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Physics > Discovery of the Electron > Flashcards

Discovery of the Electron Flashcards

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1
Q

Why does a discharge tube apply a high voltage?

A

To ionise the gas atoms in the tube

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2
Q

What happens to gas atoms after they have been ionised in a discharge tube?

A

They are positive ions and are attracted to the cathode

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3
Q

What effect does positive ions being attracted onto the cathode surface have?

A

Free electrons from the cathode surface are emitted

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4
Q

Where do the electrons come from in a discharge tube?

A

They are both emitted by the cathode and produced in the production of positive ions

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5
Q

Why were electrons called cathode rays?

A

Because they were observed to originate at the cathode

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6
Q

How do electrons cause a glow in the discharge tube?

A

By collisions with gas atoms, causing them to be excited, emitting visible light on de-excitation

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7
Q

At what point is visible light emitted in cathode ray tubes?

A

When a gas atom de-excites, after an electron has collided with it and excited it

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8
Q

When are photons emitted in cathode ray tubes?

A

When positive ions and electrons recombine

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9
Q

Which way do electrons travel in cathode ray tubes?

A

Towards the anode

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10
Q

What type of pressures are the gases in discharge tubes?

A

Low

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11
Q

What does the colour of light emitted by a discharge tube depend on?

A

The type of gas inside the tube

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12
Q

What is created when a high voltage is applied between the anode and cathode in a discharge tube?

A

A strong electric field

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13
Q

Why is the gas in a cathode ray tube kept at low pressure?

A

So there is space between gas atoms to allow positive ions to be accelerated towards the cathode

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14
Q

What is thermionic emission?

A

When a metal filament is heated by passing a current through it, and some of the free electrons gain enough kinetic energy to leave the metal surface

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15
Q

During thermionic emission, what is the anode and cathode?

A

Cathode - the wire filament

Anode - metal plate with a small hole in it

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16
Q

In thermionic emission, how are electrons accelerated through the hole in the anode?

A

A high potential difference between the cathode and anode

17
Q

In thermionic emission, what is work done on the electrons, eV, equal to?

A

Kinetic energy gained (0.5mv²)

18
Q

What does thermionic emission essentially produce?

A

An electron beam

19
Q

How is the metal heated in thermionic emission?

A

By passing an electric current through it

20
Q

Does the glass tube used in thermionic emission have gas molecules in it?

A

No - it is evacuated

21
Q

In thermionic emission, what is the equation that gives the speed of each electron in the beam from the anode?

A

eV = 1/2mv²

22
Q

Why can the speed of electrons in thermionic emission be calculated by eV = 1/2mv²?

A

Their kinetic energy doesn’t change after they pass the anode, as the electric field is no longer acting

23
Q

Which is a more effective way of freeing electrons from a metal to create electron beams, thermionic emission or cathode ray tubes?

A

Thermionic emission

24
Q

What is the work function of a metal?

A

The minimum energy needed to remove one of the free electrons to just beyond the surface of a metal

25
Q

What are the assumptions when using eV = 1/2mv²?

A
  • each electron starts from the cathode with negligible kinetic energy compared to work done on it by the accelerating potential difference
  • the speed of the electrons in the beam is much less than the speed of light in free space, so the non-relativistic formula for kinetic energy applies
26
Q

How is the specific charge on an electron found in a deflection tube using a magnetic field?

A
  • Bev = mv²/r so r = mv/Be
  • combine this with eV = 1/2mv² (rearrange r for v)
  • e/m = 2V/B²r²
27
Q

Why are electron tubes usually evacuated?

A

So that the electrons are not slowed down by collisions

28
Q

How is the specific charge on an electron found in a deflection tube using a magnetic field to deflect the beam and an electric field to balance the deflection?

A
  • radius of curvature of electron beam in magnetic field is measured
  • electron field applied to beam path is straight and forces due to fields are balanced
  • eE = BeV and E = V/d so V = E/B
  • r = mv/Be
  • so e/m = v/E
29
Q

How is the specific charge on an electron found in a deflection tube using an electric field to deflect the beam and a magnetic field to balance the deflection?

A
  • horizontal speed of electron beam found from eE = BeV and E = V/d so V = E/B
  • vertical deflection y of beam at end of plates in measured
  • s=ut + 1/2at²
  • hence e/m = ad/V where d is separation of plates
30
Q

Before the specific charge of the electron was measured, what was known to have the largest specific charge of any charged particle?

A

Hydrogen at 9.6 x 10⁷ Ckg-1

31
Q

Why could Thomson not conclude that the electron has a much smaller mass than the hydrogen atom from e/m?

A

As neither the mass nor the charge of the electron was known at that time

32
Q

How does the specific charge of an electron compare to that of a hydrogen atom?

A

The e/m of an electron is 1860 times larger

33
Q

How did Millikan measure the charge on the electron?

A
  • used oil spray and oppositely charged parallel metal plates
  • plates horizontal so field vertical
  • could make charged droplet stay stationary by adjusting potential difference between plates until electric force was equal and opposite to weight of droplet
34
Q

In Millikan’s oil droplet experiment, what equations are used?

A

QV/d = mg
so charge on droplet, Q=mgd/V

35
Q

In Millikan’s oil droplet experiment, how do the oil droplets become charged?

A

Frictional effects between the sides of the nozzle and the oil (or using X-rays)

36
Q

In Millikan ‘s oil droplet experiment, why do some droplets move up and some move down?

A
  • some have positive/negative charges
  • some are heavy/light compared to their charge and so move in different directions
37
Q

In the equation Q = mgd/V, which quantity is hard to measure? How can this be solved?

A
  • mass - use density
  • so m = 4/3πr³
  • of which radius is hard to measure so use stokes law F (mg) = 6πηrv to find r

Physics (8 decks)

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