Magnetic Fields and Motors!

Magnetic Fields and Motors!

What is a magnetic field?

You can't see a magnetic field, you can only prove its existence by its affect on other objects. It only affects objects that have the ability to be magnetised (magnetic material). Magnetic fields tend to pull or push objects so a force is exerted. The definition is:

 

A magnetic field is a region where a force is exerted on magnetic material.

 

Magnetic fields can be represented by field lines. Field lines go from north to south. The strength of the magnetic field (B, measured in Tesla) is represented by how tightly packed the lines are- the closer the field lines, the stronger the field- just like contour lines on a map!

 

• Magnetic field lines = flux measures in Tesla (B)
• Magnetic field strength =flux density measured in Webers (Wb)
   

Field Lines

 

So this picture shows different field strengths of attractive, repulsive and neutral forces.

(a) Shows a typical magnet and the direction of force runs from north to south! The spacing between the lines is the magnetic field strength (B, Tesla). Closer=stronger.

 

(b) Shows an attractive force as unlike poles attract. Again the force goes from north to south. Notice that in the centre where the magnetic field is at its strongest, the lines of force (flux) are equally spaced. This indicates a uniform field. The field strength remains constant for any part of north to south in the middle. The field strength gets weaker with distance away from the attraction!

 

(c) Here is where two magnets are repelling each other. See there is no flux (field lines) in the centre? This is because the flux has cancelled out. You can imagine that if there were lines in the middle, the flux would overlap- when they do they cancel. So, in theory they never overlap. But the field strength is zero as there is no flux so there is no field strength.

 

There is a Magnetic Field around a wire carrying Electric Current

So for anything carrying a current, there is a magnetic field around it. No current and the magnetic field disappears. It looks like this:

 

 

Left image:

The electric current if flowing towards you (the picture has a tiny cross in the middle to tell you it is flowing towards you) then the magnetic field goes counter clockwise.

 

Right image:

This should have a tiny dot in the middle to tell you it is flowing away from you, the magnetic field circulates clockwise.

 

This method helps to remember it (the right hand rule):

 

A Wire carrying a current in a magnetic field will experience a force

If you put a current carrying wire into an external magnetic field (maybe between two magnets) the field around the wire and the field from the magnets interact. The field lines from the magnet contract to form a 'stretched catapult' effect where the flux lines are closer together.

 

It looks like this:

When you put it in the magnetic field, there is a force on the wire. As you can see, there is a dot in the middle so the current is flowing away into the page. There is no force when the current is parallel to the field lines because the fields act in two opposite directions cancelling out.

 

The direction of the force is always perpendicular to both the magnetic field and the current.

 

The resultant force is upwards. If you ever want to know which direction the force is in, use the left hand rule (fleming's left hand rule):

Thumb= direction of force.

First finger (pointy finger)= direction of magnetic field

Second finger= direction of current.

 

Th (fa) = Force

First=field

SeCond=Current.

 

So in the picture above the current is flowing clockwise, to the right. Point your second finger to the right and your thumb points up for the force. The force would be downwards if your second finger was pointing to the left.

 

The size of the force is given by F=BIl

The size of the force, F, on a current carrying wire at right angles to a magnetic field is proportional to the current, I, the length of the wire in a field, l, and the strength of the magnetic field, B.

 

So that makes :

 

F=BIl (effbill)

 

It's defined by:

 

The force on one metre of wire carrying a current of one amp at right angles to the magnetic field.

 

Magnetic field strength is also called flux density and it's measured in teslas, T. Magnetic field strength is a vector quantity since it has direction and magnitude.

 

1 Tesla= Wb/m^2

 

The forces on a loop can be used to make a motor!

Imagine a wire carrying a current is made into a loop. The loop in then placed between two magnets with an attractive force (n/s). The force from the magnets will make the loop rotate.

Use the left hand rule, so for the magnet on the left, your thumb should be pointing up so the force is upwards but as the current goes past the magnet on the right, your hand should be upside down and the force is downwards because the direction of the current changes as it is in a loop. It doesn't mean the current starts flowing around the circuit the other way, it means the direction has changed from your viewpoint. Because the force is opposite for each end of the loop it rotates.

If the loop were to rotate for a half turn then the current would be in the opposite direction to what is was originally and the force would be countered stopping the rotation.

By using a split-ring commutator, the current in the loop can be reversed each time the loop becomes vertical. This makes the loop rotate steadily and it is otherwise known as a motor!