Physics (Year 12) - Electro-magnetism
We all have encountered magnets in our lives and seen how they interact with each other; like poles repel and opposite poles attract. But there is a lot more to magnetism that just that. Magnets have magnetic fields, just like how electrical point charges (from the previous sections) have electric fields. Magnetic fields are also represented by arrows and these arrows point in the direction in which a compass would point if placed at that point.
Magnetic field lines are also sometimes called magnetic flux lines. Some examples of magnetic flux lines around magnets are shown below. Just like drawing electric field lines, there are some rules that need to be followed when drawing magnetic flux lines; lines cannot intersect, the proximity of the lines represents strength of the field.
Magnetic fields due to current
When a current flows in a wire, a magnetic field is created around it. It is important to note that we are referring to conventional current; which is the flow of positive charge and flows from the positive terminal to the negative terminal.
The direction of the magnetic field around the current-carrying wire can be determined using the right hand grip rule:
Grip the current-carrying wire with your right hand and your thumb pointing in the direction of conventional current flow
The direction which your fingers curl (clockwise or anti-clockwise) is the direction of the magnetic field lines
Since not everyone is equipped with artistic talent, instead of drawing ‘3D’ variations of current-carrying wires and the magnetic fields around them (like the diagram above), we utilise dots and crosses to generate diagrams. These dots and crosses can be used to either depict the direction of current flow or the direction of magnetic field. Dots represent current coming out of the page or magnetic field coming out of the page. Whereas crosses represent current going into the page or magnetic field going into the page.
Magnetic field around a solenoid
A solenoid is a type of electromagnet with a wire arrangement where the wire is looped multiple times and the loops are placed side by side to create a coil. Due to this arrangement, the magnetic fields around the wire add together and result in a stronger magnetic effect. Also due to this arrangement, the magnetic field around a solenoid is similar to that around a simple bar magnet
To determine the direction of the magnetic field through a solenoid, we simply wrap our fingers around the coil in the direction of the conventional current. The direction our thumb points (left or right) will be the north pole of the solenoid and hence where the magnetic field lines come out of.
To increase the strength of the solenoid, the following can be done:
Increase the current through the wire
Increase the number of turns in the coil
Insert a soft iron core inside the coil
Calculating strength of magnetic field around a wire
The strength of a magnetic field is commonly referred to as the magnetic flux density, denoted by the symbol B with units of Tesla, T. The equation to calculate magnetic flux density is:
If there are two wires and you are asked to find the magnetic flux density at a point between the two wires, then you would individually find the magnetic flux density at that point from the both wires and add the two quantities together if the current in the wires are flowing in opposite directions. If they current is flowing in the same direction then you would calculate the difference between the two values.