What is an Electron?
Democritus proposed that matter is made up of small units called atoms. Slowly with time, experiments and discoveries scientists proposed that atom is not the ultimate particle. It is made up of three subatomic particles called electron, proton and neutron. Protons and neutrons are found in the nucleus of the atom while electron is found in the shells or orbits around the nucleus. Electrons are negatively charged subatomic particles while proton are positive, and neutron are neutral. Structure of an atom is given below –
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Charge on an Electron
Electric charge on an electron is -1.602 10-19C (or experimental value -1.602176634 10-19Coulombs). It is used as a standard unit of charge for subatomic particles. It is called the elementary charge. Symbol of elementary charge is e. The electrical charge on an electron is equal to the charge on proton, but with opposite sign. As the elementary charge is denoted by e, the electron is commonly symbolized by e-. Minus sign denotes negative charge. Charge on an electron is given below in the table in different units.
Mass of an Electron
The intrinsic mass of an electron is approximately 9.109 10-31 kilograms. This mass corresponds to a rest energy of 0.511MeV, according to Einstein’s principle of mass-energy equivalence. We can define its mass in relative terms of proton as well. The mass of an electron is 1/1836 of mass of proton. Mass of electron is given in the table below in different units –
As we can see mass of electron is very less so it can be taken as approximately 0amu.
Charge to Mass Ratio of Electron
As we have already discussed mass and charge of an electron. So, now its easy to calculate the charge to mass ration of electron.
Charge of an electron (e) = 1.602*10-19C
Mass of an electron (me) = 9.109 *10-31 kilograms
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If we take value up to six digits after decimal, then it will be 1.758820 1011Ckg-1.
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FAQs on Charge to Mass Ratio of an Electron
1. What is the charge-to-mass ratio of an electron, and what is its accepted value?
The charge-to-mass ratio (e/m) of an electron is the ratio of its elementary charge (e) to its mass (mₑ). It is a fundamental physical constant. The accepted value for this ratio is approximately 1.758820 × 10¹¹ Coulombs per kilogram (C/kg). This value signifies how much charge an electron carries for each unit of its mass.
2. Who first determined the charge-to-mass ratio of an electron?
The charge-to-mass ratio of an electron was first determined by the British physicist J.J. Thomson in 1897. His experiments with cathode ray tubes provided the first evidence that atoms were divisible and contained much smaller, negatively charged particles, which were later named electrons.
3. How did J.J. Thomson determine the charge-to-mass ratio (e/m) of an electron?
J.J. Thomson determined the e/m ratio using a cathode ray tube. He applied both electric and magnetic fields perpendicular to each other and to the path of the electron beam. By carefully adjusting the strengths of these fields, he could balance their effects, causing the electron beam to travel undeflected. The key steps in his method were:
- Measuring the deflection of the cathode rays under only a magnetic field or an electric field.
- Calculating the amount of force required from the opposing field to counteract the deflection and straighten the beam's path.
- Using the equations for the forces exerted by electric and magnetic fields on a charged particle, he derived an expression for the e/m ratio based on the measurable strengths of the fields and the radius of curvature of the beam.
4. Why is the charge-to-mass ratio of an electron a significant value in atomic physics?
The determination of the charge-to-mass ratio of an electron is highly significant for several reasons:
- It proved that the particles in cathode rays (electrons) were over 1,800 times lighter than the lightest atom (hydrogen), establishing electrons as the first subatomic particles.
- It demonstrated that these particles were fundamental constituents of all matter, as the e/m ratio was the same regardless of the cathode material or the gas in the tube.
- Once Robert Millikan determined the charge of the electron (e) in his oil drop experiment, the e/m ratio allowed for the precise calculation of the mass of an electron (mₑ) for the first time.
5. How does the charge-to-mass ratio of an electron compare with that of a proton?
The charge-to-mass ratio of an electron is significantly larger than that of a proton. While an electron and a proton have charges of equal magnitude (1.602 × 10⁻¹⁹ C) but opposite signs, a proton's mass is approximately 1836 times greater than an electron's mass. Because mass is in the denominator of the ratio (e/m), the electron's extremely small mass results in a much higher charge-to-mass ratio compared to the proton.
6. Why did the charge-to-mass ratio of cathode rays remain constant, irrespective of the gas in the discharge tube or the cathode material?
The charge-to-mass ratio of cathode rays remained constant because electrons are universal constituents of all atoms. No matter which gas was used in the tube or what material the cathode was made of, the particles being ejected were always the same: electrons. This consistent result was crucial evidence that atoms of different elements are all composed of these same fundamental, negatively charged particles.
7. What are the standard units for expressing the charge-to-mass ratio?
The standard SI (International System of Units) for the charge-to-mass ratio is Coulombs per kilogram (C/kg). The charge (e) is measured in Coulombs (C), and the mass (m) is measured in kilograms (kg). This unit directly represents the amount of electric charge per unit of mass.
