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Problems in Physics, May 2006

Please read the rules of the competition.


Problems with sign 'M'

Deadline expired on 12 June 2006.

M. 270. Join iron and copper wires to make a thermocouple. Use a sensible voltmeter and measure and plot the voltage-temperature graph, than measure the temperature of your hand.

(6 points)

Statistics


Problems with sign 'P'

Deadline expired on 12 June 2006.

P. 3894. A car was travelling with a speed of 50 km/h, which is the speed limit in cities, when suddenly a child step to the road in front of the car. The driver immediately began to break, and the car stopped right in front of the child. With what speed would have the car hit the children if it had moved with a speed of 70 km/h, instead of 50?

(3 points)

Statistics

P. 3895. First the image of a right-handed screw is projected to an obscure glass screen. a) Will this image be right or left-handed? b) If the image is observed from the other side of the glass screen what kind of screw do you see?

(3 points)

Statistics

P. 3896. A car starts from rest speeds up along a circular path and increases its speed at a constant rate. After covering one-fourth of a circle what is the angle between its acceleration and the radius drawn to the car at that moment.

(4 points)

Statistics

P. 3897. The figure shows two protons which start to move at the same moment with an initial speed of v. Find the smallest distance between the two protons. Data: v=106 m/s, d=10-9 m, \alpha=30o.

(4 points)

Statistics

P. 3898. A horizontal disc of mass m=0,6 kg and radius R=10 cm can be rotated easily about a vertical axle. Along a radius of the disc a sheet is fixed to the disc. (The mass of the sheet is negligible.) A horizontally fired bullet, whose mass is m1=20 g, hits the sheet perpendicularly, and the disc begins to rotate at an angular velocity of \omega=20 s-1. The distance between the axle and the place of impact is r=8 cm. What was the initial speed of the bullet, if a) it is embedded in the sheet; b) it bounced back totally elastically?

(5 points)

Statistics

P. 3899. A sample of ideal gas is taken through the cyclic process shown in the figure. At state X TX=373 K, VX=5 dm3, and at state Z VZ=12 dm3, TZ=273 K. At which state of the process Z\toX will the pressure be the same as the pressure at state Y?

(4 points)

Statistics

P. 3900. A body, whose initial temperature is T0 is put to thermal contact with the heat reservoirs of temperatures T_{1}, T_{2},\ldots, T_{n-1} and finally connected with the heat reservoir of temperature Tn=T0. Show that Clausius' statement:


\sum_{i=1}^{n}\frac{Q_{i}}{T_{i}}\le 0

holds true. Qi is the heat absorbed by the body from that heat reservoir whose temperature is Ti. Assume that the volume of the body cannot change.

(5 points)

Statistics

P. 3901. The capacitance of a parallel plate condenser is 70 pF, one of its plates is charged to +8,0.10-7 C and the other is to -3,2.10-7 C. Find the voltage between the plates.

(4 points)

Statistics

P. 3902. By what factor would the dissipated power of an electric heating element change if it was connected to an ideal diode in series and this system was connected to an alternating voltage supply of 230 V, 50 Hz? What would be the root-mean-square value of the voltage across the electric heating element? What is the ratio of this value and the maximum of the voltage?

(4 points)

Statistics

P. 3903. What can be the ratio of the linear momentum of the photoelectrons ejected by aluminum and the linear momentum of the absorbed photons?

(5 points)

Statistics


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