Question

why drift velocity of electron is opposite to external electric field?

Answer 1

Bᴇғᴏʀᴇ ᴛʜᴇ ᴘʀᴇsᴇɴᴄᴇ ᴏғ ᴇʟᴇᴄᴛʀɪᴄ ғɪᴇʟᴅ, ᴛʜᴇ ғʀᴇᴇ ᴇʟᴇᴄᴛʀᴏɴs ᴍᴏᴠᴇ ʀᴀɴᴅᴏᴍʟʏ ɪɴ ᴛʜᴇ ᴄᴏɴᴅᴜᴄᴛᴏʀ, sᴏ ᴛʜᴇɪʀ ᴅʀɪғᴛ ᴠᴇʟᴏᴄɪᴛʏ ɪs ᴢᴇʀᴏ ᴀɴᴅ ᴛʜᴇʀᴇғᴏʀᴇ ᴛʜᴇʀᴇ ɪs ɴᴏ ᴄᴜʀʀᴇɴᴛ ɪɴ ᴛʜᴇ ᴄᴏɴᴅᴜᴄᴛᴏʀ. Iɴ ᴛʜᴇ ᴘʀᴇsᴇɴᴄᴇ ᴏғ ᴇʟᴇᴄᴛʀɪᴄ ғɪᴇʟᴅ, ᴇᴀᴄʜ ᴇʟᴇᴄᴛʀᴏɴ ɪɴ ᴛʜᴇ ᴄᴏɴᴅᴜᴄᴛᴏʀ ᴇxᴘᴇʀɪᴇɴᴄᴇ ᴀ ғᴏʀᴄᴇ ɪɴ ᴀ ᴅɪʀᴇᴄᴛɪᴏɴ ᴏᴘᴘᴏsɪᴛᴇ ᴛᴏ ᴛʜᴇ ᴇʟᴇᴄᴛʀɪᴄ ғɪᴇʟᴅ

Answer 2

Before the presence of electric field, the free electrons move randomly in the conductor, so their drift velocity is zero and therefore there is no current in the conductor. In the presence of electric field, each electron in the conductor experience a force in a direction opposite to the electric field.

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