What You Will Be Able To Do

• Tell EOAs from contract accounts
• Explain smart contracts and the EVM
• Compute a fee from gas
• Send a testnet transaction
• Read the chain with web3.py

Why Ethereum

• Vitalik Buterin proposed it in 2013
• Network launched on 30 July 2015
• Goal: run programs, not only record payments
• A shared computer anyone can inspect

The World Computer, Honestly

• One shared state, one set of rules
• Every node runs every program
• Slow and costly compared with normal servers
• Value: results nobody can quietly change

World State: A Table Of Accounts

• Each block moves the state to a new state
• Account fields: address, balance, nonce
• Contracts also store code and data
• Addresses are 20 bytes, written as 0x plus 40 hex characters

0xAb...12  balance 1.5 ETH  nonce 7  (EOA)
0xCd...34  balance 0.0 ETH  nonce 1  code, storage (contract)

Two Kinds Of Account

• EOA: private key controls it, can start transactions
• Contract: code controls it, reacts when called
• Contracts have no private key
• Only EOAs begin a chain of calls

Anatomy Of A Transaction

• Recipient, value, optional data
• Gas limit and fee settings
• Nonce and signature
• Kinds: transfer, contract call, contract creation

to:        0xRecipient
value:     0.001 ETH
data:      0x (empty)
gasLimit:  21000
nonce:     7
signature: (r, s, v)

What Is A Smart Contract?

• Code plus storage at an address
• Runs exactly as written when called
• Anyone can read and call it
• Not a legal contract, just a program

The EVM

• Contracts compile to bytecode of opcodes
• A stack of 256-bit values plus memory and storage
• Every node runs the same instructions
• Same input always gives the same result

Gas: Paying For Computation

• Every opcode costs gas
• Simple transfer uses 21,000 gas
• Storage writes are costly
• Gas limit stops endless loops

Fee = Gas Used X (Base Fee + Tip)

• Base fee is set by demand and burned
• Tip goes to the validator
• Prices quoted in gwei, a billionth of ether
• London upgrade, August 2021

gas_used = 21_000
price    = 20            # gwei, base fee + tip
fee_gwei = gas_used * price
fee_eth  = fee_gwei / 1e9
print(fee_eth)           # 0.00042

Ether, Wei And Testnets

• 1 ether = 10^18 wei, 1 gwei = 10^9 wei
• Testnet follows main network rules
• Test coins come free from a faucet
• Sepolia is the common application testnet

Live: Your First Testnet Transaction

• Create a learning wallet, switch to Sepolia
• Request test ETH from a faucet
• Send 0.001 ETH to a second account
• Paste the hash into a block explorer

Reading A Transaction Page

• Status, block number, timestamp
• From, to, value
• Gas used and effective gas price
• Fee equals gas used times price

Reading The Chain With Web3.Py

• Connect through a JSON-RPC endpoint
• Reading is free and needs no key
• Replace the URL with your own Sepolia endpoint

from web3 import Web3

URL = 'https://ethereum-sepolia-rpc.publicnode.com'
w3 = Web3(Web3.HTTPProvider(URL))
print(w3.is_connected())
print(w3.eth.block_number)
block = w3.eth.get_block('latest')
print(block['baseFeePerGas'])
bal = w3.eth.get_balance('0xYourAddress')
print(Web3.from_wei(bal, 'ether'))

Recap

• Ethereum tracks accounts and runs code
• EOAs start transactions, contracts react
• The EVM runs bytecode identically everywhere
• Gas prices computation, fee is gas times price
• Testnets let you practise safely

Next Week: Your First Contract

• Solidity basics
• Compile and deploy in Remix
• Call your contract on Sepolia
• Read your contract on Etherscan

Questions And Exercise

• Send a testnet transaction
• Check the fee by hand on the explorer
• Run the web3.py script
• Explain EOA versus contract