Made an electromagnet, curious about how this works exactly.

[yanom]

Hi, I'm a sophmore in high school, and I haven't taken the Physics course yet. So I'm a bit green. Anyway, I was playing around with an electromagnet I made with a nail and some 1mm wire, and I've got a few questions about the magnetism it generates:

-First, why does that iron core need to be in there? There's no current flowing through it, and from what I understand a coil already generates a magnetic field on it's own. But I can't do much with the coil itself, I've got to have that iron core.

-If I have a fixed energy supply, what makes a stronger magnetic through the coil: high voltage, low current or low voltage, high current?

-If I understand how transformers work correctly, they're simply two coils held together where one induces a current flow in another. So shouldn't a wire held up next to a powered coil show some sort of voltage? But if I do this, my multimeter can't detect any power in that second wire. So I don't understand this right. Do transformers require an iron core too?

 

[The_General]

Hi, I'm a sophmore in high school, and I haven't taken the Physics course yet. So I'm a bit green. Anyway, I was playing around with an electromagnet I made with a nail and some 1mm wire, and I've got a few questions about the magnetism it generates:

-First, why does that iron core need to be in there? There's no current flowing through it, and from what I understand a coil already generates a magnetic field on it's own. But I can't do much with the coil itself, I've got to have that iron core.

Yes, that's true. A solenoid generates a magnetic field, but the flux intensity is greatly increased when a ferromagnetic material is introduced as the core. It happens because the magnetic field intensity is proportional to the turns of your coil and the current flowing. But, the magnetic flux density is related to the magnetic field intensity concerning the core material characteristics.

-If I have a fixed energy supply, what makes a stronger magnetic through the coil: high voltage, low current or low voltage, high current?

Maintaining the geometry and the core material, if you increase the number of turns or the current, the magnetic field intensity will increase as well. It's important to know that in real life the relation between magnetic field intensity and magnetic flux density is not linear. There are regions which can be modeled as such.

-If I understand how transformers work correctly, they're simply two coils held together where one induces a current flow in another. So shouldn't a wire held up next to a powered coil show some sort of voltage? But if I do this, my multimeter can't detect any power in that second wire. So I don't understand this right. Do transformers require an iron core too?

The ferromagnetic core has two purposes: increase the magnetic flux density magnitude and guide the flux lines within it (ideally). The voltage appears only when B (magnetic flux density) varies with time. If you are applying DC current, nothing will be noticed in the second coil, with a ferromagnetic core or not, unless you move the coil through a non uniform field, or varies the area where B is passing.

Replied inside the quote.

I'll give you some tips.
H - magnetic field intensity vector
N - turns
I - current

If you have a closed core with average length C, then you will have the following relation, excluding flux dispersion and a uniform magnetic field within the core:
|H|*C = N*I

B = u*H, considering a linear model.
u - core magnetic permeability
B - magnetic flux density vector

F = |B|*S, considering B uniform and perpendicular to the core cross section area.
F - Flux
S - core cross section area

V = d(N*F)/dt
V = voltage in a coil with N turns within a flux F.

 

[yanom]

The ferromagnetic core has two purposes: increase the magnetic flux density magnitude and guide the flux lines within it (ideally). The voltage appears only when B (magnetic flux density) varies with time. If you are applying DC current, nothing will be noticed in the second coil, with a ferromagnetic core or not, unless you move the coil through a non uniform field, or varies the area where B is passing.

So transformers only work with AC current? what do you do if you have to voltage-change in a DC-only circuit?

flux

uhh... what's that?

 

[The_General]

So transformers only work with AC current? what do you do if you have to voltage-change in a DC-only circuit?

Yes, that's true. Increasing or decreasing voltage in a DC circuit is not so simple. You can use a voltage regulator which dissipates with resistors the additional voltage (very inefficient). On the other hand, if you want to increase the voltage, a more complex system such as a switching mode power supply should be used.

About flux: http://en.wikipedia.org/wiki/Flux

 

[yanom]

ok, thanks. So flux is flow through a surface area? Like water through a certain area filter?

Wait, how does magnetism "flow"? It isn't a type of waves/radiation, is it?

 

[The_General]

ok, thanks. So flux is flow through a surface area? Like water through a certain area filter?

Exactly.

Wait, how does magnetism "flow"? It isn't a type of waves/radiation, is it?

Magnetic fields and electric fields are related by the following Maxwell Equations:

E - Electric field intensity vector
H - Magnetic field intensity vector
B - Magnetic flux density vector
Jf - Free current density

Considering a linear relation between H and B in a space free of charge, you obtain the following:

That's an homogeneous wave equation. If you decompose the vector in three coordinates and solve the equations, you will obtain three wave functions.
Therefore, you will have a wave only when E or H varies in time.