3.2 (Part 1) Understanding the Force on a Current-carrying Conductor in a Magnetic Field

8/11/2019 3.2 (Part 1) Understanding the Force on a Current-carrying Conductor in a Magnetic Field

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3.2 Understanding the Force on a Current-

carrying Conductor in a Magnetic Field(Part 1)

By Ms Nurul Ain Mat Aron

Chapter 3: Electromagnetism


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Objectives

1. describe what happens to a current -carrying conductor in amagnetic field

2. draw the pattern of the combined magnetic field due to acurrent - carrying conductor in a magnetic field

3. describe how a currentcarrying conductor in a magneticfield experiences a force

4. explain the factors that affect the magnitude of the force on acurrent - carrying conductor in a magnetic field

5. describe how a current -carrying coil in a magnetic fieldexperiences a turning force

6. describe how a direct current motor works

7. state factors that affect the speed of rotation of an electricmotor


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Force on a Current Carrying Conductor

in a Magnetic Field

We have learned that when current flows in a conductor, a magnetic field

will be generated.

When the current-carrying conductor is placed in a magnetic field, the

interaction between the two magnetic fields will produce a resultant field

known as the catapult field as shown in the figure below.


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in a Magnetic Field

The catapult field is a non-uniform field where the field at one side is

stronger than the other side.

As a result, a force is produced to move the current carrying conductor

from the stronger field to the weaker field.

The force produced by a catapult field is called the catapult force.


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in a Magnetic Field

The direction of the force can be determined by Fleming's left hand rule


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in a Magnetic Field

The strength of the force can be increased by:

Increase the current

Using a stronger magnet

using a longer wire

arranging the wire perpendicular to the direction of the magnetic field.


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Turning Effect of a Current Carrying

Coil in a Magnetic Field

If a current carrying coil is placed in a magnetic field, a pair of forces will be

produced on the coil.

This is due to the interactionof the magnetic field of the permanent magnet and the

magnetic filed of the current carrying coil.


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The diagram below shows the catapult field produced.

The direction of the force can be determined by Fleming's left hand rule.

Since the current in both sides of the coil flow in opposite direction, the forces

produced are also in opposite direction. The 2 forces in opposite direction

constitute a couple which produces a turning effect to make the coil rotate.


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Application of the Force on a Current Carrying

Conductor in a Magnetic Field

(1) Moving Coil Meter

Light Indicator

A light indicator which has lower inertia is used to increase the sensitivity of

the meter.


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Moving Coil Meter

Linear Scale

Due to the radial magnetic field and the cylindrical soft-iron core, a linear

scale is produced.

A linear scale is more accurate and easier to be read.

MirrorA mirror is used to prevent parallax error.

When the observer's eye is exactly above the indicator, the indicator will

cover its own image on the mirror.

This can used to prevent parallax error.

Curved Permanent Magnet

A curved permanent magnet is used to produce a radial field.

A radial field is a magnetic field where the field lines are either pointing

away or toward the center of the field.

A radial can be focused by a cylindrical soft-iron core.


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Moving Coil Meter

Rectangular Coils

When a current flows through the coils, a force will be generated due to the

interaction between the magnetic field of the permanent magnet and the coil.

The force will turn the coils, which in turn move the indicator.

Cylindrical Soft-Iron Core

A cylindrical soft iron core is placed inside the radial field produced by the curved

magnet.

A soft-iron core can focus the magnetic field of the permanent magnet.

Hair Spring

The deflection of the coil and the indicator stops when the force is balanced by the

opposing force from the hair spring.

The angle of deflection is directly proportional to the magnitude of the current in

the coil.


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Direct Current Motor

It consist a rectangular coil of wire placed between 2 permanent magnets.

The coil are soldered to a copper split ring known as commutator. 2 carbon

brushes are held against the commutator.

The function of the brush is to conduct electricity from the external circuit to thecoil and allow the commutator to rotate continuously.

The function of the commutator is to change the direction of the current in the coil

and hence change the direction of the couple (the 2 forces in opposite direction) in

every half revolution.

This is to make sure that the coil can rotate continuously.


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DC versus AC

same function of converting electrical energy into mechanical energy,

they are powered, constructed and controlled differently.

A.C. motors are powered from alternating current (A.C.) while D.C. motors are

powered from direct current (D.C.), such as batteries.

D.C. power supplies or an AC-to-DC power converter. D.C wound field motors are

constructed with brushes and a commutator, which add to the maintenance, limit

the speed and usually reduce the life expectancy of brushed D.C. motors.

A.C. induction motors do not use brushes; they are very rugged and have long life

expectancies.

The speed of a D.C. motor is controlled by varying the armature windingscurrent

while the speed of an A.C. motor is controlled by varying the frequency, which iscommonly done with an adjustable frequency drive control.


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Force between 2 Current-Carrying Conductors

When 2 current carrying conductors are placed close to each other, a force will

be generated between them.If the current in both conductors flow in the same direction, they will attract each

other, whereas if the current are in opposite direction, they will repel each other.

This force is due to the interaction between the magnetic field of the 2 conductor.

The figure below shows the catapult field produced by 2 current carrying

conductors when their current is in the same direction or opposite direction.

3.2 (Part 1) Understanding the Force on a Current-carrying Conductor in a Magnetic Field

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