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The Electromagnet

The Electromagnet

A simple electromagnet can be created by wrapping a coil of wire around a soft iron core, such as a large nail

The electromagnet is a type of temporary magnet in which its magnetic field is produced by electric current and to concentrate the magnetic field, the wire of an electromagnet is wound into a coil.

We now know from the previous tutorials that a straight current carrying conductor produces a circular magnetic field around itself at all points along its length and that the direction of rotation of this magnetic field depends upon the direction of current flow through the conductor, the Left Hand Rule.

In the last tutorial about Electromagnetism we saw that if we bend the conductor into a single loop the current will flow in opposite directions through the loop producing a clockwise field and an anticlockwise field next to each other. The Electromagnet uses this principal by having several individual loops magnetically joined together to produce a single coil.

Electromagnets are basically coils of wire which behave like bar magnets with a distinct north and south pole when an electrical current passes through the coil. The static magnetic field produced by each individual coil loop is summed with its neighbour with the combined magnetic field concentrated like the single wire loop we looked at in the last tutorial in the centre of the coil. The resultant static magnetic field with a north pole at one end and a south pole at the other is uniform and a lot more stronger in the centre of the coil than around the exterior.

Lines of Force around an Electromagnet

Electromagnetic Coil

 

The magnetic field that this produces is stretched out in a form of a bar magnet giving a distinctive north and south pole with the flux being proportional to the amount of current flowing in the coil. If additional layers of wire are wound upon the same coil with the same current flowing, the magnetic field strength will be increased.

It can be seen from this therefore that the amount of flux available in any given magnetic circuit is directly proportional to the current flowing through it and the number of turns of wire within the coil. This relationship is called Magneto Motive Force or m.m.f. and is defined as:

magneto motive force equation

Magneto Motive Force is expressed as a current, I flowing through a coil of N turns. The magnetic field strength of an electromagnet is therefore determined by the ampere turns of the coil with the more turns of wire in the coil the greater will be the strength of the magnetic field.

The Magnetic Strength of the Electromagnet

We now know that were two adjacent conductors are carrying current, magnetic fields are set up according to the direction of the current flow. The resulting interaction of the two fields is such that a mechanical force is experienced by the two conductors.

When the current is flowing in the same direction (the same side of the coil) the field between the two conductors is weak causing a force of attraction as shown above. Likewise, when the current is flowing in opposite directions the field between them becomes intensified and the conductors are repelled.

The intensity of this field around the conductor is proportional to the distance from it with the strongest point being next to the conductor and progressively getting weaker further away from the conductor. In the case of a single straight conductor, the current flowing and the distance from it are factors which govern the intensity of the field.

The formula therefore for calculating the “Magnetic Field Strength”, H sometimes called “Magnetising Force” of a long straight current carrying conductor is derived from the current flowing through it and the distance from it.

Magnetic Field Strength for Electromagnets

the electromagnet magnetising force

  • Where:
  • H – is the strength of the magnetic field in ampere-turns/metre, At/m
  • N – is the number of turns of the coil
  • I – is the current flowing through the coil in amps, A
  • L – is the length of the coil in metres, m

Then to summarise, the strength or intensity of a coils magnetic field depends on the following factors.

  • The number of turns of wire within the coil.
  • The amount of current flowing in the coil.
  • The type of core material.

The magnetic field strength of the electromagnet also depends upon the type of core material being used as the main purpose of the core is to concentrate the magnetic flux in a well defined and predictable path. So far only air cored (hollow) coils have been considered but the introduction of other materials into the core (the centre of the coil) has a very large controlling effect on the strength of the magnetic field.

the electromagnet

Electromagnet using a nail

If the material is non-magnetic for example wood, for calculation purposes it can be regarded as free space as they have very low values of permeability. If however, the core material is made from a Ferromagnetic material such as iron, nickel, cobalt or any mixture of their alloys, a considerable difference in the flux density around the coil will be observed.

Ferromagnetic materials are those which can be magnetised and are usually made from soft iron, steel or various nickel alloys. The introduction of this type of material into a magnetic circuit has the effect of concentrating the magnetic flux making it more concentrated and dense and amplifies the magnetic field created by the current in the coil.

We can prove this by wrapping a coil of wire around a large soft-iron nail and connecting it to a battery as shown. This simple classroom experiment allows us to pick-up a large quantity of clips or pins and we can make the electromagnet stronger by adding more turns to the coil. This degree of intensity of the magnetic field either by a hollow air core or by introducing ferromagnetic materials into the core is called Magnetic Permeability.

Permeability of Electromagnets

If cores of different materials with the same physical dimensions are used in the electromagnet, the strength of the magnet will vary in relation to the core material being used. This variation in the magnetic strength is due to the number of flux lines passing through the central core. if the magnetic material has a high permeability then the flux lines can easily be created and pass through the central core and permeability (μ) and it is a measure of the ease by which the core can be magnetised.

The numerical constant given for the permeability of a vacuum is given as: μo = 4.π.10-7 H/m with the relative permeability of free space (a vacuum) generally given a value of one. It is this value that is used as a reference in all calculations dealing with permeability and all materials have their own specific values of permeability.

The problem with using just the permeability of different iron, steel or alloy cores is that the calculations involved can become very large so it is more convenient to define the materials by their relative permeability.

Relative Permeability, symbol μr is the product of μ (absolute permeability) and μo the permeability of free space and is given as.

Relative Permeability

relative permeability equation

 

Materials that have a permeability slightly less than that of free space (a vacuum) and have a weak, negative susceptibility to magnetic fields are said to be Diamagnetic in nature such as: water, copper, silver and gold. Those materials with a permeability slightly greater than that of free space and themselves are only slightly attracted by a magnetic field are said to be Paramagnetic in nature such as: gases, magnesium, and tantalum.

Electromagnet Example No1

The absolute permeability of a soft iron core is given as 80 milli-henries/m (80.10-3). Calculate the equivalent relative permeability value.

relative permeability

 

When ferromagnetic materials are used in the core the use of relative permeability to define the field strength gives a better idea of the strength of the magnetic field for the different types of materials used. For example, a vacuum and air have a relative permeability of one and for an iron core it is around 500, so we can say that the field strength of an iron core is 500 times stronger than an equivalent hollow air coil and this relationship is much easier to understand than 0.628×10-3 H/m, ( 500.4.π.10-7).

While, air may have a permeability of just one, some ferrite and permalloy materials can have a permeability of 10,000 or more. However, there are limits to the amount of magnetic field strength that can be obtained from a single coil as the core becomes heavily saturated as the magnetic flux increases and this is looked at in the next tutorial about B-H curves and Hysteresis.

119 Comments

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  • Bashar Dhiyab

    Good elaboration, simple and straightforward

  • Noam

    does placing a diamagnetic material around the coil boost the magnetic field strength in the middle of the coil or reduces it?

    • Wayne Storr

      Diamagnetic materials, such as copper, have a relative permability of less than one (1) as they are not affected by any magnetic field inside since they tend to reject lines of flux to the outside. That is a strong magnetic field will have no affect or attraction on a diamagnetic material at all.

      Thus we believe covering a wound coil with a diamagnetic material would have little or no effect on the magnetomotive force strength of the magnetic field in the centre of the coil of wire. More current flow in the turns of the coil, the stronger the magnetic field. While the more turns of wire, the more concentrated the lines of force as the ampere-turns increases.

  • Dwayne Schultz

    How many times can a electromagnet be charged and discharged before it needs to be replaced?

    • Wayne Storr

      Indefinately, and depends on what you mean by being “replaced”. An electromagnet is basically a solenoid with a soft iron core used as the inner core. It behaves as a magnet only when an electric current flows through its coil windings. However, due to the permeability of the core material used, it is possible that some of the magnetic domains remain orientated relative to the magnetic field generated by the core once the electric current is removed. This produces the effect of residual magnetism and demagnetisation of the core can remove, or at least greatly reduce the effects of residual magnetism.

  • Dana White

    Awesome and so clear teaching!!!

  • John Muller

    Hello.
    If I were to make a hollow magnetic coil so a steel rod can pass through it, by coiling copper wire in layers, will the more layers increase the strength of the magnetic field?

    • Wayne Storr

      A magnetic field is produced when an electric current passes through a coil of wire creating an electromagnet. The strength of the magnetic field around the coil can be increased by increasing the current flowing through the coil (this will increase the flux) or by increasing the number of coil turns (the N*L turns of wire) which will also increase the flux Φ.

  • Asif

    Gustafson

  • Luke van Duren Watak

    How would you calculate the magnetic field outside of the electromagnet?

  • Omer

    On two parallel clnductore with opposing current flow, Is it correct to state that the direction of the magnetic field opposes the magnetic force?

    • tony chu

      As implies by the article, same current direction attracts each other, and opposite direction repels each other and this agrees with the Fleming left hand rule for motor.

  • DE

    Suppose I have a constant voltage driving a coil. The resistance of the coil is proportion to the total length of the wire and hence proportional to N, the number of turns. Let’s write R=kN. If the voltage applied is V, then I = V/R = V/(kN). This means H=IN/L=VN/(kNL)=V/(kL), and this is independent of N. So how come if I have more turns, I get greater magnetic force? Perhaps I am confused about the relation between H and force.

    • tony chu

      DE, I think you ve misinterpreted the the length of COIL L in formula H=IN/L as equal to the length of the WIRE, that’s why you have made that substitution ending up cancel of the term N in you calculation, which is wrong. Please take note L is the physical length/dimension of the SOLENOID COIL itself but not the UNWOUND WIRE length, you can wrap as many turns as you like within that length L, which have no relationship with the wire length you have supposed in your assumption.

    • Wayne Storr

      The magnetic field strength of an electromagnet or solenoid, depends on the number (N) of coil turns, the strength of current flowing through the coil in amperes, and its length L. This gives us: H = (NI)/L where NI represents the ampere-turns value. Clearly then for a given coil length, increasing the number of turns, or increasing the current flow (by increasing voltage as I = V/R) will result in a higher magnetic field intensity.

      Thus a coil with an N of 1000 turns would produce a greater magnetic field intensity than one with only 100 turns. Having said that, the electromagnet could equally consist of one single turn (1N) of flat copper wire or bar of width L carrying sufficient current of 1N amperes evenly distributed across its width.

  • Aidan Eby

    Hello, I am a student wanting to run this experiment with an iron nail. I plan to run multiple trials with different intensities of current. Do I need to change the nail each time? Will it affect the nail properties and results if I keep using it? Thank you

  • Conrad

    discuss the behavior and effect of electromagnetic lines of force in a coiled wire and a straight wire if there is a current flowing through respectively

  • Sanjay Kaul

    I am into industrial blade grinding machine building business , I made an electromagnetic chuck , 1000×200 , the no. Of poles is 21 . I used 18 gauge 1 mm enameled wire .

  • Tyler Hughes

    Does anyone know how the ground floor of a building can be made to emit an AC magnetic field from every square inch? I’m living on the first floor in that type of situation and have walls and appliances emitting RF at different intensities and frequencies throughout the day.

  • amelia

    A 100 Turn electromagnet creates a 20 Gauss magnetic field. If the number of loops is doubled to 200 turns, what will be the new magnetic field of the magnet?

  • Pooja Koijam

    A copper wire is coiled around a soft iron piece. The free ends of the wire are connected to a battery of similar cells. In which of the following combination will the strength of the electromagnet formed be the greatest?
    a. 10 turns of copper wire, one cells
    b. 15 turns of copper wire, four cells
    c. 20 turns of copper wire, eight cells
    d. 25 turns of copper wire, eight cells

  • Dwayne G Schultz

    This may seem like an odd question, but if I wind say 200 coils around 1/2 of a soft iron rod that is say 6 inches long, and only 50 coils of wire arond the other half, would the 200 produce a stronger say Positive (N) lines than the other half (S) ?

    • Wayne Storr

      Clearly it would depend on how the coils are wound on the same rod core. As the same series current flows through each coil, their ampere-turns will be the same so their magnetising force will be the same. The field intensity around each coil will depend on how they are wound, series-aiding or series-opposing as well as their unit length as the coil of 200 turns wound over 4cm will be the same as the coil of 50 turns wound over 1cm.

  • Dwayne G Schultz

    This is close to what i need to Know, but instead of a batterey being used to magnetize the coils can I use a Strong Permanent (like Neodium) Magnet?

  • M.Hofman

    How does the distance between the coils of an electromagnet affect the strength? It would seem logical that the further the coils are apart, the weaker the field strength but is there a formula for this?

  • Mk heera

    How many turns in a volt should a Hague decide? In a electromagnet.

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