Explain what is meant by internal energy

An electric hot water heater has a power rating of $9.0 \mathrm{kW}$ The water is heated as it passes through the heater. Water flows through the heater at a speed of $1.2{\mathrm{ms}}^{-1 }$through pipes that have a total cross-sectional area of $4.8\times {10}^{-5}{\mathrm{m}}^2$ The temperature of the water entering the heater is $15°\mathrm{C}.$

 Calculate the mass of water flowing through the heater each second

An electric hot water heater has a power rating of $9.0 \mathrm{kW}$ The water is heated as it passes through the heater. Water flows through the heater at a speed of $1.2{\mathrm{ms}}^{-1 }$through pipes that have a total cross-sectional area of $4.8\times {10}^{-5}{\mathrm{m}}^2$ The temperature of the water entering the heater is $15°\mathrm{C}.$

Calculate the temperature at which the water leaves the heater. 

An electric hot water heater has a power rating of $9.0 \mathrm{kW}$ The water is heated as it passes through the heater. Water flows through the heater at a speed of $1.2{\mathrm{ms}}^{-1 }$through pipes that have a total cross-sectional area of $4.8\times {10}^{-5}{\mathrm{m}}^2$ The temperature of the water entering the heater is $15°\mathrm{C}.$

 State any assumptions you have to made in the calculation of mass of water flowing through the heater each second and temperature at which the water leaves the heater. 

An electric hot water heater has a power rating of $9.0 \mathrm{kW}$ The water is heated as it passes through the heater. Water flows through the heater at a speed of $1.2{\mathrm{ms}}^{-1 }$through pipes that have a total cross-sectional area of $4.8\times {10}^{-5}{\mathrm{m}}^2$ The temperature of the water entering the heater is $15°\mathrm{C}.$

 It is possible to adjust the temperature of the water from the heater. Suggest how the temperature of the water could be increased without increasing the power of the heater. (Density of water= $\mathrm{l}000 {\mathrm{kgm}}^{-3};$ specific heat capacity of water$=4200\mathrm{J} {\mathrm{kg}}^{-1}°{\mathrm{C}}^{-1}.$ )