6.1 - Gravitational Force & Fields

6.3 Magnetic Force and Field

Magnetic PolesEvery magnet has two poles (North & South) and is therefore called a DipoleUnlike 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.NSSNSNNSNSNSNSEarths Magnetic FieldBecause magnets will turn so that UNLIKE poles come together, the poles are really called North seeking poles or South seeking polesCompasses contain small magnets which turn towards the Earths poles.NS

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

Magnetic Field Lines

http://www.walter-fendt.de/ph14e/mfbar.htm Magnetic FieldThis is a region of space where a test magnet experiences a turning forceField LinesThey 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)Until we know more about Magnetism it isnt possible to define The Magnetic Field Density (B) in the same way as we do for Gravitational Field Strength (g) and Electric Field Strength (E)The unit of Magnetic Flux Density (B) is the Tesla (T) and like the other field strengths it is a Vector.

A good way to think about it is that it is just a measure of how many Field Lines there are in a certain area. A magnetic field is often called a B Field

Fields Caused by CurrentsIt 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.walter-fendt.de/ph14e/mfwire.htm

The fingers show the direction of the field.

Field inside a coil

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

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.Field inside a solenoidSN

Force on a current carrying conductor review# of 29 Boardworks Ltd 20079Boardworks GCSE Additional Science: Physics Motors and GeneratorsTeacher notesThis virtual experiment can be used as an introduction to the motor effect or as a revision exercise.

The direction of the force acting on a wire in an electromagnetic field can be reversed by:The direction of the force is therefore relative to both the direction of the magnetic field and the current.Changing the direction of the force review

Reversing the CurrentReversing the Magnetic Field# of 29 Boardworks Ltd 200710Boardworks GCSE Additional Science: Physics Motors and Generators

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. Flemings left-hand rule is used to do this.Flemings left-hand rule reviewthuMb = MotionFirst finger = magnetic FieldseCond finger = Current

# of 29 Boardworks Ltd 200711Boardworks GCSE Additional Science: Physics Motors and GeneratorsTeacher notesStudents should be made aware that the direction of a magnetic field is usually taken as being N-S rather than S-N, and the direction of current is the direction of conventional current, i.e. positive to negative.Increasing the size of the force review

# of 29 Boardworks Ltd 200712Boardworks GCSE Additional Science: Physics Motors and GeneratorsForce in a Magnetic Field equation

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

B Magnetic Flux DensityI current in the wirel 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 equationB Magnetic Flux DensityI current in the wirel length of wire

1.2.3.4.Charges in Magnetic Fields

= B Field coming out of page.= B Field going away into pageImagine an arrow coming towards you or going away from you.In the picture above, the electron is moving to the right, so Conventional Current (I) is moving to the left.

From Flemings Left Hand Rule the electron experiences a force downwards at right angles to its motion. Its the sum of all the forces on all the electrons that gives the total force on the wire.

1. Electrons moving in a wire2. Electrons moving freely through a magnetic field

The force is always perpendicular to its motion, so it ends up moving in a circle.

The Magnetic Field provides the Centripetal Force.

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 Flemings Left Hand Rule to see what the force is on I1

FNow repeat for the other wire:

F

FForce caused by I2 on I1Force caused by I1 on I2The two wires move together!

If the currents are flowing in opposite directions:IIIf 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.mpgAurora Borealis

Knut Birkeland (18671917) is on the 200 Norwegian kroner note. He was a Physicist best known for his studies on the aurora borealis.

Timelapse of the Aurorahttp://vimeo.com/16917950 Aurora Borealis the PhysicsThe 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 Physicshigh 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.

Charged particles from space experience a force on them from the earths 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 theearths magnetic field we wouldbe constantly bombarded with