[Music] hi and welcome back to freesciencelessons.co.uk by the end of this video you should be able to describe how a current flowing through a wire generates a magnetic field you should then be able to explain how a solenoid can increase the magnetic effect of a current and finally you should be able to draw the magnetic field pattern for a straight wire carrying a current and for a solenoid okay i'm showing you here an electrical circuit with an open switch when we close the switch an electric current flows around the circuit now this diagram shows the conventional current if you don't know what that means then you should go back and watch the video on that topic the key factor is that when a current flows through a conducting wire a magnetic field is produced around the wire and i'm showing the magnetic field here now we can prove it as a magnetic field around the wire by using a compass like this when the current is turned off then the compass needle lines up with the earth's magnetic field however if we turn the current on again then the compass needle deflects like this and this proves that there's a magnetic field around the wire now the strength of the magnetic field depends on the size of the current a larger current produces a stronger magnetic field the magnetic field is also strongest closer to the wire as we move further from the wire the strength of the magnetic field decreases if we change the direction of the current then we change the direction of the magnetic field so in this case we've reversed the direction of the conventional current and you can see that the direction of the magnetic field has reversed a compass placed near this wire would deflect in the opposite direction to before now in the exam you could be asked to work out the direction of the magnetic field produced by a wire to do that we use the right hand grip rule and i'm showing you that here we place our right hand so that the thumb is pointing in the direction of the conventional current now the fingers are pointing in the direction of the magnetic field coming up we're going to look at another way we can increase the strength of the magnetic field in a wire carrying an electric current [Music] okay now one other way to increase the strength of the magnetic field is to coil the wire like this scientists call this shape as solenoid and you do need to learn that name when we turn on the current we get a strong and uniform magnetic field inside the solenoid the magnetic field around the solenoid has a similar shape to the magnetic field around a bar magnet and i'm showing you that here again in the exam you could be asked to look at the direction of the magnetic field and again we can apply the right hand grip rule to make this easier we're going to remove the magnetic field lines i'm also showing you the direction of the conventional current in the solenoid in this case place the fist of your right hand so the fingers curl in the same direction as the conventional current your thumb now points in the direction of the north pole now there are three ways we can increase the strength in the magnetic field produced by a solenoid and you need to learn these firstly if we increase the size of the current then we increase the strength of the magnetic field secondly the strength of the magnetic field increases if we increase the number of turns of the coil this solenoid has more turns than the previous one so the strength of the magnetic field is greater and finally if we place a piece of iron inside the solenoid then we also increase the strength of the magnetic field scientists call this an iron core now a solenoid containing an iron core is called an electromagnet and i'm showing you one of these here electromagnets are extremely useful as we can change the strength of the magnetic field by changing the size of the current we can also turn an electromagnet on or off remember you'll find plenty of questions on electromagnets in our vision workbook and you can get that by clicking on the link above you