Section F: Magnetism and Electromagnetism

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Magnetism and Electromagetism

MAGNETICALLY HARD MATERIALS (STEEL) RETAIN MAGNETISM ONCE MAGNETISED

MAGNETICALLY SOFT MATERIALS (IRON) LOSE MAGNETISM EASILY AND SO ARE USEFUL FOR TEMPORARY MAGNETS

USE IRON FILINGS TO FIND SHAPE, USE PLOTTING COMPASS TO FIND DIRECTION

WHEN DRAWING THEM MAKE SURE NORTH FLUX LINES GO TO CLOSEST SOUTH POLE

!WHEN A CURRENT FLOWS THROUGH A WIRE, A MAGNETIC FIELD IS CREATED!

USE RIGHT HAND GRIP RULE TO FIND DIRECTION OF MAGNETIC FIELD LINES

STRENGTH OF MAGNETIC FIELD AROUND A CURRENT-CARRYING WIRE CAN BE INCREASED BY - INCREASING CURRENT IN WIRE, WRAPPING WIRE INTO A SOLENOID

STRENGTH OF MAGNETIC FIELD AROUND A SOLENOID CAN BE INCREASED BY - INCREASING CURRENT, INCREASING NUMBER OF TURNS, WRAPPING SOLENOID AROUND A MAGNETICALLY SOFT MATERIAL (IRON)

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Electric Motors & Motor Effect

USE FLEMING'S LEFT HAND RULE TO FIND DIRECTION OF FORCE

Motor Effect explanation:

When a current flows through a wire, a magnetic field is created. If we place this wire inside the magnetic field of a permanent magnet these two magnetic fields interfere with each other and the wire feels a force which results in the movement of the wire. This is called the motor effect.

Electric Motor explanation:

The current flows through the circuit connected to the carbon brushes. When the carbon brushes contact the split-ring commutator, the current flows through the coiled wire and a magnetic field is created. This magnetic field interfers with the two permanent magnets on either side of the coil and the so the wire feels a force. The pivot allows the wire to rotate and the split-ring commutator allows the current to swap direction every half turn in order to keep the coil rotating.

To increase the rate at which motor turns we can - increase number of loops of wire, increase strength of permanent magnets (increase strength of magnetic field), increase current flowing

Practical motors - perm. magnets replaced with curved electromagnets(stronger), single loop replaced with several loops (smoother), coils wrapped around soft iron core (efficent, powerful) 

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Electromagnetic Induction

!WHEN A WIRE MOVES THROUGH A MAGNETIC FIELD, A VOLTAGE IS INDUCED!

Size of induced voltage (and current) can be increased by - moving wire quicker, using stronger magnet, wrapping wire into a coil so more pieces of wire move through the magnetic field

!WHEN A MAGNET IS PUSHED INTO A COIL OF WIRE, A VOLTAGE IS INDUCED!

Size of induced voltage (and current) can be increased by - moving magnet quicker, using a stronger magnet, increasing number of turns, using a coil with a larger cross-sectional area

Faraday's Law of Electromagnetic Induction - "The size of induced voltage across the ends of a wire is proportional to the rate at which the flux lines are being cut."

 

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Generators

Bicycle Dynamos 

The wheel is connected to a rotating magnet above a coil so when the bicycle wheel turns, the magnetic field lines of the magnet cut those of the coil which induces a current to flow through the circuit connected to the bicycle lights. This causes the light to shine.

Alternators

Alternators generate alternating current. When the coil is rotated between the two permanent magnets, the magnetic field lines of the coil cut those of the magnetic field lines which induces a voltage / current. Alternating current is produced because the slip rings allow the coil to flow in one direction and then the other.

Transformers

A transformer has two coils which are linked by a soft iron core. This soft iron core concentrates the two magnetic fields. When alternating current is passed through the coil the magnetic field around it constantly changes. As they two magnetic fields cut, a voltage is induced. We can use transformers to step-down a voltage(more coils on primary) or step-up a voltage(less coils on primary). Transformers are used in the UK National Grid. 

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