Fields, motors and induction
Đã giới thiệu| English | Ý nghĩa |
|---|---|
| induction/ɪnˈdʌkʃn/ | hiện tượng cảm ứng |
| transformer/trænsˈfɔːmə/ | máy biến áp |
What would explain this observation?
- A magnet beside a wire does not always produce a current. An induced electromotive force depends on changing magnetic flux linkage.
Build the model
- A field describes a force effect at positions in space.
- A current-carrying conductor in a magnetic field can experience a force.
- Electromagnetic induction 电磁感应 occurs when magnetic flux linkage changes.
- induction: Creation of emf by changing flux linkage.
- transformer 变压器: A device transferring energy between coils through changing flux.
Which gives a continuing emf in a coil?
Độ khó 1/5 · 100 xu + một phần thưởng nhỏ cho lần hoàn thành đầu tiên trong ngày
Electromagnetic induction needs CHANGE: an emf is induced only while the flux linkage through the coil is changing. A constant flux, however large, induces nothing, and a stationary magnet beside a stationary coil changes no flux.
Match each technical term to its precise meaning.
Độ khó 2/5 · 140 xu + một phần thưởng nhỏ cho lần hoàn thành đầu tiên trong ngày
Use the definitions to distinguish related quantities and processes.
Choose evidence that can test it
- Changing field strength, coil area, orientation or relative motion can change flux linkage.
- Lenz law describes an induced effect opposing the change producing it, consistent with energy conservation.
In the lab: Use a coil and sensitive meter to compare magnet motion in each direction. Record that a stationary arrangement gives no induced signal. Use approved low-voltage supplies for motor demonstrations.
Which two habits make the investigation or model in this case more defensible?
Độ khó 2/5 · 140 xu + một phần thưởng nhỏ cho lần hoàn thành đầu tiên trong ngày
Stating the conditions and assumptions lets others repeat the work exactly, and recording the supporting evidence lets them check the conclusion. Hiding disagreeing results removes the evidence that could challenge a wrong conclusion, and one measurement can never establish a wider claim.
Work from known quantities
- Known: a transformer has 200 primary turns, 50 secondary turns and 12 V primary voltage.
- For an ideal transformer, Vs/Vp = Ns/Np.
- Vs = Vp Ns/Np = 12×50/200 = 3.0 V.
- Real transformers also dissipate energy.
Your turn: An ideal transformer has Np=400, Ns=100 and Vp=20 V. Find Vs.
An ideal transformer has Np=400, Ns=100 and Vp=20 V. Find Vs.
Độ khó 2/5 · 140 xu + một phần thưởng nhỏ cho lần hoàn thành đầu tiên trong ngày
The result is 5 V. Known: Np = 400, Ns = 100, Vp = 20 V, ideal transformer. Use Vs/Vp = Ns/Np. Rearranging: Vs = Vp x Ns/Np = 20 x 100/400 = 5 V. Fewer secondary turns step the voltage down.
Check the conclusion and its limits
- A transformer needs changing magnetic flux; a steady DC input does not provide continuous transformer action.
- Magnetic field direction is not automatically the direction of force.
Not true: A steady DC input produces continuous ideal transformer action. A transformer works by electromagnetic induction, which requires changing flux. Steady DC produces constant flux, so after the brief switch-on change there is no induced emf and no continuous transformer action.
- A field describes a force effect at positions in space.
- Changing field strength, coil area, orientation or relative motion can change flux linkage.
- Watch for: A transformer needs changing magnetic flux; a steady DC input does not provide continuous transformer action.
Write from a secondary voltage of 8 V
- An ideal transformer has 250 primary turns, 100 secondary turns and a primary voltage of 20 V.
- Find the secondary voltage. A steady DC input does not keep an ideal transformer working.
A steady DC input produces continuous ideal transformer action.
Độ khó 1/5 · 100 xu + một phần thưởng nhỏ cho lần hoàn thành đầu tiên trong ngày
A transformer works by electromagnetic induction, which requires changing flux. Steady DC produces constant flux, so after the brief switch-on change there is no induced emf and no continuous transformer action.
Creation of emf by changing flux linkage: write the technical term.
Độ khó 1/5 · 100 xu + một phần thưởng nhỏ cho lần hoàn thành đầu tiên trong ngày
induction means Creation of emf by changing flux linkage.
An ideal transformer has 250 primary turns, 100 secondary turns and a primary voltage of 20 V. What is the secondary voltage, and why does a steady DC input not produce continuous transformer action?
For an ideal transformer the voltage ratio equals the turns ratio. Induction needs a changing magnetic flux.
What is the secondary voltage?
Độ khó 1/5 · 100 xu + một phần thưởng nhỏ cho lần hoàn thành đầu tiên trong ngày
20 times 100 divided by 250 is 8 V. Other options — 5 V (the lesson example's result for 400 primary turns, 100 secondary turns and 20 V); 20 V (the primary voltage).