If there is a cavity inside a conductor and a charge is placed within it then what exactly happens?
I've read in one book that the charge in the cavity induces a charge (the induced charge is opposite in nature obviously) on the outer surface surface of the cavity. Here is the exact picture. (It is from a pdf which I downloaded from the internet, by MIT). Here is the link https://www.google.com/url?sa=t&source=web&rct=j&url=http://web.mit.edu/sahughes/www/8.022/lec05.pdf&ved=2ahUKEwjQio-zt-XrAhXBjOYKHencDu0QFjABegQICxAH&usg=AOvVaw02X1KKVhOE12qLIW7Rw9FN
But when I read it from another book it said that the inner surface cannot be charge free and if a charge +Q is placed in the cavity, there must be a charge -Q on the inner surface of the conductor. Here is the picture in my book.
So which diagram and explanation is correct?
Also, in the second picture why is there +Q charge on the outer surface of the conductor?
Please help me