Physics F 5: Electromagnet

3.1 Magnetic effect of a current-carrying conductor

Learning outcomes

Student should be able to :1. State what an electromagnet is2. Draw the magnetic field pattern due to a current in a straight wire, a coil and a solenoid3. Plan and conduct experiments to study factors that affect the strength of the magnetic field of an electromagnet 4. Describe applications of electromagnets

WHAT IS ELECTROMAGNET?

ELECTROMAGNET

The magnetic effect cause by a current-carrying conductor

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

Magnetic field pattern

1. A magnetic field can be presented by field lines that show the shape of the field.2. Magnetic field lines which are close together represent a strong field.3. The field direction is defined as the direction indicated by a compass needle placed in the magnetic field.4. “Right-hand grip rule” can be used to determine the direction of the magnetic field around a current-carrying wire. (either a straight wire or a coil)

MAGNETIC FIELD PATTERN DUE TO A CURRENT IN A:

1) STRAIGHT WIRE

A wire carrying a current is passed through the centre of a manila card. Iron filling are then sprinkled on the card and formed a pattern of circles around the wire.

A small compass is placed on the pattern to indicate the direction of the filed.

By using the Right-hand Grip Rule, grip the wire using the right hand, with your thumb pointing the direction of the current.

The other fingers point around the wire in the direction of the magnetic field.

The strength of the magnetic field produced by the wire is increased if:

the current flowing through the wire is increased

the number of wires carrying the current is increased

2) COIL

The field is identical to the fields of two straight wires, one carrying current upwards and the other downwards.

In the centre of the coils, the field pattern is a straight line, while both sides are curves.

The direction of the magnetic field at any point can be determined using the right-hand grip rule.

The strength of the magnetic field of a circular coil is increased by

3) SOLENOID3) SOLENOID

A solenoid is a long coil made up of several turns of wire like a cylinder.

When a current flows through the solenoid, the pattern of magnetic field is identical to that of a bar magnet.

1. The polarity of the solenoid can be worked out by using right-hand rule for a solenoid with the thumb points to the N-pole if the fingers of the right hand grip the solenoid in the direction of the current.

Experiment

Aim : To study the factors that affect

the strength of the magnetic field of an electromagnet.

Procedure: As shown in the diagram

Study the relationship between :a. the size of current carried by the coilb. the number of turns on the coilc. the use of a soft iron core

and magnetic field strength.

Submit the report within one week

A

WORKSHEET

1. Draw the magnetic field

a)

SOLENOID

b) c)

COILSTRAIGHT WIRE

APPLICATIONS OF THE ELECTROMAGNETS

The soft ion armature is attracted to the electromagnetThe clapper hits the gong

The electromagnet loses its magnetism The armature spring back to its original positionThe current is restored in the circuit and the whole process is repeated.

The armature breaks the circuit when it is pull away from the contact point.The current cut off

Switch is pressed

Bell ring as long as

switch is pressed

• Current flow through the solenoid• The iron cores become an electromagnet

2.Arrange these statement in correct order:i) Soft iron coil lost the magnetism and

the angker return to the original position

ii) The clapper hits the gongiii) The current cut offiv) While the switch is pressed, the circuit

is completed. The soft iron coil is magnetism.

Current flow through the solenoid• Iron core becomes an electromagnet

Soft iron armature attracted to the electromagnet

• Switch Q is closed• The high circuit is completed

Current flows in the solenoid being cut off• Iron core losses its magetism• Switch Q is open and current in high

voltage circuit is cut off

Switch P is Closed

Switch P is open

Relay

An Electromagnetic RelayA device that uses a small current (Circuit 1) to switch on another circuit that carries a bigger current

(Circuit 2). It thus acts like a switch.When the switch is closed, the soft-iron core becomes electromagnet.The iron rocker arm is attracted to the electromagnet on the central pivot and pushes two contacts

together. This switches on Circuit 2. When the switch is opened, no current flows through the solenoid and the iron rocker arm loses all its

magnetism.The iron rocker arm is pulled by a spring back to its original position. Hence, the contact is broken and no current flows in the Circuit 2. Circuit 2 is now switched off.The advantages of using a relay are :

The Circuit 1 may contain light detecting resistor (LDR) only uses small currentsOnly the Circuit 2 with a large current needs to be connected with thick wire.

When one speaks through a microphone change the mechanical energy (sound) into electrical energy.

The varying current of the electrical energy passes through the solenoid then, the solenoid become an electromagnet of varying magnetic strength.

The varying magnetic strength causes the diafragm to vibrate

The vibration of the diafragm causes the air column in front to be compressed an stretched – sound produced

A Telephone EarpieceInside a telephone earpiece, two electromagnets are fixed on a permanent

magnet.When a varying electric current which carries the sound signal from the mouth-

piece passes through the solenoids, the strength of the electromagnets changes.

The soft-iron diaphragm is pulled by electromagnets with a changing force. Hence the diaphragm vibrates at a same frequency of the sound signal.

The vibration of the diaphragm causes air molecules to vibrate and hence a sound wave is transmitted to the earpiece.

Evaluation

• Answer all the Formative Quiz • Complete The Topical Excercises