4 (B420U20-1) 3. Abigail investigates how the power dissipated in a variable resistor varies as its resistance is altered. The diagram shows the circuit that Abigail uses (meters not shown). The variable resistor is connected to a battery of emf, E, and internal resistance, r. © WJEC CBAC Ltd. Abigail obtains the following data as the resistance is varied from 0.5 Ω to 6.0 Ω. Resistance, R / Ω Power dissipated in R / W 0.5 2.5 1.0 3.3 2.0 3.8 3.0 3.8 4.0 3.7 5.0 3.6 6.0 3.5 E r R (B420U20-1) Turn over. B 420 U201 0 5 5 Examiner only (a) Plot a graph of power dissipated in R (on the y-axis) against resistance (on the x-axis) and draw a smooth curve through the data. [3] © WJEC CBAC Ltd. 6 (B420U20-1) Examiner only (b) The emf of the battery is 6.0 V and the resistance, R, is now set at 4.5 Ω. (i) State what is meant by an emf of 6.0 V. [2] (ii) Calculate the current through the battery using data from your graph. [3] (iii) Calculate the internal resistance, r, of the battery. [3] (c) Abigail repeats the experiment but with a battery of the same emf but smaller internal resistance. Explain how the graph would change. [2] © WJEC CBAC Ltd. 13 (B420U20-1) Turn over. 7 Examiner only 4. (a) Calculate the mean drift velocity of the free electrons in a wire, which has a diameter of 0.50 mm and carries a current of 2.8 A. Assume each aluminium atom contributes 3 free electrons, and there are 6.0 × 1028 atoms per m3 of aluminium. [4] (b) The wire is thinner in a small section as shown below. © WJEC CBAC Ltd. Paula claims that within the thinner section the mean drift velocity of the free electrons will be greater. Evaluate whether she is correct. [3] (c) (i) State what is meant by a superconductor. [1] (ii) State and explain one advantage and one disadvantage of using superconductors to carry large currents. [4] Advantage:................................................................................................................................................................... Disadvantage:............................................................................................................................................................ 12 8 (B420U20-1) Examiner only 5. The apparatus shown below is used to produce a visible interference pattern on the screen. © WJEC CBAC Ltd. Slits B and C act as coherent sources. (a) (i) Explain what is meant by this statement. [1] (ii) Explain why destructive interference is observed at certain points on the screen. [3] (b) (i) The separation of the slits, B and C is 0.090 mm and the perpendicular distance between the slits and the screen is 3.60 m. The slits are illuminated with light of wavelength 4.4 × 10-7 m. A point X on the screen is 52.8 mm away from the central bright fringe at O. Brian states that a bright fringe will be formed at X. Evaluate whether or not he is correct. [4] Laser Screen B X O C (B420U20-1) Turn over. 9 Examiner only (ii) Determine the distance from X to the next dark fringe formed. [2] (c) Explain the historical significance of this experiment. [2] (d) A beam of electrons is fired at a thin sheet of aluminium. A pattern of light and dark circular fringes is observed as shown below. © WJEC CBAC Ltd. (i) Explain how this pattern is formed. [2] (ii) The thin sheet of aluminium is replaced with a foil of copper. The distance between the copper atoms is smaller than in aluminium. Describe how you expect the pattern to change. [2] 16 10 (B420U20-1) Examiner only 6. (a) Describe an experiment using a range of LEDs to determine the Planck constant. [6 QER] (b) A university research group uses a new experimental method to determine a value for the Planck constant that is higher than the accepted value of 6.63 × 10–34J s. Explain how other scientists would attempt to confirm this result. [2] © WJEC CBAC Ltd. 8 (B420U20-1) Turn over. 11 Examiner only 7. When sunlight shines on the atmosphere of Mars, carbon dioxide molecules at a height of 75 km behave like the amplifying medium in a laser. The energy levels involved are shown in the following diagram. A population inversion occurs between energy levels E1 and E2. © WJEC CBAC Ltd. Pumped level E2 = 0.289 eV E1 = 0.165 eV E0 = 0 (a) (i) State what is meant by a population inversion. [1] (ii) Explain why a population inversion is needed for laser action. [2] (b) Calculate the output wavelength of this naturally occurring laser and determine the region of the electromagnetic spectrum in which it lies. [3] TURN OVER FOR THE LAST PART OF THE QUESTION 12 (B420U20-1) Examiner only (c) The Llanesco crater is on the surface of Mars and has a cross-sectional area of 2 290 km2. Carbon dioxide molecules in the Mars atmosphere collide with the surface and give rise to a surface pressure of 600 Pa. The mass of a carbon dioxide molecule is 7.3 × 10–26 kg and the perpendicular component of the speed of the molecules to the surface is 550 m s–1 before and after they collide with the surface. Calculate the number of carbon dioxide molecules that collide with the Llanesco crater in 1 s. [5] END OF PAPER © WJEC CBAC Ltd. 11