Transformer is a device which is used for changing the voltage of an AC power source to higher or lower value.
Principle of transformer in physics
It is based on principle of mutual induction that is an emf is induced in a circuit due to a current changing in a neighbouring circuit.
Surface tension is a property of liquor. Due to this property liquid surface behave like a membrane and hold object on it. One of popular application of this topic is spherical shape of rain drop. We discuss this in detail in our post surface tension in physics.
Ideal voltage source is a two terminal device, which supply voltage constantly and continuously to load connect across it. We can also define ideal current source as this.