# Regents Physics Circuits Unit Part I Resistivity and Ohm’s Law.

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Regents Physics Circuits Unit Part I Resistivity and Ohm’s Law

Electric Current Electric current is amount of charge that passes a given point in a circuit Current is the flow of charge Current moves in an Electric Circuit, which is a closed path along which charged particles move

Current SI unit for current is the ampere (A) 1 A = 1 C / s = the amount of charge that passes a point per second I = qq t  q = the amount of charge in Coulombs that passes a point when a current of one ampere flows for 1 second

What is Voltage? Voltage is the potential difference between two points and represents the amount of work required to move one coulomb of charge from point A to point B Without a potential difference, current will not flow Example: The Pipe! Voltage is measured with a voltmeter

How Does Charge Move? Positive charges tend to move from points of higher potential to points of lower potential, or From positive potential to negative potential Negative charges tend to move in the opposite direction The direction of a current can be defined as either of these directions

Conventional current is the direction of positive charge flowing from positive to negative terminals However… Since most current consists of electron flow, in Regents Physics we say current is the flow of negative charge from negative to positive terminals How Does Charge Move?

Conductivity in Solids For a current to exist in an electric circuit, the circuit must consist of materials through which charge can move The ability of a material to conduct electricity depends on the number of free charges per unit volume and their mobility Conductivity – is a property of a material that depends on the availability of charges that are relatively free to move under the influence of an electric field

Conductivity in Solids Different materials have difference conductivity values Pure metals have many electrons, and these electrons are not bound, or are only loosely bound, to any particular atom So metals are good conductors since their electrons move easily Nonmetals are not since their electrons are tightly bound – called insulators

Resistance and Ohm’s Law Electrical Resistance, R, is the opposition that a device or conductor offers to the flow of electrical current The resistance of a conductor is the ratio of the potential difference applied to it ends and the current that flows through it Resistance is measured in ohms  R = Voltage (V) Current (I) Pipe

Factors Affecting Resistance of a Conductor The resistance of a wire increases with the increasing length of a wire.. Because the current (electrons) encounter and collide with an increasing number of atoms Resistance varies directly with length of wire R  L Resistance varies inversely with thickness of wire or cross-sectional area

Resistivity p Resistivity, p, is a characteristic of a material that depends on its electronic structure and temperature The resistance of a wire is directly proportional to its resistivity R  p Good conductors have low resistivities Good insulators have high resistivities

Resistance in a Conductor Summary Atoms stay still so current can pass Atoms vibrate slowing down current Larger cross-sectional areas allow for move movement of current = less resistance click

SI unit is the ohm – meter  - m As the temperature of a conductor increases, its resistivity also increases We can find the resistivities of selected metals at 20 Celsius in our RT Resistivity and Resistance R= pLpL A L and A are in meters R is resistance end

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