Transcript Magnetic Field

Magnetic Field
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Magnets have both a
north pole and a south
pole.
The magnetic lines of
force flow from pole to
pole as shown in the
sketch.
It is easy to feel the
attraction or repulsion
when one plays with two
magnets.
Electromagnetism
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A magnetic field is produced when an
electric current flows through a coil of
wire. This is the basis of the
electromagnet.
We can make an electromagnet stronger
by doing these things:
wrapping the coil around an iron core
 adding more turns to the coil
 increasing the current flowing through the coil.
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A Simple Electromagnet:
Using electromagnets
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Many objects around you
contain electromagnets.
They are found in electric
motors and loudspeakers.
Very large and powerful
electromagnets are used
as lifting magnets in scrap
yards to pick up, then
drop, old cars and other
scrap iron and steel.
They are better than
magnets because the
magnetism can be turned
off and on.
Electromagnet animation
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http://www.bbc.co.uk/schools/ks3bitesi
ze/science/physics/magnetism_4.shtml
Electric bells
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Electric bells like the ones used in most
schools also contain an electromagnet.
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When the current flows through the circuit, the
electromagnet makes a magnetic field.
The electromagnet attracts the springy metal arm.
The arm hits the gong, which makes a sound and
the circuit is broken.
The electromagnet is turned off and the springy
metal arm moves back.
The circuit is complete again.
Animation of electric bell
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http://www.bbc.co.uk/schools/ks3bitesize/
science/physics/magnetism_5.shtml
Gravitational Fields
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All objects have a force that attracts
them towards each other. This is called
gravity. Even you attract other objects
to you because of gravity. But you have
too little mass for the force to be very
strong. Gravity only becomes noticeable
when there is a really massive object
like a moon, planet or star.
Gravitational Field
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The Earth has more mass than the Moon, so
the gravitational force is greater on the
Earth than it is on the Moon.
The Earth's gravitational force pulls objects
towards the centre of the Earth.
"Down" is towards the
centre of the Earth,
wherever you are on
the planet
Gravitational Field
Earth’s gravitational force
makes the ball move down
towards the centre of the
Earth.
Sun’s gravitational force
makes the Earth move in an
orbit around the Sun.
Electric Fields
 An electric field shows us how electric
charges behave around each other.
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See what elec field looks like:
http://www.colorado.edu/physics/2000/
waves_particles/wavpart3.html
Electric Fields
 Positive and negative
charges attract each
other.
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What do two
positive charges do
to each other?
Electric Fields
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Lightning discharges
an excess of positive
and negative charge
within clouds,
between clouds, or
between clouds and
the ground.
Extension:
Physics textbook:
Read p78-79 – Gravity
Try questions 4 & 5 on p81.
Read p139/140 and try questions.
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