TOPIK 8 (T5)

TOPIC 3: ELECTROMAGNET

 3.1 MAGNETIC EFFECT OF A CURRENT-CARRYING CONDUCTOR 

ELECTROMAGNET

•  An electromagnet is a type of magnet in which the magnetic field is produced ba a flow of electric current. the magnetic field dissappears when the current ceases.

• An electromagnet can be made by sending an electric current through a coil of wire wound around an iron core.
• When a current flows through the coil, it produces a magnetic field. 
• The soft iron core becomes temporarily magnetized when the current is switched on and attract the paper clips.
• When the current is switched off, it loses its magnetism (demagnetizes).

 

magnetic field pattern

• A magnetic field pattern can be represented by field lines that show the shape of the field.
• 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.

• The field direction can also be determined using The Right-Hand Grip Rule.

 The Right-Hand Grip Rule.

 

• 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.
• Top view

 The direction of the magnetic field around a coil

• The plotting compasses show the magnetic field pattern due to current in a circular coil.
• Top view

 

The direction of the magnetic field around a solenoid

• Magnetic field pattern similar to a magnetic bar
• Field lines in the centre are close – strong field
• One end acts as north pole, whereas the other end acts as a south pole
• To determine the pole of magnetic field, use:

Right Hand  Grip Rule

• Field lines move out from North pole and re-enter the South pole

Solenoid  Rule

• Look at the direction of current flow at the end of the solenoid. 

 

FACTORS THAT AFFECT THE STRENGTH OF ELECTROMAGNETIC FIELD

Number of turns of the coil:

• Strength of the electromagnetic field increases with the number of turns of the coil.

Current:

• Strength of the electromagnetic field increases with current

application of Electromagnet

1. When the switch is pressed,

- a current flows in the coils of the electromagnet

- causing it to be magnetized.

2. The magnetized electromagnet

- attracts the soft-iron armature,

- causing the hammer to strike the gong.

3. The movement of the armature

- breaks the contact

- and causes the electromagnet to lose it magnetism

4. The spring pulls the armature back,

- remaking the contact

- and completing the circuit again.

Magnetic Relay

1. Circuit 1 requires only a small current.

2. When the switch is closed,

- small current flows in the coil,

- causing the soft-iron core to be magnetized and attracts the armature

3. The movement of the iron armature

- closes the contacts in the second circuit.

- Circuit 2 is now switched on.

4. Circuit 2 may have a large current flowing through it to operate powerful motors or
very bright lights.

5. The advantage of using a relay:

- a small current (Circuit 1) can be used to switch on and off a circuit with a large
 current (Circuit 2).

6. This is useful for two reasons:

- Circuit 1 may contain a component such as a light detecting resistor (LDR)
which uses small currents.

- Only the circuit with a large current needs to be connected with thick wire.

Telephone Ear-piece

1. The varying current from the microphone flows through the coils of an
 electromagnet in the earpiece.

2. This pulls the diaphragm towards the electromagnet by a distance which depends 
on the current.

3. As a result, the diaphragm moves in and out and produces sound waves that
are replicas of those that entered the microphone.

Circuit Breaker

1. Acts as an automatic switch that breaks open a circuit when the current
becomes too large.

2. In a household circuit, the current may become excessive when there is a short   
circuit or an overload.

3. The strength of the magnetic field of the electromagnet increases suddenly.

4. The soft iron armature is pulled towards the electromagnet.

5. This results in the spring pulling apart the contacts. The circuit is broken and 
the current flow stops immediately.

6. After repairs have been made, the reset button is pushed to switch on the supply  
again.

3.2 FORCE ON A CURRENT-CARRYING CONDUCTOR IN A MAGNETIC FIELD

• As the conductor is placed into the magnetic field of the permanent magnet, this
• results in the combination of two magnetic fields

- Resultant magnetic field at the left is stronger than the on the right.

- Magnetic force is exerted on the conductor to the right, thus the conductor is
  pushed to the right.

- The force is called catapult force.

- The force is caused by the combination of the magnetic fields due to the current

  carrying conductor ant the permanent magnets.

FLEMING’S LEFT- HAND RULE

- Forefinger, second finger and the thumb of the left hand are extended at right angles

  to each other, 

- Forefinger in the direction of the magnetic field, the second finger in the direction of

  the current, then the thumb will point the direction of the force, F or motion

Exercise 

 

Determine the direction of magnetic force act on current carrying conductor PQ,using

Fleming’s Left - Hand Rule

DC MOTOR

  

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.

Magnetic field of two current-carrying conductor interact with magnetic field of

permanent magnet

a)    Catapult field formed

b)   PQ experience downward force

c)    RS experience upward force

Force on PQ and RS are

a)    equal in magnitude

b)   opposite direction

 

 

 

The coil rotate in anticlockwise direction.

 
Factors which affect the speed of rotation of the motor 

Application of the Force on Current carrying conductor in a Magnetic field

a)    Moving coil loudspeaker

b)   Moving coil meter

Exercise

3.3 ELECTROMAGNETIC INDUCTION

Electromagnetic Induction in a straight wire

 

 1.    Current is induced in a straight conductor when it moves and cuts the
      magnetic field lines.

2.      The motion of the copper rod must be perpendicular to the direction of 
      the magnetic field lines so that an induced current will be produced.

Electromagnetic Induction in a solenoid
 

Current is induced in a solenoid when there is relative motion between
the solenoid and a magnet.

  

 Indicate the direction of the induced current in a straight wire

Fleming’s right-hand rule:

 

The thumb and the first two fingers on the right hand are held at right angles
to each other

a)    the first finger pointing in the direction of the magnetic field and

b)   the thumb in the direction of the motion,

c)    then the second finger points in the direction of the induced current.

Wire PQ is moved vertically downwards in a magnetic field. Applying Fleming’s

right-hand rule, the induced current will flow from P to Q.

 

Indicate the direction of the induced current in a solenoid.

Lenz’s Law:

The direction of the induced current in a solenoid is such that its magnetic 
effect always oppose the change producing it.

Magnet is moved towards the solenoid

 

 Magnet is moved away from the solenoid

Factors that affect the magnitude of the induced current

Faraday’s Law:

The size of the induced e.m.f is directly proportional to the rate at which the 
conductor cuts through the magnetic field lines.

The size of the induced current increased by:

1. increasing the speed of moving magnet or solenoid

2. increasing the number of turns on the solenoid

3. increasing the strength of the magnetic field through the use of a

    stronger magnet.

applications of electromagnetic induction

Current Generator

• Current generator functions by converting mechanical energy to electrical energy.

• Current generator works based on electromagnetic induction and uses the

 Fleming’s Right hand rule.

• Current generator is divided into: direct current generator and alternate

 current generator.

Direct Current Generator

Commutator: 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

Alternating Current Generator

 The two ends of the coil are connected to two slip rings which rotate with the coil.

• Each slip ring is always in contact with the same carbon brush.

The output current generated is an alternating current because the current 

changes direction in the external circuit each time the coil passes the 

vertical position. 

• Assume the current flows from P to Q is positive and the current flows from

Q to P is negative.

• The current changes magnitude and direction after every half rotation.

Compare direct current and alternating current

Direct current (d.c)	Alternating current (a.c)
Flows in one direction only in a circuit	Flows to and fro in two opposite directions in a circuit. Changes its direction periodically
Can flow through a resistor but cannot flow through a capacitor.	can flow through both a resistor and a capacitor.
Both the direct current and alternating current have a heating effect on the filament of a bulb and can light up the bulb.

Peak current and peak voltage

The current increases from zero to a maximum value of  +I₀  (at A), and back 

to zero at B. It then reverses direction and increases to  -I₀  (at C) and back

to zero again.

I₀ = peak current,

V₀ = peak voltage

• The time taken for a complete cycle from O to D is called one period, T.

•  Frequency of the current is f where  f =  1

                                                                       T

• In Malaysia, the frequency of the a.c supply is 50 Hz. Hence, the period 

of the a.c  is : T = 1/50 = 0.02 seconds

exercise

Figure shows an alternating current with a magnitude that changes with time.

(a) What is the peak current?

(b) What is the period of the a.c. current?

(c) What is the frequency of the a.c current? 

 3.4 TRANSFORMER

A transformer is an electrical device which increases or decreases an 

alternating voltage based on the principle of electromagnetic induction.

 

Structure of a simple transformer.

- A transformer consists of two coils of wire wound round separately on a 

laminated soft-iron core.

- The coil connected to the input voltage is called the primary coil. The coil 

connected to the output voltage is called the secondary coil.

- The purpose of the common iron core is to provide a magnetic field linkage 

in the secondary coil. 

Symbol of a simple transformer

 

Operating principle of a simple transformer

 

The changing magnetic field produced by primary coil induces an alternating 

current in the secondary coil.

 

- A transformer works on the principle of electromagnetic induction.

- When a.c voltage, Vp, is applied to the primary coil of transformer, an 

alternating current flows through the coil. The soft-iron core is magnetized

in one way and then the other.

- e.m.f is induced across it to produce an a.c voltage, Vs in the secondary 

coil and a.c current flows through the second coil.

- The frequency of the secondary voltage Vs is the same as that of the 

primary voltage, Vp.

- The magnitude of the secondary voltage, Vs, depends on the ratio of the 

number of turns of the primary and secondary coils.

Relationship between number of turns in coils with voltage in a transformer,

(Vp, Np, Vs and Ns)

According to Faraday’s law: