Electric current

Electric current

1. Electric Charge

   • Is
     • The Coulomb (abbreviated C).
   • Made up
     • Of atoms which have positively charged nuclei and negatively charged electrons surrounding them
   • Charge is
     • Quantized as a multiple of the electron or proton charge
   • The influence of charges
     • Is
       • Is characterized in terms of the forces between them (Coulomb's law) and the electric field and voltage produced by them. One Coulomb of charge is the charge which would flow through a 120 watt lightbulb (120 volts AC) in one second. Two charges of one Coulomb each separated by a meter would repel each other with a force of about a million tons!
   • The rate of flow of electric charge
     • Is
       • Called electric current and is measured in Amperes
   • The proton
     • The proton has 1836 times the mass of the electron, but exactly the same size charge, only positive rather than negative.

2. Conventional Electric Current

   • Is
     • Some texts reverse this convention and take electric current direction as the direction the electrons move, an obviously more physically realistic direction, but the vast majority of references use the conventional current direction and that convention will be followed in most of this material.
   • The ratio of voltage to current is called the resistance
     • If
       • The ratio is constant over a wide range of voltages, the material is said to be an "ohmic" material
         • If
           • The material can be characterized by such a resistance, then the current can be predicted from the relationship::

3. Voltage Law

   • Bonus
     • The voltage changes around any closed loop must sum to zero.
     • Since voltage is electric potential energy per unit charge, the voltage law can be seen to be a consequence of conservation of energy.
     • The voltage law has great practical utility in the analysis of electric circuits. It is used in conjunction with the current law in many circuit analysis tasks.
     • The determining of the voltages and currents associated with a particular circuit along with the power allows you to completely describe the electrical state of a direct current circuit.

4. Current Law

   • The electric current in amperes that flows into any junction in an electric circuit is equal to the current which flows out
   • Is one
     • Of the main tools for the analysis of electric circuits, along with Ohm's Law, the voltage law and the power relationship.
       • The determining of the voltages and currents associated with a particular circuit along with the power allows you to completely describe the electrical state of a direct current circuit.

5. Resistors in Parellel

   • These resistors are said to be in parallel, and as before, it is possible to consider them as one single equivalent resistor R eq . To find this equivalent resistor, we exploit the fact from charge conservation that I = I1 + I2 . Using again Ohm's law (17.3), as well as the point that the potential difference across R1 and R2 is the same, we find this becomes

6. Kirchhoff's Laws

   • Bonus
     • Although useful to be able to reduce series and parallel resistors in a circuit when they occur, circuits in general are not composed exclusively of such combinations.
   • There are two such laws:
     • The 1 st law: for a given junction or node in a circuit, the sum of the currents entering equals the sum of the currents leaving. This law is a statement of charge conservation
     • The 2 nd law: around any closed loop in a circuit, the sum of the potential differences across all elements is zero. This law is a statement of energy conservation, in that any charge that starts and ends up at the same point with the same velocity must have gained as much energy as it lost.

7. Is

   • The rate of charge flow past a given point in an electric circuit.
     • Measured
       • Coulombs/second which is named Amperes.
     • Standard abbreviations for the units
       • Are
         • 1 A equal to 1C/s
     • when
       • In most DC electric circuits, it can be assumed that the resistance to current flow is a constant so that the current in the circuit is related to voltage and resistance by Ohm's law

8. Circuit Elements

   • Are
     • Considered to be made up of localized circuit elements connected by wires which have essentially negligible resistance.
   • The three basic circuit elements
     • are
       • Resistors, capacitors, and inductors.

Bilder und Dateien der Mindmap