When particles are charged, electricity is created. Some particles are negatively charged (electrons), while others are positively charged. The opposite charges attract while particles with similar charges repel one another. Although an electron has a mass two thousand times that of a proton, its electrical charge is the same as that of a proton. Many elements, especially metals, have electrons that are easy to knock off from their parent atoms. They can also wander between atoms. Unbalanced charges are necessary to create electric current. The circuit must be completed before electric current can flow. A wire connects an electrical source, such as a battery, to a resistance. When the circuit is finished back to the source, or to ground, the free electrons are released into a wire creating an electric potential difference or voltage. Because they share the same electrical charge, the electrons bounce off other electrons in wire. The electrons bounce against other electrons in the wire and cause an electric current. The electrons will flow to the far end if there is a place for them to go, such as inside a light bulb or motor.