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In the same reference frame, kinetic friction is always in the direction opposite the motion, and does negative work. [79] However, friction can do positive work in certain frames of reference. One can see this by placing a heavy box on a rug, then pulling on the rug quickly.
If a cloth rubs against an object, the object will display an effect called friction electricity. The object becomes charged due to the rubbing process, and now possesses an static electrical charge, hence it is also called static electricity. There are two main types of electrical charge: positive and negative.
During the solidification the positive and negative ions may not be equally distributed between the liquid and solid. [119] For instance, in thunderstorms this can contribute (together with the waterfall effect) to separation of positive hydrogen ions and negative hydroxide ions, leading to static charge and lightning. [120]
The positive-to-negative convention is widely used to simplify this situation. An electric arc provides an energetic demonstration of electric current. The process by which electric current passes through a material is termed electrical conduction , and its nature varies with that of the charged particles and the material through which they are ...
When two objects were touched together, sometimes the objects became spontaneously charged (οne negative charge, one positive charge). Corona effect — Build-up of charges in a high-voltage conductor (common in AC transmission lines), which ionizes the air and produces visible light, usually purple.
The ancient Greek understanding of physics was limited to the statics of simple machines (the balance of forces), and did not include dynamics or the concept of work. During the Renaissance the dynamics of the Mechanical Powers, as the simple machines were called, began to be studied from the standpoint of how far they could lift a load, in addition to the force they could apply, leading ...
The phenomenon of static electricity requires a separation of positive and negative charges. When two materials are in contact, electrons may move from one material to the other, which leaves an excess of positive charge on one material, and an equal negative charge on the other. When the materials are separated, they retain this charge imbalance.
Electric charge is a conserved property: the net charge of an isolated system, the quantity of positive charge minus the amount of negative charge, cannot change. Electric charge is carried by subatomic particles. In ordinary matter, negative charge is carried by electrons, and positive charge is carried by the protons in the nuclei of atoms ...