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<b>THẬP ĐẠI CAO THỦ ‒ LIVE 4H30 SÁNGSỐ 06</b>

<b>Câu 1: [VNA] </b>Trên một sợi dây đàn hồi căng ngang, dài 120 cm, hai đầu cố định đang có sóng dừng ổn định với phương dao động là phương thẳng đứng. Người ta quan sát thấy chỗ rộng nhất của bụng sóng trên phương dao động có bề rộng là 4a. Biết rằng khoảng cách ngắn nhất giữa hai điểm trên dây dao động cùng pha và có cùng biên độ bằng a là 20 cm. Số bụng sóng trên dây là

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mạch có độ lớn là

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<b>Câu 3: [VNA] </b>Điện năng được truyền từ trạm phát điện A có cơng suất khơng đổi đến nơi tiêu thụ

ở B có lắp một máy hạ áp lí tưởng với tỉ số vòng dây của cuộn sơ cấp và thứ cấp bằng k, dịng điện ln cùng pha với điện áp. Nếu điện áp hiệu dụng ở hai cực của máy phát là U và k = 20 thì cơng

hai cực máy phát là 3U và giá trị của k là

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thị mơ tả động năng (đường liền nét) và thế năng (đường đứt nét) theo thời gian như hình vẽ. Biên độ dao động của vật là 6 cm. Khối lượng của vật là

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<small>W</small>_t<small>, W</small>_đ<small> (mJ)</small>

<small>t (s)O</small>

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<b>Câu 5: [VNA] </b>Đặt điện áp <i>u U 2 cos ωt</i>=

(

+<i>φ</i>

)

(U không đổi, ω thay đổi được) vào hai đầu đoạn mạch AB gồm điện trở thuần R, cuộn dây có độ

tự cảm L, điện trở trong r và tụ điện có điện dung C mắc nối tiếp như hình vẽ. Khi ω = 90π rad/s thì hệ số công suất của đoạn mạch AM là 0,8; của đoạn mạch AB là 0,6 và cường độ dòng điện sớm pha

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<small>MA</small>

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<b>Câu 6: [VNA] </b>Hạt nhân <i>²¹⁰₈₄Po phóng xạ α thành hạt nhân ₈₂²⁰⁶Pb . Ban đầu, mẫu chất chứa ²¹⁰₈₄Po </i>

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<b>Câu 7: [VNA] </b>Ba nguồn sáng phát ra ba bức xạ đơn sắc có bước sóng là 400 nm, λ<small>1</small> và λ<small>2</small>, với

  <i>₁₂</i>

<i>390 nm λλ760 nm</i>được sử dụng trong thí nghiệm Y-âng về giao thoa ánh sáng. Chiếu sáng hai khe hẹp đồng thời bằng bất kì hai trong ba bức xạ trên thì trên màn quan sát đều thấy: điểm O là vị trí vân sáng trung tâm, tại điểm N luôn là vị trí trùng nhau gần O nhất của hai vân sáng. Nếu chiếu sáng hai khe đồng thời bằng cả ba bức xạ trên thì trong khoảng ON (khơng kể O và N) có 24

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<b>Câu 8: [VNA] </b>Một con lắc lị xo gồm một lị xo nhẹ có độ cứng k = 100 N/m, đầu trên gắn với vật

trị nào sau đây?

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<b>Câu 9: [VNA] </b>Đặt điện áp <i>u U 2 cos</i>=

( )

<i>ωt</i> , (U không đổi, ω thay đổi được) vào hai đầu đoạn mạch AB mắc nối tiếp gồm điện trở thuần R, cuộn cảm có độ tự cảm L, điện trở trong r và tụ điện có điện dung

thời gian như hình bên. Khi đó, vơn kế chỉ 234 V, cơng suất tiêu thụ

nào sau đây?

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<small>0u (V)</small>

<small>tN</small>

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<b>Câu 10: [VNA] </b>Hai nguồn phát sóng kết hợp tại A, B trên mặt nước cách nhau 12 cm phát ra hai dao động điều hòa cùng tần số 20 Hz, cùng biên độ và cùng pha ban đầu. Xét điểm M trên mặt nước cách A, B những đoạn lần lượt là 4,2 cm và 9 cm. Tốc độ truyền sóng trên mặt nước là 32 cm/s. Muốn M là một điểm dao động với biên độ cực tiểu thì phải dịch chuyển nguồn tại B dọc đường nối A, B từ vị trí ban đầu ra xa nguồn A một đoạn nhỏ nhất là

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