Question 19N.3.SL.TZ0.3
| Date | November 2019 | Marks available | [Maximum mark: 8] | Reference code | 19N.3.SL.TZ0.3 |
| Level | SL | Paper | 3 | Time zone | TZ0 |
| Command term | Calculate, Deduce, Explain, State | Question number | 3 | Adapted from | N/A |
One of the two postulates of special relativity states that the speed of light in a vacuum is the same for all observers in inertial reference frames. State the other postulate of special relativity.
[1]
laws of physics are the same for all observers
OR
laws of physics are the same in all «inertial» frames ✔
NOTE: OWTTE
A long straight current-carrying wire is at rest in a laboratory. A negatively-charged particle P outside the wire moves parallel to the current with constant velocity v relative to the laboratory.
In the reference frame of the laboratory the particle P experiences a repulsive force away from the wire.
State the nature of the force on the particle P in the reference frame of the laboratory.
[1]
magnetic ✔
Deduce, using your answer to part (a), the nature of the force that acts on the particle P in the rest frame of P.
[2]
«from 3a»
force must still be repulsive ✔
for P there is no magnetic force AND force is electric/electrostatic
OR
since P is at rest the force is electric/electrostatic ✔
Explain how the force in part (b)(ii) arises.
[2]
protons and electrons in the wire move with different velocities «relative to P»
OR
speed of electrons is greater ✔
«for P» the density of protons and electrons in wire will be different «due to length contraction»
OR
«for P» the wire appears to have negative charge «due to length contraction» ✔
«hence electric force arises»
NOTE: Do not award mark for mention of length contraction without details.
The velocity of P is 0.30c relative to the laboratory. A second particle Q moves at a velocity of 0.80c relative to the laboratory.
Calculate the speed of Q relative to P.
[2]
✔
✔
NOTE: Accept 0.89c if all negative values used. Accept –0.89c even though speed is required.