Câu hỏi 7

Ta có : 

\(\left\{ \matrix{ {T_1} = 2\pi \sqrt {{l \over {g'}}} = {{\Delta t} \over 6} \hfill \cr {T_2} = 2\pi \sqrt {{l \over g}} = {{\Delta t} \over 5} \hfill \cr} \right. \Rightarrow {T_1} < {T_2} \Rightarrow g' > g \Rightarrow g' = g + {{\left| q \right|E} \over m} \Rightarrow \overrightarrow P \uparrow \uparrow \overrightarrow {{F_d}} \Rightarrow q < 0\)

\( \Rightarrow {{{T_1}} \over {{T_2}}} = \sqrt {{g \over {g'}}}  = {5 \over 6} \Leftrightarrow {g \over {g + {{\left| q \right|E} \over m}}} = {{25} \over {36}} \Rightarrow \left| q \right| = {{11.g.m} \over {25E}} = {{{{11.10.10.10}^{ - 3}}} \over {{{25.11.10}^4}}} = {4.10^{ - 7}} \Rightarrow q =  - {4.10^{ - 7}}C\) 

Chọn A