phy呀><

小霞的fans · 發表於 9/1/2010 01:19 PM

1.
有1個solenoid .. N POLE係左邊,, S POLE係右邊..
口代表指南針..求指南針磁針北極的指向。
(箭頭表示指南針磁汁的北極)

N-----口------S

magnetic field係由N去S,,所以指南針應該跟住呢個方向,,姐係指向右,,
但係點解係指向左既,,咁咪姐係magnetic field由S去N !?不解..

2.magnetic force同resultant magnetic force有分別!?
例如:a rectangular coil ispulled to the rightwith uniform speed in a uniform magnetic field pointing into the paper.which of the following descriptions about the current induced in the coil and the magnetic force acting on the coil is correct?(the coil is leaving the magnetic field)

點解D.a current is induced in the coil and a resultant magnetic force is acting on the coil to the right 唔岩..
舊野比人pull去right..佢既magnetic force咪係右囉!?唔係咩..

超夢夢的夢境 · 發表於 9/1/2010 10:05 PM

本帖最後由超夢夢的夢境於 9/1/2010 10:08 PM 編輯

1. Geographical North(South) Pole of the Earth is approximately the S(respectively N) pole as a magnet

2.As long as the whole coil stay in the uniform magnetic field, neither Faraday's Law nor Lenz's Law applies. There is no change in the magnetic field and thus no current is induced.
Also, you need to notice that pulling doesn't always means there is a net force, and in this case the force is balanced.
And you need to distinguish between force applied and magnetic force which is exerted due to the magnetic field.

小霞的fans · 發表於 10/1/2010 01:08 PM

1. Geographical North(South) Pole of the Earth is approximately the S(respectively N) pole as a magnet

2.As long as the whole coil stay in the uniform magnetic field, neither Faraday's Law nor Lenz ...
超夢夢的夢境發表於 9/1/2010 02:05 AM

登錄/註冊後可看大圖

但係呀..好似第1題咁,,
我係書度缺既exercise個指南針都係由N指去S..
幾時指調轉!?分唔到..
至於第2題呢..個COIL入面既右邊係空白,左邊係POINTING IN to paper 既magnetic field..
佢係比一個向右既力拉,,但係點解resultant magnetic force係向左既!?答案係向左= =好不明..咁拉佢果個向右既力唔係resultant magnetic force,,係咩黎- - ?

simon199392 · 發表於 10/1/2010 06:31 PM

本帖最後由 simon199392 於 10/1/2010 06:39 PM 編輯

1.
╭--->------>----- ╮
╰N--<--口---<--S ╯
所以個指南針指向左, 請注意magnectiv field 的方向
並不是
N-->--口--->--S

2.
向right ge力係有act on the coil令佢產生induced current 繼而產生一個oppose你 ge force(ie. magnetic force) by Lenz Law, 即然要oppose你向right ge力, 當然produced出黎ge要向左.

小霞的fans · 發表於 10/1/2010 09:05 PM

1.
╭--->------>----- ╮
╰N----S

2.
向right ge力係有act on the coil令佢產生induced current 繼而產生一個oppose你 ge force(ie. magnetic force) by Lenz Law, 即然要oppose你向right ge力, 當然produc ...
simon199392 發表於 9/1/2010 10:31 PM

登錄/註冊後可看大圖

第2題我明左,,thx^^~.第1題唔明
其實係唔係同"箭頭表示指南針磁汁的北極"呢句野有關!?
我始終唔明點解唔係NS/-...

simon199392 · 發表於 11/1/2010 05:02 PM

本帖最後由 simon199392 於 11/1/2010 05:03 PM 編輯

第2題我明左,,thx^^~.第1題唔明
其實係唔係同"箭頭表示指南針磁汁的北極"呢句野有關!?
我始終唔明點解唔係NS/-...
小霞的fans 發表於 10/1/2010 09:05 PM

首先, 針南針係會受外來磁場影響, 本身只受地球磁場影響, 因外來磁場近/勁過地球磁場, 所以指針方向有唔同. (以上明?

下面比4個cases 你
Case1:
╭----->---->---->----╮
╰N---<--[<--]---<---S╯
指針指向solenoid的N, 方向是向左
Case2:
╭-----<----<----<----╮
╰S--->--[-->]--->---N╯
指針還是指向solenoid的N, 不過這次方向是向右
Case3:
N---╮
| |
︹ |
^ \/
︸ |
| |
S---╯
指針還是指向solenoid的N, 不過這次方向是向上
Case4:
S---╮
| |
︹ |
\/ ^
︸ |
| |
N---╯
指針還是指向solenoid的N, 不過這次方向是向下

其實是可以歸納為「正常情況下, 若箭頭表示指南針磁汁的北極, 箭頭就是指向北極」總之你要知個magnetic field係咩樣(N繞到S, 不是N直去S), 支針係跟住個magentic field走.

注意, solenoid的N=\=地球的N,可能solenoid的N正正是地球的S/E/W.
不過有exception, 就是你solenoid的N同地球的N一樣方向.

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