Chapter 3 Electromagnetic

Chapter 3 Electromagnetic

3.1 ANALYSING THE MAGNETIC EFFECT OF CURRENTCARRYING CONDUCTOR ELECTROMAGNETS 1. 2.

Electromagnetism is the study of the relationship between electricity and magnetism. A bar magnet produces the magnetic fields around it. Draw the pattern of the magnetic fields produced around a bar magnet below

N

S

Magnetic fields of a bar magnet 3. 4. 5. 6. 7.

An electromagnet is a temporary magnet. A simple electromagnet consists of a solenoid with a soft iron core inside. Soft iron is used because it has the following characteristics; (a) Easily magnetised when the current in the solenoid is switched on. (b) Loses all its magnetism when the current is switched off. If the soft iron is replaced with a steel core, the steel core becomes a permanent magnet when current flows in the solenoid. Figure 1 shows the action of an electromagnet.

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Chapter 3 Electromagnetic

Figure 1 8.

Using the diagram above, complete the steps to switch on the magnetism effect. Switch is closed

9.

Current flows

Magnetic filed

Soft iron core is magnetised

Attracts pins

The electromagnet uses the magnetic effect of an electric current. A currentcarrying wire produces a magnetic field in the region around the wire.

THE MAGNETIC FIELD DUE TO A CURRENT IN A STRAIGHT WIRE

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Chapter 3 Electromagnetic 1. 2.

The magnetic field lines are concentric circles as shown in figure 2. The direction of magnetic field can be determined using “right-hand grip rule”. - Thumb pointing in the direction of the current. - The other four fingers show the direction of the magnetic field round the wire.

Diagram above shows the symbols of direction of currents out of or into a paper.

THE MAGNETIC FIELD DUE TO A CURRENT IN A COIL

1. 2.

Figure 3 shows the magnetic field produced by a current in circular coil. The magnetic lines are closest to each other at the centre of the coil with the magnetic field at the centre is the strongest.

The magnetic field pattern by the current in the circular coil 4 Physics Department SMK Sultan Ismail

Chapter 3 Electromagnetic 3.

The strength of the magnetic field increases when, (a) the current is increased (b) the coil has more number of turns (c) a coil of smaller radius is used

THE MAGNETIC FIELD DUE TO A CURRENT IN A SOLENOID

Figure 4 1. 2. 3.

The magnetic field for a solenoid has a similar pattern to the magnetic field of a bar magnet as shown in figure 4. One end of the solenoid is a North Pole (N) while the other end is a South Pole (S). The polarity at the ends of the solenoid can be determined by;

Right-hand grip rule

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Chapter 3 Electromagnetic

Looking at the ends of the solenoid. A clockwise indicates a South Pole while an anti-clockwise indicates a North Pole.

USE OF ELECTROMAGNETS Electric Bell

Figure 5 1. 2. 3. 4. 5. 6.

In an electric bell, the electromagnet is switched on and off very rapidly by making and breaking contact. When you press the switch, current flows in the coil, creating an electromagnet. The electromagnet then attracts the hammer towards the gong to hit it. When the hammer moves towards the gong, the contact opens. The circuit is broken and the current stops flowing. The coil loses its magnetism and the hammer returns to its original position, completing the circuit again. In this way, the hammer hits and lifts off from the gong repeatedly, making the bell ring as long as the switch is on.

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Chapter 3 Electromagnetic

ELECTROMAGNETIC RELAY

Figure 6 1. 2.

3.

In circuit 1 when the switch is closed, current flows and the iron rocker arm is attracted to the electromagnet and pushes the contacts together. Circuit 2 is now switched on. Circuit 2 may have a large current flowing through it to operate powerful motors or very bright lights. When the switch is opened, the electromagnet releases the rocker arm and the spring moves the contact apart. Circuit 2 is now switched off. The advantage of using a relay is that a small current (circuit 1) can be used to switch on and off a circuit with a large current (circuit 2).

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Chapter 3 Electromagnetic

TELEPHONE EAR-PIECE

Figure 7 1. 2. 3. 4.

A permanent magnet in figure 7 attracts the magnetic alloy diaphragm with a constant force. When the caller at the end of the line talks into the microphone, the ear-piece receives a varying current from the telephone line. The varying current in the coils of the electromagnet changes the strength of the magnetic field and causes the diaphragm to be attracted by a varying force. This causes the diaphragm to vibrate and reproduce the sound.

FACTORS THAT AFFECTS THE STRENGTH OF AN ELECTROMAGNET 1.

The strength of the magnetic field can be increased by: (a) using a shorter wire to reduce the resistance. (b) using a thicker wire to reduce the resistance. (c) reducing the resistance of the rheostat (d) the current is increased (e) the coil has more number of turns (f) a coil of smaller radius is used

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Chapter 3 Electromagnetic

3.1 THE FORCE ON A CURRENT-CARRYING CONDUCTOR IN A MAGNETIC FIELD 1. 2.

Electrical motors are found in many household items such as the fan, blender and hairdryer. The rotation is due to forces that are produced by using a current and a magnetic field.

Magnetic field around a wire

Magnetic field between two magnadur magnet

The force experienced by a current-carrying wire in a magnetic field. Figure 6 3.

4.

When a current carrying conductor is placed in the magnetic field of a permanent magnet as shown in figure 6, the interaction between the two magnetic fields (magnetic field of the permanent magnet and magnetic field produced by the current-carrying conductor) produced a force on the conductor. This produced a resultant field known as a catapult field as shown in figure 7.

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Chapter 3 Electromagnetic

+

Magnetic field produced by the current-carrying conductor

Magnetic field of the permanent magnet .

A catapult field is produced. The direction of the magnetic filed lines causes the wire to move from the strong magnetic field to the weaker filed Figure 7 5.

The relationship between the direction of the current, magnet field and the force acting on the conductor can be easily determined with Fleming’s lefthand rule.

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Chapter 3 Electromagnetic 6.

The direction of the force on the conductor is perpendicular to the direction of the current and the direction of the magnetic field.

THE TURNING EFFECT OF A CURRENT-CARRYING COIL IN A MAGNETIC FIELD 1. 2.

Consider a current-carrying coil ABCD placed between the poles magnet as shown in the figure below. As the current flows through the coil from A to D, an upward (1) force acts on the arm CD whereas a downward (3) force acts on the arms AB according to Fleming’s Left Hand rule. N

Coil S

1

B

4

Carbon brush

A

C 2 3 Magnet D

current

commutator

Electrical energy

3.

Kinetic energy

The interaction between the magnetic field of the current and the magnetic field of the permanent magnet produced a resultant magnetic field as shown in figure 7.

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Chapter 3 Electromagnetic

Figure 7 4.

The two forces which are in opposite directions constitute a couple which produces a turning effect and the coil rotates in a clockwise.

AMMETER

Figure 9 1. 2. 3. 4.

Figure 9 shows the internal parts of the ammeter or voltmeter. When a current passes through the coil, equal and opposite parallel forces act respectively on the sides of the coil beside the poles of the permanent magnet. This pair of forces causes the coil to rotate until it is stopped by the control springs. When there is no current flow, the forces no longer exist. The control springs bring the coil back to its original position and the pointer goes back to zero deflection.

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Chapter 3 Electromagnetic

DIRECT CURRENT MOTOR

1. 2. 3. 4. 5. 6.

The direct current motor (d.c motor) uses the turning effect on a currentcarrying coil in a magnetic field. A d.c motor consists of a rectangular coil of wire placed between two permanent magnets. Both ends of the coil are soldered to a commutator made of two semicircular copper rings. The use of a commutator is to enable a smooth change of direction of the current flow in the coil continuously rotates in one direction every half turn of the coil. Two carbon brushes are held against the commutator with a slight pressure with the aid of spring. The current flows from the brush P to the coil and out of the coil via the brush Q.

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Chapter 3 Electromagnetic

Stage 1 Horizontal position

Switch is closed, current flows in the direction abcd. - The force on the side ab is downwards - The force on the side cd is upwards. - These two forces produce a couple which rotates the coil in clockwise direction. Stage 3 Horizontal position

Stage 2 Vertical/Upright position

-

-

Carbon brush P touches Y and carbon brush Q touches X.

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-

The contact between the carbon brushes P and Q with the commutator X and Y is broken. No current flows in the coil therefore no turning force.

Stage 4 Vertical/upright position

-

The contact between the carbon

Chapter 3 Electromagnetic -

The current flows in the direction dcba. The force on the side ab is upwards while the force on the side cd is downwards.

-

brushes P and Q with the commutator X and Y is broken. No current flows in the coil therefore no turning force. The above process are repeated and the motor continues to rotate until the current is cut off.

FACTORS THAT AFFECT THE SPEED OF ROTATION OF AN ELECTRIC MOTOR 1.

The magnitude of force acting on a conductor in a magnetic increases by: (a) increasing the current (b) using a stronger magnet (c) increasing the number of turns on the coil.

8.2 ANALYSING ELECTROMAGNETIC INDUCTION 1. 2.

The production of an electric current by a changing magnetic field is called electromagnetic induction. The induced current is produced when, (a) a conductor cuts across a magnetic flux (b) a change of magnetic flux linkage with a coil

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Chapter 3 Electromagnetic 3. 4.

From the observations of the above activities, it is found that the movement of the magnet or the copper wire produces an induce current in the wire. The existence of the induced current is detected by the galvanometer. The direction of the induced current can be determined using Fleming’s righthand rule.

Fleming’s right-hand rule

INDUCED E.M.F BY COIL

Magnetic field lines are being cut. Current induced

No deflection on the galvanometer No current is induced

Moving the coil towards a magnet also induces current Current induced in opposite direction 16 Physics Department SMK Sultan Ismail

Chapter 3 Electromagnetic

1. 2.

In the above activities, an induced current is produced when there is relative motion between the bar magnet and the solenoid. The induced current produced in this case is due to the change of magnetic flux linkage with the solenoid.

FARADAY’S LAW 1. 2.

Faraday’s law state that the magnitude of the induced e.m.f. is directly proportional to the rate of change of magnetic flux experience by the conductor. The magnitude of the induced current or e.m.f increases with (a) the relative motion between the magnet and the coil is increased (b) the number of turns on the coil is increased (c) the cross-sectional area of the coil is increased (d) a stronger magnet is used

LENZ’S LAW 1. 2. 3.

4.

Lenz’s law is used to determine the polarity of the ends of the solenoid. Fleming’s right hand grip rule is then used to determine the direction of the induced current in the solenoid. Lenz’s Law states that the direction of the induced e.m.f is such that its magnetic effects always oppose the change producing.

Lanz’s law is an example of the Principle of Conservation of Energy. When the magnet or solenoid is moved against the opposing force, work is done. Therefore mechanical energy is converted to electrical energy.

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Chapter 3 Electromagnetic

APPLICATION OF ELECTROMAGNETIC INDUCTION 1. 2.

A generator is basically the inverse of a motor. A generator consists of many coils of wire wound on an armature that can rotate in a magnetic field. The coil is rotated by an external force (e.g. falling water, steam, etc) so that the coil cuts the magnetic field/flux lines as it rotates.

DC GENERATOR

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Chapter 3 Electromagnetic

Graph of output current from the dc generator

When coils is at its horizontal position 900 and 2700. The Change of rates of magnetic flux is maximum and Induced e.m.f is maxsimum.

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When coils is at its vertical position 00, 1800 and 3600. No changes of magnetic flux and No e.m.f is induced

Chapter 3 Electromagnetic

AC GENERATOR 1.

Two slip rings are used to obtain an alternating current output.

2.

When coils is at its horizontal position 900 and 2700. Change of rates of magnetic flux is maximum and Induced e.m.f is maxsimum. When coils is at its vertical position 00, 1800 and 3600. No changes of magnetic flux and no e.m.f is induced.

3.

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Chapter 3 Electromagnetic

ALTERNATING AND DIRECT CURRENT (a.c/d.c) Aspect

Direct Current (d.c)

Alternating current (a.c)

Current-time graph

Characteristics

Effect on a capacitor Effect on a bulb

- Magnitude of current is - Magnitude of current is constant. change with time. - Current flows in one direction only. The bulb does not light up The bulb lights up Note: The bulb light up for a very Note: Capacitor allows alternating short while after the switch is current but not direct current to closed. pass through. The bulb lights up Note: Both currents produce heating of the filament

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Chapter 3 Electromagnetic

3.4 ANALYSING TRANSFORMER

1. 2. 3. 4.

In Malaysia, our electricity for domestic supplied at a voltage of 240 V a.c. However, most of home appliances at home use lower than or higer than 240V. A transformer is a device which used to rise or lower the potential difference of an alternating current. It works on the principles of electromagnetic induction.

OPERATING PRINCIPLE OF A TRANSFORMER 1.

A simple transformer consists of two coils wound on a laminated iron core.

A transformer 2. 3.

Symbol

The coil which is connected to the power supply whose voltage is to be raised or lowered is called the primary coil. The supply voltage is called the primary (input) voltage, Vp. The other coil which is connected to the electrical equipment or resistor is called the secondary coil.

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Chapter 3 Electromagnetic

4. 5. 6. 7.

The voltage across the secondary coil is called the secondary (output) voltage, V s. The operating principle of a transformer is based on electromagnetic induction. When the alternating current in the primary coil flows, it produced a flux or magnetic field lines. The change of the magnetic flux in the primary coils causes the magnetic flux to cut the secondary coil. The changing of the magnetic flux produces an induced e.m.f across the secondary coil with the same frequency as the electrical supply. Note: The primary coil must be connected to an a.c supply an not d.c supply. This is because an induced e.m.f will not be produced in the secondary coil. The d.c is constant does not create a changing magnetic flux in the secondary coil.

STEP-UP AND STEP-DOWN TRANSFORMERS Step-down transformer

Step-up transformer

The number of turns in the secondary coil is less than the number of turns in the primary coil. The voltage across the secondary coil is less than the voltage across the primary coil.

The number of turns in the secondary coil is greater than the number of turns in the primary coil. The voltage across the secondary coil is greater than the voltage across the primary coil.

Ns < Np & Vs < Vp

Ns > Np & Vs > Vp

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Chapter 3 Electromagnetic Vs N s = Vp N p

EFFICIENCY OF A TRANSFORMER 1. 2. 3.

The efficiency of a transformer in normally less than 100% that is the power of the output is less than the power of the input. A transformer transfers electrical energy from one circuit to another circuit by electromagnetic induction. In the process, a fraction of the electrical energy is lost as heat energy. Formula for the efficiency of a transformer: Output power Efficiency = x 100% Input power P = o x 100% Pi

IDEAL TRANSFORMER 1.

Ideal transformer is no loss of energy in a transformer, all the energy supplied to the primary coil will be transferred to the secondary coil which has an efficiency of 100%. ∴ Output power = Input power Po = Pi VsIs = VpIp Ip Vs = Vp Is

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Chapter 3 Electromagnetic

FACTORS THAT AFFECT THE EFFICIENCY OF A TRANSFORMER AND WAYS TO IMPROVE THE EFFICIENCY OF A TRANSFORMER Factor Resistance of coil Energy is lost as heat energy in the coil because of the heating effect of current flow in a conductor. Magnetisation and demagnetisation of iron core The core is continually magnetised and demagnetised by the changing magnetic field. Some energy is transformed into heat in the core. This loss is called hysteresis.

Ways to minimise energy losses

Thick copper wire is used to reduce the resistance of the coil.

Using soft iron core. Easily magnetised and demagnetised.

Using laminated (layered) iron core. Eddy currents in iron core The core is itself a conductor, so the changing magnetic flux in the iron core induced currents in it. Eddy currents generate heat in the iron core. Leakage of magnetic flux Electrical energy is lost when a fraction of the magnetic flux produced by the primary coil does not link with the secondary coil. As result, there is a reduction in e.m.f induced in the secondary coil.

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The iron core should be a closed loop of iron with the secondary coil wound over the primary coil.

Chapter 3 Electromagnetic

Examples 1 Transformer that has 2000 turns in its primary coil is used to operate a 12 V, 24 W lamps from the 240 V mains, as shown in figure 1. If energy losses in transformer can be neglected and the lamps is operated at its ratings, find (a) the number of turns in the secondary coil (b) the current in the secondary coil, (c) the current in the primary coil.

Examples 2 A transformer enables a 12 V lamp to be used with a 240 V mains supply. If there are 600 turns on the primary coil, calculate (a) the number of turns in the secondary coil, (b) the secondary current if the primary current is 0.5 A. (Assume no energy loss)

Examples3 A transformer connected to the 240 V mains has 1000 turns on the secondary coil. The efficiency of the transformer is 80% and it is used to light a 12 V, 48 W lamps. If the lamp lights up with normal brightness, calculate (a) the number of turns in the primary coil (b) the primary current

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