How switch knows that ip phone is connected ?

As you connect ip phone with the switch, switch start sending some voltage on the link, which is used by ip phone for initialisation.

Now as IP Phone will be functional.

By default power supply by Sw will be 15.4Watts.

By default, each switch port can automatically detect the presence of a PoE-capable device before applying power.

Switch(config-if)# power inline {auto | static} [max milliwatts]

With the auto keyword, the connected device can request power through CDP or LLDP
and the switch will attempt to deliver it, up to a default maximum of 30W—as long as
there is enough power available from the switch’s power supply.

You can use the static keyword instead, to preallocate a fixed amount of power to a device.

Add the max keyword to specify a maximum amount of power to offer on the interface,
regardless of what the device requests.