# Homework Help: What direction does a compass point to in a magnetic field?

1. May 22, 2017

### Notaphysicsmajor

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Hello

Today we did a lab and one of the activities was having a compass underneath a wire while the current was off. The wire was straight so that it was directly aligned to be with the needle of the compass. One the current was flowing, the compass arranged itself in that it was not exactly 90 degrees perpendicular to the wire, but pretty close.

To help visualize the set up pretend that the wire was horizontal and the current was moving from left to right. The needle would then move once then current was moving. From what I understand is that magnetic field lines move inward toward with the south end, and outwards from the north end. Can someone clarify to me what these north and south ends are, my guess is that it would be a magnet? Also when a current is turned on, which of the needles on the compass is pointing in the direction of the magnetic field produced by the current on the wire?

2. May 22, 2017

### scottdave

The needle of a compass is a magnet. If a magnet is in another magnetic field, a force which tries to align the two fields is exerted. This is a standard experiment. It is easily googled. I still remember it in high school Physics class. To give you a hint, the right hand rule will tell you which way the magnetic field is around a current carrying conductor.

3. May 22, 2017

### Tenebris

It's not a magnet, but an electromagnetic field, generated by the electrons in the wire. there is a right-hand rule to find the direction of the field. here's a picture below

4. May 22, 2017

### kuruman

To elaborate on what @Tenebris said, the very long wire has "North" and "South" poles all the way to infinity. More realistically, the distance at which you put your needle sensor is much smaller than the length of the wire. The needle lines up with the magnetic field, locally, so that if you imagine an arrow with its tail at the "south" pole of the needle and its tail at the "north" pole of the needle, the arrow will point in the direction of the magnetic field.
I assume you meant to say "a torque is exerted". A force will accelerate the center of mass of the magnet.

5. May 22, 2017

### sophiecentaur

What do we mean by a North or South pole? Historically it's to do with the fact that a 'magnetised' rod / needle will align itself in a N/S direction. The North SEEKING pole* (with the red paint on or the letter N) will point roughly towards the Earth's North Pole and the South SEEKING pole will point in the opposite direction. The field can often be described in terms of lines of force and, where the lines of force are close together (e.g. at the ends of a bar magnet) is called a Pole. But these lines have no ends; they form a closed curve, passing down inside the magnet and coming out of the other pole as well as spreading out and joining the poles outside. An electric current flowing through a straight wire produces circular field lines and a compass will point its N end in the direction of the field - as shown in the picture above. We could have chosen any direction for the conventional field direction but magnets were first used in conjunction with the Earth's poles so we say that lines of force go from the Earth's South Pole to the Earth's North pole because that's the way the N on a bar magnet points. (A kind of circular argument but it's consistent and it's basically an arbitrary choice.)
* I was not listening to that bit when first told about this in school and was confused for a long time because I missed out the word "seeking" in my mind.

6. May 22, 2017

### scottdave

Yeah, a 'couple' of forces, actually. Force in one direction at the North end, and in the other direction at the South, creating a torque. Nice catch.

Last edited: May 22, 2017