What causes alpha particles to deflect backwards?
The alpha particles carry a charge of +2e. When they are fired at a gold atom most pass through without really interacting with the nucleus (which is alsp positively charged). A small amount will interact, however, and scatter elastically. The nucleus and apha particle feel a mutual repulsion force, and the alpha particle is deflected. Think of it like if you stuck a stick vertically onto some floorboards, then put a carpet over the stick, it would make a wig-wam sort of shape in the carpet. Now roll tennis balls at this shape. If you roll them far from the stick, they’ll not be affected. If you roll them near it, they’ll be deflected slightly and if you roll them perfectly at it, they’ll go up the hill and come straight back to you. In this analogy the tennis ball is the alpha particle and the deformed carpet represents a spherical 1/r electric potential field which the nucleus causes.
