# Physics Internal energy數(40)

1)A pack of oat flakes of 650g provides 387.4 kacl of energy for every 100g

(a) Given that 1 kcal = 4200 J, find the energy provided by the whole pack in joules

(b) The temperature of a certain amount of water is raised from 25 °C to 90 °C using the energy in (a). Find the mass of the water.

2)Jack takes out a pack of sushi of mass 500g from a 2 °C refrigerator, but he just leaves on a table. Germs breed so quickly at 22 °C that once the sushi reaches this temperature, it will be unsuitable for eating. If the specific heat capacity of the sushi is 3 kJ kg^-1 °C^-1 and it gains heat at an average rate of 7 W at room temperature of 25 °C, within what time must Jack eat the sushi after taking it out of the refrigerator?

**Please list out the formular and SI unit used

THX

Rating

1. (a) Energy provided by a pack of flakes = (650/100) x 387.4 kcal

= 2518.1 kcal = 2518.1 x 4200 J = 1.06 x 10^7 J

(b) Let m (in kg) be the mass of water

m x 4200 x (90-25) = 1.06 x 10^7

where 4200 J/kg-K is the specific heat capacity of water

solve for m

2. Let t (in seconds) be the required time

Heat gained by the sushi before reaching 25 C= 7.t

thus, (500/1000) x 3000 x (22-2) = 7t

solve for t

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