Date: 1-April-2009 - Help.com

Date: 1-April-2009

Title: Demonstrate an electromagnet capable of lifting 10 grams from a distance of 1cm.

Theory:
i) Hysteresis Curve
Hysteresis:
When a ferromagnetic material is magnetized in one direction it will still not relax back to zero magnetization when the magnetizing field is removed. It needs to be driven back to zero by a field in opposite direction. If an alternating (ac) magnetic field is applied to the material, its magnetization will trace out a loop called a hysteresis loop. The lack of retrace-ability of the magnetizing curve is the property called hysteresis and is related to the existence of the magnetic domains in the material. Once the magnetic domains are re-oriented it takes some energy to turn them back again. This is useful as a “magnetic memory”. It is the use of chromium oxide and iron oxide used on audio tapes.

H – represents the magnetizing force
B – represents the flux density in the material
The Hysteresis curve:
Parameters that determine the magnetzing force, H and the magnetic flux density B

a)

Assume magnetic core is un-magnetized as B = 0.
As the magnetizing force, H, increases from zero, the flux density, B increases proportionally as indicated by the curve in (a)

b)

When H reaches a certain value (called Hsat or Hsaturation) the value of B levels off to a value (called Bsat or Bsaturation) once, saturation is reached further increase in H will not change the value of B.

The shape of a hysteresis curve gives information relative to the material
c)

Remove H, the magnetizing force, and B will start to fall back along a different path to a residual value, BR, as shown on the curve. This shows that the material still retains magnetism even though the magnetizing force has been removed i.e. H = 0. This is the ability of a material to maintain a magnetized state without the presence of a magnetizing force is called retentivity

Retentivity of the material represents the maximum flux that can be retained when H=0 after it has been magnetized to saturation and is indicated by the ratio of BR to BSat.
d)

Reversal of the magnetizing force is represented by negative values of H on the curve – an increase in H in the negative direction causes saturation to occur at a value of (- HSat) where the flux density is at its maximum negative value (- BSat)
e)

When it is reversed again the flux density will go to its negative residual value (- BR)
f)

As H is applied again in the positive direction the curve will follow the pattern in (f) back to the initial positive saturation point
g)

As the magnetizing force is alternated, the hysteresis loop follows the same pattern.

Wide B has:
 Low permeability
 Higher retentivity
 Higher coericinity
 Higher Reluctance

Suitable for Pernament magnets – magnet recording

Narrow B

Narrow B has:
Higher permeability
Lower Retentivity
Lower reductance
Lower Coercivity

Suitable for transformers of motor cores to minimize energy dissipation with AC application.

Medium B

Medium B has the in-between qualities **
3.3 Electromagnetic Induction
Electromagnetic Induction refers to the phenomenon where by a change in the magnetic field is passing through the magnetic field passing through a coil creates an e.m.f. in the coil.
Moving a magnet towards a coil causes the needle to deflect – a current must now be flowing in the coil. If the North Pole is moved the towards the coil the needle is seen to move one direction, and if the south pole is moved towards the coil, the needle is seen to move in the opposite direction.
E = induced e.m.f. = Current created
3.4 Factors affecting size of induced e.m.f.
1. E B: The greater the strength of the magnet, the greater the size of the induced e.m.f
2. E N: The greater the number of turns, the greater the size of the induced e.m.f
3. E v: The greater the velocity, the greater the size of the induced e.m.f
If both the magnets and the coil are kept stationary, no e.m.f. is induced. Therefore, whenever there is a magnetic field that is changing (in size or direction) an e.m.f. is induced. The principle is used in the bicycle dynamo.
3.5 Magnetic Flux (pho)
Factors affecting Size of Force
1. F I The stronger the current, the stronger the force
2. F l, The length of the conductor in the magnetic field
3. F B, The magnetic, flux density (the strength of the magnet)
 F = BIl
Magnetic Flux Density (B):
Magnetic Flux Density is a vector quantity, whose magnitude is equal to the force that would be experienced by a conductor of length 1m, carrying a current of 1A at right angles to the field, and whose direction is that of a north pole at that point. Its unit is the Tesla, T.
Force on a moving charge in a magnetic field:
Both positive charges and negative charges moving in a magnetic field will experience a force F = Bqv,
F = force, B = Magnetic flux density, q = charge, v = velocity of charge

Magnetic Flux – Magnetic Flux Density * Area, i.e. = BA. The unit of magnetic flux is the Weber (Wb).
3.6 Faraday’s Law of Electromagnetic Induction:

3.7 Lenz’s Law:
Lenz’s Law states that the direction of an induced current is always opposing the charge that caused it. By moving the magnet towards the coil as shown the current that flows in an anti-clockwise, direction at the end on the right. This has the effect of creating a north pole here and therefore tries to oppose the movement of the magnet towards it. E.g. When a cylinder magnet, and, a cylinder metal are moved down copper pipe at exactly the same time, it is found that the cylindrical magnet falls through.
Explanation:
As the magnet flows through the copper pipe/ the copper pipe experiences a changing magnetic field. According to faraday’s Law this then induces an emf. According to Lenz’s law the direction of the induced emf’s is such that they oppose the movement of the magnet through the piping, i.e. they slow down the movement of the magnet through the pipe.
ii) Alignment of magnets:
Magnets always align themselves E.g. a compass, if you spin the magnet at the centre (then the second magnet is the earth polarity) it will align itself until its facing on the right direction (north) this will make friction like force that will stop the magnet from spinning so its impossible to make free-energy out of house “held” items (except with nature help, using a dynamo motor and a flap like propeller then sunk underneath the water falls, isn’t that free?) and its also impossible to use or harvest energy from the outer space!
ii) Maxwell’s Equations

iii) Turns in a coil
Apparatus:
Nail, battery, wire, battery holder, paper clips, steel nails, iron nail (6 inch), wire strippers, multi-meter

Diagram:

Method:
1. Gather required apparatus.
2. Get the coil of wire and wrap it around the nail x number of times, depending on what you want the magnetic flux density to be.
3. Use a wire strippers to strip the top and bottom of the wire bare leaving only a small amount of wire, just enough for there to be a connection between the battery and the wire.
4. Get the battery and place it in the circuit (preferably using a battery holder for better connection)
5. Using the multi-meter calculate the current flowing through the circuit.
6. Now that you have R, I, µ, and the number of turns, use the following equation:

Demonstration:

Flaws with above experiment:

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