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Electromagnetic Induction Making Electrical Current Michael Faraday • Faraday figured out that electrical current moving through a wire created a magnetic field. • We made electromagnets! • Faraday also figured out that moving a magnetic field through electrical wires created current. • This is called induced current! Induced Current Electromotive Force • Changing a magnetic field creates emf or electromotive force. • Faraday looked further into electromagnetic induction. • There was emf when a magnetic field was moved through wires. • There was also an emf when wires were moved around a magnetic field. • The motion only had to be relative for emf. Motional EMF • 𝜀 = 𝐵ℓ𝑣 • B is the magnetic field strength in teslas. • ℓ is the length of the wire moving through the field. • v is the speed moving through the field in m/s. • 𝜀 is in Volts. • There must be motion for emf to be created! Induced Current Electromagnetic Flux ΦB Electromagnetic Flux ΦB ΦB = B⏊ A = B A cos θ ΦB = magnetic flux B⏊ = component of the magnetic field perpendicular to loop of the wire. A = area of the loop. Electromagnetic Flux ΦB ΦB = B⏊ A = B A cos θ B = strength of the magnetic field A = area of the loop. cos θ = angle of the magnetic field to the surface of the loop. Electromagnetic Flux ΦB ΦB = B⏊ A = B A cos θ ΦB = tesla·meter2 = weber (Wb) θ = 90° Φ=0 θ = 45° ΦB > 0 θ = 0° ΦB = max Faraday’s Law of Induction • Electromotive Force (emf) is created by moving an electromagnetic field through a loop. • The faster the ΦB is moved in a less amount of time the more emf is created. ℰ= ̶ ΔΦB Δt Lenz’s Law • A current produced by an induced emf moves in a direction so that its magnetic field opposes the original change in flux. • WARNING!!! CUIDADO!!! • We are talking about two magnetic fields!! • The magnetic field of the changing magnetic flux!! • The magnetic field of the induced current!! Lenz’s Law Lenz’s Law Lenz’s Law Lenz’s Law The End Solenoids Remember? • A coil of wire with electrical current running through it is called a solenoid. • The magnetic field of each loop adds to the magnetic field of other loops. • The South Pole is where current flows in. • The North Pole is where the current flows out. Magnitude of Magnetic Field of a Solenoid interior. B = μ0 NI L B is the magnetic field strength in teslas (T). μ0 is the magnetic permeability of a vacuum. μ0 = 4π × 10-7 N A-2 N is the number of turns in the wire. I is the current in amperes (A). L is the length of the solenoid. Faraday’s Law of Induction • If you add N loops the emf of each loop is added together. • The minus sign is to show us the direction of the loop. ΔΦB ℰ= ̶ N Δt Generators Generators Generators Generators • Electrical current can be induced by spinning a coil of wires inside a magnetic field. • The reverse also works, a magnetic field can be turned inside a coil of wires. • Using steam to cause turbines to spin is the basic idea of how all generators work. • The only thing that changes is the energy source to generate the steam (coal, gas, nuclear, hydroelectric, solar, wind, etc.) Generator Equation ℰ = NBω A sin ωt Generator Equation ℰ = NBω A sin ωt ℰ is electromotive force N is number of turns in the wire. B is magnetic field strength. A is area of the loop. ω is angular velocity in radians/s. t is time. Transformers Transformers Transformers • Transformers are used to increase or decrease voltage. (Technically AC voltage but we will get to that later.) • They have primary and secondary coils linked by an iron core. • Both the coils and the core are insulated to guard against the effects of magnetic fields (we will get to eddy currents later). Transformers • A step-down transformer has more primary coil loops than secondary coil loops. The voltage going out decreases. • Np ˃ Ns • A step-up transformer has more secondary coil loops than primary coil loops. The voltage going out increases. • Ns ˃ Np Transformers Transformers Transformers Transformer Equation I Vs Ns = N Vp p Everything is insulated ΦB can be ignored. Vp / Vs are voltage of primary and secondary coils. Np / Ns are the number of turns of primary and secondary coils. Transformer Equation II Is Ip Np = N s Everything is insulated ΦB can be ignored. Ip / Is are current of primary and secondary coils. Np / Ns are the number of turns of primary and secondary coils. The End