Lecture sheets for exam 2

• What property of particles source the electric force? In other words, what is the source of the electric field?

  The charges of particles source the electric field and electric force.
• What particles in atoms carry charge?

  The atomic particles that carry charge include protons and electrons. Neutrons do not carry charge.
• Like charges repel and opposite charges attract .
• What causes a current to flow in a wire?

  
  

  An external electric field causes current to flow in a wire. Typically this electric field is sourced by a battery.
• What must charges do in order to produce a magnetic field? What is the source of the magnetic field in “magnets”?

  Charges that move make magnetic fields. Magnetic fields in “magnets” are caused by electrons that spin and orbit in the atoms.
• Motors demonstrate which EM phenomenon?

  Motors demonstrate the magnetic force law on currents.
• Motors demonstrate which EM phenomenon?

  Motors demonstrate the magnetic force law on currents.
• A changing magnetic field produces an electric field . A changing electric field produces a magnetic field .
• How does the Hertz’s experiment work?

  The transmitter of Hertz’s apparatus produces EM waves using oscillating charges. The EM waves travels to a distant receiver and oscillates its charges.
• How did Maxwell know that light was an EM wave?

  Maxwell predicted that EM waves travel at the speed of 3.00×108 m/s, which is equal to the speed of light.
• Ether is the medium through which light waves propagate.
• What are the properties of the ether?

  It is everywhere, tenuous, and stiff.
• Swimming with a current makes a swimmer swim faster . Swimming against a current makes a swimmer swim slower . To swim perpendicular to a current, the swimmer should swim faster to counter the water flow
• What does an interferometer measure directly between the light rays along the two arms? What observable is used to make this measurement?

  The interferometer measures shifts in the path time difference between the light rays. A change in the time path difference is measured by a shift in fringes.
• What does the path time difference tell you about the Earth’s motion? What did the null result (zero path time difference) mean?

  The path time difference measures the velocity of Earth relative to the ether. The null results imply that Earth is not significantly moving relative to the ether. In particular, this experiment limits the Earth’s speed to a value smaller than allowed by previous observations.
• What is the principle of relativity? 


  The laws of physics are the same in all uniformly moving reference frames.
• What is the speed of light in my reference frame if I’m moving at constant velocity relative to the source? If the light is on a ship moving away from me at half the speed of light?

  3 × 10^8 m/s in both cases. It is the same in all inertial reference frames, regardless of the motion of the source.
• What are the laws of physics?

  Newton’s Laws of Motion, Maxwell’s Equations, and the speed of light
• What constitutes an event? Give an example. 


  An event must have a specific location and a time. An example would be a ball hitting the center of the blackboard in Meyer 121 at 12:14 pm on Thursday.
• . How can you compare the aging of a person moving in your frame with that of a person who is at rest in your frame? What if the person’s motion (in your frame) approaches the speed of light?

  The person moving in your frame will age more slowly than the person at rest. If the person’s motion in your frame approaches the speed of light, they will stop aging
• What happens to the length of an object moving in your frame? What happens if the object’s speed (in your frame) approaches the speed of light?

  The length of an object moving in your frame will be shorter than it would be at rest. If the object’s speed approaches the speed of light, its length will approach to zero (very short).
• In what inertial reference frame (IRF) will the object be the longest?

  An object will be longest in the IRF where it is not moving.
• Two twin observers are traveling at relative to each other. Do they agree on who ages slower? Why not?

  They do not agree on who ages slower because for each observer, the proper time is measured in the other person’s frame.
• What is the “twin paradox”? How is it resolved?

  • The traveling twin moves at a constant velocity, except the “turning back” motion. From the perspective of the twin on Earth, the traveling twin is younger because of the time dilation.
  • The traveling twin, on the other hand, claims the time dilation on the clock of the twin on Earth.
  • This disagreement on aging is called “twin paradox”.
  • It’s resolved by realizing that the traveling twin does not stay in an IRF because of the “turning back” motion.
• What are “simultaneous events”? What does it mean when two clocks are “synchronized”?

  Two events are simultaneous if they occur at the same time. Two clocks are synchronized if they are started with the same reading at the same time
• Two events are simultaneous in one inertial reference frame (IRF). Are they generally simultaneous in

  another IRF? Can two clocks stay synchronous in all IRFs?

  Two events simultaneous in one IRF are generally not simultaneous in another IRF. Two clocks

  cannot be synchronous in all IRFs (unless they are at the same position).
• . What is the car-garage problem? How does relativity of simultaneity resolve the car-garage problem?

  two observers’ disagreement on the relative lengths between the car and the garage. It arises because simultaneous events chosen by an observer to compare the lengths are not simultaneous in the other observer’s frame. In order to measure the length of an object the coordinates of the object’s both ends should be read at the same time. Therefore comparison of lengths in one reference frame is invalid in the other reference frame. In other words, both observes are right in their own frame.
• What happens to the pitch of a siren when it is moving towards you? Away from you?

  The pitch of a siren increases when it is moving towards you; it decreases when it is moving away from you
• What happens to the color (frequency) of a light source when it is moving towards you? Away from you?

  The light becomes “bluer” (frequency increases) if it is moving towards you. The light becomes “redder” (frequency decreases) if it is moving away from you.
• What happens to light rays from a source moving toward us? What is an astronomical example of this effect?

  They come closer together, appearing brighter. The phenomena is called relativistic beaming. The apparent brightness of the quasar’s jet is an example of it. The jet moving towards Earth appears brighter, while the jet moving away from Earth appears dimmer.