E&M~~~~

1. If the wires are connected in series, then total resistance = 4R
Else if all of them are connected in parallel, then resultant resistance = R/4
Besides, some other possible combinations for connecting 4 resistances are:
3 parallel plus 1 in series --> resultant resistance = 4R/3
2 parallel in series with another 2 in parallel --> resultant resistance = R

2. First of all, the B-field inside a long solenoid is given by B = unI where
u = permeability, n = no. of turns per unit length and I = current.
所以 constant I 都可以話某程度上令 B = constant 因為 u 同 n 都唔係話改就隨便改到.

3. 如上題, constant I 會令致 constant B, 咁 varying I 當然會令致 varying B.

4. 應該話: single loop 的 area 會在變化中的 B 跟住改變, 其改變幅度會隨著 B 的變率而唔同. 如果個 loop 在一個固定值的 B 下, 因為無 induced current 在其中, 所以線與線之間無因電流而產生的磁力, 所以面積係會不變的.

1. 咁要睇佢地係並聯定係串聯，並聯 = 4/R ，串聯 = 4R