Hướng dẫn giải Bài 8 (Trang 10 SGK Vật lý 11)

Hai quả cầu nhỏ mang hai điện tích có độ lớn bằng nhau, đặt cách nhau <span id="MathJax-Element-1-Frame" class="MathJax" style="box-sizing: border-box; display: inline; font-style: normal; font-weight: normal; line-height: normal; font-size: 16px; text-indent: 0px; text-align: left; text-transform: none; letter-spacing: normal; word-spacing: normal; overflow-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; padding: 0px; margin: 0px; position: relative;" tabindex="0" role="presentation" data-mathml="<math xmlns="http://www.w3.org/1998/Math/MathML"><mn>10</mn><mi>c</mi><mi>m</mi></math>">10cm trong chân không thì tác dụng lên nhau một lực là <math xmlns="http://www.w3.org/1998/Math/MathML"><mn>9</mn><mo>.</mo><msup><mn>10</mn><mrow><mo>-</mo><mn>3</mn></mrow></msup><mi>N</mi></math>. Xác định điện tích của hai quả cầu đó. Giải <math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>q</mi><mn>1</mn></msub><mo>=</mo><msub><mi>q</mi><mn>2</mn></msub><mo>=</mo><mi>q</mi><mo>;</mo><mo> </mo><mo> </mo><mo> </mo><mo> </mo><mo> </mo><mo> </mo><mo> </mo><mo> </mo><mi>r</mi><mo>=</mo><mn>10</mn><mo> </mo><mi>c</mi><mi>m</mi><mo>=</mo><msup><mn>10</mn><mrow><mo>-</mo><mn>1</mn></mrow></msup><mi>m</mi><mo>;</mo><mo> </mo><mo> </mo><mo> </mo><mo> </mo><mo> </mo><mo> </mo><mo> </mo><mo> </mo><mo> </mo><mi>F</mi><mo>=</mo><mn>9</mn><mo>.</mo><msup><mn>10</mn><mrow><mo>-</mo><mn>3</mn></mrow></msup><mi>N</mi><mo>;</mo><mo> </mo><mi>ε</mi><mo>=</mo><mn>1</mn><mo>.</mo></math>                            <math xmlns="http://www.w3.org/1998/Math/MathML"><mi>F</mi><mo>=</mo><mi>k</mi><mo>.</mo><mfrac><mfenced open="|" close="|"><mrow><msub><mi>q</mi><mn>1</mn></msub><mo> </mo><msub><mi>q</mi><mn>2</mn></msub></mrow></mfenced><mrow><mi>ε</mi><msup><mi>r</mi><mn>2</mn></msup></mrow></mfrac><mo>=</mo><mi>k</mi><mo>.</mo><mfrac><msup><mi>q</mi><mn>2</mn></msup><mrow><mi>ε</mi><msup><mi>r</mi><mn>2</mn></msup></mrow></mfrac></math>                                        <math xmlns="http://www.w3.org/1998/Math/MathML"><mo>⇒</mo></math><math xmlns="http://www.w3.org/1998/Math/MathML"><msup><mi>q</mi><mn>2</mn></msup><mo>=</mo><mfrac><mrow><mi>F</mi><mo>.</mo><mi>ε</mi><mo> </mo><msup><mi>r</mi><mn>2</mn></msup></mrow><mi>k</mi></mfrac><mo>=</mo><mfrac><mrow><mn>9</mn><mo>.</mo><msup><mn>10</mn><mrow><mo>-</mo><mn>3</mn></mrow></msup><mo>.</mo><mn>1</mn><mo>.</mo><msup><mrow><mo>(</mo><msup><mn>10</mn><mrow><mo>-</mo><mn>1</mn></mrow></msup><mo>)</mo></mrow><mn>2</mn></msup></mrow><mrow><mn>9</mn><mo>.</mo><msup><mn>10</mn><mn>9</mn></msup></mrow></mfrac><mo>=</mo><msup><mn>10</mn><mrow><mo>-</mo><mn>14</mn></mrow></msup></math>                         <math xmlns="http://www.w3.org/1998/Math/MathML"><mo>⇒</mo><mi>q</mi><mo>=</mo><mo>±</mo><msup><mn>10</mn><mrow><mo>-</mo><mn>7</mn></mrow></msup><mfenced><mi>C</mi></mfenced></math>.