6.3 – Magnetic Force and Field

Magnetic Poles

Every magnet has two poles (North & South) and is therefore called a Dipole Unlike Electric Fields it is impossible to have a Monopole.

If you cut a magnet in half you end up with another dipole.

Unlike poles attract, Like poles repel.

These magnets will turn so that UNLIKE poles come together.

           Earth’s Magnetic Field    

Because magnets will turn so that UNLIKE poles come together, the poles are really called ‘North seeking poles’ or ‘South seeking poles’

Compasses contain small magnets which turn towards the Earth’s poles.

http://phet.colorado.edu/en/simulation/magn ets-and-electromagnets

Magnetic Field Lines

Magnetic Field

This is a region of space where a test magnet experiences a turning force

http://www.walter-fendt.de/ph14e/mfbar.htm

Field Lines

They point from the North Pole to the South Pole.

Magnetic Flux Density (B)

The strength of the magnetic field seems linked to the density of the magnetic field lines.

There is a stronger field at the poles where there are more field lines.

Magnetic Flux Density (B) This is the equivalent of:

g for Gravitational Fields (Nkg^-1) E for Electric Fields (NC^-1)

Fields Caused by Currents

It turns out that if a small compass is placed near a wire carrying a current it experiences a weak turning force.

This led scientists to realise that Magnetism is actually caused by moving charges.

The field is strongest closest to the wire.

The direction of the field can be found using the Right Hand Corkscrew Rule.

http://www.walterfendt.de/ph14e/mfwire.htm

Field inside a coil

When a current flows      All these circles add        This makes a really around a circular loop       together. strong field in the

the magnetic field    centre of the circular forms circles.

Field inside a solenoid

A solenoid is a coil of wire, carrying a current.

The field that is created by a solenoid is just like that of a bar magnet but the field lines go through the centre.

Force on a current carrying conductor review

Changing the direction of the force review

The direction of the force acting on a wire in an electromagnetic field can be reversed by:

                 Reversing the Current                  Reversing the Magnetic Field

The direction of the force is therefore relative to both the direction of the magnetic field and the current.

Fleming’s left-hand rule review

It is possible to predict the direction of the force acting on a wire – its motion – if the direction of the current or the magnetic field are known. Fleming’s left-hand rule is used to do this.

First finger  =  magnetic Field

seCond finger  = Current

Increasing the size of the force review

Force in a Magnetic Field equation

As you have just seen the size of the force depends on:

B – Magnetic Flux Density I – current in the wire l – length of wire

If the field is not at Right Angles to the wire then the perpendicular component of the field is used and the equation is:    

Force in a Magnetic Field alternative equation

B – Magnetic Flux Density I – current in the wire l – length of wire

Charges in Magnetic Fields

1. Electrons moving in a wire

In the picture above, the electron is moving to the right, so Conventional Current (I) is moving to the left.

From Fleming’s Left Hand Rule the electron experiences a force downwards at right angles to it’s motion. It’s the sum of all the forces on all the electrons that gives the

What forces are there between two current carrying wires?

Step 1 – What does I₂ do to I₁ ?

Use the Right Hand Corkscrew rule to see what the field lines do.

Step 2 – Which way does I₁ move?

Use Fleming’s Left Hand Rule to see what the force is on I₁

Now repeat for the other wire:

If the currents are flowing in opposite directions:

If the currents are flowing in opposite directions:

What would happen to a coil?

What happens to the shape of the coils?

What would happen to a coil?

http://ocw.mit.edu/ans7870/8/8.02T/f04/visualizations/magnetostatics/15 -MagneticForceAttract/MagForceAtt_640.mpg

Aurora Borealis

Knut Birkeland (1867–1917) is on the 200 Norwegian kroner note.

He was a Physicist best known for his studies on the aurora borealis.

Timelapse of the Aurora

http://vimeo.com/16917950

Aurora Borealis – the Physics

The Earth has a magnetic field caused by currents in its core, which channels charged particles from solar flares and from our upper atmosphere towards the poles

Aurora Borealis – the Physics

Charged particles from space experience a force on them from the earth’s magnetic field which makes them spiral around the magnetic field lines and head towards the poles.

As they meet air molecules they excite the molecules causing them to give out light.

Without the protection of the earth’s magnetic field we would be constantly bombarded with high energy particles.

This is one of the reasons that Space flight is so difficult. Astronauts report white flashes in their vision as Cosmic rays pass straight through their heads.

Without shielding missions to Mars will be impossible.