Direction of Magnetic Field
This page is here to provide an explanation for finding the direction of magnetic field. Since magnetic field is a cross product, finding its direction can be confusing at times, and you'll be asked many questions involving the direction of the magnetic field due to a moving charge, a current carrying wire, a current carrying coil, a bar magnet, etc. Hopefully this should work as a resource to come to when confused about anything regarding direction of magnetic fields!
The Main Idea
You previously learned about electric fields, and how a charged particle, or group of charged particles create a field in a specific pattern due to those particles' orientations. Magnetic field works in a similar fashion, but remember that it doesn't come from simply a charged particle, but a moving charged particle, with a velocity [math]\displaystyle{ \vec{v} }[/math]. Due to this new concept, we start to see some new patterns that you should want to get very familiar with.
A Mathematical Model
Recall that the equation for magnetic field due to a moving charged particle is [math]\displaystyle{ \vec{B} =\frac{\mu_0}{4\pi} \frac{(q\vec{v} \times \hat{r})}{|\vec{r}|^2} }[/math], otherwise known as the good-ole Biot-Savart law. The most important thing to note about this in regards to direction of magnetic field is that this is a cross product between the charge of the moving particle times its velocity, [math]\displaystyle{ q\vec{v} }[/math] and the unit distance vector from the particle to the observation location, [math]\displaystyle{ \hat{r} }[/math]. This Biot-Savart law is the basis for every orientation of magnetic field you will encounter in this class (wires, magnetic dipoles, etc), and its cross product is what makes finding the direction of magnetic field much trickier than finding the direction of electric field.
A Moving Charged Particle
The first scenario you need to be familiar with is when a particle with a charge [math]\displaystyle{ q }[/math] has a velocity [math]\displaystyle{ v }[/math]. What does this look like? Check out the following image:
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