International Physics Olympiad

IPhO 1977

Hradec Králové, Czechoslovakia · 3 theory problems with complete solutions

  1. Problem 1: Otto cycle of a four-stroke combustion engine

    The compression ratio of a four-stroke internal combustion engine is \(\varepsilon = 9.5\). The engine draws in air and gaseous fuel at a temperature \(27^\circ\mathrm{C}\) at a pressure \(1\ \mathrm{atm} = 100\ \mathrm{kPa}\). Compression follows an adiabatic process from point 1 to point 2, see Fig. 1. The pressure in the cylinder is doubled during the mixture ignition (2–3). The hot exhaust gas expands adiabatically to the volume \(V_2\) pushing the piston downwards (3–4). Then the exhaust valve opens and the pressure gets back to the initial value of \(1\ \mathrm{atm}\). All processes in the cylinder are supposed to be ideal. The Poisson constant (i.e. the ratio of specific heats \(C_p/C_V\)) for the mixture and exhaust gas is \(\kappa = 1.40\). (The compression ratio is the ratio of the volume of the cylinder when the piston is at the bottom to the volume when the piston is at the top.)

    Figure 1

    Figure 1:

    a) Which processes run between the points 0–1, 2–3, 4–1, 1–0?

    b) Determine the pressure and the temperature in the states 1, 2, 3 and 4.

    c) Find the thermal efficiency of the cycle.

    d) Discuss obtained results. Are they realistic?

    Official English text of the problem.

  2. Problem 2: Colours of a soap film

    Dipping the frame in a soap solution, the soap forms a rectangle film of length \(b\) and height \(h\). White light falls on the film at an angle \(\alpha\) (measured with respect to the normal direction). The reflected light displays a green color of wavelength \(\lambda_0\).

    a) Find out if it is possible to determine the mass of the soap film using the laboratory scales which has calibration accuracy of \(0.1\ \mathrm{mg}\).

    b) What color does the thinnest possible soap film display being seen from the perpendicular direction? Derive the related equations.

    Constants and given data: relative refractive index \(n = 1.33\), the wavelength of the reflected green light \(\lambda_0 = 500\ \mathrm{nm}\), \(\alpha = 30^\circ\), \(b = 0.020\ \mathrm{m}\), \(h = 0.030\ \mathrm{m}\), density \(\varrho = 1000\ \mathrm{kg\,m^{-3}}\).

    Official English text of the problem.

  3. Problem 3: Electron beam aimed at a target by a magnetic field

    An electron gun \(T\) emits electrons accelerated by a potential difference \(U\) in a vacuum in the direction of the line \(a\) as shown in Fig. 2. The target \(M\) is placed at a distance \(d\) from the electron gun in such a way that the line segment connecting the points \(T\) and \(M\) and the line \(a\) subtend the angle \(\alpha\) as shown in Fig. 2. Find the magnetic induction \(B\) of the uniform magnetic field

    Figure 1

    Figure 2:

    a) perpendicular to the plane determined by the line \(a\) and the point \(M\)

    b) parallel to the segment \(TM\)

    in order that the electrons hit the target \(M\). Find first the general solution and then substitute the following values: \(U = 1000\ \mathrm{V}\), \(e = 1.60 \cdot 10^{-19}\ \mathrm{C}\), \(m_e = 9.11 \cdot 10^{-31}\ \mathrm{kg}\), \(\alpha = 60^\circ\), \(d = 5.0\ \mathrm{cm}\), \(B < 0.030\ \mathrm{T}\).

    Official English text of the problem.