Điện áp xoay chiều u = 220 cos100t (V) có giá trị hiệu dụng là: 

Một khung dây dẫn quay đều xung quanh một từ trường đều có cảm ứng từ vuông góc với trục quay của khung. Từ thông qua khung biến đổi theo phương trình Φ = Φ₀cosωt Wb. Suất điện động cực đại trong khung được tính bằng công thức:

Dòng điện xoay chiều có cường độ i = 2cos5πt A. Dòng điện này có cường độ hiệu dụng bằng :

Cho dòng điện có cường độ  (i tính bằng A, t tính bằng giây) chạy qua một đoạn mạch xoay chiều. Chọn kết luận đúng.         

Chọn phát biểu đúng:

Suất điện động cảm ứng do một máy phát điện xoay chiều một pha tạo ra có biểu thức  (t tính bằng giây). Tần số góc của đoạn mạch là:

Một dòng điện xoay chiều có cường độ A. Chọn phát biểu sai?

Đặt điện áp xoay chiều vào hai đầu đoạn mạch R,L,C nối tiếp thì dòng điện qua đoạn mạch có biểu thức . Độ lệch pha của điện áp u so với dòng điện i bằng:

Cho đoạn mạch điện xoay chiều gồm tụ điện và cuộn cảm thuần mắc nối tiếp. Gọi u, i lần lượt là điện áp tức thời giữa hai đầu mạch và cường độ dòng điện tức thời chạy qua mạch. U0, U là điện áp cực đại và điện áp hiệu dụng giữa hai đầu mạch. I₀, I là giá trị cực đại và giá trị hiệu dụng của cường độ dòng điện chạy qua mạch. Biểu thức liên hệ nào dưới đây không đúng?

Một đèn nêon được đặt dưới hiệu điện thế xoay chiều có dạng u = 100sin100πt (V). Đèn sẽ tắt nếu hiệu điện thế tức thời đặt vào đèn có giá trị nhỏ hơn hoặc bằng 50V. Khoảng thời gian đèn tắt trong mỗi nửa chu kỳ của dòng điện xoay chiều là bao nhiêu?  

Một khung dây dẫn phẳng dẹt, hình chữ nhật có 500 vòng, diện tích mỗi vòng là 220 cm2. Chọn khung quay đều với tốc độ là 50 vòng/s quanh một trục đối xứng nằm trong mặt phẳng khung. Hệ thống đặt trong từ trường đều có vectơ cảm ứng từ vuông góc với trục quay và có độ lớn . Suất điện động xuất hiện trong khung dây có giá trị cực đại bằng:

Khi dùng đồng hồ đa năng hiện số có một núm xoay để đo điện áp xoay chiều, ta đặt núm xoay ở vị trí:        

Đặt điện áp  (u tính bằng V, t tính bằng s) vào hai đầu đoạn mạch AB. Biết ở thời điểm t, điện áp tức thời giữa hai đầu AB có giá trị 200 V, ở thời điểm , cường độ dòng điện tức thời qua đoạn mạch bằng không và đang giảm. Điện áp u và cường độ dòng điện trong mạch lệch nhau một góc là:

Cường độ dòng điện i = 2cos100πt A có giá trị cực đại là:  

Một bóng đèn neon được mắc vào nguồn xoay chiều có điện áp u = 220cos(100πt) V. Đèn chỉ bật sáng khi điện áp đặt vào đèn vượt quá giá trị 100 V. Trong 1 giây đèn này bật sáng bao nhiêu lần?

Cường độ dòng điện tức thời chạy qua một đoạn mạch xoay chiều là i = 2cos(100πt – π/2)A, t đo bằng giây. Tại thời điểm t₁ nào đó, dòng điện đang giảm và có cường độ bằng 1A. Đến thời điểm t = t₁ + 0,005s, cường độ dòng điện bằng:

Một khung dây dẫn, phẳng dẹt có 200 vòng, mỗi vòng có diện tích 600 (cm²). Khung dây quay đều quanh trục nằm trong mặt phẳng khung, trong một từ trường đều có vecto cảm ứng từ vuông góc với trục quay và có độ lớn là . Suất điện động e trong khung có tần số 50 (Hz). Chọn gốc thời gian là lúc pháp tuyến của mặt phẳng khung dây cùng hướng với vecto cảm ứng từ. Biểu thức tức thời của e là:

Cho biểu thức của cường độ dòng điện xoay chiều là i = I₀ cos (ωt + φ) A. Cường độ hiệu dụng của dòng điện xoay chiều đó là:

Một dòng điện xoay chiều đi qua điện trở R = 25 Ω trong thời gian t = 120 s thì nhiệt lượng tỏa ra trên điện trở là Q = 6000 J. Cường độ hiệu dụng của dòng điện xoay chiều là:

Một đoạn mạch gồm điện trở thuần R = 100Ω, tụ điện C, cuộn cảm thuần L mắc nối tiếp. Đặt vào hai đầu đoạn mạch một điện áp xoay chiều u = U₀cos314t thì U_R = U_L = U_C và mạch tiêu thụ công suất 200 W. Thay tụ C bằng tụ C’ thì công suất tiêu thụ của mạch bằng 50 W. Giá trị C’ bằng:

Điện áp xoay chiều u = 220cos100πt(V) có giá trị hiệu dụng là ?          

Dòng điện chạy qua một đoạn mạch có biểu thức . Trong khoảng thời gian từ 0 đến 0,01s cường độ dòng điện tức thời có giá trị bằng vào những thời điểm.  

Một khung dây dẫn phẳng quay đều với tốc độ góc quanh một trục cố định nằm trong mặt phẳng khung dây, trong một từ trường đều có vectơ cảm ứng từ vuông góc với trục quay của khung. Suất điện động cảm ứng trong khung có biểu thức . Tại thời điểm t =0, vectơ pháp tuyến của mặt phẳng khung dây hợp với vecto cảm ứng từ một góc bằng:             

Dòng điện xoay chiều chạy qua một đoạn mạch có biểu thức , t tính bằng giây (s). Vào một thời điểm nào đó, dòng điện đang có cường độ tức thời bằng  thì sau đó ít nhất là bao lâu để dòng điện có cường độ thức thời bằng  ?

Suất điện động cảm ứng do một máy phát điện xoay chiều một pha tạo ra có biểu thức  (t tính bằng giây). Tần số góc của đoạn mạch là:

Dòng điện chạy qua một đoạn mạch có cường độ A (T > 0). Đại lượng T được gọi là:

Một dòng điện xoay chiều có biểu thức i = 4 cos (100pt + ) (A). Chọn phát biểu đúng ? 

Tác dụng của cuộn cảm đối với dòng xoay chiều là:

Một khung dây quay đều quanh trục D trong một từ trường đều có véc tơ cảm ứng từ vuông góc với trục quay. Biết tốc độ quay của khung là 150 vòng/phút. Từ thông cực đại gửi qua khung là F0 = 10/π (Wb). Suất điện động hiệu dụng trong khung có giá trị là :        

Một đoạn mạch xoay chiều gồm điện trở thuần mắc nối tiếp với cuộn cảm thuần. Khi trong mạch có dòng xoay chiều thì điện áp hiệu dụng hai đầu cuộn cảm bằng một nửa điện áp hiệu dụng hai đầu đoạn mạch. Hệ số công suất của mạch bằng bao nhiêu?

Điện áp xoay chiều u = 220 cos100t (V) có giá trị hiệu dụng là: 

Một dòng điện chạy trong một đoạn mạch có cường độ . Đại lượng f gọi là:

Một chiếc đèn nêôn đặt dưới một điện áp xoay chiều 119V – 50Hz. Nó chỉ sáng lên khi điện áp tức thời giữa hai đầu bóng đèn lớn hơn 84V. Thời gian bóng đèn sáng trong một chu kỳ là bao nhiêu?         

Scientists have discovered the bones of what may be the largest meat-eating dinosaur ever to walk the earth. The discovery was made by a team of researchers from Argentina and North America in Patagonia, a desert on the eastern slopes of the Andes in South America. Besides the interesting fact that the dinosaur was huge and horrifying, it is even more astounding that the bones of a number of the dinosaurs were found together. This discovery challenges the prior theory that the biggest meat-eaters lived as loners and instead indicates that they may have lived and hunted in packs. The Tyrannosaurus Rex lived in North America and was believed to hunt and live alone. The newly discovered meat-eater appears to be related to the Giganotosaurus family, being as closely related to it as a fox would be to a dog. It is actually not of the same family at all as the Tyrannosaurus Rex, being as different from it as a cat is from a dog. The fossilized remains indicate that the animals lived about 100 million years ago. With needle-shaped noses and razor sharp teeth, they were larger than the Tyrannosaurus Rex, although their legs were slightly shorter, and their jaws were designed to be better able to dissect their prey quickly and precisely.

Question 36: The author states that the newly discovered dinosaur remains are evidence that it was the author         

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Under certain circumstances, the human body must cope with gases at greater-than-normal atmospheric pressure. For example, gas pressures increase rapidly during a drive made with scuba gear because the breathing equipment allows divers to stay underwater longer and dive deeper. The pressure exerted on the human body increases by 1 atmosphere for every 10 meters of depth on seater, so thay at 39 meters in seawater a diver is exposed to pressure of about 4 atmosphere. The pressure of the gases being breathed must equal the external pressure applied to the body, otherwise breathing is very difficult. Therefore all of the gases in the air breathed by a scuba diver at 40 meter are present at five times their usual pressure. Nitrogen, which composes 80 percent of the air we breathe, usually causes a balmy feeling of well-being at this pressure. At a depth of 5 atmosphere, nitrogen causes symptoms resembling alcohol intoxication, known as nitrogen narcosis. Nitrogen narcosis apparently results from a direct effect on the brain of the large amounts of nitrogen dissolved in the blood. Deep dives are less dangerous if helium is substituted for nitrogen, because under these pressures helium does not exert a similar narcotic effect. As a scuba diver descends, the pressure of nitrogen on the lungs increases. Nitrogen then diffuses from the lungs to the blood, and from the blood to body tissues. The reverse occurs when the diver surfaces, the nitrogen pressure in the lungs falls and the nitrogen diffuses from the tissues into the blood, and from the blood into the lungs. If the return to the surface is too rapid, nitrogen in the tissues and blood cannot diffuse out rapidly enough and nitrogen bubbles are formed. They can cause severe pains, particularly around the joints. Another complication may result if the breath is held during ascent. During ascent from a depth of 10 meters, the volume of air in the lungs will double because the air pressure at the surface is only half of what it was at 10 meters. This change in volume may cause the lungs to distend ang even rupture. This the rise of the exhaled air bubbles, and must exhale during ascent.

Question: The word “They” in bold on paragraph 2 refers to ________.         

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Under certain circumstances, the human body must cope with gases at greater-than-normal atmospheric pressure. For example, gas pressures increase rapidly during a drive made with scuba gear because the breathing equipment allows divers to stay underwater longer and dive deeper. The pressure exerted on the human body increases by 1 atmosphere for every 10 meters of depth on seater, so thay at 39 meters in seawater a diver is exposed to pressure of about 4 atmosphere. The pressure of the gases being breathed must equal the external pressure applied to the body, otherwise breathing is very difficult. Therefore all of the gases in the air breathed by a scuba diver at 40 meter are present at five times their usual pressure. Nitrogen, which composes 80 percent of the air we breathe, usually causes a balmy feeling of well-being at this pressure. At a depth of 5 atmosphere, nitrogen causes symptoms resembling alcohol intoxication, known as nitrogen narcosis. Nitrogen narcosis apparently results from a direct effect on the brain of the large amounts of nitrogen dissolved in the blood. Deep dives are less dangerous if helium is substituted for nitrogen, because under these pressures helium does not exert a similar narcotic effect. As a scuba diver descends, the pressure of nitrogen on the lungs increases. Nitrogen then diffuses from the lungs to the blood, and from the blood to body tissues. The reverse occurs when the diver surfaces, the nitrogen pressure in the lungs falls and the nitrogen diffuses from the tissues into the blood, and from the blood into the lungs. If the return to the surface is too rapid, nitrogen in the tissues and blood cannot diffuse out rapidly enough and nitrogen bubbles are formed. They can cause severe pains, particularly around the joints. Another complication may result if the breath is held during ascent. During ascent from a depth of 10 meters, the volume of air in the lungs will double because the air pressure at the surface is only half of what it was at 10 meters. This change in volume may cause the lungs to distend ang even rupture. This the rise of the exhaled air bubbles, and must exhale during ascent.

Question: What does the passage mainly discuss?         

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Under certain circumstances, the human body must cope with gases at greater-than-normal atmospheric pressure. For example, gas pressures increase rapidly during a drive made with scuba gear because the breathing equipment allows divers to stay underwater longer and dive deeper. The pressure exerted on the human body increases by 1 atmosphere for every 10 meters of depth on seater, so thay at 39 meters in seawater a diver is exposed to pressure of about 4 atmosphere. The pressure of the gases being breathed must equal the external pressure applied to the body, otherwise breathing is very difficult. Therefore all of the gases in the air breathed by a scuba diver at 40 meter are present at five times their usual pressure. Nitrogen, which composes 80 percent of the air we breathe, usually causes a balmy feeling of well-being at this pressure. At a depth of 5 atmosphere, nitrogen causes symptoms resembling alcohol intoxication, known as nitrogen narcosis. Nitrogen narcosis apparently results from a direct effect on the brain of the large amounts of nitrogen dissolved in the blood. Deep dives are less dangerous if helium is substituted for nitrogen, because under these pressures helium does not exert a similar narcotic effect. As a scuba diver descends, the pressure of nitrogen on the lungs increases. Nitrogen then diffuses from the lungs to the blood, and from the blood to body tissues. The reverse occurs when the diver surfaces, the nitrogen pressure in the lungs falls and the nitrogen diffuses from the tissues into the blood, and from the blood into the lungs. If the return to the surface is too rapid, nitrogen in the tissues and blood cannot diffuse out rapidly enough and nitrogen bubbles are formed. They can cause severe pains, particularly around the joints. Another complication may result if the breath is held during ascent. During ascent from a depth of 10 meters, the volume of air in the lungs will double because the air pressure at the surface is only half of what it was at 10 meters. This change in volume may cause the lungs to distend ang even rupture. This the rise of the exhaled air bubbles, and must exhale during ascent.

Question: The word “exert” in bold on paragraph 1 is closest in meaning to ________.         

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Life originated in the early seas less than a billion years after the Earth was formed. Yet another three billion years were to pass before the first plants and animals appeared on the continents. Life’s transition from the sea to the land was perhaps as much of an evolutionary challenge as was the genesis of life. What forms of life were able to make such a drastic change in lifestyle? The traditional view of the first terrestrial organisms is based on megafossils-relatively large specimens of essentially whole plants and animals. Vascular plants, related to modern seed plants and ferns, left the first comprehensive megafossil record. Because of this, it has been commonly assumed that the sequence of terrestrialization reflected the evolution of modern terrestrial ecosystems. In this view, primitive vascular plants first colonized the margins of continental waters, followed by animals that feed on the plants, and lastly by animals that preyed on the plant-eaters. Moreover, the megafossils suggest that terrestrial life appeared and diversified explosively near the boundary between the Silurian and the Devonian periods, a little more than 400 million years ago. Recently, however, paleontologists have been taking a closer look at the sediments below this Silurian-Devonian geological boundary. It turns out that some fossils can be extracted from these sediments by putting the rocks in an acid bath. The technique has uncovered new evidence form sediments that were deposited near the shores of the ancient oceans- plant microfossils and microscopic pieces of small animals. In many instances the specimens are less than one-tenth of a millimeter in diameter. Although they were entombed in the rocks for hundreds of millions of years, many of them fossils consist of the organic remains of the organism. These newly discovered fossils have not only revealed the existence of previously unknown organisms, but have also pushed back these dates for the invasion of land by multicellular organisms. Our views about the nature of the early plant and animal communities are now being revised. And with those revisions come new speculations about the first terrestrial life-forms.

Question: According to the passage, what happened about 400 million years ago?         

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Life originated in the early seas less than a billion years after the Earth was formed. Yet another three billion years were to pass before the first plants and animals appeared on the continents. Life’s transition from the sea to the land was perhaps as much of an evolutionary challenge as was the genesis of life. What forms of life were able to make such a drastic change in lifestyle? The traditional view of the first terrestrial organisms is based on megafossils-relatively large specimens of essentially whole plants and animals. Vascular plants, related to modern seed plants and ferns, left the first comprehensive megafossil record. Because of this, it has been commonly assumed that the sequence of terrestrialization reflected the evolution of modern terrestrial ecosystems. In this view, primitive vascular plants first colonized the margins of continental waters, followed by animals that feed on the plants, and lastly by animals that preyed on the plant-eaters. Moreover, the megafossils suggest that terrestrial life appeared and diversified explosively near the boundary between the Silurian and the Devonian periods, a little more than 400 million years ago. Recently, however, paleontologists have been taking a closer look at the sediments below this Silurian-Devonian geological boundary. It turns out that some fossils can be extracted from these sediments by putting the rocks in an acid bath. The technique has uncovered new evidence form sediments that were deposited near the shores of the ancient oceans- plant microfossils and microscopic pieces of small animals. In many instances the specimens are less than one-tenth of a millimeter in diameter. Although they were entombed in the rocks for hundreds of millions of years, many of them fossils consist of the organic remains of the organism. These newly discovered fossils have not only revealed the existence of previously unknown organisms, but have also pushed back these dates for the invasion of land by multicellular organisms. Our views about the nature of the early plant and animal communities are now being revised. And with those revisions come new speculations about the first terrestrial life-forms.

Question: With which of the following conclusions would the author probably agree?  

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Life originated in the early seas less than a billion years after the Earth was formed. Yet another three billion years were to pass before the first plants and animals appeared on the continents. Life’s transition from the sea to the land was perhaps as much of an evolutionary challenge as was the genesis of life. What forms of life were able to make such a drastic change in lifestyle? The traditional view of the first terrestrial organisms is based on megafossils-relatively large specimens of essentially whole plants and animals. Vascular plants, related to modern seed plants and ferns, left the first comprehensive megafossil record. Because of this, it has been commonly assumed that the sequence of terrestrialization reflected the evolution of modern terrestrial ecosystems. In this view, primitive vascular plants first colonized the margins of continental waters, followed by animals that feed on the plants, and lastly by animals that preyed on the plant-eaters. Moreover, the megafossils suggest that terrestrial life appeared and diversified explosively near the boundary between the Silurian and the Devonian periods, a little more than 400 million years ago. Recently, however, paleontologists have been taking a closer look at the sediments below this Silurian-Devonian geological boundary. It turns out that some fossils can be extracted from these sediments by putting the rocks in an acid bath. The technique has uncovered new evidence form sediments that were deposited near the shores of the ancient oceans- plant microfossils and microscopic pieces of small animals. In many instances the specimens are less than one-tenth of a millimeter in diameter. Although they were entombed in the rocks for hundreds of millions of years, many of them fossils consist of the organic remains of the organism. These newly discovered fossils have not only revealed the existence of previously unknown organisms, but have also pushed back these dates for the invasion of land by multicellular organisms. Our views about the nature of the early plant and animal communities are now being revised. And with those revisions come new speculations about the first terrestrial life-forms.

Question: Which of the following resulted from the discovery of microscopic fossils?         

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Life originated in the early seas less than a billion years after the Earth was formed. Yet another three billion years were to pass before the first plants and animals appeared on the continents. Life’s transition from the sea to the land was perhaps as much of an evolutionary challenge as was the genesis of life. What forms of life were able to make such a drastic change in lifestyle? The traditional view of the first terrestrial organisms is based on megafossils-relatively large specimens of essentially whole plants and animals. Vascular plants, related to modern seed plants and ferns, left the first comprehensive megafossil record. Because of this, it has been commonly assumed that the sequence of terrestrialization reflected the evolution of modern terrestrial ecosystems. In this view, primitive vascular plants first colonized the margins of continental waters, followed by animals that feed on the plants, and lastly by animals that preyed on the plant-eaters. Moreover, the megafossils suggest that terrestrial life appeared and diversified explosively near the boundary between the Silurian and the Devonian periods, a little more than 400 million years ago. Recently, however, paleontologists have been taking a closer look at the sediments below this Silurian-Devonian geological boundary. It turns out that some fossils can be extracted from these sediments by putting the rocks in an acid bath. The technique has uncovered new evidence form sediments that were deposited near the shores of the ancient oceans- plant microfossils and microscopic pieces of small animals. In many instances the specimens are less than one-tenth of a millimeter in diameter. Although they were entombed in the rocks for hundreds of millions of years, many of them fossils consist of the organic remains of the organism. These newly discovered fossils have not only revealed the existence of previously unknown organisms, but have also pushed back these dates for the invasion of land by multicellular organisms. Our views about the nature of the early plant and animal communities are now being revised. And with those revisions come new speculations about the first terrestrial life-forms.

Question: The word “entombed” is closest in meaning to ______

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Life originated in the early seas less than a billion years after the Earth was formed. Yet another three billion years were to pass before the first plants and animals appeared on the continents. Life’s transition from the sea to the land was perhaps as much of an evolutionary challenge as was the genesis of life. What forms of life were able to make such a drastic change in lifestyle? The traditional view of the first terrestrial organisms is based on megafossils-relatively large specimens of essentially whole plants and animals. Vascular plants, related to modern seed plants and ferns, left the first comprehensive megafossil record. Because of this, it has been commonly assumed that the sequence of terrestrialization reflected the evolution of modern terrestrial ecosystems. In this view, primitive vascular plants first colonized the margins of continental waters, followed by animals that feed on the plants, and lastly by animals that preyed on the plant-eaters. Moreover, the megafossils suggest that terrestrial life appeared and diversified explosively near the boundary between the Silurian and the Devonian periods, a little more than 400 million years ago. Recently, however, paleontologists have been taking a closer look at the sediments below this Silurian-Devonian geological boundary. It turns out that some fossils can be extracted from these sediments by putting the rocks in an acid bath. The technique has uncovered new evidence form sediments that were deposited near the shores of the ancient oceans- plant microfossils and microscopic pieces of small animals. In many instances the specimens are less than one-tenth of a millimeter in diameter. Although they were entombed in the rocks for hundreds of millions of years, many of them fossils consist of the organic remains of the organism. These newly discovered fossils have not only revealed the existence of previously unknown organisms, but have also pushed back these dates for the invasion of land by multicellular organisms. Our views about the nature of the early plant and animal communities are now being revised. And with those revisions come new speculations about the first terrestrial life-forms.

Question: What is the following paragraph likely to discuss?  

Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions:

Under certain circumstances, the human body must cope with gases at greater-than-normal atmospheric pressure. For example, gas pressures increase rapidly during a drive made with scuba gear because the breathing equipment allows divers to stay underwater longer and dive deeper. The pressure exerted on the human body increases by 1 atmosphere for every 10 meters of depth on seater, so thay at 39 meters in seawater a diver is exposed to pressure of about 4 atmosphere. The pressure of the gases being breathed must equal the external pressure applied to the body, otherwise breathing is very difficult. Therefore all of the gases in the air breathed by a scuba diver at 40 meter are present at five times their usual pressure. Nitrogen, which composes 80 percent of the air we breathe, usually causes a balmy feeling of well-being at this pressure. At a depth of 5 atmosphere, nitrogen causes symptoms resembling alcohol intoxication, known as nitrogen narcosis. Nitrogen narcosis apparently results from a direct effect on the brain of the large amounts of nitrogen dissolved in the blood. Deep dives are less dangerous if helium is substituted for nitrogen, because under these pressures helium does not exert a similar narcotic effect. As a scuba diver descends, the pressure of nitrogen on the lungs increases. Nitrogen then diffuses from the lungs to the blood, and from the blood to body tissues. The reverse occurs when the diver surfaces, the nitrogen pressure in the lungs falls and the nitrogen diffuses from the tissues into the blood, and from the blood into the lungs. If the return to the surface is too rapid, nitrogen in the tissues and blood cannot diffuse out rapidly enough and nitrogen bubbles are formed. They can cause severe pains, particularly around the joints. Another complication may result if the breath is held during ascent. During ascent from a depth of 10 meters, the volume of air in the lungs will double because the air pressure at the surface is only half of what it was at 10 meters. This change in volume may cause the lungs to distend ang even rupture. This the rise of the exhaled air bubbles, and must exhale during ascent.

Question: It can be inferred from the passage that which of the following presents the greatest danger to a diver?