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Magnetic Fields

Magnetic Fields. The area over which the effects of a magnet can be felt is called the magnetic field . Its prescence is indicated by the use of field lines . They show you the way a free north pole would move in the field. If they are bunched together, the field is strong. N. S.

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Magnetic Fields

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  1. Magnetic Fields The area over which the effects of a magnet can be felt is called the magnetic field. Its prescence is indicated by the use of field lines. They show you the way a free north pole would move in the field. If they are bunched together, the field is strong.

  2. N S Permanent Magnets Certain materials (iron nickel and cobalt) contain small areas of magnetic moments called domains. If unmagnetised, they are randomly ordered. If magnetised, there is some degree of order. This domain theory explains magnetic saturation and the way in which magnets are both destroyed and made.

  3. N N S N There is no net force at the neutral point. Neutral Point Attraction Repulsion What field pattern is produced when two south poles are brought together?

  4. Fleming’s Left Hand Rule Motion The wire carrying the current is at right angles to the field. It experiences a force at right angles to both the current and the field. • Remember: • Field is fron N to S • Currentis + to - Field Current

  5. Magnetic Flux Density B (Tesla T) Defined by B = Force per unit current element ie B=F/IL Consider a charge Q moving at a velocity v through a Field of flux density B: F = BIL but for a steady current I = Q/t so F = B(Q/t)L = BQL/t but L/t = v since L is in the direction of I.  F=BQv

  6. + + + + + + + + + + + + + + + + Charge on electron An electron of mass m and charge e, travelling at a speed v enters an area of uniform flux density B. Show that it will travel in a circle of radius r given by r = mv/Be B field into board Electron gun F For circular motion, F = mv2/r and we know that F = Bqv = Bev so Bev = mv2/r i.e. r = mv/Be

  7. The D C Motor S N negative positive

  8. The Magnetic Effect of Current Moving charge creates a magnetic field i.e. current produces magnetism. The right hand grip rule. negative positive

  9. - + The flux density B, a distance r from the wire is given by; r I The field can be “concentrated” a little by forming the wire into a flat coil of one loop. This end of the coil looks like a south pole

  10. Plan View - + This end of the coil looks like a south pole

  11. A force acts between parallel, current carying wires. There will also be a force on wire A due to the current in B. Wire A Wire B Field due to current in A The wire B is in a magnetic field due to the current in wire A Wire B Resultant Force Field due to A

  12. I1 I2 i.e. they attract each other F F Oh yes! Newton’s 3rd Law

  13. F F They repel

  14. Are the forces in each wire the same? Newton’s 3rd law says they must be. So how does that work? The force on each wire is given by F = BIL for on and F=BIL for the other Proportional to I Proportional to I F II F II i.e. F and F are proportional to II

  15. The End

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