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Section 2: Electric Current • This section discusses electrical pressure • Charges flow from high voltage areas to low voltage areas • Voltage—like an electrical pressure that pushes charge (Section 2 cont’d) • Just as water or air must have a pressure difference to flow, voltage difference must be present for electric charges to flow • Voltage difference—the push that causes charges to move and is measured in volts (V) (fig. 11 pg. 202) Closed Circuits • The flow of charges through a wire or any conductor is called electric current • The electric current in a circuit is measured in amperes (A) • Circuit—a closed, conducting path for the flow of electrons (Section 2 cont’d.) • Current is almost always the flow of electrons • In order to keep current moving continuously through a circuit a power source (voltage source) is needed • One common source--battery Dry-Cell Batteries • Individual batteries used in flashlights, etc.—dry cell batteries • Made of a zinc container that surrounds a moist chemical paste w/a solid carbon rod suspended in the middle • Provides a voltage difference between +/terminals (Dry-cell batteries cont’d) • When the two terminals of a dry cell battery are connected in a circuit, a reaction occurs. • The voltage difference between these 2 terminals causes current to flow through a closed circuit Wet-cell batteries • Wet-cell battery—contains two connected plates made of different metals or metallic compounds in a conducting solution – Ex: car batteries (lead plates in sulfuric acid) – The chemical reaction in ea/cell provides a voltage difference More on Batteries • In addition to batteries a voltage difference is provided at electrical outlets • Household devices are designed for 120 V (standard) • Some wall sockets supply 240 V – Ex: electric ranges, clothes dryers, etc. Resistance • Resistance—the tendency for a material to oppose the flow of electrons, changing electrical energy into thermal energy and light • All materials have some resistance • Resistance is measured in ohms *type of material & wire size effect resistance Controlling the flow of electricity • So far: Voltage difference causes charges to flow • An electrical resistance restricts the movement of charges ex: water flowing in a pipe Ohm’s Law: • Current = Voltage difference / resistance • OR • I(A) = V (V) / R (ohms) • OR • V=IR • Ohm’s Law Diagram—pg. 207