Electro

13) The US penny was formerly made mostly of copper and is now made of copper-coated zinc. Can a calorimetric experiment be devised to test for the metal content in a collection of pennies? Ifso, describe the procedure you would use.
Answer
To try to analyze the metal content of coins in the collection mentioned could conduct an experiment of thermal equilibrium. Where have themass of coins and the specific heat of copper may find the content of zinc in the coins.
By heating the currencies at a certain temperature and place in a glass container with water at acertain temperature and knowing the specific heat of both together with the copper can perform operations with the formula:
m1c1(te – t1) = m2c2(t2 – te) * m3c3(t3 – te)

thus substituting thevalues ​​we solve the mass of metal in order to obtain the result.

31) Consider the cycle process described in figure P20.30 if Q is negative for the process BC and ∆Eint is negative for theprocess CA, what are the signs of Q, W and ∆Eint that are associated with each process?

Answer
| Q | W | ∆Eint |
a) A B | 0 | + | + |
b) B C | + | 0 | +|
c) C A | 0 | - | - |

(a) Because the pumping is rapid, no energy enters or leaves the system by heat. Because W > 0 when works is done on the system, it is positivehere. Thus, we see that U = Q + W must be positive. The air in the pump is warmer.

(b) Since it is an isometric process, the volume is constant and there is no work done either on or bythe system, but energy transfers by heat, making both Q and U positive.

(c) Again no energy transfers into or out of the system by heat, but the air molecules escaping do work on thesurrounding air molecules as they push them out of the way. Thus W is negative and U is negative. The decrease in internal energy is evidenced by the fact that the escaping air becomes cooler.