LED circuit loss of weight due to mass of photons

[NG Neer]

Let's suppose you have a big capacitor (1 farad or so) charged up, and
you connect an LED across it. It will keep it glowing for several
minutes at least. While this is going on, does the circuit overall
(LED, cap, wire...) lose an infinestimately small amount of weight due
to the photons leaving the LED?

Is the loss in weight because some of the electrons stored in the cap
were actually *converted* to photons, or is the photon production
explained by energy being directly converted to mass (E=MC^2)? Thanks.

 

[Tim Williams]

Photons are massless. The mass gets converted to energy.

The assembly will weigh about 5 x 10^-14 grams less (about 32 billion proton
masses). Sadly, you won't be able to measure this, because evaporation and
oxidation of the casing, adsorbtion of air and pollutants on external
surfaces, etc. are all many orders of magnitude greater in effect, let alone
the practical matter of a parts-per-quadrillion scale to weigh it on.

Tim

 

[NG Neer]

A hot brick is
heavier than a cold brick

I never considered that. Is the weigth increase due to heating of the
brick purely giverned by E=MC2? (ie would adding one Joulse of energy
really increas the weight by 1/C^2 Kg?)

I always thought that quantum physics like E=MC2 only take effect in
extreme cases (ie fission or fusion). Could you actually measure (on a
*very* sensitive scale) the weight difference of hot/cold bricks?
Thanks...

 

[Tim Williams]

I always thought that quantum physics like E=MC2

*Cough* relativity. Quantum physics is the diametrical opposite of
relativity. Go ahead, ask Einstein yourself and find out. ;-)

Tim

 

[z]

Let's suppose you have a big capacitor (1 farad or so) charged up, and
you connect an LED across it. It will keep it glowing for several
minutes at least. While this is going on, does the circuit overall
(LED, cap, wire...) lose an infinestimately small amount of weight due
to the photons leaving the LED?

Is the loss in weight because some of the electrons stored in the cap
were actually *converted* to photons, or is the photon production
explained by energy being directly converted to mass (E=MC^2)? Thanks.

gotta give ya points for a good and original question.

 

[Don Klipstein]

Let's suppose you have a big capacitor (1 farad or so) charged up, and
you connect an LED across it. It will keep it glowing for several
minutes at least. While this is going on, does the circuit overall
(LED, cap, wire...) lose an infinestimately small amount of weight due
to the photons leaving the LED?

Is the loss in weight because some of the electrons stored in the cap
were actually *converted* to photons, or is the photon production
explained by energy being directly converted to mass (E=MC^2)? Thanks.

The latter. The electric field in a charged capacitor has mass. The
capacitor will lose mass as it discharges, though every electron that
leaves it from one lead is replaced by an electron going in the other
lead.

- Don Klipstein ([email protected])

 

[Don Klipstein]

Photons are massless.

That part is not true. Although photons have no rest mass - but nobody
can slow down a photon. Photons have no matter, but energy has mass, and
photons have energy.

Any successful decelleration of a photon will reduce its momentum
without reducing its velocity. And photons do have momentum with finite
velocity.

The mass gets converted to energy.

The assembly will weigh about 5 x 10^-14 grams less (about 32 billion proton
masses). Sadly, you won't be able to measure this, because evaporation and
oxidation of the casing, adsorbtion of air and pollutants on external
surfaces, etc. are all many orders of magnitude greater in effect, let alone
the practical matter of a parts-per-quadrillion scale to weigh it on.

- Don Klipstein ([email protected])

 

[Tim Williams]

That part is not true. Although photons have no rest mass - but nobody
can slow down a photon. Photons have no matter, but energy has mass, and
photons have energy.

Ok, mass-energy, but not mass /per se/, by which I meant RME. 'Cuz they're
never at rest. Chock it up to ambiguous phrasing?

Any successful decelleration of a photon will reduce its momentum
without reducing its velocity. And photons do have momentum with finite
velocity.

....And no mass. That sometimes fries people -- "but p = mv!". Ah, but at
exactly the speed of light, what is mass?

Ain't relativity great? ;-)

Tim

 

[JosephKK]

There was a young lady named Bright
Whose speed was much greater than light
She set out one day
In a relative way
And arrived the previous night.



Jim

And the young man named Fisk
whose fencing was exceedingly brisk
in a tourney one day
in a relative way
reduced his rapier to a disk.

 

[JosephKK]

There was a young lady named Bright
Whose speed was much greater than light
She set out one day
In a relative way
And arrived the previous night.



Jim

I will have to try again, getting the limerick right this time:

There was a young man named Fisk,
Whose fencing was exceedingly brisk.
So fast was his action
that the Fitzgerald contraction
reduced his rapier to a disk.

- unknown

 

[Rich Grise]

...
There was a young man named Fisk,
Whose fencing was exceedingly brisk.
So fast was his action
that the Fitzgerald contraction
reduced his rapier to a disk.

I think this scans better:

There once was a young man named Fisk,
Whose fencing's exceedingly brisk.
So fast was his action
Fitzgerald contraction
Reduced his epee to a disk.

;-)
Rich

 

[JosephKK]

As others have said - the charge.
Also worth noting is that photons have momentum.
My keyring torch weighs some 40g, and outputs some 1000J of photons.

This would impart a momentum of some 1000J/C - or 3*10^-6Kgm/s, or the
heady speed of 75 microns/second, or over the hour and a half or so of
discharge, 20cm or so, ending up at a speed of some 2Km/year.

1000 Joules in how much time?