Physics Electromagnsadsadetisc Topic Notes

7/23/2019 Physics Electromagnsadsadetisc Topic Notes

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6.1 use the following units: ampere (A), volt (V), watt (W)

Ampere (A): a measure of current

Volt (V): a measure of potential difference / voltage

Watt (W): a measure of power

6. understand that magnets repel and attract othermagnets and attract magnetic su!stances

"ost magnets have two poles: the north and the #outh $ole

#imilar poles repel % north repels north

&pposite poles attract % north attracts south

"agnets can attract magnetic o!'ects % o!'ects made from iron, steel, nicel and co!alt

6. descri!e the properties of magneticall* hard and softmaterials

"agneticall* hard materials can !e permanentl* magneti+ed % it creates its own permanent

magnetic field % these are materials that have a larger portion or iron / steel / co!alt / nicel

"agneticall* soft materials (such as iron) are not suita!le for permanent magnetism

% the* can easil* lose their magnetism

% the* have a weaer magnetic field

Allo*s containing iron are called ferrous, and allo*s not containing iron are called non%

ferrous. $ure iron is magneticall* soft, !ut some allo*s containing iron (ferrites) are

magneticall* hard
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6. understand the term -magnetic field line

"agnetic field lines visuali+e the main features of a magnetic field

he*:

% show the shape of the magnetic field

% show the direction of the magnetic field (field lines travel from north to south)

% show the strength of the magnetic field % where the field lines are closest is where the

magnetic field is strongest

6.0 understand that magnetism is induced in some

materials when the* are placed in a magnetic field

"agnetism can !e induced in some non%magnetic materials when the* are placed in a

magnetic field (materials containing iron, nicel or co!alt)

his is !ecause the magnetic field encourages the electron line up and form poles inside

the material
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n magneticall* soft materials, the magnetism will disappear after the magnet is moved

awa* (iron is a magneticall* soft material)

6.6 descri!e e2periments to investigate the magneticfield pattern for a permanent !ar magnet and that!etween two !ar magnets

"agnetic field lines can !e seen !* sprinling iron filings a!ove and surrounding the

magnet

his can !e repeated with two !ar magnets, with lie / opposite poles near to each other

6.3 descri!e how to use two permanent magnets toproduce a uniform magnetic field pattern
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When magnetic field lines are the same distance awa* from each other, it forms a uniform

magnetic field pattern

When two opposite poles are put ne2t to each other, the magnetic field lines will all !e going

from the north to #outh $ole, perpendicular to the magnets

6.4 understand that an electric current in a conductorproduces a magnetic field round it

When a current flows through a wire, a magnetic field is created around it

% the field is 5uite wea

% the field is circular in shape

his is called electromagnetism

6. descri!e the construction of electromagnets
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Wrapping the wire around an iron core also maes the whole contraption an electromagnet

Ways to strengthen the magnetic field:

- adding more turns to the coil strengthens the field

% increasing the current flowing through the

he magnetic field disappears when the circuit is switched off

6.17 setch and recogni+e magnetic field patterns for astraight wire, a flat circular coil and a solenoid when eachis carr*ing a current
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he direction of the magnetic field / current can !e determined !* the right hand grip rule

he magnetic field of a solenoid loos a !it lie a magnetic field (red arrows show direction

of current and !lue show magnetic field lines)


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"agnetic field of a flat coil

6.1 understand that a force is e2erted on a current%carr*ing wire in a magnetic field, and how this effect isapplied in simple 8.#c. electric motors and loudspeaers

f a current through a piece of wire is held at right angles to a magnetic field, the wire will

move (force is created)

When a current flowing through a wire is put through a magnet, the magnetic field lines

overlap.
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n some places, the direction of the fields are the same, and the magnetic field is stronger.

n other places, the direction of the fields are in opposite directions, so the magnetic field is

weaer.

he wire is pushed from the strong part of the field to the wea part (the magnetic field lines

dont lie !eing s5uished together)

Electric motors

he magnetic on the two sides of the coil are at different directions. &ne side feels the force

pushing it downwards, whilst the other is !eing pushed upwards, so the coil rotates.

he direction current on each side of the coil is switched when the coil is vertical !* the

commutator, so the forces acting on each side is switched and the rotation direction is

maintained

o increase rate of motor turns:

% increase the num!er of turns or loops of wire (to mae a coil)

% increase the strength of the magnetic field

% increase current flowing through the loop of wire


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6.11 understand that there is a force on a chargedparticle when it moves in a magnetic field as long as itsmotion is not parallel to the field

f an* charged particle (egg. a current running through a wire), it will e2perience a force from

the magnetic field.

he introduction of a new magnetic field causes the magnetic field lines to overlap, and the

stronger field lines push the particle to an area of weaer field lines (see more

information here)

f the wire is running parallel to the field, the force will !e felt along the wire, not

perpendicular to it, so it does not move

6.1 use the left hand rule to predict the direction of theresulting force when a wire carries a currentperpendicular to a magnetic field

9se the leming left hand rule to predict motion, current or field direction when the other

two are given
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6.1 descri!e how the force on a current%carr*ingconductor in a magnetic field increases with the strengthof the field and with the current

orce on a current%carr*ing conductor increases if:

% "agnetic field increase

% current increases

6.10 understand that voltage is induced in a conductor ora coil when it moves through a magnetic field or when amagnetic field changes through and descri!e the factorswhich affect the si+e of the induced voltage

f a wire is moved across a magnetic field at right angles, a voltage is induced in the wire. f

the wire is part of a complete circuit, a current flows. his can also occur if a magnet is

moved through a coil of wire

When the magnet goes into the coil, the current is positive, and when it moves out again,

the current is negative. f the polarit* of the magnets is reversed, direction of current isreversed as well

f the magnet is stationar*, (whether in or out of the coil, no current is produced)

his is called electromagnetic induction
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actors that affect amount of induced current:

% speed that the magnet / wire is moved

% strength of the magnet

% the num!er of turns in the coil (surface area that moves through the magnetic field)


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6.16 descri!e the generation of electricit* !* the rotationof a magnet within a coil of wire and of a coil of wirewithin a magnetic field and descri!e the factors which

affect the si+e of the induced voltage

"otion causes the coil inside the magnetic field to turn

As it rotates, the wires cut through the magnetic field lines, and a current is induced in the

wires

As a result, the current flows to the slip ring, (one side of the wire is connected to one slip

ring)

When the force acting on each side of the wire is changed (ever* half rotation), the direction

of the current changes

his maes an arc. current, and this generator is called an alternator.

Factors that affect how much voltage is induced

% speed that the magnet / wire is moved

% strength of the magnet

% the num!er of turns in the coil (surface area that moves through the magnetic field)

6.13 descri!e the structure of a transformer, andunderstand that a transformer changes the si+e of an
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alternating voltage !* having different num!ers of turnson the input and output sides

When there is an alternating current, the shape of the magnetic field around it increases

and decreases with current.

When an ac current%carr*ing coil is placed near another wire coil, the changing magnetic

field from the first coil is also applied to the second coil

As the magnetic field is constantl* going in and out of the coil, the direction of current is

constantl* changing (alternating output)

he si+e of the induced voltage depends on the num!er of coils. f there are more coils,

more voltage is induced and vice versa

6.14 e2plain the use of step%up and step%downtransformers in the large scale generation andtransmission of electrical energ*

ransformers that increase voltage are called step%up transformers. ransformers that

decrease voltage are called step%down transformers

ransformers are reall* close to !eing 177; energ* efficient
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n step%up transformers, as voltage increases !ut power sta*s constant, the current

decreases. Vice%versa for step%down transformers.

As current passes through a wire, energ* is lost as heat. his is wh* after generation,

currents are passed through step%up transformers. 8uring transmission of energ* across

long distances, a minimum amount of energ* is lost.

When it enters towns and cities, the suppl* is passed through a step%down transformer so it

can !e used in homes.

6.1 now and use the relationship !etween input(primar*) and output (secondar*) voltages and the turnsratio for a transformer

nput (primar*) voltage / output (secondar*) voltage < primar* turns / secondar* turns

9p / Vs < =p / =s

or e2ample

A transformer has 177 turns on its primar* coil and 077 turns on its secondar* coil. f an

alternating voltage of V is supplied across the primar*, what is the voltage across the

secondar* coil>

177 / 077 < V / #

(077 2 ) ? 177 < #

# < 17V

6.7 now and use the relationship: input power < output

power

nput power < output power

#ince power < current 2 voltage
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9p 2 < Vs 2 s

p < primar*, s < secondar*

When a voltage of 1V is applied across the primar* coil of a step%down transformer, a

current of 7.A flows through the primar* coil. @alculate the current flowing through thesecondar* coil if the voltage induced across it is V. Assume that the transformer is 177;

efficient

1V 2 7. < 2 s

< (1 2 7.) /

< .A

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