CHAP 4: Current, Resistance, and Electromotive Force

• An electric current consists of charges in motion from one region to another. If the charges follow a conducting path that forms a closed loop, the path is called an electric circuit.
• Is any motion of charge from one region to another.

  Current (I)
• A conventional current is treated as a flow of positive charges, regardless of whether the free charge in the conductor are positive, negative, or both.
• In a metallic conductor, the moving charges are electrons but the current still points in the direction positive charges would flow.
• The same current can be produced by:

  (a) Positive charges moving in the direction of the electric field or;

  (b) The same number of negative charges moving at the same speed in the direction opposite to the electric field.
• Current is not a vector.
• In a current-carrying wire, the current is always along the length of the wire, regardless of whether the wire is straight or curved. No single vector could describe motion along a curved path, which is why current is not a vector.
• SI unit of current

  Ampere (A) 1A = 1/Cs
• Current per cross-sectional area.

  Current density (J)
• The ratio of the magnitudes of electric field and current density. It is the measure of the ability of a material to oppose the flow of current.


  Resistivity (p)
• Metals are good conductors of electricity. Hence, they have low resistivity.
• The insulators like rubber, glass, graphite, plastics, etc. have very high resistivity when compare to the metallic conductors.
• The third type is the semiconductor which comes in between the conductors and insulators. Their resistivity decreases with the increase in temperature and is also affected by the presence of impurities in them.
• The greater the resistivity, the greater the field needed to cause a given current density.
• SI unit of resistivity


  Ohm-meter (m)
• Reciprocal of resistivity. It is an intrinsic property of a material which is defined as the measure of the amount of electrical current a material can carry.


  Conductivity
• When an electric current flows through a bulb or any conductor, the conductor offers some obstruction to the current and this obstruction is known as resistance and denoted by R. Every material has resistance and this is the reason why conductors give out heat when current passes through it.
• The measure of opposition applied by any object to the flow of the electric current. It is the ratio of V to I for a particular conductor.


  Resistance
• The electrical resistance of a conductor is dependent on the following factors:

  (a) The cross-sectional area of the conductor

  (b) Length of the conductor

  (c) The material of the conductor

  (d) The temperature of the conducting material
• SI unit of resistance

  Ohm
• A circuit made to have a specific value of resistance between its ends. 


  Resistor
• Resistors in the range of 0.01 to 10^7 can be brought off the shelf. Individual resistors used in electricity circuitry are often cylindrical, a few millimeters in diameter and length, with wires coming out of the ends.
• indicate the digits


  First two bonds
• a power-of-10 multiplier

  Third band
• indicated the precision (tolerance) of the value; no band means 20%, a silver band 10%, and a gold band 5%.


  Fourth band
• For a conductor to have a steady current, it must be part of a path that forms a closed loop or complete circuit.
• Is a closed loop or path, forming a network of electrical components where electrons can flow. This path is made using electrical wires and is powered by a source, like a battery.


  Electric circuit
• The influence that makes current flow from lower to higher potential. It is the energy supplied by a battery or a cell per coulomb of charge passing through it.


  Electromotive force (emf)
• emf is not a force by an energy-per-unit-charge quantity, like potential. The SI unit of emf is the same as that for potential, the volt.
• a device that provides emf. Examples are batteries, electric generators, solar cells, thermocouples, and fuel cells.


  Source of emf
• The resistance within a battery or other voltage sources that causes a drop in the source voltage when there is a current.


  Internal Resistance
• For a real source of emf, the terminal voltage equals the emf only if no current is flowing through the source.