physic plz help it is simple i need serious help. splzz show me all your work plzzz tytytyty?

Question

C: Celsius
1 How much heat is needed to raise the temp. of 90kg of water from 18c to 80c?
2 if a 1MJ of heat is transferred to 10kg of water initially at 15c what will its final temp be?
3 if 12kg of water cools from 100c down to room temp. (20c) how much heat will it release to the environment?
4. how much heat would be needed to warm 1.6kg of ice from -15c to its melting point of 0?
5. A 5kg lock of lead at 250c cools down to 20c. How much heat does it give off in doing so?
6. if you must do 500J of work to operate a pulley system, and the pulley system lifts a 50N load to a height of 3m, how efficient is the pulley system?
7. For every megajoule of chemical potential energy in the fuel used to run a certain truck only 120kJ of useful work is done by the truck in making itself move. How efficient is the truck? Where are some of the places that the energy from the fuel is wasted?

Answer 1

Someone needs to get their homework done! I'll help on the first one, but you've got to do the rest.

All of these heat problems have to do with heat going in or heat going out (endothermic vs. exothermic). You get to determine the amounts of heat needed. Sounds like a hoot, no?

Water is a good substance to use because of that handy metric system. You see, the unit of heat in the metric system, the calorie is based on the amount of heat needed to raise the temperature of one gram of water, one degree C. In other words...

1 calorie raises 1g of water 1 degree C.

Another handy item to know is that regardless of whether you're talking about mass or volume, they're the same.

1 g water = 1 mL water

(All of these 1's multiplied together = math done in your head)

90 kg of water converted to grams - 90 kg * (1000g / 1 kg) = 90*1000 g = 90,000 g water (canceling out kg) (note: metric system = math done in head)

90,000 g water heated from 18 degrees C to 80 degrees C.
Change in temp = 80 - 18 = 62 degrees C (still... in my head)

Okay, I'm still typing this, but consider that I'm going to write the conversion from grams to calories like this... 1 calorie = 1 gram * 1 degree C. Since I want to cancel grams and degrees C and leave only calories, I'll write it like this... (1 cal / 1g * 1 degree C).

Alright - I'm ready to wrap this up.
90,000 g water * 62 degrees C * (1 cal / 1 g * 1 degree C)
g's & degree's C cancel and you're left with cals

Answer - 5580000 cals and it was done... in my head.

Yes, I'm a teacher and no, I'm not going to spoonfeed you your answers and yes, I dare you to copy/paste this onto your assignment.

Good Luck!

Answer 2

And for a little extra help, since your questions use J or MJ as units - not calories:

1 calorie = 4.186 Joules

And the specific heat of ice is 0.5 cal/gram/degC and that of lead is 0.0305 cal/gram/degC.

Have fun!