# Magnetic feild question

1. Sep 20, 2010

### Caelus

if you turn on an electromagnet, how long does it take the feild to apear?

2. Sep 20, 2010

### pallidin

I'm not 100% positive, but I think it is at the speed of light.

3. Sep 20, 2010

### Drakkith

Staff Emeritus
As far as i know, it is the speed of light. Also, PLEASE edit your title and change Feild to Field. It's really bugging me lol! And it helps with the search function if someone wants to search for a Field.

4. Sep 20, 2010

### stevenb

Hmmm, the question can be a little misleading (particularly the word appear!). I agree with the above that it is the speed of light if you are asking about the propagation delay.

Another way to interpret your question is in terms of circuit theory assuming infinite propagation speed. Then you are really talking about the circuit time constant. This then depends on the circuit. However, if you basically have an inductive circuit with resistance, (as might be typical), then the time constant is

$$\tau={{L}\over{R}}$$,

where R is the resistance and L is the inductance. This is a first order response and it takes about five time constants to effectively reach the steady state field. Steady state is when the field is effectively constant which seems to be a different meaning than "appear".

5. Sep 20, 2010

### pallidin

Ah, good call steven.

Perhaps the OP is talking more about the circuit itself.
That is, "I press the power button for an electromagnet. How long does it take for the magnetic field to start propagating?"

6. Oct 9, 2010

### Caelus

Im finding the question hard to word. if you had two spaceships 600,000,000 meters from each other and one turned on a realy powerful electormagnet, how long would it take the other spaceship to detect it (ignoring the resistance in the magnet)?

7. Oct 9, 2010

### granpa

well the speed isnt infinite or antennas wouldnt work.

8. Oct 9, 2010

### Newai

We can edit our titles?

9. Oct 9, 2010

### pallidin

Converts to: 372,822.7 miles.
As the speed of light(also magnetism) is 186,282 miles per second, this equates to:
2 seconds.

10. Oct 10, 2010

### Caelus

Magnetic feilds arn't affected directly by gravity right?

11. Oct 10, 2010

### pallidin

The full nature of gravity is not understood.
It does seem to affect everything, even massless photons.
However, the SIGNIFICANCE, or magnitude, of effect is most important.

Gravity seems to be an immutably pervasive but VERY weak force.
My initial thoughts are that, yes, magnetism is affected by gravity.