Gaya pada konduktor pembawa arus
| English | Bahasa Indonesia |
|---|---|
| magnetic flux density/mæɡˈnetɪk flʌks ˈdensɪti/ | densitas fluks magnet |
| Fleming's left-hand rule/ˈflemɪŋz left hænd ruːl/ | aturan tangan kiri Fleming |
| tesla/ˈteslə/ | tesla |
The definition hiding inside a motor
- A wire carrying a current across a magnetic field jumps sideways. Every electric motor ever built is that jump, arranged to keep repeating.
- Look at the force closely and something else falls out of it. Nobody can measure a magnetic field directly, but this force can be measured, and it is proportional to $B$.
- So the motor effect is not only an application; it is how the strength of a magnetic field is defined.
- This lesson is $F = BIL\sin\theta$, the definition of magnetic flux density 磁通密度, and Fleming's left-hand rule 弗莱明左手定则.
The force on a current-carrying wire
- A wire of length $L$ carrying current $I$ at angle $\theta$ to a field of flux density $B$ feels a force:
- The force is largest at right angles, where $\sin\theta = 1$ and $F = BIL$, and zero when the wire lies along the field, where $\sin\theta = 0$.
- The force is at right angles to both the current and the field, which is why a motor turns rather than pulling itself apart.
Gaya pada kawat pembawa arus dalam medan magnet adalah:
Terbesar ketika kawat tegak lurus terhadap medan ($\sin 90^{\circ} = 1$, sehingga $F = BIL$).
Gaya pada kawat nol ketika sejajar dengan medan.
Dengan kawat sejajar medan, $\theta = 0$ dan $\sin\theta = 0$, sehingga tidak ada gaya.
Kawat $0.10\ \text{m}$ membawa arus $2.0\ \text{A}$ tegak lurus terhadap medan $0.50\ \text{T}$. Berapakah gayanya?
$F = BIL = 0.50 \times 2.0 \times 0.10 = 0.10\ \text{N}$.
Magnetic flux density
- Rearranged for a wire at right angles, the same equation defines $B$:
- Magnetic flux density is the force per unit current per unit length on a wire at right angles to the field.
- Its unit is the tesla 特斯拉: $1\ \text{T} = 1\ \text{N/(A m)}$. One tesla is a strong field; the Earth's is about 50 microtesla.
Kepadatan fluks magnet diukur dalam ____.
$1\ \text{T} = 1\ \dfrac{\text{N}}{\text{A}\cdot\text{m}}$ — gaya per satuan arus per satuan panjang.
Apa itu kepadatan fluks magnet?
B = F/(IL) untuk kawat tegak lurus, dalam tesla. Definisi ini berasal langsung dari gaya efek motor, yang merupakan cara B diukur.
Fleming's left-hand rule
- Hold the thumb and first two fingers of your left hand mutually at right angles.
- First finger is the Field, seCond finger is the Current, thuMb is the force, or Motion.
- Left hand for the motor effect, where a current produces motion. The right hand is for the generator effect, where motion produces a current, and mixing them up reverses every answer.
Three quantities, three directions, all mutually perpendicular
Rasakan gaya pada kawat
Arus dalam medan magnet mengalami gaya F = BIL tegak lurus terhadap keduanya — balikkan arus atau balikkan magnet, dan gaya akan berbalik arah.
Dalam aturan tangan kiri Fleming, ibu jari menunjukkan:
Jari pertama = Medan, jari kedua = Arus, ibu jari = gaya/Pergerakan.
Pasangkan setiap jari aturan tangan kiri Fleming dengan apa yang diwakilinya.
First-Medan, seCond-Arus, thuMb-Pergerakan. Tangan kiri untuk efek motor; tangan kanan untuk efek generator.
Worked example: force on a wire
- A wire of length $0.10\ \text{m}$ carries $2.0\ \text{A}$ at right angles to a field of flux density $0.25\ \text{T}$. Find the force, and then the force if the wire is turned to $30°$ to the field.
- At right angles: $F = BIL = (0.25)(2.0)(0.10) = 0.050\ \text{N}$.
- At $30°$: $F = BIL\sin 30° = 0.050 \times 0.5 = 0.025\ \text{N}$, half as much.
- If the wire were turned parallel to the field the force would be zero. The angle is measured between the wire and the field, not between the wire and anything else.
Sebuah kawat 0.10 m dialiri arus 2.0 A pada sudut 30 derajat terhadap medan magnet 0.25 T. Berapakah gaya yang bekerja padanya, dalam satuan newton?
F = BIL sin30 = 0.25 x 2.0 x 0.10 x 0.5 = 0.025 N, setengah dari nilai tegak lurus. Jika sejajar dengan medan, gayanya akan nol.
Two parallel currents
- Each wire sits in the other's field, so each feels a force. Apply the left-hand rule twice and the result is symmetric.
- Currents in the same direction attract. Currents in opposite directions repel.
- This is worth remembering because it is the reverse of what charges do: like charges repel, but like currents attract.
Dua kawat sejajar yang membawa arus searah saling tarik-menarik.
Setiap kawat berada dalam medan kawat lain; menerapkan aturan tangan kiri pada masing-masing menghasilkan tarikan. Ini kebalikan dari muatan sejenis yang tolak-menolak.
Worked example: measuring a field
- A stiff wire on a top-pan balance carries a current through a magnetic field. Explain how this measures $B$.
- The wire is placed at right angles to the field, so the force is $F = BIL$.
- The magnet sits on the balance, and by Newton's third law the force on the magnet is equal and opposite to the force on the wire, so the balance reading changes by $F/g$ in kilograms.
- Then $B = F/(IL)$, using the measured force, the current from an ammeter and the length of wire actually in the field.
- The last detail is the one that catches people: $L$ is the length inside the field, not the whole wire.
Urutkan langkah-langkah mengukur kepadatan fluks magnet dengan neraca arus.
Hukum ketiga Newton membuat neraca merasakan reaksi terhadap gaya pada kawat. L adalah panjang di dalam medan, bukan seluruh kawat.
Marks that slip away
- $\theta$ is the angle between the wire and the field. The force is zero when they are parallel, not maximum.
- $L$ is the length of wire in the field.
- Left hand for the motor effect, right hand for the generator effect. Say which you are using.
- Parallel currents in the same direction attract, which is the opposite of like charges.
You've got it
- $F = BIL\sin\theta$: maximum at right angles, zero along the field, and the force is perpendicular to both current and field
- rearranged, it defines magnetic flux density: the force per unit current per unit length on a wire at right angles, in tesla
- Fleming's left-hand rule: First finger Field, seCond finger Current, thuMb Motion, for the motor effect
- parallel currents attract when in the same direction and repel when opposite