Please answer like this :
1 :
2 :
3 :
4 :

1: a system is a collection of two or more things/objects. but if you want to use a tennis ball for an example it would be considered an object but at an atomic level we could say it is a system. why? because, it is made up of millions of atoms and each and every atom could be considered an object
2: phone systems, ecological systems, space systems
3: the internal energy of the system can be understood by examining the simplest possible system. example: an ideal gas
4: electricity- by an electrical current.