A. is the answer
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We write DE = q+w, where DE is the internal energy change and q and w are heat and work, respectively.
(b)Under what conditions will the quantities q and w be negative numbers?
q is negative when heat flows from the system to the surroundings, and w is negative when the system does work on the surroundings.
As an aside: In applying the first law, do we need to measure the internal energy of a system? Explain.
The absolute internal energy of a system cannot be measured, at least in any practical sense. The internal energy encompasses the kinetic energy of all moving particles in the system, including subatomic particles, as well as the electrostatic potential energies between all these particles. We can measure the change in internal energy (DE) as the result of a chemical or physical change, but we cannot determine the absolute internal energy of either the initial or the final state. The first law allows us to calculate the change in internal energy during a transformation by calculating the heat and work exchanged between the system and its surroundings.
Before starting, we will convert all the givens into standard units as follows:
1.8 * 10^14 KJ/sec = 1.8 * 10^14 * 1000 = 1.8 * 10^17 J/sec
24 hours = 24 * 60 * 60 = 86400 second
Now, we are given that:
The earth receives 1.8 * 10^17 J/sec. We will begin by calculating the total energy received by the earth in 24-hours
Total energy = 1.8 * 10^17 * 86400 = 1.5552 * 10^22 Joules
Then we will get the mass from the following rule:
E = m * c^2 where:
E is the total energy = 1.5552 * 10^22 Joules
m is the mass we need to find
c is the speed of light = 3 * 10^8 m/sec
Substitute with the givens to get the mass as follows:
1.5552 * 10^22 = m * (3*10^8)^2
m = 172800 kg
The answer to this is A i think.
