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1

TP Chapter 1 Q) What is a discharge tube?

* 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.

2

TP Chapter 1 Q) Explain thermionic emission

* 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.

3

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

* 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.

4

TP Chapter 1 Q) What causes that positive glow?

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

5

TP Chapter 1 Q) What causes the negative glow?

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

6

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

* 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.

7

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

* 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.

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?

* 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.

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?

* 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.

10

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

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

11

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

* 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.

12

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

* 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.

13

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

* A) eVA = 1/2mv2

14

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

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

15

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

* A) 1.76x1011 Ckg-1

16

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

* A) 9.58x107 Ckg-1

17

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

* 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.

18

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

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

19

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

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

20

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

* A) Q/m = V/B2rd = E/B2r

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?

* A) v = E/B

22

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

* A) Q/m = 2VA/B2r2

23

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

* 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.

24

TP Chapter 1 Q) What is Stokes' Law?

* A) F = 6πrηv

25

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

* 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.

26

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

* 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.

27

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

* A) r2 = 9ηv/2ρg

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?

* 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.

29

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

* A) v = Bqr/m

30

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

* 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.