Chapter 1 turning points

Question 1

TP Chapter 1 Q) What is a discharge tube?

Answer 1

• A) The magnetic force acts at right angles to the velocity of the particles and magnetic field. This is a centripetal force that causes the particle to move in a circle according to FLHR.

Question 2

TP Chapter 1 Q) Explain thermionic emission

Answer 2

• A) A metal filament is heated due to a current through it. The temperature of the filament rises. The electrons gain enough kinetic energy to leave the metal surface.

Question 3

TP Chapter 1 Q) How are the charged particles in a discharge tube created?

Answer 3

• A) Electrons are removed from atoms to cause positive ions and free electrons. Ionisation through collision of fast moving electrons can also occur. Both positive ions and free electrons conduct.

Question 4

TP Chapter 1 Q) What causes that positive glow?

Answer 4

• A) Electrons - which are created through ionisation or collisions with the cathode.

Question 5

TP Chapter 1 Q) What causes the negative glow?

Answer 5

• A) Photons are emitted when free electrons and ions recombine.

Question 6

TP Chapter 1 Q) Why is there a gradient on the positive glow?

Answer 6

• A) The glow is stronger when there are more free electrons - which increases the chance of collisions/recombination. However when this occurs the electrons lose KE and therefore the energy needed to cause exciation. So there is less chance of a photon being emitted - hence the glow becomes fainter.

Question 7

TP Chapter 1 Q) Why does the discharge tube have to be at a low pressure?

Answer 7

• A) If the gas was at a higher pressure, there would be more chance of electrons and ions combining. This means they would not accelerate to high enough speeds to cause a glow through excitation. The particles need enough KE to cause excitation.

Question 8

TP Chapter 1 Q) Why does the specific charge of the particles moving towards the negative electrode (positive rays) depend on the choice of gas?

Answer 8

• A) As these are determined by the positive ions. Which mass will change with different elements chosen for the gas. Heavier gases will have a lower specific charge due to m being larger in Q/m.

Question 9

TP Chapter 1 Q) Why does the specific charge of the particles moving towards the positive electrode (negative rays) not depend on the choice of gas?

Answer 9

• A) As these are caused by electrons. These are a substructure of all elements. It does not matter which gas is used as electrons will be contained in all elements.

Question 10

TP Chapter 1 Q) When Crookes put a paddle wheel in the discharge tube - why did it move? What does this show?

Answer 10

• A) The paddle moved due the momentum of the electrons.

Question 11

TP Chapter 1 Q) How is the paddle affect when air particles are absorbed by the walls of the tube?

Answer 11

• A) Fewer electrons in the cathode rays as some have been absorbed. So the wheel rotates less as there’s a smaller change of momentum.

Question 12

TP Chapter 1 Q) What conclusions did Thomson make from his experiments?

Answer 12

• A) Cathode rays were negatively charged particles. Particles had much smaller mass than hydrogen due to much larger specific charge. Particles were part of substructure of atoms as particles were the same from every element.

Question 13

TP Chapter 1 Q) What is the electron gun equation?

Answer 13

• A) eVA = 1/2mv2

Question 14

TP Chapter 1 Q) When would the electron gun equation not apply?

Answer 14

• A) When the speeds of the electrons are near the speed of light and Newtonian mechanics no longer applies.

Question 15

TP Chapter 1 Q) What is the specific charge of the electron?

Answer 15

• A) 1.76x1011 Ckg-1

Question 16

TP Chapter 1 Q) What is the specific charge of a hydrogen ion?

Answer 16

• A) 9.58x107 Ckg-1

Question 17

TP Chapter 1 Q) What is the significance of the specific charge of the electron?

Answer 17

• A) It was many times greater (x 1800) the specific charge of hydrogen - which had the largest specific charge at the time. This implied the particle was many times lighter than hydrogen.

Question 18

TP Chapter 1 Q) State three methods that was used to determine the specific charge of the electron

Answer 18

• A) Using an electric field, using a magnetic field and using both and electric and magnetic field.

Question 19

TP Chapter 1 Q) Derive an equation for the specific charge using a uniform electric field

Answer 19

• A) Q/m = ad/Vp, where a = 2y/t2, and t = L/v

Question 20

TP Chapter 1 Q) Derive an equation for the specific charge using a magnetic field

Answer 20

• A) Q/m = V/B2rd = E/B2r

Question 21

TP Chapter 1 Q) How can you determine the velocity of a charged particle going in a straight line in a magnetic and electric field?

Answer 21

• A) v = E/B

Question 22

TP Chapter 1 Q) Derive an equation for the specific charge using a both electric and magnetic fields

Answer 22

• A) Q/m = 2VA/B2r2

Question 23

TP Chapter 1 Q) Why does a beam of electrons follow a parabolic motion when fired between two electric plates?

Answer 23

• A) As there is a constant force acting vertically on the electrons. Vertical component of the electrons velocity increases but the the horizontal component of velocity is unchanged.

Question 24

TP Chapter 1 Q) What is Stokes’ Law?

Answer 24

• A) F = 6πrηv

Question 25

TP Chapter 1 Q) Outline the components of Millikan's experiment

Answer 25

* A) Charge oil droplets using X-Rays. Release the oil droplets and measure their terminal velocity. Using this and Stoke's law calculate the radius of the droplet (using r2 = 9ηv/2ρg). Apply a pd to so electric field is strong enough to cause the oil droplet to become stationary. m can be calculated if r is known. Can calculate Q using Q = mgd/V. Take many repeats to calculate Q.

Question 26

TP Chapter 1 Q) What was the significance of Millikan's results?

Answer 26

* A) Charge of oil droplets is always an integer number of 1.6x10-19 C. This charge is the charge of the electron. Charge is quantised in these amounts.

Question 27

TP Chapter 1 Q) What equation states the radius of the oil droplet in Millikan's experiment?

Answer 27

* A) r2 = 9ηv/2ρg

Question 28

TP Chapter 1 Q) Why does an electron move in a circle in a magnetic field at right angles to the direction of travel of the electron?

Answer 28

* A) The magnetic force acts at right angles to the velocity of the particles and magnetic field. This is a centripetal force that causes the particle to move in a circle according to FLHR.

Question 29

TP Chapter 1 Q) How do you calculate the velocity of an electron in circular motion in a magnetic field?

Answer 29

* A) v = Bqr/m

Question 30

TP Chapter 1 Q) Why is no work done on an electron in circular motion due to a magnetic field?

Answer 30

* A) No work is done as the kinetic energy is not increased as force is not in same direction as the motion of the particles.

Question 31

TP Chapter 1 Q) What is the momentum of an electron accelerated by 30 keV?

Answer 31

* A) p = √2eVm = 9.35x10-23 kgms-1

Question 32

TP Chapter 1 Q) If the radius of the oil droplet is known, how can the mass of the oil be determined?

Answer 32

* A) m = 4πr3ρ/3