Saw the plastic pipes in half along their length. There should now be two lengths of half circles.These represent the cross section of an electrical wire or resistor. These can be used as models at the front of the classroom.
Place a length of half circle on the bench at the front of the class and prop it up on top of the wooden block, so it is at an incline.
Demonstrate what happens when a single plastic ball is placed at the top of the length of half circle; the ball rolls down. Ask the children what it is that stops the ball moving faster. It should be straightforward to see that the ball knocks into the walls and this slows it down. This is one of the mechanisms of electrical resistance. Electrons knock off the sides of the wire -- a mechanism known as surface scattering.
Demonstrate what happens when many balls are all placed rolling down the length of a half circle. It should be pointed out there are now two mechanisms that slow down the balls. Firstly, the balls knock off the sides, as before. Secondly, the balls knock into each other and this also slows them down. This second mechanism is known as electron-electron scattering.
Explain how resistors may be made. Tell the children that resistors are just pieces of wire of slightly different length. Imagine if the length of the half circle was wider. In this case, the balls would not knock into each other as often -- there is a low resistance. The reverse argument can be used to explain high resistance wires using thinner lengths of half circles.