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CHAPTER 8: PUSAT TUISYEN ALFA ELECTROMAGNETISM

8.1 Effect of a Magnet on a Current-carrying Conductor 8.1 Kesan Magnet ke atas Konduktor yang Membawa Arus 8.1.1 Straight wire/ Wayar lurus

Magnetic fields are circular Medan magnet adalah membulat

Field is strongest close to the wire Medan yang paling kuat adalah yang paling dekat dengan wayar

Increasing the current increases the strength of the field Peningkatan arus meningkatkan kekuatan medan

8.1.2 Circular coil/ Gegelung membulat The magnetic field is similar to two straight lines

carrying current in opposite directions Medan magnet adalah sama dengan dua garis lurus yang membawa arus dalam arah yang bertentangan

The magnetic field between the wires are straight lines whereas the ones near the wire are circular Medan magnet di antara wayar adalah garis lurus di mana satu wayar yang terdekat adalah membulat.

To determine the direction of the circular magnetic fields, use the right hand rule Untuk menentukan arah medan magnet,

gunakan hukum tangan kanan

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8.1.3 Solenoid/ Solenoid

When the number of turns on a coil is increased, it becomes a solenoid

Apabila bilangan putaran pada gegelung meningkat, ia menjadi solenoid.

The magnetic fields are similar to a bar magnet, i.e. magnetic poles on either end Medan magnet adalah sama dengan magnet bar, cth. kutub magnet__________________

To determine the polarity of the solenoid/ Untuk menentukan kutub solenoid:

Method 1: Observe the directions of the current flow on either end Perhati arah arus mengalir____________

Method 2: Right-hand grip rule Genggaman tangan kanan (Ibu jari menunjukkan Utara)

8.1.4 Electromagnet/ Keelektromagnetan

An electromagnet is a magnet made by winding a coil of insulated wires around a soft iron core, so that a magnetic field is produced when a current passes through the coil Suatu elektromagnet adalah magnet yang diperbuat oleh ______________

To increase the strength of the electromagnet: Untuk meningkat kekuatan electromagnet: Increase the current/ Meningkatkan arus Increase the number of turns on the coil/ Meningkatkan bilangan lilitan pada gegelung Insert a soft iron core in the middle of the solenoid/ Menggunakan teras besi lembut di

bahagian tengah solenoid

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8.1.5 Applications of Electromagnets/ Aplikasi elektromagnet Electromagnetic lifter/ Pengangkat elektromagnet

Used to lift heavy objects made of steel or steel alloys, e.g. cars Digunakan untuk mengangkat objek berat yang diperbuat daripada keluli atau aloi keluli, cth. kereta

Consists of a soft iron core wrapped by insulated copper coils Terdiri daripada teras besi lembut yang dibalut dengan penebat gegelung kuprum

Soft iron is used as the core because its magnetism will disappear almost instantaneously when the current is turned off Besi lembut digunakan sebagai teras kerana kemagnetannya akan hilang dengan serta merta apabila arus dimatikan.

When the current is switched on, the coils become an electromagnet Apabila arus dihidupkan, gegelung menjadi elektromagnet

Electric bell/ Loceng elektrik

When the switch is pressed, the direct current flows through the circuit, turning the solenoid into an electromagnet. Apabila suis dihidupkan, arus terus mengalir melalui litar, solenoid bertukar menjadi electromagnet.

The springy metal is attracted towards the electromagnet, and the hammer strikes the gong. Logam berspring ditark ke electromagnet, dan penukul mengetuk gong itu.

The contacts are no longer in contact, so the current stops flowing and the solenoid is demagnetized. Sesentuh tidak lagi dalam keadaan bersentuhan, jadi arus berhenti mengalir dan solenoid dinyahmagnetkan.

The springy metal will return to its original position, and completes the circuit again. Logam berspring akan kembali kepada kedudukan asal, dan melengkapkan litar itu semula.

The process is repeated as long as the switch is pressed Proses diulang selagi suis dihidupkan.

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Electric relay/ Geganti elektrik

When the switch is turned on, the input circuit is completed and the solenoid becomes an electromagnet. Apabila suis dihidupkan, input litar itu menjadi sempurna dan solenoid itu menjadi elektromagnet.

The electromagnet attracts the iron armature which pushes contacts C together. Elektromagnet itu menarik plat besi dimana menolak sesentuh C.

This closes the output circuit and the electric motor is started. Ini menutup output litar dan motor elektrik berputar.

Circuit breaker/ Pemutus litar

When too much current flows through the wire, the electromagnet becomes strong enough to attract the iron catch. Apabila terlalu banyak arus mengalir melalui wayar, elektromagnet itu menjadi cukup kuat untuk menarik ..

This disconnects the contacts and stops current from flowing through. Ini memutuskan sesentuh dan memberhentikan arus daripada mengalir.

The reset button is pushed to release the iron catch from the electromagnet and to close the contacts so that current can flow through again. Butang tetapan semula ditekan untuk melepaskan .. daripada elektromagnet dan untuk merapatkan sesentuh supaya arus boleh mengalir lagi.

Ticker timer/ When connected to a 50 Hz A.C. source, the thin metal strip becomes an electromagnet with changing polarity. Apabila disambungkan dengan sumber 50 Hz A.U., jalur besi nipis menjadi elektromagnet dengan kutub yang berubah-ubah.

The thin metal strip will be attracted to the permanent magnetic pole of opposite polarity. Jalur besi lembut itu akan ditarik ke kutub magnet kekal yang bertentangan kutub.

Because of the constantly changing polarity, the iron strip will vibrate with a frequency of 50 Hz.

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Telephone earpiece/ Cuping telinga telefon Consists of a permanent magnet clamped between two soft iron cores which are wound with copper wires in opposite directions. Melibatkan magnet kekal diletakkan di antara dua teras besi lembut di mana_____________________________

A thin iron diaphragm is fixed in front of the soft iron core. Diafragma besi lembut diletakkan di hadapan teras besi lembut.

The microphone converts audio signals to electrical energy with fluctuating frequency. Mikrofon menukaran isyarat audio kepada tenaga elektrik dengan frekuensi yang berubah-ubah.

When the fluctuating current flows through the solenoid, an electromagnetic field with fluctuating intensity is produced. Apabila arus yang berubah-ubah mengalir melalui solenoid, suatu medan elektromagnet dengan keamatan yang berubah-ubah dihasilkan.

The fluctuating pull of the magnetic field on the diaphragm causes the diaphragm to vibrate.

The vibrating diaphragm results in compression and rarefaction of air in its vicinity, producing sound of the same frequency as that received by the microphone.

Cassette recorder/ Perakam kaset A cassette recorder has two horseshoe-shaped electromagnets known as cassette heads. Suatu perakam kaset mempunyai dua bentuk tapak kuda elektromagnet dikenali sebagai kepala kaset.

One head functions as the erasing head which demagnetizes the tape whereas the other acts as the recording head which records information onto the tape. Satu kepala berfungsi sebagai kepala-pemadam di mana menyahmagnetkan pita di mana yang lain bertindak sebagai kepala perakam yang merakam informasi ke atas pita.

The magnetic tape is made from a resilient plastic material coated with a magnetic substance such as magnetic oxides or ferrites. Pita magnet diperbuat daripada ___________bahan plastic disalut dengan bahan bermagnet seperti oksida magnet

When recording, audio signals in the form of fluctuating electric impulses flow through the cassette head, producing a magnetic field with fluctuating intensity.

The cassette head magnetizes the portion of the magnetic tape that goes through the narrow slit with uniform speed.

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8.2 Interaction Between Current-carrying Conductor and the Magnetic Field 8.2 Interaksi di Antara Konduktor yang Membawa Arus dan Medan Magnet 8.2.1 Interaction of magnetic fields of a current-carrying conductor and permanent magnets 8.2.1 Interaksi medan magnet konduktor yang membawa arus dan magnet kekal

Permanent magnet Magnet kekal

Current-carrying conductor Konduktor yang membawa arus

Elastic field Medan elastik

8.2.2 Determining the direction of the induced force (Flemings Left Hand Rule) 8.2.2 Menentukan arah daya aruhan (Hukum Tangan Kiri Fleming)

The force is increased if/ Daya akan meningkat jika:

Current is increased/ Arus meningkat

A stronger magnet is used/ Magnet yang lebih kuat digunakan

The length of wire in the field is increased/ Panjang wayar dalam medan meningkat 8.2.3 D.C. Motor/ Motor A.T.

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8.3 Electromagnetic Induction/ Aruhan Elektromagnet

8.3.1 Inducing e.m.f. and current/ Aruhan d.g.e dan arus

E.m.f and current can be induced by/ D.g.e dan arus boleh diaruhkan oleh:

Moving a magnet bar in and out of a solenoid Magnet yang bergerak masuk dan keluar solenoid

Moving a conductor across a magnetic field Konduktor yang bergerak merentasi medan magnet

E.m.f. and current can only be induced when there is relative motion between a conductor and magnetic fields that are perpendicular to each other. D.g.e dan arus hanya boleh diaruhkan apabila adanya gerakan relatif di antara konduktor dan medan magnet yang bersudut tepat dengan satu sama lain. 8.3.2 Determining the direction of the induced current 8.3.2 Menentukan arah arus aruhan 8.3.2.1 Right-hand Slap Rule Point all four fingers in the direction of the field and the thumb in the direction of the motion. The direction of the slapping action gives the direction of the induced current.

8.3.2.2 Flemings Right Hand Rule/ Hukum Tangan Kanan Fleming

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8.3.2.3 Lenzs Law/ Hukum Lenz Lenzs Law used to determine the direction of the current flow Hukum Lenz digunakan untuk menentukan arah arus mengalir The solenoid will always resist any movement of the magnet relative to the solenoid Solenoid akan sentiasa menentang mana-mana pergerakan magnet relatif kepada solenoid

When the bar magnet is inserted into the solenoid, the solenoid will try to repel the bar magnet. Therefore, the polarity of that end of the solenoid will be the same as the bar magnets. Apabila magnet bar dimasukkan ke dalam solenoid, solenoid akan cuba untuk menolak magnet bar itu. Maka, kutub pada hujung solenoid akan menjadi sama dengan kutub magnet bar.

When the bar magnet is removed from the solenoid, the solenoid will try to attract the bar magnet. Therefore, the polarity of that end of the solenoid will be the opposite of the bar magnets. Apabila magnet bar dialihkan daripada solenoid. Solenoid akan cuba menarik magnet bar. Maka, kutub pada hujung solenoid akan bertentangan dengan kutub magnet bar.

Method 1: Based on direction of current flow observed at the either end of the solenoid

Berdasarkan arah arus mengalir diperhatikan pada hujung solenoid

Method 2: Using the right-hand grip rule/ Menggunakan hukum genggaman tangan kanan

8.3.3 Determining the magnitude of the induced current (Faradays Law) 8.3.3 Menentukan magnitud arus aruhan (Hukum Faraday) Faradays Law: the magnitude of the induced e.m.f. is directly proportional to the rate of change magnetic flux through a coil or alternatively the rate of the magnetic flux being cut. Hukum Faraday: magnitud aruhan d.g.e berkadar terus dengan kadar perubahan flaks magnet melalui gegelung atau kadar flaks magnet dipotong berulang kali. If there is no relative motion between a magnet and a solenoid, there is no electromagnetic induction. Jika tiada pergerakan relatif di antara suatu magnet dan solenoid, tiada aruhan elektromagnet berlaku. To increase the e.m.f. and current/ Untuk meningkatkan d.g.e dan arus:

Increase the relative motion/ Meningkat pergerakan relatif

Increase the number of turns on the coils/ Meningkatkan bilangan lilitan gegelung

Increase the magnetic strength/ Meningkat kekuatan magnet

Increase the cross-section area of the wire/ Meningkatkan luas keratan rentas wayar

Insert a soft iron core in between the coils of the wire Memasukkan teras besi lembut di antara gegelung wayar

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8.3.4 Direct Current & Alternating Current/ Arus Terus & Arus Ulang-alik

Direct current/ Arus Terus Alternating current/ Arus Ulang-alik

Direct current is usually supplied by acid-based batteries or dry cells Arus Ulang-alik biasanya dibekalkan oleh sel kering.

Direct current is current flowing in one fixed direction in a circuit Arus terus adalah arus yang mengalir pada arah yang tetap dalam litar.

Alternating current is generated from a.c. generators, e.g. hydroelectric power generators Arus ulang-alik dihasilkan daripada penjana arus ulang-alik, cth. penjana kuasa hidroelektrik.

Alternating current is current whose magnitude and direction changes periodically with time Arus ulang-alik adalah arus di mana magnitud dan arah berubah mengikut masa.

D.C. Generator A.C. Generator

Factors that affect the magnitude of the induced current in the generators: Faktor yang menyebabkan magnitud arus teraruh pada penjana:

Magnetic field strength Kekuatan medan magnet

Number of windings on the armature Bilangan putaran pada angker

The presence of an iron core in the armature Kehadiran gegelung keluli pada angker

The speed of rotation of the armature Kelajuan putaran angker

Area of the armature Luas angker

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8.3.4.1 Effective voltage/ Voltan berkesan

When two identical light bulbs are connected to a direct current and an alternating current of the

same e.m.f., it is found that the light bulb connected to the d.c. shines with brighter intensity. Apabila dua mentol yang sama disambungkan ke arus terus dan arus ulang-alik dengan d.g.e. yang sama, ia didapati mentol lampu yang disambungkan dengan a.t. lebih terang keamatannya.

To get the light bulb in the a.c. circuit to shine with the same intensity (or the same heating effect), the voltage of the power source has to be increased. Untuk mendapatkan mentol lampu pada litar a.u. dengan keamatan yang sama dengan a.t., sumber kuasa voltan perlu ditingkatkan.

Effective voltage is typically known as root mean square voltage whereas the actual voltage is known as peak voltage.

Voltan berkuat kuasa biasanya dikenali sebagai voltan punca-min-kuasa di mana voltan

sebenar dikenali sebagai voltan puncak.

8.3.5 Operating Principles of Current-measuring Devices 8.3.5 Hukum Operasi Alat Pengukur Arus Moving coil meter/ Gegelung meter bergerak

Built on the principle of electromagnetism Dicipta mengikut hukum keelektromagnetan

When current flows through the coil, the mutual interaction between the magnet and the coil forms a rotating force that turns the coil and hence deflects the indicator. Apabila arus mengalir melalui gegelung, magnet dan gegelung saling berinteraksi untuk membentuk daya putaran yang menukarkan gegelung dan memesongkan penunjuk.

Sensitivity can be increased by/ Meningkatkan kepekaan dengan: Using a stronger magnet

Menggunakan magnet yang lebih kuat Increasing the windings on the coil

Meningkatkan putaran pada gegelung Increasing the area of the coil

Meningkatkan luas gegelung Using a smaller spring constant

Menggunakan suatu spring yang lebih kecil Using a lighter indicator

Menggunakan penunjuk yang lebih ringan

Only for measuring direct current Hanya untuk mengukur arus terus

Vrms = root mean square voltage [V]/ voltan punca-min-kuasa [V] Vpeak = peak voltage [V]/ voltan puncak [V]

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Hot wire meter/ Meter wayar panas

Built on the principle of heating effect of electric current Dicipta mengikut hukum kesan pemanas arus elektrik

When current flows through the wire AB, the wire heats up and expands. Apabila arus mengalir melalui wayar AB, wayar itu akan menjadi panas dan mengembang

This causes the thread to be taut and the pulley turns causing the indicator to deflect Ini menyebabkan benang itu menjadi tegang dan takal itu memesongkan penunjuk.

The rate of heating is not directly proportional to the magnitude of the current, therefore a nonlinear scale is used. Kadar pemanasan adalah tidak berkadar terus dengan magnitud arus, maka skala tak linear digunakan.

For measuring direct current and alternating current Untuk mengukur arus terus dan arus ulang-alik

Moving iron meter/ Meter besi bergerak

Built in the principles of electromagnetism Dicipta mengikut hukum keelektromagnetan

When current flows through the solenoid, the solenoid is magnetized, causing iron rods P and Q to be magnetized with the same polarity. Apabila arus mengalir melalui solenoid, solenoid dimagnetkan, menyebabkan batang besi P dan Q dimagnetkan dengan kutub yang sama.

Therefore both rods repel each other and Q rotates, causing the indicator needle to deflect. Maka kedua-dua batang menolak antara satu sama lain dan Q berputar, menyebabkan jarum penunjuk terpesong.

For measuring direct current and alternating current Untuk mengukur arus terus dan arus ulang-alik

Only can measure large magnitudes of current because small currents are unable to induce a magnetic field because it is strong enough to magnetize the two iron rods Hanya boleh mengukur arus magnitud yang besar kerana arus yang kecil tidak dapat untuk mengaruh suatu medan magnet kerana ia cukup kuat untuk memagnetkan dua rod besi.

8.3.5.1 Modifications to a moving coil meter/ Modifikasi untuk menggerakkan gegelung meter

To an ammeter To a voltmeter

A shunt resistor (very low R)

Added in parallel

A multiplier (very high R)

Added in series

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8.4 Transformer/ Transformer 8.4.1 Basics of a transformer/ Asas transformer

Transformers are used to change the potential difference of an a.c. source. Transformer digunakan untuk mengubah beza keupayaan suatu sumber a.u.

Vp = primary voltage / input voltage [V] Vs = secondary voltage / output voltage [V] Np = number of turns on primary coil Ns = number of turns on secondary coil

8.4.2 Operating principle of a transformer/ Operasi hukum suatu transformer

Input circuit must be connected to a.c. Input litar mesti disambung dengan a.u.

D.c. is uniform in magnitude and has a fixed direction. Therefore the induced e.m.f. is not produced in the secondary coil which depends on change in the magnetic flux. A.t. sentiasa sama dalam magnitud dan mempunyai arah tetap. Maka aruhan d.g.e tidak dihasilkan dalam gegelung sekunder di mana bergantung kepada arah dalam flaks magnet.

A.c. always has changing direction and magnitude. Therefore the direction and magnitude of the induced magnetic field in the primary coil also changes, inducing e.m.f. in the secondary coil. A.u. sentiasa mempunyai arah dan magnitud yang berubah-ubah. Maka arah dan magnitud aruhan medan magnet dalam gegelung primer juga berubah, mengaruh d.g.e dalam gegelung sekunder.

8.4.3 Types of transformers/ Jenis transformer Step-up transformer Transformer menaik

Step-down transformer Transformer menurun

Increase the potential difference Meningkatkan beza keupayaan

Reduce potential difference Mengurangkan beza keupayaan

Number of turns in the secondary coil is greater than in the primary coil Bilangan lilitan dalam gegelung sekunder lebih banyak berbanding gegelung primer

Number of turn in the secondary coil is less than in the primary coil Bilangan lilitan dalam gegelung sekunder lebih sedikit berbanding gegelung primer

Current in primary coil is greater than in secondary coil Arus dalam gegelung primer lebih tinggi berbanding gegelung sekunder

Current in primary coil is less than in secondary coil Arus dalam gegelung primer lebih rendah berbanding gegelung sekunder

Vp = voltan primer / input voltan [V] Vs = voltan sekunder / output voltan [V] Np = bilangan lilitan gegelung primer Ns = bilangan lilitan gegelung sekunder

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8.4.4 Efficiency/ Kecekapan

Sebab P = IV,

Vp = primary voltage / input voltage [V] Vs = secondary voltage / output voltage [V] Ip = current in primary coil [A] Is = current in secondary coil [A]

If the transformer is said to be ideal, the efficiency = 100%. Therefore Jika suatu transformer digelar unggul, kecekapannya = 100%. Maka

8.4.5 Factors that affect the efficiency of a transformer 8.4.5 Faktor yang menyebabkan kecekapan suatu transformer

Factors/ Faktor Methods to increase efficiency Kaedah meningkatkan kecekapan

Heating effect of current in coil Kesan pemanasan arus dalam gegelung Power lost as heat P = I

2R

Kehilangan kuasa sebagai haba P = I2R

Use thicker copper wires of low resistance Guna wayar kuprum tebal untuk rintangan yang rendah.

Use coolant to decrease the temperature of the transformer Guna penyejuk untuk menurunkan suhu transformer.

Heating effect of induced eddy currents* Kesan pemanasan arus pusar teraruh Eddy currents are generated within the iron core Arus pusar dijanakan dengan teras besi.

Use a laminated iron core where each layer is insulated with enamel paint to prevent flow of eddy currents Guna teras besi berlamina di mana setiap lapisan disalut dengan cat untuk menghalang aliran arus pusar.

Magnetization of the iron core Kemagnetan teras besi Energy used in the magnetization and demagnetization of the iron core everytime the current changes its direction is known as hysterisis. This energy is lost as heat which subsequently heats up the iron core. Tenaga digunakan dalam pemagnetan dan penyahmagnetan teras besi pada setiap kali arus berubah arahnya dikenali sebagai

Use a soft iron core that is easily magnetized and demagnetized Menggunakan teras besi lembut dimana mudah dimagnetkan dan dinyahmagnetkan

Flux leakage Kebocoran flaks Some of the induced magnetic flux from the primary coil is not transferred to the secondary coil Sesetengah flaks magnet yang teraruh (yang dihasilkan) daripada teras primer tidak

Secondary coils are intertwined tightly with the primary coils Gegelung sekunder dililit dengan ketat bersama gegelung primer

Iron core should resemble a closed loop Teras besi patut menyerupai gelung tertutup

Vp = voltan primer/ voltan input [V] Vs = voltan skunder/ voltan output [V] Ip = arus dalam gegelung primer [A] Is = arus dalam gegelung skunder [A]

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8.5 Generation and Transmission of Electricity 8.5 Cara-cara menjanakan kuasa elektrik.

8.5.1 Generation of electricity

Energy source

Process Advantages Disadvantages

Hydro Dammed waterL gravitational potential energy kinetic energy (rotates turbine)

Abundant and free

Clean, non-polluting

Water from dams can be used to:

Irrigate crops Flood control Rear fish Recreaction Tourist spot

Ecosystem imbalance

Safety of nearby residents threatened

Natural gas and diesel

Natural gas heats up water steam rotates turbine

Easy to transport High cost

Costly and complicated extraction processes

Byproducts: sulfur dioxide, carbon dioxide

Coal Coal heats up water under high pressure steam rotates turbine

Can be used to generate electricity in themal stations

Easy to transport

Air pollution

Non-renewable

Byproducts: carbon dioxide

Nuclear Fission of uranium heats up water steam rotates turbine

Small amounts used

No unwanted byproducts

Serious environmental implications

Enormous investment and technological input

Requires highly trained personnel

Spend nuclear rods still highly radioactive and indestructible

Biomass Biomass heats up water steam rotates turbine

Renewable

Can substantially reduce wastage of organic material

Storage requires large space

Unpleasant odours

Solar Solar cells convert sunlight to electricity

Free and easily available

Clean, non-polluting

Renewable

Absorption of sunlight inefficient at moment

Requires large are

Expensive equipment

Intensity of sunlight highly dependent on latitude, seasons, weather conditions

Wind Wind rotates turbines Clean, non-polluting

Suitable for isolated regions because of no cabling costs

Construction of wind turbines require large space

Noisy

Radio and microwaves disruption

Efficient generation only at coastal or mountainous regions

Inconsistent wind speed

8.5.2 Transmission of electricity/ Pancaran elektrik

To reduce power lost through transmission, electricity is sent at very high voltage through thick cables of low resistance Untuk mengurangkan kehilangan kuasa melalui pancaran, elektrik dihantar pada voltan yang sangat tinggi melalui kabel tebal dengan rintangan yang rendah.

When voltage , current Apabila voltan , arus

Based on P = I2R, when current , power loss

Berdasarkan daripada P = I2R, apabila arus ,

kuasa hilang .

*Eddy currents: circulating electrical currents that are induced in electrically conductive elements when exposed to changing magnetic fields, creating an opposing force to the magnetic flux

Power station P = IV

Power lost through cables

P = I2R

Current, I

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8.5.3 National electricity grid system/ Elektrik grid sistem nasional

8.6 Electrical Appliances/ Peralatan Elektrik