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Electromagnetism: 10.3 How can you increase the magnetic field strength of a solenoid coil?

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10.3 How can you increase the magnetic field strength of a solenoid coil?

What factors affect the strength of an electromagnet?

As we have seen, due to the alignment of the parallel lines of force, the magnetic field inside the solenoid is very uniform and very strong, but only in the coil and the poles at the ends of the solenoid, so how can we increase the magnetic effect i.e. make a stronger electromagnet?

(a) Increasing the current flow through the coil

The strength of the magnetic field is increased overall by increasing the current.

Any stream of moving electrically charged particles naturally creates a magnetic field.

The more charged particles moving through the wire, the greater the magnetic field effect.

(Don't say by 'increasing the p.d.' without saying to increase the current!)

(b) Increasing the number of coils of wire of the solenoid coil

Many solenoids consist of hundreds of turns of thin insulated copper wire.

The more coils packed tightly together, the greater the strength of the magnetic field.

You can do this by (i) increase the number of coils using the same length of wire OR (ii) you can both increase the number of coils AND the total length of wire.

(c) Using an soft iron core - this makes a practical temporary electromagnet

If you place a rod of magnetic material like iron, inside the solenoid, the iron becomes an induced magnet and the magnetic lines of force are intensified through it.

 

As with the 'empty' solenoid its self, the magnetic flux is greatest at the ends of the solenoid, which now coincides with the ends of the magnetically 'soft' iron rod (left diagram).

The solenoid plus the iron rod are effectively make a strong 'bar magnet' (right diagram).

As long as the current is flowing the electromagnetic effect will work.

Switch off the current and the magnetic effect goes.

This means you can use this system as an on/off temporary electromagnet that has many useful applications.

(d) Decreasing the length of the solenoid (not the total length of the wire!)

If you can compact the solenoid to a shorter length for the same number of insulated coils of wire you increase the intensity of the magnetic field - making the lines of force closer together.

I don't consider this an important factor since the insulated coils of wire are usually packed as tightly together as possible and length might be determined by how it fits into some device.

 

In most applications it is factors (b) and (c) that are employed to increase the effectiveness of the solenoid.


coil1.gifspace.gifcoil2.gif

 

Coil 1. Just a plain solenoid coil, producing a relatively weak magnetic field.

Coil 2. This solenoid with a soft iron core, produces a much greater strength of magnetic field due to the addition of the iron rod.

Coil 3. Using two soft iron rods, or one thicker one, the field strength is increased even more.

Coil 5. Unlike coils 2. and 3., which are temporary electromagnets (on/off with current), coil 5 would make a steel rod a permanent magnet.

Coil 5. could not be used as an on/off electromagnet, but it is a way of making permanent magnets.

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Keywords, phrases and learning objectives on electromagnetism

Be able to know, describe and explain, how to increase magnetic field strength of solenoid coil by increasing the current flowing through it, more turns of wire wound around, using a soft iron core and making the coil more compact.


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