Pruebas objetivas

   EJEMPLIFICACIONES DE PRUEBAS OBJETIVAS

Se ha tenido muy en cuenta la adecuación de los problemas o preguntas formuladas al programa de la asignatura de Física en segundo de Bachillerato, sin olvidar la preparación de la PAU.

Por ello, se han traducido al inglés algunos de los problemas incluidos en los exámenes de Selectividad o PAU en años anteriores, en lugar de escoger directamente problemas ya escritos en inglés en materiales publicados en otros países. Aunque los ejercicios que aparecen en estos textos suelen tratar los mismos temas que los que se trabajan en nuestro país, pueden tener un nivel de exigencia bastante diferente, que en el caso de páginas web norteamericanas, suele ser inferior. Al final de cada prueba se indica la fuente de la que se ha obtenido el problema o la pregunta, de forma que los alumnos sean conscientes de que se enfrentan a cuestiones similares a las que se le plantearán en la PAU, aunque formuladas en inglés.

  Modelos de pruebas objetivas.

Logroño, Septiembre de 2005
Profesor: Enrique Garralaga Robres
Centro: IES "HERMANOS D'ELHUYAR"
Alumno..................................
Asignatura: Physics       Curso: 2º Bachillerato       Fecha: Dec, 7th, 2004

1.-

    a) Demonstrate that line forces are always perpendicular to equipotential surfaces.

    b) Prove that the total energy of any body of mass m, orbiting around another body of mass M, is given by the expression: ET = - (G. M . m) / 2 r Where r is the distance between the center of masses of the two bodies.

PROBLEMS:

2.-

A wooden block of mass 1 kg is linked to a spring, resting on a frictionless ground, such as we can see above. A bullet (whose mass is 200 g) has been fired horizontally with a speed v = 100 m/s against the block. After the stroke, the bullet becomes embedded in the wooden block. The value of the elastic constant of the spring is: K = 200 N/m. Calculate:

 
    •  a) The total linear momentum, before and after the stroke.

    •  b) The initial speed of the block-bullet system, right after suffering the impact.

    •  c) The length that the spring has shortened.

3.- A satellite wishes to orbit the earth at a height of 100 km above its surface. Determine the acceleration and orbital period of the satellite.
                                                            (from "the Physics Classroom, study works on line")

4.- Calculate the energy that should be communicated to a satellite, whose mass is 100 kg, in order to elevate it from the earth surface to a height of 8 km above it. Once the satellite has reached this height, calculate as well the velocity that should be communicated to it, in order to keep it orbiting around the earth.
                                                                                        (Selectividad, La Laguna ,1993)

5.- A satellite, whose mass is 2000 kg, describes a circular orbit around the earth, being its total radius 8000 km. Calculate:

    a) The angular momentum of the satellite, related to the center of the orbit.
    b) Its kinetic, potential and total energy.
                                                                                            (Selectividad, Madrid, 1996)


Logroño, Septiembre de 2005
Profesor: Enrique Garralaga Robres
Centro: IES "HERMANOS D'ELHUYAR"
Alumno..................................
Asignatura: Physics       Curso: 2º Bachillerato       Fecha: March, 11th, 2005

1.- A and B are two long parallel conductors, placed at a distance of 10 cm. Conductor A carries an intensity I A = 30 Amp, and conductor B carries an intensity I B = 40 Amp, in the opposite sense. Find:

    •  The value of the resultant magnetic field on a straight line contained in the plane of the two conductors, parallel to them, and at the same distance from both.

    •  b) Idem on a straight line, parallel to the two conductors, situated at 5 cm from A and 15 cm from B.

2.- A coil of conducting wire, that has 500 loops of radius 0.05 m, is placed in a uniform magnetic field of value 0.1 T., in such a way that the magnetic flux through it has a maximum value.

    •  Find the induced electromotive force in the coil if, in a time interval of 0.02 s., the magnetic field doubles its value.

    •  b) Find the induced electromotive force if the coil turns an angle of 180º with respect to an axis that passes through its centre, and is perpendicular to the magnetic field, in a time interval of 0.02 s. The value of the magnetic field is 0.01 T.

3.- A harmonic wave propagates itself along an elastic medium, according to the equation:

y (x, t) = 24 sin (2,000t -5x) (Metric units)

Determine:

    •  The values of its amplitude, frequency and wavelength.

    •  b) The phase difference between two points separated 0.2 m along the direction of the propagating wave.

    •  c) The equation of an identical wave that propagates itself in the opposite sense.

                                                                                   (Selectividad,Oviedo)

4.-

    a) THEORY : Stationary waves in strings.
    b) PROBLEM : The equation that describes the stationary A B wave of the string that we can see in the figure is:

    y = - 0,001 cos (100 p t) sin (kx) (Metric units)

      •  b 1 ) Write down the equation of the tranverse travelling wave that, after starting in point A, and suffering subsequent reflection in B, originates the
      stationary wave.

      •  b 2 ) Find the speed of the travelling wave.







Logroño, Septiembre de 2005
Profesor: Enrique Garralaga Robres
Centro: IES " HERMANOS D'ELHUYAR "
Alumno ................................
Asignatura : Physics Curso: 2º Bachillerato Fecha : May, 24 th , 2005

1.-

    a) Define the Intensity I of an ondulatory movement. How does I depend on the Amplitude A? ( Selectividad, I. Baleares )

    b) What is the loudness level (sonoridad) of a wave whose Intensity is 3 . 10-4 W / m2?

    ( Selectividad, Murcia )

    c) Light propagates through a certain medium at a speed of 120,000 Km / s. Find out what the minimum incident angle of a light ray must be in order to suffer a total reflection when it leaves this medium and comes out into air.

    ( Selectividad, Murcia )

2.- We want to see an image of our own face, in order to get shaved or for doing make up. The image must be upright, virtual and 1.5 times bigger, if our face is located at a distance of 25 cm from the mirror.

    a) What type of mirror should we use?

    b) What its focal distance must be?

    c) Now we want that our image be twice bigger, using the same mirror. Where should our face be placed? (Selectividad, I. Baleares)

3.- A beam (un haz) of light of wavelength l = 400 nm (nanometers), has an Intensity of 100 W/m 2 .

    a) What is the energy of every foton of the beam?

    b) How much energy arrives every minute to a surface of 1 cm2 perpendicular to the beam?

    c) How many fotons per second arrive to that surface?

    DATA:1 nm = 10-9m. Planck's constant: h = 6.63 x 10-34 J.s

                                                                          (Selectividad, País Vasco)

4.-
a) Define the Activity of a radioactive portion of matter. Is the Activity a characteristic constant of every element or not? Just explain.

    b) An Isotope of Bi has a Decay time (vida media) t = 60.5 minutes. Initially, how many atoms are disintegrated per second in a sample (muestra) of 50 g. of the isotope of Bismuth?

    DATA: Avogadro's Number N A = 6,022 x 1023molecules / mol Atomic mass of Bismuth = 209 a.m.u.

                                                                    ( Selectividad, Valencia )