Posted by Rachel | Posted in Tea and Accessories | Posted on 21-07-2009
Tags: electric, heater, kettle, tea, tea water heater electric, water
please help me on physics?
I need help on these two problems.i cant find the formulas in the book and it just doesn’t make sense to me. can anyone help me solve these?
1) Think about using the conversion of gravitational potential energy to thermal energy in order to heat water for showers. Assume that 1.0L of water(or 1000g of water). initially at 12.7 celsius falls over a 50m waterfall. if all the potential energy of the water is converted to kinetic energy as the water is falling, and the kinetic energy is converted into thermal energy when it reaches the bottom, wat temp. difference exists btwn the bottom and the top of the waterfall? Will this be sufficient to produce a warm shower?
2) An electric immersion heating coil is used to bring 90.0mL of water to a boil, for a cup of tea. if the immersion heater is rated 200W, find the time needed to bring this amount of water, initially at 21 celsius to the boiling point.
1) You know to know that 4.2 x 10^3 joules of any kind of energy is needed to raise the temperature of 1 kg of water by I˚ C
Energy delivered by the falling water = m . g . ∆h = 1 . 9.8 . 50 = 490 Joules
Heat energy produced = 490 J = m . sp ht . temp rise
490 = 1 . 4.2 . 10^3 . temp rise
temp rise = 0.116 = 0.12˚C
2) Heat needed = m . sp ht . ∆t = 90 . 10^–3 . 4.2 . 10^3 . 79 = 29862 J
200 W = 200 joules per second so the time taken to supply the required heat = 29862 / 200
= 149.3 s = 150 s = 2 and a half minutes.
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