EDIT: If the stuff below seems more complicated than you wish to be involved in, feel free to ignore it.
The Mac will send packets bound for the Internet to the Top-Most ACTIVE interface listed in the left side of the box at:
System Preferences > Network
If you set the 'Service Order' to have Ethernet at the top, whenever Ethernet is working, Wi-Fi will be ignored.
Set Service order is available using the gear Icon at the bottom of that box.
So all my Internet traffic is going over my Ethernet connection, even though Wi-Fi is still connected and nominally "active".
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