in case of signals we always deal with voltage why not current?
First of all we don't, but I will come to that.
A signal is about change of something.
Think about circuits, what opposes change of current and change of voltage in a circuit?
Which can achieve the greatest rate of change with real components?
Change of current is associated with inductance, whereas change of voltage is associated with capacitance. Alternatively you could say that voltage change is about electric effects and current change is about magnetic effects.
Traditionally inductors have been harder to provide than capacitors, except in the earliest days when wound components were actually easier.
Either way current signals are used when magnetic effects are required, for instance in waveguides, the magnetic sweep circuits in cathode ray tubes, etc.
The signal is also current when considering power. It is the current which varies with load in mains supply, not the voltage. The voltage contains no information about the power directly, the current contains it all and is therefore the signal.
but in reality what is the difference between current and voltage?
I don't understand.
If you don't know the difference between voltage and current why did you ask your original question?
actually current cant exist without voltage the,then indirectly we are dealing with voltage.
Of course it can.
There's lots of threads about this here with lots of examples, do a forum search.
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