magnetism

what is magnetic induction

the process whereby an object made of a magnetic material becomes

a magnet when it is near or in contact with a magnet.

what is a magnetic field

The space surrounding a magnet where it produces a magnetic force

how are ferromagnetic materials attracted to a magnet

Magnetic domains can grow or shrink in response to magnetic fields. Domains in a ferromagnetic material can easily realign themselves to be attracted to an external magnetic pole.

w hat is a magnetic domain

atoms with similar magnetic orientations line up with neighbouring atoms in groups

how to electrically magnetise?

an object made of a ferromagnetic material can be placed in a very strong magnetic field (such as a d.c. carrying solenoid)

the magnetic domains will become very aligned, even after the removal of an external magnetic field

how to electrically demagnetise?

an object made of a ferromagnetic material can be placed in a very strong changing magnetic field (such as a d.c. carrying solenoid)

domains become unaligned and magnetism will weaken

differences between temporary magnets and permanent magnet

magnetism: A magnetic material that loses their magnetism easily (after the removal of an external magnetic field) VS The domains of the ferromagnetic material remain aligned after the removal of the external magnetic field

uses: used for electromagnetic devices VS hard magnets

how can a magnetic field be produced

moving electrical charges

how are strength of magnetic field indicated with field lines

the denser the field lines, the stronger the magnetic field

Right hand grip rule

point thumb of right hand in the same direction as the conventional current through the wire, when the right hand is gripping the wire. the fingers curl in the same way as the field

why is soft iron core used for making electromagnets

it is magnetised and demagnetised easily, making magnetism easily controlled

what is a solenoid and what does it produce

a coil. made by wrapping turns of insulated wire closely together but without overlapping

produces strong uniform magnetic fields inside them

how to determine force when current + magnetic field

Fleming’s left hand rule

use FLHR to explain F / B / I

using Fleming’s Left Hand rule, when the thumb is pointed ___, and is aligned with the magnetic force, and the first finger is pointe ___ aligning with the magnet’'s magnetic field, the 2nd finger is pointed ____

so, the ___ is pointing ___

or explain it any other way

how is a magnetic force produced with a magnet and a conductor

current runs trough the conductor, with the moving charges through __ creating a magnetic field around the conductor. the magnetic field around the wire and the magnetic field between the poles of the magnet interact with each other, creating a magnetic force that acts on the conductor

how does a split ring commutator enable continuous rotation in one direction (suppose its turning in a clockwise direction)

1. if the left hand rule is applied to the coil, side AB experiences an upwards force an side cd a downward force

2. these two forces create a net turning effect about the axis of rotation which rotates the coil in a clockwise direction until it is vertical

3. The brushes are then in line with the gaps in the commutator and the current stops.

4. However, because of its inertia, the coil overshoots the vertical and the commutator halves change contact from one brush to the other.

5. This reverse the current through the coil and also the directions of the force on its sides. Side ab is on the right now, acted on by a downward force, while cd is on the left with an upward force. The coil thus carries on rotating clockwise.

how to increase rate of turning of motors

• The more turns there are on the coil

• the larger the current through it

  • the greater is the turning effect on the coil and the faster it turns.

• the strength of the magnetic field is increased by using a stronger magnet

what is electromagnetic induction

the process by which an e.m.f. is induced when a conductor experiences a changing magnetic field.

The magnitude of the induced e.m.f. increases when

1. greater speed of relative motion between conductor and magnetic field

2. strength of the magnetic field

3. number of turns in the circuit

4. soft iron core

5. increasing SA of plane of coil

Faraday’s law of electromagnetic induction states that…

Induced electromotive force across a conductor due to a changing magnetic field is directly proportional to the rate of change of magnetic field through the circuit / coil

Lenz’s law says that….

When a changing magnetic field induces a current in a conductor, the magnetic field from that current opposes the change that produces it.

direction of induced current is such that its magnetic effect always opposes the change producing it

when is greatest EMF generated in an a.c. circuit

1. when movement of conductor is perpendicular to magnetic field lines in magnetic t

2. OR the sides of the conductor are moving through the horizontal field at the quickest rate

briefly, how does the transformer work

An alternating voltage applied to the primary coil creates an alternating magnetic field which passes through an iron core to the secondary coil, where it induces an e.m.f..

properties of a transmission line

1. its long

2. thick

3. made out of metals of relatively low resistivity

pl/A

why transmission lines need to have high voltage and low current?

because of P = I2R, if I is too high, power loss is going to be high

transformers reduce current when voltage is stepped up

reasons why real transformers will not be 100% efficient

• some energy is transferred to internal energy as there are currents through resistance of the coils (heating effect of currents through wires)

• chanigng B field induces EMF across soft iron core, inducing current through core, which leads to energy loss in terms of heat and internal energy as eddy currents through the core. To limit this effect the core is usually laminated (made from many insulated layers)

• induced EMF across secondary coil also creates a changing magnetic field, inducing current in primary coil which opposes the existing current in primary coil (energy lost)

• some energy is involved when the core is repeatedly magnetised and demagnetised.

how are eddy current s formed in a conductor experiencing a magnetic force due to magnetic field

When any piece of metal experiences a changing magnetic flux, eddy currents will be induced and circulate within the solid metal (transferring energy to internal energy). The eddy currents will produce their own magnetic field which will oppose the change that produced them ( Lenz’s law).

how are ac current generated (briefly)

by rotating a coil in a magnetic field and using slip rings to deliver induced current in coil to an external circuit

explaining induced EMF questions

1. there is relative motion between ___

2. EMF induced across conductor (regardless of if its a closed circuit)

3. if its a closed circuit, current will be induced

why soft iron core should pass through both coils in a transformer

• common soft corn core carries and concentrates the changing magnetic field produced by the primary coil so that this changing field also passes through the secondary coil, inducing an EMF in it, improving the efficiency in inducing EMF / current in secondary coil

fundamental principle of electric generator by magnetic induction

• in a closed circuit, moving magnetic field causes magnetic field to change with time due to relative emotion between magnet and conductor

• EMF is inched inn conductor

• current is induced in conductor, causing electricity to be generated