# Surface charge demo

(In the 4th edition this material is covered in Chapter 18, not Chapter 19 as stated in the title screen of the video.)

It is possible to see mechanical effects of surface charge in a high-voltage circuit. In an ordinary circuit with a 1.5 volt battery, the electric field in the wires is only a few volts per meter, and the amount of surface charge buildup sufficient to produce such small fields is so small that it is difficult to see mechanical effects of that surface charge. (Notice that electrostatic fields can be very large: the field required to make a spark in air is about three million volts per meter.)

The video shows a circuit driven by a 10,000 volt power supply. The power supply is arranged so as to apply +5000 volts to one end of a chain of four resistors, and -5000 volts to the other end. The electric field in each resistor is large, so the gradient of surface charge is large along the circuit, with a large positive surface charge at the right end of the resistor chain and a large negative surface charge at the left end. Given the symmetrical arrangement, we expect zero surface charge on the wire at the center of the chain of resistors.

We discharge a hanging piece of aluminum foil and bring it near the -5000 end of the circuit. We see the foil attracted to the wire, then repelled. The foil was polarized, attracted, then charged negative by contact. Each time we try this, we first touch the foil to distribute any charge all over the body, so the foil is neutral. We rub a plastic pen, charging it negative, and bring it near the foil, which is repelled. This shows that the foil did charge negative by contact as expected.

Next we try the +5000 volt end of the chain of resistors. We again see that the neutral foil is attracted due to polarization, charges by contact, then is repelled. We see that the foil is now attracted by the negative pen, showing that it charged positively as expected.

When we bring the foil near the center of the chain of resistors, we see as expected that nothing happens, because the surface charge there should be zero.

This demonstration of the electric effects of the surface charge was suggested by members of the science education group at the Weizmann Institute in Israel.