 and so what makes magnetic fields? Well the answer is moving charges. So we saw that if we have an infinitely long wire carrying a current then the magnetic field a distance r away from that would have a strength given by... but the direction is more complicated you have to use the right hand rule so you put your right hand thumb in the direction of the current and then your fingers point in the direction that the magnetic field is going to go so in this case it's going to go into the page which we usually denote by a circle with a cross in it which looks like the back of an arrow. The arrow is going into the page so we see the fletching at the back of the arrow and a common example of something that is definitely not an infinitely long straight wire that we've looked at is the solenoid with a magnetic field due to each piece of current in each piece of wire here so we've got all these little bits of wire that we have to take into account separately because they're all pointing in different directions but if we add up the magnetic field due to all those pieces of wire then we end up with a surprisingly constant magnetic field through the middle of the solenoid and the field is otherwise very similar to a bar magnet. Now a bar magnet's magnetic field is also made by moving charges though in this case it's the microscopic quantum mechanical spin of the individual particles inside it and if we were to isolate just a single moving charge and it was moving at a lost EV and we wanted to know the magnetic field at a position that was a distance r away then again we'd have to do a right hand rule and this moving charge here looks exactly like a current and so we use essentially the same right hand rule for a positive charge we put our right thumb along the velocity and then our fingers curl down and so we can see that the magnetic field in that case would be going again into the page. Now calculating the strength of that magnetic field or the strength of magnetic fields around magnets or the strength of magnetic fields in solenoids can be done of course it does require a little bit more complicated calculations so we're not going to worry about that now as long as we can figure out what direction we expect the magnetic field to be going we're pretty happy and we know that we have a nice long wire we can figure out how strong the magnetic field is away from that.