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If I plug in my memory stick/USB into my laptop and load it up with documents etc. will its mass/weight increase?

user36639
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You confuse between four different concepts:

  • First, between mass and weight.

Yes, mass is not weight. They are two different concepts: mass is a physical property of matter which determines the body's resistance to being accelerated by a force, for example: It requires more force to make a car move than a bicycle, it's because the car has more mass than the bicycle.

Weight is the gravitational force that pulls on mass. On the Moon, your weight is reduced compared to your weight on earth because gravity is weaker there, but your mass is still the same.

  • Second, between light and electricity.

Light is an electromagnetic wave, example: the light coming from the sun.

Electricity is not due to electromagnetic waves, but because of a negatively charged subatomic particle, namely the electron. Everything around you is made up of different kinds of atoms which are themselves composed of a dense nuclei and of electrons. Electricity is the flow of those electrons, from an atom to another. So when you charge your smartphone via USB, the electrons flow from the computer, to the wire, to the battery of the smartphone.

Light doesn't have mass but electrons do, their mass is: $9.11×10^{−31}\text{kg}$. In fact, when you load a USB with documents or when you charge a phone, its mass increase by a tiny tiny amount, around $10^{-18}\text g$. It's because of changes in the level of energy stored of the electrons on your phone, their physical number stays the same but they possess more energy, hence more mass, $E=mc^2$ as Einstein cleverly putt.

To continue in our discussion about electromagnetic waves: Light has no mass, but it do possess momentum, hence it has weight because gravity is caused by momentum and not mass.

  • You might want to add that though the rest-mass of light is zero, it does have relativistic mass in the sense that it has inertia and couples to gravity. – mcFreid Jan 02 '14 at 15:53
  • @mcFreid Adding relativistic mass to the debate will only generate confusion to the OP. –  Jan 02 '14 at 15:54
  • Perhaps, but it more closely answers his question because light does have "weight". – mcFreid Jan 02 '14 at 15:56
  • @mcFreid Okay, I will add a paragraph discussing weight. –  Jan 02 '14 at 15:56
  • I tried to simplify it as much as possible, so pardon me if it contains some impreciseness. –  Jan 02 '14 at 16:18
  • I don't think that the mass-vs-weight discussion has any relevance here. By the equivalence principle and assuming a constant Earth's gravitational field, the mass and weight are proportional - morally the same thing - with the same coefficient. If one increases, so does the other. – Luboš Motl Jan 02 '14 at 16:48
  • @LubošMotl The question's title is Does light have mass/weight? which means that the OP uses the terms mass and weight interchangeably, that's why I had to explain the difference between those two concepts. –  Jan 02 '14 at 16:51
  • Dear @Adobe, my very point is that it is right to use the concepts interchangeably for this question and many others. The fact that the OP uses mass/weight does indicate that he knows that there are two concepts; and he knows that they are related. – Luboš Motl Jan 03 '14 at 08:43
  • "because gravity is caused by momentum and not mass." Mass becomes a measure of quantity as soon as 1 kg has been defined by acceleration by gravity. Only if you understand mass in this sense, measuring quantity, it does not "cause" gravity as gravity is "sth. else" then. - What causes gravity, at all? Gravitiy is the cause for acceleration. The intricacy about rest mass zero (photons) is that photons have impulse because they have the v in m times v (impulse). As no v can be faster than c they cannot be "accelerated" by gravitiy and that is how gravity is defined: acceleration of scales, kg. – Peter Bernhard Nov 21 '22 at 08:33
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Re. 2nd question:-

"If I plug in my memory stick/USB into my laptop and load it up with documents etc. will its mass/weight increase?"

Its mass will not necessarily increase or decrease. It depends on the prior state of the memory. If there are the same average numbers of binary ones and zeros before and after the weight would be approximately the same. This is due to how the ones and zeros are stored on the memory stick. Placed electrons create charges on the chip gates to store information, but if you are just rearranging them there is no extra weight.

Ref. Wiki:- Floating Gate Transistor

"Because the floating gate (FG) is electrically isolated by its insulating layer, any electrons placed on it are trapped there and, under normal conditions, will not discharge for many years."

Chris Degnen
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