has ethernet cable connection between time capsule and imac priority over WIFI connection

At the time my iMac gets its Internet access over WLAN of a Router/modem.
The connection between airport TimeCapsule and the iMac is also wireless.
After updating to Sierra (coincidence?) I get very often the message that the backup on the TimeCapsule is corrupt, and time machine initiates a new backup. I want to try the next backup over a cable connection between the Computer and the TimeCapsule.
Questions:
1. What happens if I connect my iMac 27 2012 to the Airport Time capsule per Ethernet cable, but the WLAN on the iMac remains active? Will the connection between the iMac and the TC be through cable or through WLAN?
2. Will the iMac WLAN function?

AirPort Time Capsule (2013)

AirPort Time Capsule (2013)

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