CURRENT AND RESISTANCE compiled by: Firdiana Sanjaya Ana Alina

Electric current

 The charges passing through the surface can be positive or negative,or both. It is conventional to assign to the current the same direction as the flow of positive charge. In electrical conductors, such as copper or aluminum, the current is due to the motion of negatively charged electrons. Therefore, when we speak of current in an ordinary conductor, the direction of the current is opposite the direction of flow of electrons.

Microscopic Model of Current  We can relate current to the motion of the charge carriers by describing a microscopic model of conduction in a metal.

The speed of the charge carriers vd is an average speed called the drift speed A schematic representation of the zigzag motion of an electron in a conductor. The changes in direction are the result of collisions between the electron and atoms in the conductor.

Electrical resistance  Voltage can be thought of as the pressure pushing charges along a conductor, while the electrical resistance of a conductor is a measure of how difficult it is to push the charges along.  The resistance (R) of a material depends on its length, cross-sectional area (A), and the resistivity (the Greek letter  ), a number that depends on the material: R =  L / A Resistance is measured in ohm (  ).

 The resistivity and conductivity are inversely related. Good conductors have low resistivity, while poor conductors (insulators) have resistivities that can be 20 orders of magnitude larger.  Resistance also depends on temperature.  =  0 [ 1 +  (T – T 0 ) ] Or R = R 0 [ 1 +  (T – T 0 ) ]  Materials with no resistance produce no energy loss when currents pass through them.

Resistance  Ohm’s law For many materials (including most metals), the ratio of the current density to the electric field is a constant & that is independent of the electric field producing the current

Electrical Power

Direct Current (DC)  A battery produces direct current  The battery voltage (or emf) is constant  Generally results in a constant current flowing one way around a circuit.

Alternating Current (AC)  The current may not be constant, but it will still flow in one direction.  This is because the voltage (emf) is following a sine wave oscillation.  The voltage changes from positive to negative and back again 50 times each second (or the frequency is 50 Hz).

The graph above shows voltage as a function of time, but it could just as well show current as a function of time: the current also oscillates at the same frequency.

Root mean square  RMS = average value we use for the voltage from a wall socket is known as the root mean square, or rms, average.  The rms value, however, is obtained in this way: first, square everything (this makes everything positive) second, average third, take the square root of the average

Series circuits

Parallel circuits

Kirchhoff`s rules

Multi loop circuits  The general method for analysis of multi loop circuits

Voltmeters,ammeters,ohmmeters  Voltmeter: to measure the terminal voltage of your car battery  Ammeters:to measure the current through a resistor in a simple circuit  Ohmmeter:to measure the resistance between two points

Ammeters

Voltmeters

Ohmmeters

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