Chap 21 Electromagnetism 12 S

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Electromagnetism

 Magnetic effect of a current

a. draw the pattern of the magnetic field due to currents in straight wires and in solenoids and state the effect on the magnetic field of changing the magnitude and/or direction of the current

 Applications of the magnetic effect of a current

b. describe the application of the magnetic effect of a current in a circuit breaker

 Force on a current-carrying conductor

c. describe experiments to show the force on a current-carrying conductor, and on a beam of charged particles, in a magnetic field, including the effect of reversing

i. the current

ii. the direction of the field

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Electromagnetism

 The d.c. motor

e. describe the field patterns between currents in parallel conductors and relate these to the forces which exist between the conductors (excluding the Earth’s field) f. explain how a current-carrying coil in a magnetic field

experiences a turning effect and that the effect is increased by increasing

i. the number of turns on the coil ii. the current

g. discuss how this turning effect is used in the action of an electric motor

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Magnetic effect of current



Magnetic Field pattern

• _{Wire vertical and card}

horizontal

• _{Field lines in concentric circles}

round the wire

• _{Field lines closer to wire is}

closer to each other, ie. Magnetic field is stronger nearer to wire.

• _{When direction of current flow}

is reverse, the direction of magnetic field will also be reversed.

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Magnetic effect of current

Right-hand grip rule

• _{Grip the wire with the right}

hand such that the thumb points in the direction of conventional current.

• _{Fingers will point in the}

direction of the field lines.

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Magnetic effect of current



Flat coil

• _{Field is stronger in the}

region inside the coil



Solenoid

• _{Field is stronger in the}

region inside the solenoid

• _{Field lines inside is}

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Electromagnet



Electromagnet

• _{A solenoid is made by winding}

insulated copper wire around a soft iron core.

• _{When current flows through}

solenoid, core becomes a magnet.

• _{When no current flows, core losses}

its magnetism.



Strength of electromagnet:

• _{Size of current through solenoid} • _{Number of turns of solenoid}

• _{Inserting a core of soft magnetic}

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Application of magnetic effect of a current



Uses of temporary magnets

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Application of magnetic effect of a current



Uses of temporary magnets

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Application of magnetic effect of a current



Uses

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 Pg. 422

Interaction between current and magnetic field

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Fleming’s Left Hand Rule (Motor Rule)

 Use to predict the movement of a wire in a magnetic

field

 F - M – C

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Force on a Current Carrying Conductor

 Fleming’s left hand rule (motor rule)

• _{The are two magnetic fields}

– _{field due to the current in the}

wire

– _{field due to the magnet}

• _{When both fields acts in the}

same direction, they will combine to give a stronger field.

• _{When the fields acts in}

opposing directions, the

combined field will be weaker.

• _{The unbalanced fields results}

(13)

Force on a beam of charged particles

 Use Fleming’s

left hand rule

 Note that

electrons are negatively charged and

move in opposite direction to

conventional current

 Try Quickcheck

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Turning effect on coil with current

 A rectangular coil is suspended between two opposite poles of a magnet.

 Currents on AB and CD are in opposite directions.

 Due to the two fields, there is a resultant pair of equal and opposite forces.

 This results in the turning effect.

 The turning effect can be increased by:

•

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Principle of D.C. Motor

 Used in electrical appliance such as electric fans.

 Converts electrical energy to mechanical energy.

 Works only with direct current

 Components of the D.C motor

• _{Single coil of wire that spins between opposite poles of a magnet} • _{Ends of a coil connected to a split ring (commutator).}

• _{Split ring turns with the coil and rubs against two carbon brushes.}

(16)

Principle of D.C. Motor (P)

 Current flows from power supply to P, X, through the coil, Y and Q.

•_{Using left hand rule, left side of}

coil experiences downward force

while right side experience upward force.

•_{Coil moves until it reaches vertical position.}

•_{At this point there is no current flow (why?),}

However the momentum will move the coil off the vertical position.

•_{Y will then be in contact with P while X is}

with Q.

•_{Analyze the forces acting on the coil…..} •_{What do you notice about the direction of}

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D.C. motor (P)



Split ring commutator

• _{Purpose is to reverse the direction of the current in the}

coil whenever the current changes its contact from one carbon brush to another.

• _{This ensure that the coil will rotate in a fixed direction.}

• _{Try pg. 431, Qn 5}



Turning effect of the coil can be increased by:

(18)

Moving coil instrument

(19)

Reed Switch (Enrichment)

 When a current flows through the solenoid

• _{The two metal strips inside the glass tube will be magnetised}

(20)

Summary

Turning effect on a current-carrying

coil Turning effect is

increased by increasing

(a) number of turns (b) current

Force on a beam of charged particles in a magnetic field Force on a

current-carrying conductor in a magnetic field Fleming’s Left hand rule Electromagnetism Electric motor Electric motor

is shown by

results in helps to

determine the direction of