Welcome to Professor MAD.
Physics for Everyone.
Today we are going to talk about Magnetism.
The first thing we know about magnets is that magnets attract specific metals.
Metals that naturally attract magnets are known as Ferromagnetic Metals.
Metals will firmly stick to these magnets.
For example Iron, Cobalt, Steel, Nickel, Manganese, Gadolinium
and Lodestone are all ferromagnetic metals .
When a freely suspended magnet comes to rest,
it always rests in the north-south direction.
So the direction points to the geographic North of the earth,
is called the magnet's North Pole.
The direction pointing to the geographic South
is called the magnet South Pole.
This property is especially useful in navigation.
The magnetic compass is built based on this.
Another interesting property of a magnet is,
Opposites attract and Likes repel.
If you have two bar magnets with their ends marked north and south,
then one magnet's North End will attract the other South End.
On the other hand the south end of one magnet,
will repel the south end of the other.
And also North will repel the North.
Magnetic field is the region around a magnetic material,
within which the force of magnetism acts.
A magnetic field is represented by a series of lines around a magnet.
To have a better view, let's erase some.
The direction of magnetic field lines is always
from the North Pole to the South Pole.
We show it using arrowheads.
The total magnetic field which passes through a given area
is called the magnetic flux density.
It is denoted using the letter B.
If we consider a bar magnet, the magnetic field lines around it can be shown like this.
Let's consider area, A.
If we place it near the pull end of the magnet,
the number of magnetic fields passing through the area is high.
If we set it somewhere else like this,
the number of magnetic field lines passing through the area is low.
The SI unit of the magnetic flux density is Tesla.
It is a measurement of the magnetic field strength.
In 1820,
Danish natural philosopher Hans Christian Oersted noticed that,
when he turned on an electric current by connecting the wire to both ends of the battery,
a nearby compass needle deflected away from magnetic north,
where it normally pointed.
He discovered that,
a current carrying conductor produces a magnetic field around it.
This suggested a relationship,
between the moving electric charge and the magnetic field.
This phenomenon is called Electromagnetism.
This is a current carrying conductor wire.
The magnetic field lines around the long wire,
which carries an electric current form concentric circles around the wire.
The direction of the magnetic field is perpendicular to the wire.
There is a simple method to find out the direction of the magnetic field
around a wire.
Take your right hand and point the thumb in the direction of the current.
The direction of your other fingers curling is the direction of the magnetic field.
This is a current carrying wire.
We can find the magnetic field using the mentioned right hand screw rule.
A single straight conductor wire only creates a very weak magnetic field.
If we wound this conductor into a coil,
we can get more concentrated magnetic field lines.
So in most cases,
electromagnets are created in this way to strengthen the magnetic field
We can draw the magnetic field around a coil like this.
It's almost like the magnetic field of a bar magnet.
Electromagnets also have magnetic poles.
The direction of the magnetic field in an electromagnet
depends on the direction of the current.
There is a simple method to identify the two poles of an electromagnet.
If the electricity travels clockwise around the electromagnet,
then the end you're looking at is the South Pole.
If the electricity travels counter-clockwise around the electromagnet,
then the end you're looking at is a North Pole.
In electromagnetism,
the magnetic field is caused due to the motion of charges.
Not by the conductor itself.
If somehow we move the same number of charged particles without a conductor,
we can still observe the magnetic field.
Actually the moving charges create a magnetic field.
Now we have the basic idea of magnetism and electromagnetism.
We know the magnets attract the opposite poles and repel the likes.
This attraction or repulsion between the magnetic poles
is called magnetic force.
Here we have a uniform magnetic field,
and we are trying to move the positive charge perpendicular to this magnetic field.
Now we know the moving charge particle also creates its own magnetic field.
And these two magnetic fields interact.
As a result the charged particle experiences a magnetic force,
perpendicular to both magnetic fields.
We can observe the particle move upward.
This force can be calculated using the Lorentz force law.
Here all vectors must be perpendicular to each other.
We use the left hand rule,
to determine the direction of the magnetic force on a positive moving charge.
First,
make an L shape with your left hand's thumb and the first two fingers.
Then,
point your middle finger perpendicular to your thumb and index finger like this.
If you point your pointer finger in the direction of the positive charge is moving,
then your middle finger in the direction of the magnetic field,
and your thumb points in the direction of the magnetic force
pushing on the moving charge.
This is called the Lorenz left-hand rule.
We can use the mnemonic FBI to remember this rule.
F for the force, B for the magnetic field and I for the current,
which is in the same direction as positive charges velocity
Think we have a large magnetic field.
Let's only consider the uniform magnetic field lines.
Here we have a flat plane.
Let's place the plane perpendicular to all these field lines.
If we look at it on side A,
We will see that the field lines are coming from the plane.
We use the dots
to represent that the magnetic field lines are coming from the plane
If we look from side B,
we will see that the field lines are going into the plane.
We use the sine X to represent the magnetic field lines going into the plane.
Here we have a magnetic field.
It comes out from the shown plane like this.
Suppose a positive charge moves in this field with velocity V.
As we explained, we can find the direction using the left-hand rule like this.
We can see the particle faces of force in the right direction.
Since the force is always perpendicular to the velocity,
it does not work on the charged particle.
The particle's kinetic energy and speed thus remain constant.
Due to this force, the charge changes its direction,
but the force is always perpendicular.
As you see, the charge starts to move in a circular motion.
The magnetic force supplies the centripetal force.
If we know the charge, velocity and mass of the charged particle and the magnetic flux density,
we can use the laws of circular motion to find the radius of this circle.
Centripetal force is equal to the magnetic force.
So the radius will be M X V divided by Q X B.
A moving electric charge whose velocity is perpendicular to the magnetic field,
will feel a magnetic force that will cause it to experience centripetal acceleration.
This force will act to change the direction of the Velocity,
while keeping the magnitude of the Velocity constant.
The electric charge will therefore follow a circular pathway around some fixed point.
Now imagine instead of the perpendicular velocity,
a particle enters at Theta angle with the magnetic field.
To calculate the force and radius here,
we have to find the velocity component perpendicular to the field,
which is V Sine Theta.
Since the V Coz Theta component is parallel to the field,
it is not affected by the magnetic field.
The particle moves on a helical path.
Radius of the path is equal to the m X V Sine Theta divided by Q X B.
Now let's talk about a little experiment we can do in our school.
We need a nearly uniform magnetic field.
Then a simple setup with a movable conductor like this
when we place the conductor on the two paths,
the circuit is complete and the current starts to flow.
We discussed earlier
that a current carrying conductor has its own magnetic field around it.
Like this.
What happens when we bring our uniform magnetic field closer to the conductor?
As we discussed earlier the conductor faces a magnetic force.
To find the direction, we can use the left hand rule.
This conductor will move on to the right side.
The magnetic force of a current carrying conductor in a magnetic field
can be calculated using the current, length of the conductor and the magnetic flux density.
In this video we talked about all the basic concepts of magnetism.
Watch our next video to learn how a motor works using electromagnetic forces