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Cathode Rays

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Cathode Rays

Cathode rays, also known as e-beam or electron beams, are electron streams detected in vacuum tubes. Cathode rays were part of very important experiments used to discover the electron. Let’s find out how!

What is a cathode ray?

Cathode rays, also known as electron beams, are streams of electrons detected in discharge tubes (vacuum tubes). These discharge tubes are devices that control the electric current between a potential difference applied to the electrodes in a high vacuum.

A glow behind the positive electrode (cathode) can be observed when a potential difference (voltage) is applied to the electrodes. The electrons emitting from the cathode is what causes this glow.

To find which electrode is the cathode and the anode, we need to look at the connections between the electrodes and the voltage supply:

  • The electrode connected to the negative pole of the voltage source is the cathode.
  • The electrode connected to the positive pole of the voltage source is the anode.

Properties of cathode rays

These are some properties of cathode rays: they are negatively charged, they travel in a straight route, and they ionise the gas inside the tube. The properties of cathode rays don’t change due to the gas used in the vacuum tube.

Julius Plücker and Johann Wilhelm Hittorf first observed cathode rays in 1869, and Eugen Goldstein named them in 1876. The most important use of cathode rays was discovered by J.J. Thomson in 1897 when he concluded that cathode rays were made up of a previously unidentified negatively charged particle, the electron.

The cathode ray tube

Nowadays, we use the name cathode ray tubes. They were previously called gas discharge tubes or Crookes tubes (named after William Crookes whose experiments showed the earliest direct signs of electrons and their charge).

Cathode ray tubes consist of an evacuated glass with two metal electrodes and rarefied gas within the glass. When a potential difference (voltage) is applied across the electrodes, electrons begin to emit from the cathode towards the anode and accelerate inside the gas due to this high potential difference.

These electrons excite the gas atoms, leading to the emission of electromagnetic radiation. As a result, the route of the electrons becomes visible. The name cathode ray tube originally comes from the fact that the electrons are emitted from the cathode.

In his experiments, Crookes observed that these particles, which we now know are electrons, carry momentum. A magnet can bend their straight path in the expected route for moving negative charges (you can see this in the image below).

Cathode rays A cathode ray tube StudySmarterA cathode ray tube, Micah Sittig CC BY 2.0

Check out our explanation on Momentum.

How were cathode rays used in the discovery of the electron?

J.J. Thomson improved on Crookes’s experiments with gas discharge tubes. Through his experiments, Thomson

  • verified that cathode rays have a magnetic field and an electric field,
  • gathered the rays in a metal cup and discovered an overabundance of a negative charge, and
  • measured the ratio between an electron’s charge and its mass (qe/me), which was one of the most important steps in finding the exact charge of a single electron.

Cathode rays J.J. Thomson’s cathode ray tube StudySmarterA diagram of J.J. Thompsons cathode ray tube, Oğulcan Tezcan – StudySmarter Originals

Take a look at the image below. In Thomson’s experiment, which was used to determine the velocity of electrons, an electric field E is applied between two metal plates, and there is a magnetic field B perpendicular to the electric field because the tube is placed between the opposite poles of a magnet. Since the two forces E and B are perpendicular, they apply opposite forces to the electrons. When these forces cancel each other, the net force on the electrons disappears, which means that the velocity of the charged particle (electron) is v = E/B.

Cathode rays Thomson’s experiment scheme StudySmarterA schematic diagram showing the electron beam passing between the plates, Oğulcan Tezcan – StudySmarter Originals

Determining qe/me

To understand how Thomson determined qe/me, we look at the relation between the forces. Recall that

where F is the net force on the electron measured in newtons (N), qe is the charge of the electron measured in coulombs (C), and E is the electric field affecting the electron measured in newton per coulomb (N/C).

The vertical deflection is related to the electron’s mass, and we know from the acceleration equation that

where F is the net force on the electron (N), me is the mass of the electron measured in kilograms (kg), and a is the acceleration of the electron measured in metres per second squared (m/s2).

At that time, qe was not yet known, and since F is not known, we can apply the first equation to the previous one to see things more logically:

Now, if we solve the equation for qe/me, we have

We can calculate the deflection to obtain a and calculate E using the applied voltage and the distance between the plates.

The importance of qe/me

The importance of the electron’s charge-to-mass ratio was crucial because Thomson calculated it as -1.76 ⋅ 10-11 C/kg. This was a considerable number, and Thomson stated that this implies the electron has such a small mass. Today we know it is so small that the mass of a proton is 1836 times the mass of an electron.

Later on, Thomson conducted different experiments using different methods (such as the photoelectric effect) to emit electrons from atoms. Still, he always came up with the same properties for the electron, determining it as an independent particle.

It was only when I was convinced that the experiment left no escape from it that I published my belief in the existence of bodies smaller than atoms. - J.J. Thomson

Cathode Rays - Key takeaways

  • Cathode rays, also known as electron beams, are streams of electrons in discharge tubes (vacuum tubes).
  • Discharge tubes are devices that control electric current between a potential difference (voltage) applied across electrodes in a high vacuum.
  • When a potential difference is applied to the electrodes in a cathode ray tube, a glow behind the positive electrode (cathode) can be observed. The reason for the glow is the electrons emitting from the cathode.
  • Cathode ray tubes are also called gas discharge tubes or Crookes tubes.
  • The charge-to-mass ratio of the electron was significant because Thomson calculated it to be -1.76 ⋅ 10-11C/kg, which was a considerable number. Thomson stated that this implies that the electron has such a small mass.
  • After various experiments, Thomson concluded that the electron is an independent particle, making it the first subatomic particle discovered.

Images

Cathode ray tube. https://www.flickr.com/search/?text=cathode%20ray%20tube&license=4%2C5%2C6%2C9%2C10

Frequently Asked Questions about Cathode Rays

A cathode ray oscilloscope is used to examine the signal characteristics, oscillation distortion, and signal frequency response.

Cathode rays are produced when there is a potential difference (voltage) applied across the electrodes. The electrons begin to emit from the cathode towards the anode and accelerate inside the gas due to this high potential difference. These electrons excite the gas atoms, leading to the emission of electromagnetic radiation. Thus, the route of the electrons becomes visible.

Cathode rays are electron streams detected in vacuum tubes. These are some properties of cathode rays: they are negatively charged, they travel in a straight route, and they ionise the gas inside the tube. The properties of cathode rays don’t change due to the gas used in the vacuum tube. 

Final Cathode Rays Quiz

Question

Which of the following is not one of the names for cathode ray tubes?

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Answer

Test tubes.

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Question

Which of the following makes the route of the electrons possible in a cathode ray tube?

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Answer

Electrons exciting the gas atoms, leading to the emission of electromagnetic radiation.

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Question

Which of the following was J.J. Thomson able to determine as a result of his experiments?


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Answer

The ratio between an electron’s charge and its mass.

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Question

Which of the following is false?


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Answer

In J.J. Thomson’s cathode ray experiment, the electric and magnetic fields were parallel to each other.

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Question

Which of the following was the first subatomic particle discovered?


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Answer

Electrons.

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Question

Was J.J. Thomson able to determine the mass of a proton?


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Answer

No, J.J. Thomson did not determine the mass of a proton.

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Question

If a cathode ray tube is affected by an external source of a magnetic field, will the electron beam change direction?


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Answer

Yes.

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Question

Where does the name cathode ray tube come from?


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Answer

The name cathode ray tube comes from the fact that electrons are emitted from the cathode.

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Question

Which scientist used cathode ray tubes in his experiments where he discovered the electron?


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Answer

J.J. Thomson.

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Question

Cathode ray tubes consist of an evacuated glass with two metal electrodes and rarefied gas within the glass. True or false?


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Answer

True.

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Question

The electrode connected to the negative pole of the voltage source is the anode. True or false?


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Answer

False. The electrode connected to the negative pole of the voltage source is the cathode. 

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Question

When were cathode rays first observed?


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Answer

1869.

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Question

The electrode connected to the positive pole of the voltage source is the anode. True or false?


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Answer

True. 

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Question

What is the symbol for the electric field?


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Answer

The symbol for the electric field is E.

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Question

What is the symbol for the charge of an electron?


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Answer

The symbol for the charge of an electron is qe.

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