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Force on a current-Carrying Conductor in a Magnetic Field

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4.1:  FORCE ON A CURRENT-CARRYING CONDUCTOR  IN A MAGNETIC FIELD

At the end of this subtopic, students should be able to: Describe the effect of a  current-carrying conductor  in a magnetic field Draw the pattern of the combined magnetic field (catapult field) to indicate the direction of force on a current carrying conductor in a magnetic field. Explain factors that affect the magnitude of force on a current carrying conductor in a magnetic field. Describe the effect of a  current-carrying coil  in a magnetic field. Describe the working principle of a direct current motor. Describe the factors that affect the speed of rotation in an electric motor.

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Which can be made by sending an electric current through a coil of wire wound around an iron core

Used in motors, generators, relays, loudspeakers, hard disks, MRI machine

When a current flows through the coil,  it produces  magnetic field. The soft iron core becomes  temporarily magnetized When the current is cut off, it loses its magnetism

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A  region around a magnet or a current-carrying conductor in  which  a magnetic force will act on a magnetic material. Consists  of magnet lines or magnetic flux. Magnetic field lines which are close together represent a strong field.  The field direction is defined as the direction indicated by a compass needle placed in the magnetic field.

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How to determine the direction of magnetic field?

Grip the wire using the  right hand, with your thumb pointing in the direction of the current. Your other fingers now point round the wire in the direction of the magnetic field.  When the direction of the current is reversed, the magnetic field direction also is reversed.

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Magnetic field lines Straight wire carrying a current

Thumb points in the direction of the current Right Hand Other fingers give the direction of the field Current flow

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Image result for current carrying wire magnetic field iron filling

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Fingers curl in the
direction of current.

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Factors that affect the strength of the electromagnet

Current flowing in the solenoid Number of turns on the solenoid Distance between the turns The use of soft iron core

More paper clips attached to the solenoid!

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When the bell push is pressed, a current flows in the coils of the electromagnet, causing it to be magnetized.  The magnetized electromagnet attracts the soft-iron armature, causing the hammer to strike the gong.  The movement of the armature breaks the contact and causes the electromagnet to lose it magnetism.

The light spring pulls the armature back, remaking the contact and completing the circuit again.  The cycle is repeated so long as the bell push is pressed and continuous ringing occurs.

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Connect to 2nd cirecuit
2nd switch
Iron armature
Electromagnet
1st circuit
1 st switch

Electromagnetic  Relay (automatic switch)

When the switch is closed, current flows in the coil, causing the soft-iron core to be magnetized and attracts the  armarture . The movement of the iron armature closes the contacts in the second circuit. Circuit 2 is now switched on.  Circuit 2 may have a large current flowing through it to operate powerful motors or very bright lights .

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The varying current from the microphone flows through the coils of an electromagnet in the earpiece.  This pulls the iron diaphragm towards the electromagnet by a distance which depends on the current.  As a result, the diaphragm moves in and out and produces sound waves that are replicas of those that entered the microphone.

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This results in the spring pulling apart the contacts. The circuit is broken and the current flow stops immediately.  After repairs have been made, the reset button is pushed to switch on the supply again.

Soft iron armature pulled towards electromagnet

Acts as an automatic switch that breaks open a circuit when the current becomes too  large due to short circuit or overloading. The  strength of the magnetic field of the electromagnet increases suddenly. The soft iron armature is pulled towards the electromagnet.

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FORCE ON A CURRENT-CARRYING CONDUCTOR IN A MAGNETIC FIELD

conductor is pushed upwards Switch closed

The combination of the magnetic field due to the current in the conductor and the external magnetic field produce a  resultant magnetic field.  This resultant magnetic field produce a  magnetic force  which  later act on the current carrying conductor.

When a conductor carries an electric current through another magnetic field, a magnetic  force is exerted on the wire .

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The direction of the magnetic force, F, acting on the conductor can be determined by using  Fleming’s left-hand rule.

Force  direction Magnetic  field direction Current  direction

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Investigation on the force on a current-carrying conductor in a magnetic field

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The direction of magnetic field is parallel to the direction of current.

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How a current-carrying conductor in a magnetic field experiences a force ?

N S N S Magnetic Field of  Permanent Magnet Magnetic Field of  Current carrying conductor

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Region of weaker magnetic  field N S Region of stronger magnetic field Force   on wire

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Factors affecting the magnitude of  force on a current-carrying  conductor  in a magnetic field

Increase  current  in the conductor  The magnitude of the  force on a current-carrying conductor in a magnetic field Use  stronger permanent  magnet

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Resultant magnetic field or catapult field for a current-carrying coil in a magnetic field.

N S Magnetic Field of  Permanent  Magnet Magnetic Field of Current-carrying Coil N S F F

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N S Spring Carbon brush Commutator Permanent  Magnet Coil of wire

Spring :   Push the brush so it will always contact with the commutator.

Commutator:   Reverse the direction of current in the coil every half rotation so that the coil continues to turn in same direction

Carbon  Brush:   To contact with the commutator so the current from the battery enters the coil.

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The  working principle of the  direct current motor

When the coil gets to the upright position, the contact between the carbon brushes with the commutator is broken.  There is ________turning force on it because no current flows in the coil.  But the coil continues to rotate because of its _______________.

When current flows through the horizontal coil, magnetic field is produced around it.  The interaction between the magnetic field of the current and the magnetic field of the permanent magnet produces a _________ and 2 turning force.  The direction of the force is determined by Fleming’s left-hand rule.  The two forces  _____________the coil ______________________________.

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When the coil in a horizontal position again, the sides of the coil changes position.  The commutator _________________   the direction of the current in the coil to ensure that the forces on the coil turn the coil in one direction only.  So the coil is still rotating in the same _______________.

The above processes are repeated and the motor continues to rotate.

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FACTORS AFFECTING THE SPEED OF ROTATION OF THE MOTOR

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At the end of this subtopic, students should be able to: Describe electromagnetic induction in  straight wire and solenoid . Explain factors that affect magnitude of induced  emf Determine the  direction of induced current  in straight wire and solenoid Design a  direct current  and  alternating current generator

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What happens to the light bulb when the bicycle is moving?

What happens to the light bulb when the bicycle stops?

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It is the  production of an electric current  by a changing magnetic field as a result of  relative motion  between the conductor and the magnetic field.

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The induced current is produced only when there is  relative motion  between the conductor / coil and the magnetic field lines. The relative motion of a conductor across a magnetic field can be produced by:

Moving a permanent magnet towards one end of a solenoid or vice versa.

Moving  a straight wire quickly across a magnetic field between two flat magnets or vice versa.

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Each time the straight wire cuts across the magnetic field, or the permanent magnet moves towards the solenoid,  a  current is induced in the coil and a deflection is observed in the sensitive galvanometer.  This current is called induced current.  The  electromotive force that is produced is called the induced  e.m.f .

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a. ELECROMAGNETIC INDUCTION IN STRAIGHT WIRE

If a galvanometer shows a deflection, it means there is an _________________________ Current is induced in a straight conductor when it moves and cuts the magnetic field lines _________________________ The motion of the copper rod must be _________________ to the direction of the magnetic field lines so that an induced current will be produced.

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Push the bar magnet into the solenoid. Observe the deflection of the galvanometer. 	 Hold the bar magnet stationary in the solenoid. Note the reading of the galvanometer. 	 Pull the bar magnet out of the solenoid. Observe the deflection of the galvanometer.  Solenoid  Magnet Galvanometer Galvanometer Solenoid  Magnet Solenoid  Magnet Galvanometer

b. ELECROMAGNETIC INDUCTION IN A SOLENOID

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The galvanometer showed a  positive reading  when the bar magnet and solenoid were coming  closer  to each other. This shows that a  current is produced  in the solenoid in  certain direction .

The galvanometer showed a  negative reading  when the magnet and solenoid were moving  further away  from each other. This shows that a current is produced in the solenoid in the  opposite directions .  A  current is induced  in a solenoid when there is  relative motion  between the solenoid and a magnet.

Current is induced in a  straight conductor  when it  moves and cuts  the magnetic field lines . Current is induced in a  solenoid  when there is  relative motion  between the solenoid and a magnet.

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HOW TO INDICATE THE DIRECTION OF THE INDUCED CURRENT IN A STRAIGHT WIRE?

Current direction Magnetic field direction Force/ motion direction

If the thumb and the first two fingers on the right hand are held at  right angles to each other  with the first finger pointing in the direction of the magnetic field and the thumb in the direction of the motion, then the second finger points in the direction of the induced current.

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A wire PQ is moved vertically downwards in a magnetic field.  The induced current will flow from P  to Q .   [ Apply Fleming’s right-hand rule]

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HOW TO INDICATE THE DIRECTION OF THE INDUCED CURRENT IN A SOLENOID?

The resulting induced current always flows in the direction opposing the change in magnetic flux that produced it. Lenz’s Law

Relative motion between magnet and solenoid  Polarity at the end of the solenoid facing the magnet  Toward each other  Same polarity as the magnet  Away from each other  Opposite polarity as the  magnet

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Magnet is moved towards the solenoid
s
Magnet is moved away from the solenoid
Induced current produces a North pole to
oppose the motion of the magnet coming
towards the solenoid
Repulsion
s
Magnet is pushed
towards solenoid
Induced current produces a South pole to attract
the magnet moving away from it
s
Attraction
Magnet is pushed
away from solenoid

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The  magnitude   of the induced  e.m.f  is  directly proportional  to the rate at which the conductor cuts through the magnetic field lines. 	 Faraday’s Law:  Speed of  movement  of  the magnet or the solenoid Number of turns  on the solenoid  S trength  of the magnetic  field The magnitude of induced current in solenoid

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Show the correct direction of the induced current when the magnet is moved in the direction shown.

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reverses the connections of the coil with the external circuit after every half cycle, so that the current in the outside circuit always flows in the same direction.  Commutator N S Spring Carbon brush Permanent  Magnet Coil of wire

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Coil AB moves ______________, coil CD moves______________. When the coil rotates, its sides cut across the magnetic field lines and induced current flows in the coil from ________________. The galvanometer will deflect to ____________________.

The sides AB and CD are moving _______________ to the magnetic field and thus do not ___________ the magnetic field lines.  No ________ ______________is produced at the instant.  The galvanometer returns to  ______mark.

ELECTROMAGNETISM

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