2020 DSE Physics 物理討論

5) 你諗嘅system太複雜，唔太明你想要嘅Ans喺咩
不過，consider a ball moving in space, where background temperate is lower than the temperature of ball. The ball keeps losing energy by emitting radiation.
However, if you are considering the conservation of momentum, the velocity of the ball is constant (unless you would like to consider sth like laser cooling, the emitted photon has also momentum).

Molecular KE (part of internal energy) is definitely decreasing in this case.

KE of the ball stays constant (at least in DSE syllabus)

I am not sure if this is the answer you want.
OR someone else can help

DSE 黎講
電(流)生磁，磁生電
Magnet有magnetic property，都係因為electron.

大學黎講
你當有一堆charges moving along a wire with constant velocity.
有current => 有B
However, if you are in the reference frame of the charges (simply speaking , moving tgt with the charges),
The charges are at rest then
only electric field exists

All objects with temperature > 0 K will emit radiation

Radio個課要點溫好虛

[member]


簡單嚟講，因為g 同EM force obeys inverse square law (同1/r^2 成正比）所以去到好遠都仲有。

以下係dse out syl

quantum field theory 講粒子間的力係由於gauge bosons 嘅交換。

EM force 係因為交換Photons (massless spin 1),
gravitational force 係因為交換graviton (搵到話我知，會攞nobel prize),

而nuclear force 交換pions, 其中一款 mesons (spin 0),
而weak force 交換bosons, Z0, W+, W-

學校老師未必顧到咁多學生(你諗吓一班十幾廿個同學 個個程度都唔同)
而且教學時間亦相當有限
未必個個老師肯教/學生主動去學 so called “Out-Syl” 內容
(宜家仲停埋課Tim 雖然話就話可以Online Teaching (& Learning) 但始終有一定既限制 )
但其實只要轉個問法 就可以用 “In-Syl”內容 去解答”Out-Syl”既問題

唔係DSE嘢