The direction of the magnetic field is determined by the direction in which a north pole of a compass needle moves inside it. Conventionally, it is established that magnetic field lines emerge from the north pole and merge at the south pole.
How is the direction of the magnetic field determined, and what is the convention regarding the movement of a compass needle?
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The direction of the magnetic field is determined by the convention known as the right-hand rule. According to this rule:
1. Point your thumb: Align your thumb in the direction of the current (flow of positive charge).
2. Extend your index finger: Extend your index finger in the direction of the magnetic field (north to south).
3. Let your middle finger be perpendicular: Your middle finger, perpendicular to both your thumb and index finger, indicates the direction of the magnetic force acting on a positive charge.
In terms of the movement of a compass needle, it aligns itself with the magnetic field lines. The north pole of a compass needle points towards the Earth’s magnetic north pole, which is essentially the south pole of the Earth’s magnetic field. This means that the convention is that magnetic field lines outside of a magnet go from the north pole to the south pole, and the north pole of a compass needle points in the direction of these field lines. Inside a magnet, the field lines go from the north pole to the south pole, so the convention is maintained.