1. ?
Physics quiz for everyone: Perplexing questions about the world around us broaden your scope

2. subject: relativistic quantum mechanics
topic: particles and anti-particles, CPT theorem
Why fiction writers speculate that antiparticles are coming from the world that moves backward in time?

3. subject: modern science
topic: electronics
Why fiction writers speculate that antiparticles are coming from the world that moves backward in time?
First antiparticle, positron, was predicted by Paul Dirac. He argued that a quantum mechanical particle must be eventually found in all possible states including the state with the negative energy. He proposed that vacuum is filled with virtual particles and antiparticles that appear and annihilate in pairs so fast that they cannot be observed with classical instruments. Motion of an antiparticle is the same as motion of its counterpart particle recorded and then demonstrated in the mirror in the rewind mode.

4. Why electrons in graphene behave as if they were massless?
subject: modern science
topic: electronics
Why electrons in graphene behave as if they were massless?
Atomic structure of graphene:
carbon atoms organized
in hexagons
Graphene chip

5. Why electrons in graphene behave as if they were massless?
subject: modern science
topic: electronics
Why electrons in graphene behave as if they were massless?
Electrons and holes in graphene behave like particles and antiparticles due to the symmetry of the lattice:
space-inversion
time-inversion since electrons are in the stationary Bloch states
Energy of a free particle is proportional to its wavenumber
Similarly energy of an electron in graphene is proportional to its wave-vector because conduction and valence bands intersect at the Fermi energy level
The higher impulse an electron gets, the more energy it has to overcome binding
Since electron in graphene has linear dispersion relationship the same as free particles, it is said to be massless
Graphene lattice has
space inversion symmetry

6. Why patterns made by the frost have shapes of fractals ?
subject: thermodynamics
topic: electronics
Why patterns made by the frost have shapes of fractals ?

7. Why patterns made by the frost have shapes of fractals ?
subject: thermodynamics
topic: electronics
Why patterns made by the frost have shapes of fractals ?
Frost patterns appear as a result of the phase transition of water from liquid to solid phase. This happens at some critical temperature. Phase transitions are characterized by the order parameter that is water density in this case. When temperature approaches its critical value, the density becomes uniform function of temperature: the smaller kinetic energy, the smaller the density that explains self-similarity pattern of the molecules spatial distribution. During phase transition, motion of individual water molecules does not depend on their own interaction with the neighboring molecules. This is universality of the phase transitions. The same frost pattern can be obtained if one uses liquid with the different chemical structure.

8. What people call a Hofstadter butterfly?
subject: solid state physics
topic: electron in magnetic field
What people call a Hofstadter butterfly?

9. What people call a Hofstadter butterfly?
subject: solid state physics
topic: electron in magnetic field
What people call a Hofstadter butterfly?
A Hofstadter butterfly is a fractal structure of electron levels in a magnetic field. In classical Hall effect magnetic field produces potential difference across the conductor carrying the current. Relationship between current and produced voltage is continuous. If magnetic field is strong enough and temperature is low, conductivity of electrons can take only certain quantized values. This is called quantum Hall effect. In two-dimensional systems, conductivity of electrons and their energy becomes a uniform function of magnetic field. The distribution of energy levels for small scale repeats the same patterns on the large scale.

10. subject: atomic physics
topic: black body radiation
Why furnace glows red?
Furnace radiates the light of certain spectrum depending on its temperature. From the prospective of classical mechanics, all modes of light are equally probable. As temperature of the furnace increases, more modes gets excited and all modes are equally probable. Furnace should radiate white light. However, the spectrum of the furnace has well defined pick that displaces from infrared to blue color with the temperature. Why?

11. subject: atomic physics
topic: black body radiation
Why furnace glows red?
Furnace spectrum has well defined maximum shifting from infrared to blue color with the temperature. This is an example of black-body radiation. This was the first phenomenon explained on the basis of quantum mechanics. Light consists of particles, phonons, that are in thermal equilibrium. Probability of a phonon to have some energy, E, depends on the value of the energy. The modes of high frequency have smaller probability than the modes of law frequency.

12. subject: classical mechanics
topic: Newton’s first law
You are seating comfortably inside the train crossing a beautiful country-side. Suddenly the train stops and you feel a push forward. Why this happens?

13. subject: classical mechanics
topic: Newton’s first law
You are seating comfortably inside the train crossing a beautiful country-side. Suddenly the train stops and you feel a push forward. Why this happens?
When the train moves with a constant velocity, inside the train everything happens the same way as if it was at rest. When the train suddenly stops, its velocity changes. Objects inside tend to continue their motion forward because of the inertia. A push forward is due to the inertia force associated with decelerated motion of the train.

14. subject: classical mechanics
topic: relativity of motion
A sailor drops his hat from the top of the mast on the ship moving forward with a constant speed. Where the hat is going to land?

15. subject: classical mechanics
topic: relativity of motion
A sailor drops his hat from the top of the mast on the ship moving forward with a constant speed. Where the hat is going to land?
The hat is going to land at the base of the mast at the same spot where it would land if the ship was at rest.

16. subject: classical mechanics
topic: internal and external forces
Once upon a time baron Münchausen claimed that he pulled himself together with his horse out of the swamp . Should we believe him?

17. subject: classical mechanics
topic: internal and external forces
Once upon a time baron Münchausen claimed that he pulled himself together with his horse out of the swamp . Should we believe him?
No. The upward force exerted by baron’s arm on his hair is internal. By Newton’s third law it will produce a reaction force applied to the shoulder joint and directed downward.

18. subject: classical mechanics
topic: internal and external forces
Why you cannot push the boat out of the shore seating inside the boat? Once you get on the shore and push the boat with the same force, the boat starts moving?
A picture of Leonid Vandenko

19. subject: classical mechanics
topic: internal and external forces
Why you cannot push the boat out of the shore seating inside the boat? Once you get on the shore and push the boat with the same force, the boat starts moving?
When you are pushing a boat seating inside it, the force of your arms causes a reaction force of the boat equal in magnitude and opposite in direction. These two forces are internal and they cannot change momentum of the system. When you are staying on the shore, action-reaction force appears between you and the boat, but you are not a part of the system now. Your force is regarded as external and it can change momentum of the boat.

20. subject: classical mechanics
topic: internal and external forces
When a weight-lifter presses on the floor with a largest force: when he pushes the weight up with acceleration from his chest or when he holds it on the stretched hands?

21. subject: classical mechanics
topic: internal and external forces
When a weight-lifter presses on the floor with a largest force: when he pushes the weight up with acceleration from his chest or when he holds it on the stretched hands?
To hold the weight on the stretched hands, the weightlifter has to apply force that compensates the force of gravity. To lift the weight with acceleration, the force has also encounter for acceleration of the weight. Thus, in this case the force applied to the weight must be larger and the weightlifter presses on the floor with the larger force in this case.

22. Why physicists call the space station a freely falling system?
subject: classical mechanics
topic: freely falling system
Why physicists call the space station a freely falling system?

23. Why physicists call the space station a freely falling system?
subject: classical mechanics
topic: freely falling system
Why physicists call the space station a freely falling system?
Even thought the space station orbits around the Earth, it is called a freely falling system because the only external force acting on it is the force of gravity. Acceleration of the space station is the same as an acceleration of the elevator that moves freely under the force of gravity. The motion of the space station is not directed downward because its velocity is perpendicular to the acceleration. If the angle between velocity and acceleration would slightly deviate from 90°, the station would eventually land on the surface of the Earth. Otherwise one can say that its free fall lasts forever.

24. Why astronauts become weightless at the space station?
subject: classical mechanics
topic: apparent weight
Why astronauts become weightless at the space station?

25. Why astronauts become weightless at the space station?
subject: classical mechanics
topic: apparent weight
Why astronauts become weightless at the space station?
Students often say that astronauts become weightless at the space station because there is no gravity. This is inaccurate answer. The gravity is the only external force applied to the space-station as a whole and it defines trajectory of the space station. However, the motion of the space-station is such that there is no normal force acting on the objects inside the space-station. The normal force is a reaction force that appears at the boundary between station’s interior and exterior space. It exists both inside and outside the station. Since the space-station falls freely, the only external force acting on the space-station is gravity. Normal force is equal to zero. Therefore, objects inside have no apparent weight.

26. Is Archimedes law valid at the space station?
subject: fluid dynamics
topic: Archimedes law
Is Archimedes law valid at the space station?

27. Is Archimedes law valid at the space station?
subject: fluid dynamics
topic: Archimedes law
Is Archimedes law valid at the space station?
Archimedes law is not valid at the space-station. The law states that dynamics of an object, e. g. a cork, surrounded by the fluid depends on both acceleration of gravity g and cork's density. Inside freely falling system, acceleration of gravity g is canceled out by acceleration a and corks of different densities move the same way. But once g and a differ in the smallest possible way, the dynamics of different corks becomes distinguishable. Archimedes principle becomes valid. This is an example of spontaneous symmetry breaking. When g=a, two spatial directions up and down are equivalent. Once the symmetry disappears, dynamics changes qualitatively.

28. What will happen to a pendulum at the space- station?
subject: classical mechanics
topic: apparent weight
What will happen to a pendulum at the space- station? T v mg

29. What will happen to a pendulum at the space- station?
subject: classical mechanics
topic: apparent weight
What will happen to a pendulum at the space- station?
When the space-station is on the ground, the pendulum swings with the period that depends on its length and acceleration of gravity g. When the station moves downward with acceleration a, the objects inside experiences the force of inertia that reduces the gravity. The period of the pendulum decreases: v T v
When space-station reaches freely falling state and pendulum is at the turning point with velocity equal to zero, it is going to stay still. If it was moving with some velocity, it will rotate under the force of tension in the vertical plane as it is shown on the picture.

30. Which insect is flapping its wings faster: a fly or a mosquito?
subject: classical mechanics
topic: sound waves
Which insect is flapping its wings faster: a fly or a mosquito?

31. Which insect is flapping its wings faster: a fly or a mosquito?
subject: classical mechanics
topic: sound waves
Which insect is flapping its wings faster: a fly or a mosquito?
A mosquito flaps its wings faster because the sound that it makes flying is of the higher frequency. 


32. subject: classical mechanics
topic: sound waves
Why one can hear somebody’s voice staying next to the open door and cannot hear somebody’s whisper in the same situation?

33. subject: classical mechanics
topic: sound waves
Why one can hear somebody’s voice staying next to the open door and cannot hear somebody’s whisper in the same situation?
Surprisingly whisper consists off the sounds of the higher frequency than the normal voice. One can feel that whispering requires bigger effort of the muscles. The sound waves bend or diffract around the open door and therefore, one can hear the voice staying next to it. Since the whisper sound waves have higher frequency they cannot diffract and one cannot hear the whisper in the same situation.

34. Why waves bend around some rocks and do not bend around the others?
subject: classical mechanics
topic: diffraction

35. Why waves bend around some rocks and do not bend around the others?
subject: classical mechanics
topic: diffraction
Bending of the wave around an object that is called diffraction depends on the relationship of the size of the object and the wavelength. If the wavelength is much larger than the object, the wave front bends around the obstacle and then, gets restored due to the interference. If the wavelength is much smaller than the size of the object, the object blocks the wave propagation.
Wavelength is larger than the object
Wavelength is smaller than the object

36. subject: thermodynamics
topic: condensation
When dew is falling?

37. subject: thermodynamics
topic: condensation
When dew is falling?
The dew is falling when the air saturates proper amount of water vapor and distances between water molecules become small enough for gas-liquid phase transition to begin. The higher is the temperature, the higher density of vapor is required for condensation because molecules are moving too fast to bind together. Point of dew is a temperature at which saturated vapor condensates into liquid at the same rate at which it would be evaporating under given pressure.

38. subject: thermodynamics
topic: phase transitions
Why icebergs float?

39. subject: thermodynamics
topic: phase transitions
Why icebergs float?
Icebergs float because overcoming transition from solid to liquid phase, water reduces in the volume at the temperature ranging from 0°C to 4°C and then starts expanding. Water molecules made of two hydrogen atoms and one oxygen atom represent electric dipoles. In the solid phase one water molecule makes hydrogen bonds with four other water molecules and they form a tetrahedral structure.
In the liquid phase the hydrogen bonds break and the molecules get packed closer. The volume of water decreases. As the temperature increases the molecules acquire more kinetic energy. They vibrate that results in the volume increase above 4°C.

40. Why the night sky is dark?
subject: cosmology
topic: uniform universe
Why the night sky is dark?
The stars are distributed uniformly throughout the universe. The intensity of their light decreases with the distance, but the number of the stars in a given spatial angle increases with the distance in such a way that it should compensate for the loss of the intensity. The night sky should glow. However, it is dark. Why?
Space angle
The number of the stars increases with the distance

41. Why the night sky is dark?
subject: cosmology
topic: uniform universe
Why the night sky is dark?
One of the hypothesis is that the stars live for about 1010 years that is a long term, but it is very small relatively to the the age of our universe that is estimated to be 1024 years. The radiated light does not have enough time to fill uniformly the interstellar space. One can say that our universe has not reached yet thermal equilibrium.

42. subject: optics
topic: light scattering
Why the sky is blue?

43. subject: optics
topic: light scattering
Why the sky is blue?
This effect can be explained by elastic scattering of the light that was first described by British physicist Rayleigh. Molecules of the air absorb the light coming from the Sun and radiate the light of the same frequency. Scattering is due to nitrogen and oxygen molecules and it takes place mostly in the blue part of the spectrum.

44. Why the sky gets red at the sunset?
subject: optics
topic: light scattering
Why the sky gets red at the sunset?

45. Why the sky gets red at the sunset?
subject: optics
topic: light scattering
Why the sky gets red at the sunset?
When the sun is close to the horizon, the layer of the atmosphere through which the light travels is larger comparing to the situation when the sun is up in the sky and the blue part of the spectrum gets completely dissipated by scattering: The image of the sun is brought to us by the light that lost its blue component. This is why we see it in red and orange colors.

46. subject: optics
topic: light scattering
Why polar stratospheric clouds are so beautiful? Why one can see them only close to the Poles?

47. subject: optics
topic: light scattering
Why polar stratospheric clouds are so beautiful? Why one can see them only close to the Poles?
This is an example of Mie scattering of light. The light is scattered by drops of water which size is larger than the light wavelength. Therefore, scattering of the drops is different for different wavelength and the clouds shimmer with various colors. These clouds appear at the stratosphere that is the upper layer of the atmosphere. One can see them close to the Poles because on these altitudes the sun light can reach them after the sunset.

48. subject: electromagnetism
topic: electric charges
What is the difference between the charges that consist off integer number of electrons and charges that are a fraction of an electron?

49. subject: electromagnetism
topic: electric charges
What is the difference between the charges that consist off integer number of electrons and charges that are a fraction of an electron?
Integer charges can be separated from each other. Fractional charges called quarks cannot be separated from each other. Attraction between quarks increases with the distance. Protons and neutrons cannot be disintegrated into quarks. Separate quarks do not exist.

50. subject: electromagnetism
topic: safety rules
What can happen to you if you touch a bared wire? What you can feel and what puts your body in danger: a current or a voltage or both? Do you get a burn?

51. subject: electromagnetism
topic: safety rules
What can happen to you if you touch a bared wire? What you can feel and what puts your body in danger: a current or a voltage or both? Do you get a burn?
A current going trough your body puts your life in danger
A current less than 0.01 A can hardly be noticed
A current less than 0.02 A brings a pain. One can feel that his hand is attracted by the wire
A current higher than 0.1 A effects breathing and heart beats and puts your life in danger
One can get a severe burn if the current is higher than 0.2 A

52. If one touches the wire does electric force attracts his hand?
subject: electromagnetism
topic: safety rules
If one touches the wire does electric force attracts his hand?

53. If one touches the wire does electric force attracts his hand?
subject: electromagnetism
topic: safety rules
If one touches the wire does electric force attracts his hand?
No, the hand is not attracted by the wire. When the current passes through the muscles, they contract and therefore, one cannot unclench the fist. If one touches the wire with the back of the hand it will be repulsed from the wire.

54. subject: electromagnetism
topic: safety rules
How resistance of someone’s skin changes when it gets wet? Can you explain why this happens?

55. subject: electromagnetism
topic: safety rules
How resistance of someone’s skin changes when it gets wet? Can you explain why this happens?
Resistance of a dry skin is very large ~ Ohm. Resistance of a wet skin is very small ~1000 Ohm because water in the presence of chemicals is a very good conductor. Never do any experiments with the wet hands!

56. Why pure water is an excellent dielectric?
subject: electromagnetism
topic: safety rules
Why pure water is an excellent dielectric?

57. Why pure water is an excellent dielectric?
subject: electromagnetism
topic: safety rules
Why pure water is an excellent dielectric?
Water molecules have dipole moment. In the presence of electric field dipole moments get oriented along the field lines. Electric field produced by dipoles is called induced electric field. It makes external field smaller. Therefore, water is a good dielectric.

58. Why tap water is an excellent conductor?
subject: electromagnetism
topic: safety rules
Why tap water is an excellent conductor?

59. Why tap water is an excellent conductor?
subject: electromagnetism
topic: safety rules
Why tap water is an excellent conductor?
If a molecule has significant dipole moment, it destroys hydrogen bonding networks of water molecules. Since water molecules are relatively small, they surround additive molecule and a molecule can easily diffuse:
In the presence of electric field the dipoles move along the field lines. Thus, tap water is a good conductor. - +

60. Why a lightning has a shape of a broken line?
subject: electromagnetism
topic: electrostatic field of the Earth
Why a lightning has a shape of a broken line?

61. Why a lightning has a shape of a broken line?
subject: electromagnetism
topic: electrostatic field of the Earth
Why a lightning has a shape of a broken line?
Let us look at the process of lightning formation. Clouds contain ionized particles. Initially negative charges are concentrated in the middle of the cloud and positive charges are at the base and at the top of the cloud. First, negative charges flow to the cloud base. Due to the large potential difference between the cloud and the ground, air molecules get ionized and eventually air becomes conducting. The bright line of a lightning that we see is an electrical current. The breakdown of the air happens through several stages: a certain volume of the air gets ionized and conducts the current, then the air below the lightning gets ionized and the current keeps going down. This process is very fast therefore, we see different parts of the current simultaneously. The line is not straight because the current deviates due to the positive charges in the air.

62. What keeps electrostatic field of the Earth relatively constant?
subject: electromagnetism
topic: electrostatic field of the Earth
What keeps electrostatic field of the Earth relatively constant?

63. What keeps electrostatic field of the Earth relatively constant?
subject: electromagnetism
topic: electrostatic field of the Earth
What keeps electrostatic field of the Earth relatively constant?
Surface of the Earth is negatively charged while the upper layer of the atmosphere is positively charged. The Earth could be completely discharged in about 5 minutes by ionization of the air due to the cosmic rays and natural radioactivity of the Earth. However, this does not happen because the Earth gets a constant supply of the electrons through the lightings.

64. subject: electromagnetism
topic: circuits
How fish – an electrical scat or an electrical eel – can strike its victim with a current?

65. subject: electromagnetism
topic: circuits
How fish – an electrical scat or an electrical eel – can strike its victim with a current?
An electrical fish can produce a current of about 1 A along its body and have potential difference of 600 V between the head and the tail. It has special cells that after receiving a signal let the ions pass through the membrane. The cells acquire potential difference. They are connected into the long chains along the body that reminds serial connection of resistors. The chains meet at some points on the head and on the tail. A combination of serial and parallel connection of the cells produces such a large current and create potential difference:

66. subject: thermodynamics
topic: atmospheric pressure
German physicist Otto von Guericke put together two copper hemispheres, pumped out the air and demonstrated that one needs 16 horses to pull them apart. The experiment was done in Magdeburg in XVII century. What was holding the hemispheres together ?

67. subject: thermodynamics
topic: atmospheric pressure
German physicist Otto von Guericke put together two copper hemispheres, pumped out the air and demonstrated that one needs 16 horses to pull them apart. The experiment was done in Magdeburg in XVII century. What was holding the hemispheres together ?
The hemispheres are hold together by atmospheric pressure. The air exerts uniform force on their outer surfaces.

68. Why snowflakes have beautiful geometric shapes?
subject: thermodynamics
topic: condensation
Why snowflakes have beautiful geometric shapes?

69. Why snowflakes have beautiful geometric shapes?
subject: modern science
topic: electronics
Why snowflakes have beautiful geometric shapes?
The shapes of the snowflakes depend on the humidity, atmospheric pressure, temperature and the details of the snowflake dynamics through the cloud. The shapes get rarely replicated. However, most of the snowflakes have six-fold symmetry because the hexagonal structure of ice. Snowflakes formation starts with ice nucleus that contains particles other than water. Then it grows by deposition of water molecules in air.
Since the cloud as a closed system has to be in thermodynamic equilibrium, the deposition of the water vapor causes evaporation of the water droplets. This promotes further formation of the snowflakes at the water droplet expense.
Hexagonal structure of ice

70. How an airplane takes off?
subject: air dynamics
topic: Bernoulli’s law
How an airplane takes off?

71. How an airplane takes off?
subject: air dynamics
topic: Bernoulli’s law
How an airplane takes off?
An airplane takes off due to the lift force produced as a result of the difference of the air flow above the wing and below it. The base of the wing is flat while the top is slightly curved. The two flows are continues and have the same density. The air flow above the wing travels longer path, therefore, it has to travel at the higher speed. Bernoulli’s law tells us that the flow with the larger velocity has the lower pressure. Thus, the pressure above the wing is lower than the pressure above it that results in the net lift force. Viscosity of the air and its friction causes the flow going around the wing shown by blue line. This flow increases the lift force.