The circuit of an inverting amplifier incorporating an ideal operational amplifier (op-amp) is shown in the figure below.
Explain why point P is known as a virtual earth.
Derive an expression, in terms of the resistances 
and 
, for the gain of the amplifier circuit. Explain your working.
and 
must be almost zero. Now, the non-inverting input is connected to the zero-volt line (earth), so 
, therefore, for the amplifier not to saturate, 
