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Electromagnet consists of solenoid of many turns wound on core of soft magnetic material

The strength of a magnetic field is increased by:

- Increasing the number of turns of solenoid - Passing larger current through solenoid - Inserting soft magnetic material as a core

For creation of force by current source, we use Fleming’s Left Hand Rule.

For creation of current source by force, we use Fleming’s Right Hand Rule.

Current flowing out of page is denoted by

ʘ



For determining the magnetic field across a solenoid, we use Right Hand Grip Rule.

20.1 MAGNETIC FIELD BY CURRENT

When a compass is placed under current-carrying wire, it deflects and when placed above the wire, it also deflects, but to opposite direction. This shows magnetic field is produced.

Magnetic Field Due to Long Straight Wire

By placing a wire vertically through a horizontal card and place iron filings around it, we know the pattern of magnetic field around a straight wire. The iron filings are closer to each other when placed nearer to wire, showing stronger magnetic field. It is thus known that when we increase the current, more magnetic field lines are formed, creating stronger field. To find out the direction of magnetic field, we use plotting compass placed along line of magnetic field. Diagrams below illustrate this.

We can also deduce the direction of magnetic field by using the right-hand grip rule.

It is known that when the current is reversed (e.g. from into page to out of page), the direction of magnetic field is also reversed.

Magnetic Field of a Flat Coil

Magnetic field due to current by flat coil is stronger inside coil than outside because field from each wire side are concentrated in a small area. They’re also straight and perpendicular to plane of coil.

Right Hand Grip Rule

Use your right hand fingers to curl around the wire and your thumb pointing towards the direction of current. The direction of the curled fingers is the direction of magnetic field.

Magnetic Field of a Solenoid

The magnetic field lines resembles that of a bar magnet. Inside the solenoid, it resembles field lines in flat coil. It even has poles. To determine the polarity, look at one end of solenoid and if current is flowing in anti-clockwise direction, it’s N-pole.

If current is flowing in clockwise direction, it’s the S-pole. Repeat the experiment for the other end of the solenoid. Otherwise, use right-hand grip rule in FIGURE 19.5.

It is known that if we reverse the current, the polarity will also be reversed.

Applications of Electromagnets 1. Magnetic Relay

It consists of 2 circuits – one consisting of electromagnet to switch on another circuit. When first circuit is closed, current flowing through solenoid magnetises the iron core which then attracts the soft-iron armatur. The upper part of armature swings up and touches the contact hence closing the second circuit. The second circuit may need large current to operate, hence an electromagnetic relay can activate the second circuit by just using a small current for the first circuit.

2. Reed Switch

A reed switch is a pair of soft iron strips, known as reeds, housed inside glass tube, containing inert gas (to prevent oxidation of the reeds), with a gap between reeds. When magnetic field from bar magnet/electromagnet is brought near, say the glass is wound, reeds become temporarily magnetised and attract each other, closing contacts and allows current flowing in circuit connected to the reed switch 3. Electric Bell

When switch’s on, circuit’s closed & current flows through electromagnetic coil. Soft iron cores are magnetised & armature is attracted to the cores so that the hammer strikes the again and repeats process. The bell continues to ring as long as switch is kept on.

4. Circuit Breaker

Circuit breakers are used to cut circuit when current exceeds specified value.

When usual current’s flowing, strength of electromagnet is insufficient to separate contacts. When current’s too high, strong magnetic force pulls contacts & breaks circuit. Spring keeps contact while fault’s repaired. Contacts stay apart unless contacts are pushed back by pressing reset button.

5. Audio/Video Tapes

The tapes are coated with magnetic material. Sound/pictures are represented as varying currents. The currents causes electromagnet at the head of the tape to magnetise tape according to the picture or sound.

Force on Current-Carrying Conductor in Magnetic Field

The direction of force on conductor can be determined using Fleming’s Left Hand Rule. Look at the diagram below. Thumb shows direction of force, first finger shows direction of magnetic field and second finger shows direction of current. Position the first finger to direction of magnetic field and your second finger to direction of current flow. The thumb will show you where the force directs the conductor.

An explanation

A current-carrying conductor produces a circular magnetic field. Between two magnets there is a magnetic field produced from North to South pole. When the current-carrying conductor (e.g. wire carrying current) is brought between the magnetic field of the 2 magnets, the magnetic fields created will combine to form a stronger magnetic field.

Magnetic field will combine when the direction of the magnetic field is the same to produce stronger field. Contrawise, they repel to produce weaker field. Magnetic field created by wire has one side which coincide with the direction of the magnetic field. This side attracts the magnetic field to take this path, producing stronger magnetic field. Otherwise, the sides which don’t coincide has weaker magnetic field.

Force is exerted on the conductor from the region where field is now stronger to the region where field is now weaker to balance the unequal fields.

Force Between Two Parallel Current-Carrying Conductor Just remember:

Like current directions attract.

Unlike current directions repel.

...and also apply the theory on force on current-carrying conductor in magnetic field.

20.2 MAGNETIC FLUX OF D.C. CIRCUITS Turning effect on Current-Carrying Coil

We have learnt force between two parallel current-carrying conductor is due to attraction/repulsion between magnetic fields created by both wires.

Now, that there’s a magnet by the side of these two current-carrying conductor, flowing in opposite direction and affiliated by coil, what would happen?

Look at Figure 20.12. In (a), two parallel current-carrying coil placed between a horseshoe magnet create circular magnetic field for each side. Applying Force on Current-Carrying Conductor in Magnetic Field, the side of each fields coinciding with magnetic field of horseshoe magnet will merge and create an equal but opposite force on each side of the coil, which is called catapult field. The combination of these forces will rotate the coil.

D.C. Motors

D.C. motors works only with direct current. It consists of a coil of wire, which spins on an axle, placed between two N-pole and S-pole of a permanent magnet, and connected to a split-ring commutator, which, each half of the ring rubs against carbon brush as the coil turns to allow flow of current.

When circuit is closed, conventional current flows from positive terminal of battery towards X through P, then through coil and back to battery through Y and Q. To determine the direction of turn of d.c. motors, we use FLEMING’S LEFT HAND RULE on EACH SIDE OF THE COIL, then UPWARD-SIDE OF COIL TURNS TOWARDS DOWNWARD-SIDE OF COIL.

For example, in the diagram, upward force is experienced on right side of coil while left side of coil experiences downward force. The coil hence turns anticlockwise until it reach vertical position, where at this point current is cut because split-ring is not in contact with carbon brush. However, the momentum of the coil continues the rotation until the split-rings is, again, in contact with carbon brush.

Note that half-ring Y is now in contact with P while X is now in contact with Q, The process is then repeated. Note also that the current in coil reverses each time coil passes vertical position, i.e. the right side of coil flows towards the battery at first, but now it’s on the left side and current now flows away from the battery.

Turning effect can be increased by:

1. Increasing number of turns in coil, e.g. giving extra force to increase period 2. Increasing magnitude of current

3. Inserting soft iron core within coil to concentrate magnetic line of force.

This will create a radial field which keeps the pair of forces acting on coil constant and increases magnetic field strength hence increasing turning effect in coil,

20.3 CURRENT BY MAGNETIC FLUX 20.4 MAGNETIC FLUX OF A.C. CIRCUITS CHAPTER 21: INTRODUCTORY ELECTRONICS 21.1 CATHODE RAY OSCILLOSCOPE

21.2 CIRCUIT COMPONENTS

CHAPTER 22 – ELECTRONIC SYSTEMS