6.3 – Magnetic
Force and Field
magnet has two poles (North & South) and is therefore called a Dipole Unlike Electric Fields it is impossible to have a Monopole.
you cut a magnet in half you
end up with another dipole.
Unlike poles attract, Like poles repel.
magnets will turn so
that UNLIKE poles come together.
magnets will turn so that UNLIKE poles come together, the poles are really
called ‘North seeking poles’
or ‘South seeking poles’
contain small magnets which turn towards the Earth’s poles.
This is a
region of space where a test magnet experiences a turning force
point from the North Pole to the South Pole.
strength of the magnetic field seems linked to the density of the magnetic
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)
unit of Magnetic Flux Density (B)
is the Tesla (T) and
like the other field strengths it is a Vector.
good way to think about it is that it is just a measure of how many Field
there are in a certain area. A magnetic field is often called a ‘B Field’
Until we know more about Magnetism it isn’t
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)
turns out that if a small compass is placed near a wire carrying a current it
experiences a weak turning force.
led scientists to realise that Magnetism is
actually caused by moving charges.
field is strongest closest to the wire.
The direction of the field can be found using the Right Hand Corkscrew Rule.
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.
solenoid is a coil
of wire, carrying a current.
that is created by a solenoid is just like that of a bar magnet but the field lines go through the centre.
direction of the force acting on a wire in an electromagnetic field can be
The direction of the force is therefore relative to both the
direction of the magnetic field and the current.
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
have just seen the size of the force depends on:
– 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:
1. Electrons moving in a wire
picture above, the electron is moving to the right, so Conventional Current (I)
is moving to the left.
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
What forces are there between
two current carrying wires?
Step 1 –
What does I2 do to I1 ?
the Right Hand Corkscrew rule to see what the field lines do.
Step 2 –
Which way does I1 move?
Fleming’s Left Hand Rule to see what the force is on I1
Now repeat for the other wire:
currents are flowing in opposite directions:
What would happen
to a coil?
to the shape of the coils?
Birkeland (1867–1917) is on the 200 Norwegian kroner note.
He was a
Physicist best known for his studies on the aurora borealis.
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
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
they meet air molecules they excite the molecules causing them to give out
the protection of the earth’s magnetic field we would be constantly bombarded
with high energy particles.
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.
shielding missions to Mars will be impossible.
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