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# Electromagnetic induction (2)

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### Electromagnetic induction (2)

1. 1. Electromagnetic induction
2. 2. Important factors in inducing currents • 1. An emf is induced if the coil or the magnet (or both) move (change in flux). • 2. The size of the induced emf depends on the speed of movement. • 3. The induced emf depends on the strength of the B field. • 4. Changing the area inside the magnetic field • 5. Increasing the number of turns also changes the flux linkage, and so induces a greater emf.
3. 3. What you are going to learn today • What is magnetic flux, and magnetic flux linkage? • What must happen to a conductor (or to the magnetic field in which it’s placed) for electricity to be generated? • What factors would cause the induced emf to be greater? • What is Lenz’s law and what are the applications of this law?
4. 4. Flux - The rate of flow of energy through a given surface • flux density B (The strength of your magnetic field) • magnetic flux, Φ. Φ = ΒΑ (Α = Αrea) • Flux Linkage, N Φ – (N = number of turns)
5. 5. Lenz’s Law • Lenz’s Law states that the direction of the induced current is always such as to oppose the change that causes the current. • To include this idea in our formula, a minus sign has to be introduced, giving; •             Emf = – N x dΦ/dt
6. 6. Fleming's Right hand rule
7. 7. p133
8. 8. Kinetic energy recovery systems Toyota • http://www.youtube.com/watch?v=evZ- C8fVrP4 F1 • http://www.youtube.com/watch? v=09knBT2gqqU
9. 9. Inducing an Emf (no current yet) • Connect the coil of wire to the micro- voltmeter and place it close to the magnet. • 1. Move the magnet next to the coil. What happens? How does it depend on speed and direction of movement? • 2 .Move the coil next to the magnet. What happens? How does it depend on speed and direction of movement? • 3. Gradually unwind the coil in the magnetic field. What happens? • 4. Take the coil and crumple it up, keeping it in the field. What happens?
10. 10. Conductor in a magnetic field Metal rod, length L in a magnetic field moving with a velocity v down the page. An electron in the rod will experience a force (= Bev) that will push it towards the end Q The electrons will be pushed towards end Q leaving end p more positive an electric field E builds up until the force on electrons in the rod due to this electric field (= Ee) balances the force due to the magnetic field. Ee = Bev so E =Bv For a rod of length L, E = V/L and so V/L = Bv Hence the induced emf = BLvv = velocity E = Electric field V = Voltage B = Magnetic field
11. 11. Completing the circuit • The emf will now cause a current to flow in the external resistor R. This means that a similar current flows through the rod itself giving a magnetic force, BIL to the left • L is now the separation of the two conductors along which the rod PQ moves.) An equal and opposite force (to the right) is needed to keep PQ moving at a steady speed. • The work done in moving the rod will equal the energy dissipated in the resistor. • In a time t, the rod moves a distance d = v t • Work done (FxD) on the rod = BIL v t • Energy dissipated in R = power x time = ItV • giving BIL v t = ItV • Emf (V) = BvL
12. 12. However! You are increasing the area inside the magnetic field Emf (V) = BvL In one second the area has increased by Lv (A =Lv) induced emf = B x area swept out per second = B x A / t B x A can be called the magnetic flux, Φ. Thus induced emf = Φ / t = rate of change of magnetic flux And more generally emf = d Φ/ dt So how can you increase the induced voltage? L
13. 13. Flux Linkage (N Φ) • Increasing the number of turns of wire N in our circuit increases the emf produced • induced emf = rate of change of flux linkage • emf = N x d Φ /dt
14. 14. Sketching Flux Patterns NS NS SN – +