Upcoming SlideShare
×

# 12.1

• 935 views

• Comment goes here.
Are you sure you want to
Be the first to comment
Be the first to like this

Total Views
935
On Slideshare
0
From Embeds
0
Number of Embeds
0

Shares
0
0
Likes
0

No embeds

### Report content

No notes for slide

### Transcript

• 1. Electromagnetism Topic 12.1 Electromagnetic Induction
• 2. Induced Electromotive Force (e.m.f.)
• What is electromagnetic induction?
• The diagram shows a copper rod connected to an ammeter:
• There is no battery in the circuit.
• 3.
• What happens when you move the copper rod downwards, to cut across the horizontal magnetic field?
• The pointer on the meter makes a brief `flick' to the right, showing that an electric current has been induced.
• 4.
• What happens when you move the rod upwards?
• The meter again gives a `flick', but this time to the left.
• You have now induced a current in the opposite direction.
• 5.
• If you hold the rod stationary, or if you move the rod along the field lines, there is no induced current.
• 6. Why does electromagnetic induction occur?
• When you move the copper rod, its free electrons move with it.
• But when a charge moves in a magnetic field it experiences a force on it
• (the B Q v force).
• You can use Flemings Left hand Rule to show that the force on each electron is to the left as shown in the diagram
• (Remember that an electron moving down has to be treated like a positive charge moving up.
• 7.
• 8.
• So electrons accumulate at one end of the rod, making it negative.
• This leaves the other end short of electrons and therefore positive.
• There is now a voltage (potential difference) across the ends of the moving rod.
• If the ends of the moving rod are joined to form a complete circuit, the induced voltage causes a current to flow round the circuit as shown by the flick of the ammeter.
• 9.
• The induced voltage is a source of electrical energy ‑ an e.m.f
• When a conductor is moving in a magnetic field like this, an e.m.f is induced, even if there isn't a complete circuit for a current to flow.
• 10. Formula for a Straight Conductor
• Consider a conductor of length l that moves with velocity v perpendicular to a magnetic flux density or induction B as shown in the figure.
• 11.
• When the wire conductor moves in the magnetic field, the free electrons experience a force because they are caused to move with velocity v as the conductor moves in the field.
• F = e v B
• 12.
• This force causes the electrons to drift from one end of the conductor to the other, and one end builds‑up an excess of electrons and the other a deficiency of electrons.
• This means that there is a potential difference or emf between the ends.
• Eventually, the emf becomes large enough to balance the magnetic force and thus stop electrons from moving.
• 13.
• evB = eE ( from F = evB and F = eE)
• Therefore E = Bv
• If the potential difference (emf) between the ends of the conductor is ε then
• ε = E L (from E = V/d)
• By substitution we have
• ε = B v L
• 14. Magnetic Flux
• The magnetic flux ( Φ ) through a region is a measure of the number of lines of magnetic force passing through that region.
• Φ = AB cos θ
• where A is the area of the region and θ is the angle of movement between the magnetic field and a line drawn perpendicular to the area swept out.
• The unit of magnetic flux is the weber Wb.
• 15.
• For a single conductor in the magnetic flux density, it can be seen that
• ε = - ΔΦ / Δ t (the rate of change of flux density)
• For N number of conductors as in the case for a solenoid, the term flux‑linkage is used.
• Then
• ε = - N Δ ( Φ / Δ t)
• The minus sign shows us that the emf is always produced so as to oppose the change in flux.
• 16. Time-changing Magnetic Flux
• Therefore the production of an emf is produced by a time changing magnetic flux.
• This could be due to the wire or coil moving through a magnetic field
• Or by an increasing or decreasing magnetic field of an electromagnet next to a wire or coil.
• We know that an e.m.f. is induced when there is a change in the flux linking a conductor.
• Faraday's law makes the connection between the size of the induced e.m.f. and the rate at which the flux is changing.
• It states that:
• the magnitude of the induced e.m.f is directly proportinonal to the rate of change of magnetic flux or flux linkage.
• For a single conductor in the magnetic flux density, it can be seen that
• ε = - ΔΦ / Δ t (the rate of change of flux density)
• And ε = B v l
• Therefore - ΔΦ / Δ t = B v l
• 19. Lenz’s Law
• Faraday's law tells us the size of the induced e.m.f., but we can find its direction using Lenz's law
• The direction of the induced e.m.f is such that it will try to oppose the change in flux that is producing it.
• 20.
• 21.
• Lenz's law is illustrated in the diagrams: As you move the N‑pole into the coil, an e.m.f. is induced which drives a current round the circuit as shown.
• Now use the right‑hand grip rule
• Can you see that the current produces a magnetic field with a N‑pole at the end of the coil nearest to the magnet?
• So the coil repels the incoming magnet, and in this way the induced current opposes the change in flux.
• 22.
• Why is the current reversed as you move the N‑pole out?
• By Lenz's law, the coil needs to attract the receding N‑pole
• 23.
• Lenz's law is a result of the conservation of energy. If you move the magnet into the coil, you feel the repulsive force.
• You have to do work to move the magnet against this force.
• And so energy is transferred from you (or the system that is moving the magnet) to the electrical energy of the current.
• 24.
• 25.
• 26.
• 27.