A 50 mm X 45 mm X 20 mm cell phone charger has a surface temperature of

Question:

A 50 mm X 45 mm X 20 mm cell phone charger has a surface temperature of T_s= 33°C when plugged into an electrical wall outlet but not in use. The surface of the charger is of emissivity ϵ = 0.92 and is subject to a free convection heat transfer coefficient of h = 4.5 W/m²· K. The room air and wall temperatures are T_∞= 22°C and T_sur= 20°C, respectively. If electricity costs C = $0.18/kW · h, determine the daily cost of leaving the charger plugged in when not in use.