▁phys is or ption , ▁generally ▁speaking , ▁can ▁only ▁be ▁observed ▁in ▁the ▁environment ▁of ▁low ▁temperature ▁( ther mal ▁energy ▁at ▁room ▁temperature ▁~ 2 6   me V ) ▁and ▁the ▁absence ▁of ▁the ▁relatively ▁strong ▁chem is or ptions . ▁ ▁In ▁practice , ▁the ▁categor isation ▁of ▁a ▁particular ▁ad s or ption ▁as ▁phys is or ption ▁or ▁chem is or ption ▁depends ▁princip ally ▁on ▁the ▁binding ▁energy ▁of ▁the ▁ad s orb ate ▁to ▁the ▁substr ate . ▁ ▁Model ing ▁by ▁image ▁charge ▁ ▁To ▁give ▁a ▁simple ▁illustr ation ▁of ▁phys is or ption , ▁we ▁can ▁first ▁consider ▁an ▁ad s or bed ▁hydro gen ▁atom ▁in ▁front ▁of ▁a ▁perfect ▁conduct or , ▁as ▁shown ▁in ▁Fig . ▁ 1 . ▁ ▁A ▁nucle us ▁with ▁positive ▁charge ▁is ▁located ▁at ▁R   =   ( 0 ,   0 ,   Z ), ▁and ▁the ▁position ▁coordinate ▁of ▁its ▁electron , ▁r   =   ( x ,   y ,   z ) ▁is ▁given ▁with ▁respect ▁to ▁the ▁nucle us . ▁ ▁The ▁ad s or ption ▁process ▁can ▁be ▁viewed ▁as ▁the ▁interaction ▁between ▁this ▁hydro gen ▁atom ▁and ▁its ▁image ▁charges ▁of ▁both ▁the ▁nucle us ▁and ▁electron ▁in ▁the ▁conduct or . ▁ ▁As ▁a ▁result , ▁the ▁total ▁electro static ▁energy ▁is ▁the ▁sum ▁of ▁att raction ▁and ▁rep ulsion ▁terms : ▁ ▁The ▁first ▁term ▁is ▁the ▁attract ive ▁interaction ▁of ▁nucle us