UNIVERSITY PHYSICS II CHAPTER 21 Faraday's Law and Electromagnetic Induction 821.1 Faraday's law of electromagnetic induction 1. Experiments ① A magnet and a ② two loops of wire,a loop of wire battery and a switch A current is observed in the loop as long as the magnetic flux through the loop is changing with time
1 1. Experiments 1A magnet and a loop of wire 2two loops of wire, a battery and a switch A current is observed in the loop as long as the magnetic flux through the loop is changing with time. §21.1 Faraday’s law of electromagnetic induction
821.1 Faraday's law of electromagnetic induction 2. Electromotive force-emr ←⊕ F←田 K R purpose of emf: supply a nonstatic electric force to move the charge keep the potential difference of the two plates and the current in the circuit 821.1 Faraday's law of electromagnetic induction Mechanisn F ×/ak4F田 The work done by Fk opposing Fe converse the energy in other forms into electric K R energy Outside the F move +g from positive plate to negative plate nside the e circul el>e move +q from negative plate to positive plate
2 2. Electromotive force-emf Fe← ⊕ r ← ⊕ Fe r + − K R ←⊕→ Fk r Fe r Fe r ← ⊕ Fe r ← ⊕ R purpose of emf: supply a nonstatic electric force to move the charge, keep the potential difference of the two plates and the current in the circuit. §21.1 Faraday’s law of electromagnetic induction Mechanism: The work done by opposing converse the energy in other forms into electric energy. Fe r Fk ← ⊕ r Fe r ← ⊕ Fe r + − K R ←⊕→ Fk r Fe r Outside the circuit： Fe r move +q from positive plate to negative plate Inside the circuit： Fk Fe r r > move +q from negative plate to positive plate §21.1 Faraday’s law of electromagnetic induction
821.1 Faraday's law of electromagnetic induction energy conversion Outside the circuit: ←田 F4F←田 ←田→ F Fd/>0 Inside the circuits F d/<0 K R ∮F:d=0 How about F? F Define the nonstatic electric field Ek A=SE. di>o Not a conservative force 821.1 Faraday's law of electromagnetic induction Define the emf emf=F Ed emf is the measure of the capacity of transforming the emf= Edi other form energy to electric energy. (inside) Direction of the emf
3 energy conversion： ⋅d = 0 ∫ F l L e r r Fe← ⊕ r ← ⊕ Fe r + − K R ←⊕→ Fk r Fe r ∫ + − Fe ⋅dl < 0 r r ∫ − + Fe ⋅dl > 0 r r Fe r Outside the circuit： Inside the circuit： Fk r How about ? 0 q F E k k r r Define the nonstatic electric field = = ⋅d > 0 ∫ A E l L k r r Not a conservative force §21.1 Faraday’s law of electromagnetic induction E l L k r r emf= ⋅d ∫ Define the emf ∫ + − = ⋅ （inside） emf E dl k r r emf is the measure of the capacity of transforming the other form energy to electric energy. + − Direction of the emf §21.1 Faraday’s law of electromagnetic induction
821.1 Faraday's law of electromagnetic induction 3. Faraday's law of electromagnetic induction faraday discovered that the induced emf around a closed path is equal to the negative of the time rate of change of the magnetic flux through the same path d induced emf dt Change by magnetic field 更=B.dA Change by area of loop s 21.1 Faraday's law of electromagnetic induction dgp emf=-N“m= d(nm) dy dt dt emf em Reference direction n>0, >0,<0 >0,Q空 <0,E>0
4 3. Faraday’s law of electromagnetic induction Faraday discovered that the induced emf around a closed path is equal to the negative of the time rate of change of the magnetic flux through the same path t Φm d d induced emf = − §21.1 Faraday’s law of electromagnetic induction ∫ Φm = B⋅ A r r d Change by magnetic field Change by area of loop 0, 0 d d > 0, > ε < t Φ Φ m m 0, 0 d d > 0, < ε > t Φ Φ m m t Ψ t NΦ t Φ N m m m d d d d( ) d d emf = − = − = − §21.1 Faraday’s law of electromagnetic induction emf emf + + Reference direction
821.1 Faraday's law of electromagnetic induction enz’slaw Is there a easy way to determine the direction of the induced current or emf? The induced current will always be directed so as to oppose the change in the magnetic flux that is taking place. mf em s 21.1 Faraday's law of electromagnetic induction Emf arising from moving conductor Equilibrium state: B f △V 4 vB=gE=q △V=B Lorentz force is the nonstaticelectric force qν×BE v×B q
5 Lenz’s law Is there a easy way to determine the direction of the induced current or emf? The induced current will always be directed so as to oppose the change in the magnetic flux that is taking place. §21.1 Faraday’s law of electromagnetic induction emf emf + + ①Emf arising from moving conductor mf l + − e f d c v r → B r ⊕ × ∆U × × × × × × × × × × × ∆V = Blv Fm = Fe l V qvB qE q ∆ = = Equilibrium state: Lorentz force is the nonstaticelectric force. FK Fm qv B r r r r = = × v B q F E m K r r r r = = × §21.1 Faraday’s law of electromagnetic induction