Questions I Got Wrong In Past Papers 2

Question 1

Explain why no emf is generated when a ring perpendicular to a magnetic field is moved in the same direction as the field lines?

Answer 1

There is no change in magnetic flux and therefore no emf induced.

Question 2

A sealed bag of air is inside a container which also contains air.

Air is released from the container and the sealed bag of air expands.

Explain why the bag of air expands when air is removed from the container.

Answer 2

effect on gas in the box eg

pressure in the box falls (because p = NkT/V) 

particle density in box falls 

fewer particles hitting outside of bag (per second);

reason why bag volume increases eg

 pressure inside initially greater than outside 

more (frequent) collisions from gas inside bag than outside 

V rises so that p falls until same as in box; 

force pushing bag outwards (from particle collisions) initially greater than force pushing bag inwards

Question 3

The rate of rotation of a galaxy can be calculated by measuring the wavelength of light from each side of the galaxy.

Explain why there is a difference in redshift of the light from the left hand side from the right hand side of the galaxy.

Answer 3

(red shift different because) one side moves away from us faster than the other side (because of rotation); red shift / wavelength increases for increased (relative) velocity;

ignore blue shift

Question 4

The electrical behaviour of a thermistor can be modelled as follows:

• most electrons are bound to atoms
• those few electrons with extra energy E are able to move.

Use the ideas about Boltzmann factor to explain why the conductance of a thermistor increases with increasing temperature.

Answer 4

any three of the following, [1] each:

• BF gives proportion / probability / fraction of electrons with energy ε / able to move;
• BF increases with increasing temperature
• average energy per particle increases with increasing temperature
• energy exchanged between atoms / electrons at random
• electrons more likely to / more electrons obtain energy ε as temperature increases
• current is flow of free electrons

Question 5

The time dilation factor for a space craft travelling relative to Earth is 1.7 .

Here are some possible times in year for the round trip according to the observers on the space craft. Put a ring around the correct value:

6.0 , 8.0 , 10 , 17

Answer 5

6. 0.
   - Remember: If we choose the people on space ship as the observers then its as if they are stationary and Earth is moving relative to them.

Question 6

When liquid bromine is released into a tube full of air at atmospheric pressure, it can take up to an hour for the brown colour to completely fill up tube. This is because the bromine molecules follow a random walk through the air.

Explain what is meant by the random walk and why the bromine molecules in the air filled tube undergo a random walk.

Answer 6

description of random walk eg

 path is a string of straight lines with varying length and direction;

reasons for random walk

 bromine molecules collide with air molecules;

 (random) change of direction/velocity on each collision;

Question 7

The average displacement of a particle which follows a random walk of N steps is proportional to N^0.5. Explain how this rule can be used to justify the relationship x = C x t^0.5 , where x is the average displacement of a bromine molecule in a time t and C is a constant.

Answer 7

EITHER average speed (and separation) of molecules constant OR average distance between collisions constant; (number of collisions) N proportional to (elapsed time) t;

x proportional to N^0.5

N proportional to t

so x = C x t^0.5