Is the wide part on a plug positive?

What you are refering to is a polarized plug. The wide connector forces the proper orientation in the outlet. This is so that the hot and neutral connectors in the plug, match the hot and neutral (cold) prongs on the plug. This forces a switch to operate on the "live" or hot wire. If a switch interrupted the flow of electricity in the neutral wire, the appliance would still shut off but the plug itself will remain "hot" whcih is a shock hazard. No, the wide blade on a plug is the neutral connection.