# Question #5fabf

Feb 27, 2017

You scratch my back, I scratch yours. Gender equality. All Lives Matter. Bringing balance to the force. What goes around comes around.

#### Explanation:

1st Law of Thermodynamics:
$\Delta E = \Delta U + \Delta W$

$\Delta E$ is the total energy given to the system that you are interested in. Eg: Energy to heat a hot potato.

$\Delta U$ is the internal energy of the system. Eg: Heat energy, potential energy.

$\Delta W$ is the work done by the system.

Some conservation of energy examples:

I give $10 J$ of energy to heat up a balloon. Lets say the balloon gets hotter by $1 K$ per Joules of energy. So I'd expect an increase of $10 K$ to my balloon but in practice, I see perhaps an increase in $5 K$.

That's $\Delta E = 10 J$ & $\Delta U = 5 J$. What happened to the rest? Back to the 1st law, if we solve the equation for $\Delta W$, We get $\Delta W = \left(10 - 5\right) J = 5 J$. The work done by the gas is positive which causes gas in the balloon to expand. Hence, hot gas expands.

Another example is dropping a rocket from high altitude to sea level. If you watched space documentaries, space shuttles heat up upon reentry. In a perfect vacuum, we'd expect all the gravitational potential energy to change into purely kinetic energy, the energy of motion. In the atmosphere, we have air resistance. The kinetic energy is not as much as we'd expect. If I had $\Delta E = 10 J$ of gravitational potential at my disposal, $\Delta W = 8 J$, work done by the rocket (kinetic energy) while $\Delta U = 2 J$, heat energy.

Purely on theoretical physics and mathematics;
According to Noether's theorem , laws of physics are unchanging with time hence energy being the conjugate pair of time is conserved.

Another conjugate pair is displacement and momentum . Hence momentum is conserved.

Cheers.