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When electricity is passed in a solenoid it makes an electromagnet. But the more current you want the solenoid has to have more coils have to be there. But we know that more coils increase resistance. So how can this resistant wire coil let more current flow?

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    $\begingroup$ Are you talking about mutual inductance? Your question is unclear. $\endgroup$ – Yashas Mar 5 '17 at 15:24
  • $\begingroup$ Having more coils does not increase current. It increases the magnetic field. $\endgroup$ – sammy gerbil Mar 6 '17 at 1:19
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To increase the current flowing through the electromagnet you would simply increase output voltage of your PSU. However I suspect this is not what you are talking about in your question.

One could say that magnetic field of a solenoid depends on two parameters - electric current $I$ and the number of wire turns per unit length $n$: $$B = \mu n I$$ I think you really are talking about the product $n I$, which corresponds to electric current flowing through the solenoid per unit length.

Perhaps an example could help. Let's assume you have made a $10cm$ long solenoid by winding a copper wire around a cylinder in a single layer. If you took a $1mm$ wire, the solenoid would have approximately 100 turns with turn density 10 turns per cm. Your have a current source in your disposal, which outputs $I_0 = 1A$ at all times. If you hook it up to the solenoid, $I=1A$ will flow through the wire. In terms of current per unit length it equals 10 Amperes per cm. You can calculate the magnetic field inside the solenoid using the formula above.

What would happen if you took another piece of the same wire and added another layer of turns to the solenoid, then connected it in series with the original solenoid? Your current source would still manage to push $I=1A$ though the winding, roughly at 2 times higher voltage then before. The linear density of electric current ($\frac{A}{cm}$) would have increased by a factor of 2, since now you have 20 turns of wire per length cm: electric current flowing through that 1cm of solenoid is now $20\times 1A$ instead of $10 A$ in the previous case. Now the magnetic field inside your solenoid has also increased by a factor of 2.

I hope the above makes some sense to you.

Edit: It is imperative that you add another layer of turns to the solenoid, not increase its length. Additional turns at the end of the solenoid will only significantly increase the field of a short but wide coil, not that of a rather long solenoid.

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It is true that resistance increases with wire length, but the resistance of a conducting wire is usually very small so as to make this increase in resistance negligible. Even accounting for this, a conductor in a circuit will always allows current to flow, so adding more turns to the coil results in a proportional increase in the number of current loops contributing to the net magnetic field. The coil may heat a little more because if the added resistance, but this is never an obstacle to increasing the magnetic field.

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  • $\begingroup$ How does increasing the number of coils increase the current? You wrote that increasing the length of wire increases resistance, which reduces current. $\endgroup$ – sammy gerbil Mar 6 '17 at 1:16
  • $\begingroup$ @sammygerbil clarified my answer (I hope) following your comment. $\endgroup$ – ZeroTheHero Mar 6 '17 at 1:24
  • $\begingroup$ How is it that the resistance of a conducting wire is usually very small so as to make this increase in resistance negligible. When we know that inside a incandescent light bulb the tungsten wire is coiled up and this resistance is so much that it is used to create heat and light in the bulb. $\endgroup$ – avito009 Mar 6 '17 at 6:39
  • $\begingroup$ This is an interesting observation. However, the wiring inside an incandescent bulb is designed to become very hot as to glow, whereas normal wiring does not (otherwise houses would immediately be set on fire by the electrical wiring). Hopefully when you buy longer wires (to connect the speakers of your hifi system or for cabling your DVD player) you do not see glowing wires or a significant decrease in the quality of the signal, illustrating the difference between the use of wires to transmit currents or generate magnetic fields, and the use of wires as part of an incandescent bulb. $\endgroup$ – ZeroTheHero Mar 6 '17 at 9:06
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I think I have the answer. As the number of coils are increased the magnetic field will become stronger, because each coil has its own magnetic field, so the more coils there are the more field lines there are which means it would be a stronger electromagnet. The electromagnet will become stronger if we add more coils because there are more field lines in a loop then there is in a straight piece of wire. In a solenoid there are a lot of loops and they are concentrated in the middle, as more loops are added the field lines get larger, therefore making the electromagnet stronger. The magnetic field becomes stronger because the magnetic field around a wire is circular and vertical to the wire, but the magnet fields from each of the turns in the coil add together, so the total magnetic field is much stronger. The magnetic field around a solenoid is much stronger than a bar magnets because each coil acts like a magnet when a current is passed through it, when the coils are repeated several times it is like having several mini magnets in a row, making it more stronger than bar magnet.

I cant take credit for this answer, so here is the link http://www.markedbyteachers.com/as-and-a-level/science/how-does-the-number-of-coils-on-an-electromagnet-affect-its-strength-1.html

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