Question

В чайник налита вода при температуре T=19∘C, которая закипает на электроплитке за время t0=8 мин. За какое время с начала кипения треть воды выкипит? Ответ выразить в мин, округлив до целых. Удельная теплоёмкость воды составляет c=4200 Дж/(кг⋅∘C), а удельная теплота парообразования воды − L=2300 кДж/кг. Теплоёмкостью чайника, а также потерями тепла в окружающую среду пренебречь. Мощность электроплитки считать постоянной. Конденсацией воды на стенках и крышке чайника в процессе кипения пренебречь.

Answer 1

Это физика скорее. )
Для кипения воду надо нагреть до 100 градусов, т е на 81 градус. 
Для этого надо подвести тепла Q=4200*81*массу воды (m).
Это было сделано за 8 минут. Значит тепловой поток 340200*m/8/60=708.75m в секунду.
Для испарения одной трети надо подвести тепла Q=2300000* m/3=766666m
Требуемое время t= (766666*m)/(708.75*m)= 1082 секунды = 18 минут
 

Answer 2

Problem Analysis

We are given the initial temperature of water in a kettle, the time it takes for the water to boil, and we need to determine the time it takes for one-third of the water to evaporate after boiling starts. We are also provided with the specific heat capacity of water and the specific heat of vaporization. We need to calculate the time it takes for one-third of the water to evaporate, assuming no heat loss to the surroundings and neglecting condensation on the walls and lid of the kettle.

Solution

To solve this problem, we can use the concept of heat transfer. The heat required to raise the temperature of the water from its initial temperature to the boiling point is given by:

Q1 = m * c * (T1 - T0)

where: - Q1 is the heat transferred to raise the temperature of the water, - m is the mass of the water, - c is the specific heat capacity of water, - T1 is the boiling point temperature, and - T0 is the initial temperature of the water.

The heat required to evaporate one-third of the water is given by:

Q2 = m * L

where: - Q2 is the heat required for vaporization, - L is the specific heat of vaporization.

Since the heat transferred during boiling is equal to the heat required for vaporization, we can equate Q1 and Q2:

m * c * (T1 - T0) = m * L

Simplifying the equation, we find:

T1 - T0 = L / c

Now, we can calculate the time it takes for one-third of the water to evaporate. Let's assume the total time for boiling is t0. The time it takes for one-third of the water to evaporate, t1, can be calculated using the following equation:

t1 = (t0 * (T1 - T0)) / (3 * (T1 - T0))

where: - t1 is the time it takes for one-third of the water to evaporate, - t0 is the total time for boiling, - T1 is the boiling point temperature, and - T0 is the initial temperature of the water.

Now, let's substitute the given values into the equation and calculate the time.

Calculation

Given: - T0 = 19°C - t0 = 8 minutes - c = 4200 J/(kg⋅°C) - L = 2300 kJ/kg

First, let's convert the specific heat of vaporization from kJ/kg to J/kg:

L = 2300 kJ/kg = 2300 * 1000 J/kg = 2,300,000 J/kg

Next, let's calculate the boiling point temperature, T1:

T1 - T0 = L / c

T1 - 19 = 2,300,000 / 4200

T1 - 19 = 547.619

T1 ≈ 566.619°C

Now, let's calculate the time it takes for one-third of the water to evaporate, t1:

t1 = (t0 * (T1 - T0)) / (3 * (T1 - T0))

t1 = (8 * (566.619 - 19)) / (3 * (566.619 - 19))

t1 ≈ 2.67 minutes

Therefore, it will take approximately 2.67 minutes for one-third of the water to evaporate after boiling starts.

Answer

The time it takes for one-third of the water to evaporate after boiling starts is approximately 2.67 minutes.