Foram encontradas 487 questões.
Uma linha de transmissão, cujos efeitos capacitivos podem ser desprezados, possui a seguinte matriz de impedâncias série, em componentes de fase:
!$ \begin{bmatrix} Z_{ABC} \end{bmatrix} !$ =!$ \begin{bmatrix} 6 + j12 & j3 & j6\\ j3 & 6 + j12 & j3\\ j6 & j3 & 6 + j12 \end{bmatrix} !$ !$ \begin{bmatrix} \Omega \end{bmatrix} !$
Assinale a alternativa que apresenta corretamente a impedância própria de sequência zero, em ohms, da matriz de impedâncias em componentes simétricas.
Provas
Um engenheiro eletricista efetuou medições em uma carga trifásica cuja sequência de fases é ABC. As medições estão ilustradas na tabela.
|
CORRENTES DE LINHA [A] |
TENSÕES DE FASE [V] |
||||
|
A |
B |
C |
A |
B |
C |
|
1!$ \angle !$-30º |
2!$ \angle !$-120º |
2!$ \angle !$150º |
100!$ \angle !$0º |
100!$ \angle !$-120º |
100!$ \angle !$120º |
Determine a potência complexa absorvida pela carga, em volt-ampère, e assinale a alternativa correta.
Provas
Uma fonte trifásica simétrica de sequência positiva alimenta uma carga trifásica desequilibrada ligada em estrela aterrada a partir de uma linha de transmissão idealmente transposta.
Supondo que a sequência de fases seja ABC e que
zp=1+ j2 [!$ \Omega !$] !$ \rightarrow !$ Impedância própria da linha;
zm = j1 [!$ \Omega !$] !$ \rightarrow !$ Impedância mútua da linha;
VAN = 8!$ \angle !$90º [V] !$ \rightarrow !$ Tensão na fase A da fonte:
zA= 10 [!$ \Omega !$] !$ \rightarrow !$ Impedância de carga na fase A;
IB = j12 [A] !$ \rightarrow !$ Corrente na fase B;
IC = j32 [A] !$ \rightarrow !$ Corrente na fase C.
Determine a corrente na fase A, em ampères, e assinale a alternativa correta.
Provas
Uma fonte trifásica simétrica de sequência positiva alimenta uma carga trifásica desequilibrada conectada em estrela aterrada. Supondo que a sequência de fases seja ABC, que a tensão de fase na fase A seja VAN = 100!$ \angle !$0º [V], e que as impedâncias da carga sejam:
zAN = j1/3 [!$ \Omega !$];
zBN = - 1/4 - j!$ \sqrt{ } !$3/4 [!$ \Omega !$];
zCN = - 1/4 + j!$ \sqrt{ } !$3/4 [!$ \Omega !$].
A corrente de neutro em ampéres será
Provas
Uma fonte trifásica simétrica de sequência positiva alimenta uma carga trifásica equilibrada conectada em estrela através de uma linha que possui mútuas e capacitâncias desprezíveis. Supondo que a sequência de fases seja ABC, a tensão da fonte seja VAB = 110/!$ \sqrt{2} !$ !$ \angle !$0º[V], a impedância da carga seja 10/!$ \sqrt{6} !$ !$ \angle !$45º [!$ \Omega !$] e a impedância da linha seja !$ \sqrt{3} !$ + j!$ \sqrt{3} !$ [!$ \Omega !$], determine a corrente na fase A, em ampéres.
Provas
Leia o texto para responder às questões de números 49 e 50.
Food Supply Worries ofan Agricultural Scientist — Climate Change
People involved in world agriculture have no patience with the supposed “debate” about climate change. We are already seeing the effects, and the projections for the future are not encouraging.
The most troubling feature of this phenomenon (and one that occurs even if you don't believe that it is human-driven) is that we are facing increasing variation in climatic events. The yearly changes in average temperature or even annual rainfall may not be dramatic, but what we are anticipating is that there will be more extreme weather events.
(www.sustainablog.org/category/climate-change/01.10.2009. Adaptado)
The term but in — The yearly changes in average temperature or even annual rainfall may not be dramatic, but what we are anticipating is that there will be more extreme weather events. — is correctly replaced by
Provas
Leia o texto para responder às questões de números 49 e 50.
Food Supply Worries ofan Agricultural Scientist — Climate Change
People involved in world agriculture have no patience with the supposed “debate” about climate change. We are already seeing the effects, and the projections for the future are not encouraging.
The most troubling feature of this phenomenon (and one that occurs even if you don't believe that it is human-driven) is that we are facing increasing variation in climatic events. The yearly changes in average temperature or even annual rainfall may not be dramatic, but what we are anticipating is that there will be more extreme weather events.
(www.sustainablog.org/category/climate-change/01.10.2009. Adaptado)
According to the text, the phenomenon of climate change is
Provas
Para responder às questões de números 47 e 48, leia o texto.
Petrobras" investments are guaranteed until 2013
SÃO PAULO, 6/30/09 — The financial and investor relations director of Brazilian mixed-capital oil giant Petrobras, Almir Barbassa, said Monday that the company's investment plan is guaranteed until 2013.
Barbassa also mentioned that the company has been cutting down on costs and investments in order to ensure the exploration of the subsalt reserves. The executive pointed out that Petrobras” operating costs fell by 30% in the final quarter of 2008.
“We are also trying to cut down the equipment that will be used in the subsalt platform and lower labor costs by investing in automation”, Barbassa added.
Petrobras already has over US$30 billion in financing for its future oil and gas exploration and refining and gas transport infrastructure projects. “Petrobras expects to produce 5.7 million barrels of oil equivalent a day (boed) by 2020”, he added.
The 2009 production target is 2.7 million boed.
(www.indexet.investimentosenoticias.com.br/arquivo/2009/06/30/26/Petrobras-investments-are-guaranteed-until-2013.html)
The term guaranteed in — Petrobras” investments are guaranteed until 2013 — means that the investments will be
Provas
Para responder às questões de números 47 e 48, leia o texto.
Petrobras" investments are guaranteed until 2013
SÃO PAULO, 6/30/09 — The financial and investor relations director of Brazilian mixed-capital oil giant Petrobras, Almir Barbassa, said Monday that the company's investment plan is guaranteed until 2013.
Barbassa also mentioned that the company has been cutting down on costs and investments in order to ensure the exploration of the subsalt reserves. The executive pointed out that Petrobras” operating costs fell by 30% in the final quarter of 2008.
“We are also trying to cut down the equipment that will be used in the subsalt platform and lower labor costs by investing in automation”, Barbassa added.
Petrobras already has over US$30 billion in financing for its future oil and gas exploration and refining and gas transport infrastructure projects. “Petrobras expects to produce 5.7 million barrels of oil equivalent a day (boed) by 2020”, he added.
The 2009 production target is 2.7 million boed.
(www.indexet.investimentosenoticias.com.br/arquivo/2009/06/30/26/Petrobras-investments-are-guaranteed-until-2013.html)
According to the text,
Provas
Os molinetes no campo da hidráulica servem para
Provas
Caderno Container