How to send ether from eoa to contract - ethereum

Question

I need to send ether from externally owned account to contract.
All code I found so far looks like this
contract Contract {
mapping (address => uint) balances;
event Transfer(address indexed _from, uint256 _value);
function deposit() public returns (uint) {
balances[msg.sender] += msg.value;
Transfer(msg.sender, msg.value);
return balances[msg.sender];
}
}
But I can not understand how does it work. I thought it should look like this: we should run some send function which will take current contract address, sender address and amount.
Can someone explain me the logic behind it?
I also found the solution which correspond to this logic
contract Contract {
function pay() payable {}
}
and than call it from console like that
var contract
Contract.deployed().then(function(instance) { contract = instance; })
contract.pay.sendTransaction({from: eoa_address,to: contract_address,value: web3.toWei(amount,"ether"), gas:1000000})
But in this case sendTransaction function called outside of contact.
Is there any way of calling it like from this perspective inside contract?

Answer 1

to send Ether to a contract :
we could create a payable function if we need to pay to execute this function
contract Contract {
function do_somthing() payable {
action1
action2
...
}
}
if we want to just send ethers to the contract without executing any function we define the fallback function as you presented in your question :
contract Contract {
function pay() payable {}
}
the example you have provided before :
contract Contract {
mapping (address => uint) balances;
event Transfer(address indexed _from, uint256 _value);
function deposit() public returns (uint) {
balances[msg.sender] += msg.value;
emit Transfer(msg.sender, msg.value);
return balances[msg.sender];
}
}
is recording the balances sent by the users to the contract (this function needs to be declared as payable for the recent compiler : function deposit() public payable returns)
function deposit() public payable returns (uint) {
balances[msg.sender] += msg.value;
emit Transfer(msg.sender, msg.value);
return balances[msg.sender];
}