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# Magnetic Fields Review

Review of Magnetic Fields topic for the Edexcel A2 Physics specification.
by

## Matt Evans

on 5 November 2012

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#### Transcript of Magnetic Fields Review

Where can we find magnets? Magnets have poles! Magnetic Fields Review Refrigerators Roller coasters Jewelry Toys Junkyards Can you think of some more examples? -Magnets have a North pole and a South pole How do
magnets
stick? For
example,
how do
magnets
stick to
the fridge? Different Types of Magnets opposite points on the surface of a magnet where magnetic forces are strongest

any influence that causes an object to move

to push away by a physical force

to pull in by a physical force Poles: Repel: Attract: Key Definitions Now I can... Draw a picture of two magnets repelling and two magnets attracting Define the terms attract and repel Magnets attract or repel other magnets Force: Magnetic Flux A measure of the quantity of magnetism

Dependent on the strength of the magnetic field

Scalar quantity A product of the average magnetic field and the perpendicular area that it penetrates

The number of field lines that 'cut through an area'

Measured in webers, Wb Magnetic Flux Density, B The magnetic field is stronger if there are more field lines in a given area

The density of flux is larger

Measured in Tesla, T The three magnetic elements are:
Iron - Fe
Nickel - Ni
Cobalt - Co

Only materials made of these elements will experience a magnetic force Moving Charges Moving charges create a magnetic field

All wires have a magnetic field around them when a current is flowing

The direction of the magnetic field can be found using the right-hand grip rule Current-Carrying Conductors & Fleming's Left-Hand Rule When a wire carries a current it has a magnetic field around it

Placing a wire into a magnetic field results in the two magnetic fields interacting

The wire will either be attracted or repelled from the fixed magnet The force experienced is at a maximum
when the current flowing is perpendicular
to the field lines of the magnet The force experienced is at a minimum
when the current flowing is parallel
to the field lines of the magnet The size of the force on the wire can be increased by: Increasing the strength of the magnetic field it experiences
Increasing the size of the current flowing through it
Increasing the number of turns of wire experiencing the magnetic field Charges & Magnetic Fields All charges produce a magnetic field

If a charged particle is placed in a magnetic field then it will experience a force

Fleming's Left-Hand rule can be used to determine the direction of this force The maximum force is when the particle is traveling perpendicular to the flux The 2nd finger represents the velocity vector of positive charge movement

When there is a negative charge the direction is opposite to the velocity vector of that particle! The minimum force is when the particle is traveling parallel to the flux Now I can... Apply Fleming's Left-Hand rule to charges Define the terms flux and magnetic flux density Apply Fleming's Left-Hand rule to currents Electromagnetic Induction Moving a magnet into a coil induces a voltage across the ends of the wire - a current is induced in the wire Faraday's Law
The size of the induced voltage depends upon the 'rate of cutting of magnetic flux lines' Using a Stronger Magnet The magnetic field lines are closer together

More lines of flux are cut in a given time If the Coil Face has a Bigger Area The total flux intercepted by the coil would be larger Increasing the Number of Turns More turns mean that more lines of flux will be 'cut' Increase the Speed of the Magnet More lines of flux are 'cut' in a shorter space of time Induced emf & Fleming's Right Hand Rule The direction of the induced emf can be found using Fleming's Right-Hand Rule Lenz's Law This is called the Dynamo Effect Caused by movement between a conductor and a magnetic field The induced emf must cause a current to flow in such a direction as to oppose the change in flux linkage that produces it Flux Linkage The number of magnetic flux lines passing through a coil of wire

Dependent on the number of turns, N, on the coil and the quantity of the magnetic field

Measured in weber-turns Now I can... Define the term Flux Linkage

Describe and explain Faraday's Law & Lenz's Law

Explain the factors that affect the emf induced in coil

Apply Fleming's Right-Hand rule to determine the direction of an induced emf
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