnetic field produces electric current 2/19/2021 Lecture VIII 3

Magnetic flux • If magnetic field is rain – magnetic flux is the amount of water in a bucket accumulated per unit of time: • Magnetic flux is measured in Weber: 1 Wb=1 T. m 2 2/19/2021 Lecture VIII Three ways to change flux • Change B • Change A • Change q 4

Faraday’s Law of induction • Changing magnetic flux induces emf (voltage), • Acts like a battery! 2/19/2021 Lecture VIII 5

Lenz’s Law: conservatism of nature • An induced emf always gives rise to a current whose magnetic field opposes the original change in flux: – Flux decreases internal Bint (created by the induced current) is in the same direction as the external Bext – Flux increases Bint – in the opposite direction to Bext – Determine current direction using 1 st right hand rule 2/19/2021 Lecture VIII 6

Moving conductor • A conducting rod is moving to the right on a U-shaped conductor in a uniform magnetic field flux is increasing • Induced emf: 2/19/2021 • Induced electric field: Lecture VIII 7

Electric generator • Electric generator: – conductor loop rotating in magnetic field produces – Alternating electric current AC 2/19/2021 Lecture VIII 8

Alternating current (AC) • Emf changes sign current changes the direction I Req ~ 2/19/2021 Lecture VIII 9

Transformer • If all flux goes through the core: 2/19/2021 Lecture VIII 10

Transformer • If energy is conserved • If efficiency e<100% 2/19/2021 Lecture VIII 11

Power transmission • High voltage low current power loss in power line resistance is lower: Power is dissipated only in resistors!!! 2/19/2021 Lecture VIII 12

Power transmission • 65 k. W is transmitted over two 0. 100 Ohm lines. • V=120 1200 V 120 V, e=99% • Compare losses to power transmission at 120 V 2/19/2021 Lecture VIII 13

Moving conductor – eddy currents • A increases F increases Induced current creates Bint opposite to external Bext • I down • Now we have a current in magnetic field there is a force acting on it • The direction of this force is opposite to v • Conservatism of nature • Currents created in conductors moving through the magnetic field – eddy currents – work to resist the change 2/19/2021 Lecture VIII 14

Electric generator –counter torque • Loop is rotating cw • Induced currents experience force in magnetic field resultant torque on the loop ccw – counter torque • Nature resists change. 2/19/2021 Lecture VIII 15

Electric motor – counter emf • Current loop in magnetic field • Magnetic field creates a torque that rotates the loop • Changing flux emf • Based on conservatism – this emf will try to create a current in the opposite direction to the original current - counter emf. • Current is large at the beginning and is decreased later on. 2/19/2021 Lecture VIII 16