Properties of conductors

Question

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

Answer 1

So which diagram and explanation is correct?

Both of them are correct. They just are drawn in different ways.

Figure 1 shows how the positive charge in the middle attracts electrons in the conductor around it. The minuses, representing a negative charge (electrons), are drawn onto the conductor. It leaves out the fact, that by having the electrons move, there will be less electrons on the outer surface of the conductor.

In Figure 2 the electrons that are attracted to the positive charge in the center are indicated by minus symbols that are outside the conductor. That also is a perfectly reasonable representation. Figure 2 also is more detailed, as it points out that there will be a positive charge (fewer electrons) on the outside of the conductor.