It's easy to imagine that, after charging your phone, its weight will have increased. The argument goes something like this: recharging the battery is replacing the electrons which have been used to power the phone.
Since electrons have a mass (though extremely small), you must have added weight to the phone.
We can calculate the amount of this added mass.
Most phone batteries have a maximum electrical charge of around 12,000 coulombs.
A single electron has a charge of about 1.6×10−19 coulombs, meaning that the battery must contain something like 7.5×1022 electrons.
Each electron has a mass of 9.1×10−31 kg, so altogether they contribute 0.00006825 grams to the mass of the phone. This is about the mass of a single grain of pollen.
However, there is something fundamentally wrong with this argument. The lithium-ion batteries in your phone work by storing negatively-charged electrons at the cathode and positively-charged lithium ions at the anode.
When the phone is switched on, the electrons travel from the cathode through the phone’s circuitry before combining with the lithium ions at the anode.
When the phone is being charged, the electrons are forced back to the battery’s cathode, ready to power the circuit again.
This process doesn’t add any more electrons to the phone’s battery; it just redistributes those already present. So, charging your phone actually doesn’t add any additional weight.
This article is an answer to the question (asked by Dick Hooker, via email) 'Does my phone weigh more when it's fully charged?'
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