Question

A mass of 15 kg of air in a piston-cylinder device is heated from 25 o C to 77 o C by passing current through a resistance heater inside the cylinder. The pressure inside the cylinder is help constant at 300 kPa during this process and a heat loss of 60 kJ occurs. Calculate the electric energy supplied in kWh.

Answer 1

Answer:

The electrical energy supplied is 0.233 kWh

Explanation:

Given :

Mass of air $m = 15$ kg

Initial temperature $T _{1} =$ 298 K

Final temperature $T_{2} =$ 350 K

Heat loss $Q _{out} = 60$ kJ

From the first law of thermodynamics,

     $\Delta U = W_{in} - W_{out} - Q_{out}$

We know that internal energy is proportional to difference of temperature,

    $W_{in} = m (T_{2} - T_{1} ) + 60$

    $W_{in} = 15 \times (350-298) + 60 }$

Now we need answer into kWh so ( 1 kWh = 3600 kJ ).

So we need to multiply ( $\frac{1}{3600}$ )

     $W_{in} = [15 \times (350-298) + 60 ] \times \frac{1}{3600}$ kWh

     $W_{in} = 0.233$ kWh

Therefore, the electrical energy supplied 0.233 kWh