What is load capacitance ? How does it affect the circuit ?

First, a "load" is an impedance to which power from a circuit is

delivered (and either dissipated in the load or reflected backed to

the source circuit). Impedance is, very approximately, "The AC

equivalent of DC resistance with phase shifting added, both of which

change of over frequency". Examples could include antennas, loud

speakers, AC motors and appliances, lights, logic gate inputs, etc.

If you can assume only linear circuit components are used, you can

reduce *every* circuit, without approximation to just three

components: a power source (voltage or current), a source impedance

and a load impedance. The power source and source impedance

combinations are also known as Thevenin and Norton sources, thus every

circuit can be reduced to two components: a Thevenin/Norton source and

a load impedance. If you can't assume linear, you can still reduce to

three or two, but with certain limitations. Components like

resistors, capacitors and inductors are practically linear.

Semiconductors and vacuum tubes are definitely non-linear, thus there

are limitations.

If the load impedance has a primarily capacitive reactance, or

negative imaginary impedance, then you have capacitive load, which can

be modeled as a load capacitance. The load could also be inductive

(positive imaginary impedance), or resistive (zero imaginary

impedance). You can have combinations also forming

resistive-capacitive or resistive-inductive impedances. I'm assuming

you know complex numbers, or at least, trigonometry and right

triangles: impedance is always the hypoteneuse, resistance one side

and (inductive - capacitive) reactance the other side.

The specific effect on a circuit depends on what the circuit is. In

digital circuits a capacitive load acts to slow down pulse rise times.

In RF/microwave it creates a phase shifted reflection of incident

power back into the power source. In audio it can create a low-pass

filter effect. In power electronics (e.g. AC motors) it creates a

phase shift for starting a motor or for compensating power factor

shift caused by the motor. Sometimes these are intentional effects;

sometimes not. A common thread is that power is not fully used to its

theoretical maximum: only purely resistive loads dissipate any power;

"reactive" power is reflected back.

MM