Electrodynamics

• The science of electrodynamics deals with electrical charges in motion.
• A closed conducting path where charges flow is called an electric circuit.
• There are 3 essential elements in an electric circuit. These are the electric current, resistance and voltage or potential difference.
• Electric current is the flow of charged particles in a specified direction.
• Charged particles are often referred to as current carriers. In a common conductor such as Cu, the current is due to the motion of negatively charged electrons, so the direction of the current is opposite to the direction of motion of the electrons.
• A simple circuit consists of the following:

  1. Source of electrical energy to drive the electric charge around the circuit; example: battery.
  2. Load which uses the electrical energy; example: bulb
  3. Switch to break or complete the circuit.
• An electric circuit is a complete or closed path through which charges can flow from one terminal of an electrical source to its other terminal.
• Where:

  I = current ; the unit is ampere ( A ) ; 1 A = 1C/s N= no. of electrons

  q = charge ; the unit is coulomb ( C )

  e = constant no. ( 1.6 x 10-19 C/electron )

  t = time ; the unit is second ( s )
• The Ampere was chosen as the SI base unit of current in honor of Andre Marie Ampere.
• The amount of current in a circuit is measured with an ammeter.
• This instrument must be connected in series to the circuit.
• In a circuit, electrons need a push to flow. This push comes from a battery or generator, which creates an electromotive force (EMF). EMF is like a pressure difference that forces electrons to move through the wire
• The electrical pressure difference caused by the battery is called potential difference or voltage.
• The voltage across two points in a circuit can be measured with a voltmeter.
• An opposition to the electron flow along a conductor is called resistance.
• factors affecting resistance of a wire.

  • length,
  • cross- sectional area or thickness
  • and temperature
• The longer the wire is, the greater its resistance.
• A thicker wire, however, has less resistance because a thick wire offers more room for an electric current to pass through than a thin wire does.
• The higher the temperature of a wire the greater its resistance and the lower the temperature of a wire, the lesser its resistance.
• Resistance has SI units of volts / ampere or Ohms (Ω ).
• Materials that offer a low resistance to current are called conductors, while materials which offer a high resistance to current are called as insulators.
• A resistor is a passive electronic component that introduces resistance to the flow of electric current in a circuit.
• Why We Use Resistors:

  • Limit Current
  • Control Voltage
  • Signal Voltage Division
• Types of Resistors:

  • Fixed Resistors
  • Variable Resistors
• Variable Resistors: These allow you to adjust the resistance value, like a volume knob on a stereo. Examples include potentiometers and rheostats.
• According to Ohm's Law, current increases proportionally with voltage increase (assuming constant resistance). In other words, if you double the voltage, the current will also double.
• According to Ohm's Law, current is inversely proportional to resistance (assuming constant voltage). This means if you double the resistance, the current will be cut in half.
• Where: I = current ; the unit is A or V/Ω or C/s

  V = potential difference or voltage; the unit is V or A.Ω or J/C

  R = resistance ; the unit is Ω or V/A
• Increasing the voltage will increase the current.
• Increasing the Resistance, the current will decrease.
• = q/t ;

  q = It ;

  t = q/I ;

  N=q/e
• I = current ; the unit is ampere ( A ) ;

  1 A = 1C/s N= no. of electrons q = charge ;

  the unit is coulomb ( C )

  e = constant no. ( 1.6 x 10-19 C/electron )

  t = time ; the unit is second ( s )
• 1 E = 1.6*10^-19