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Current Electricity

Electric Current - The Electric current is a flow of electric charge through a conductive medium.

In electric circuits this charge is often carried by moving electrons in a wire. It can also be carried by ions in an electrolyte, or by both ions and electrons such as in a plasma.

The SI unit for measuring the rate of flow of electric charge is the ampere, which is charge flowing through some surface at the rate of one coulomb per second. Electric current is measured using an ammeter.

The conventional symbol for current is I, which originates from the French phrase intensité de courant, or in English current intensity.

Resistance- An electron traveling through the wires and loads of the external circuit encounters resistance. Resistance is the hindrance to the flow of charge. For an electron, the journey from terminal to terminal is not a direct route. Rather, it is a zigzag path that results from countless collisions with fixed atoms within the conducting material. The electrons encounter resistance - a hindrance to their movement. While the electric potential difference established between the two terminals encourages the movement of charge, it is resistance that discourages it. The rate at which charge flows from terminal to terminal is the result of the combined affect of these two quantities.

Ohm’s Law- Ohm's Law was named after Bavarian mathematician and physicist Georg Ohm.

Ohm's Law deals with the relationship between voltage and current in an ideal conductor. This relationship states that:

The potential difference (voltage) across an ideal conductor is proportional to the current through it.

 The constant of proportionality is called the "resistance", R.

Ohm's Law is given by:

    V = I R

where V is the potential difference between two points which include a resistance R. I is the current flowing through the resistance. For biological work, it is often preferable to use the conductance, g = 1/R; In this form Ohm's Law is:

    I = g V

Conductance - Conductance is an expression of the ease with which electric current flows through a substance. In equations, conductance is symbolized by the uppercase letter G. The standard unit of conductance is the siemens (abbreviated S), formerly known as the mho.

Conductance is inversely related to resistance. If R is the resistance of a component or device (in ohms), then the conductance G (in siemens) is given by:

G = 1/R

Specific Conductivity- The specific conductivity is the reciprocal of the specific resistance of a solution measured between two electrodes 1 cm2 in area and 1 cm apart.  Conductivity is thus measured by placing two electrodes (with opposite electrical charge) in the water.  For a known electrical current, the voltage drop across the electrodes reveals the water’s resistance.   Since the resistance of aqueous solution changes with temperature (resistance drops with increasing temperature), the resistance is corrected to the resistance of the solution at 25 ÂșC.

Combination of Resistance

Resistance can be joined to each other by two ways:
    1. Series combination
    2. Parallel combination

Series Combination
1. If different resistances are joined with each other such that there is only one path for the flow of        electric current then the combination of such resistances is called Series Combination.
    2. In series combination current through each resistor is constant.
    3. In series combination Potential difference across each resistor is different depending upon the value of         resistance.
    4. Equivalent resistance of circuit is equal to the sum of individual resistances.

Re = R1 + R2 + R3 + R4 + …………….. Rn

    If one component is fused, then the other components of circuit will not function.

Parallel Combination
    1. If there are more than one path for the flow of current in a circuit then the combination of resistances     is called Parallel Combination.
    2. In parallel combination current through each resistor is different.
    3. Potential difference across each resistor is constant.
    4. Equivalent resistance of circuit is always less than either of the resistances included in the circuit.

    In parallel combination of resistors, if one component of circuit (resistor) is damaged then rest of the     component of the circuit will perform their work without any disturbance. It is due to the presence of     more than paths for the flow of electric current.

Equivalent Resistance In Parallel Combination
    Consider three resistances R1 , R2 & R3 connected in parallel combination with a power supply of voltage V and current in each resistor is I1, I2 and I3

Then Voltage V = 1/R1 + 1/R2 + 1/R3

Electric Power - Electric power is the rate at which electric energy is transferred by an electric circuit. The SI unit of power is the watt, one joule per second.

Kilowatt Hour- The kilowatt hour, or kilowatt-hour, (symbol kW•h, kW h or kWh) is a unit of energy equal to 1000 watt hours or 3.6 megajoules. For constant power, energy in watt hours is the product of power in watts and time in hours. The kilowatt hour is most commonly known as a billing unit for energy delivered to consumers by electric utilities.

Ammeter- Ammeter is an electrical measuring device, which is used to measure electric current through the circuit. It is the modified form of galvanometer. The resistance of an ideal ammeter is zero.

Voltmeter-   Voltmeter is an electrical measuring device, which is used to measure potential difference between two points in a circuit. The resistance of an ideal Voltmeter is infinite.

Galvanometer - A galvanometer is a type of ammeter: an instrument for detecting electric current. It is an analog electromechanical transducer that produces a rotary deflection of some type of pointer in response to electric current flowing through its coil in a magnetic field.

Shunt - A shunt is a device which allows electric current to pass around another point in the circuit by creating a low resistance path. The term is also widely used in photovoltaics to describe an unwanted short circuit between the front and back surface contacts of a solar cell, usually caused by wafer damage. The origin of the term is in the verb 'to shunt' meaning to turn away or follow a different path.

Transformer- A transformer is a power converter that transfers AC electrical energy through inductive coupling between circuits of the transformer's windings. A varying current in the primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic flux through the secondary winding. This varying magnetic flux induces a varying electromotive force (EMF), or "voltage", in the secondary winding. This effect is called inductive coupling.

AC Generator or Dynamo - An 'AC generator' or 'dynamo' is a machine which produces AC from mechanical energy. Actually, it is an alternator which converts one form of energy into another.

Electric Motor- An electric motor is an electromechanical device that converts electrical energy into mechanical energy.

Most electric motors operate through the interaction of magnetic fields and current-carrying conductors to generate force. The reverse process, producing electrical energy from mechanical energy, is done by generators such as an alternator or a dynamo; some electric motors can also be used as generators, for example, a traction motor on a vehicle may perform both tasks. Electric motors and generators are commonly referred to as electric machines.

Microphone- Microphones are transducers which detect sound signals and produce an electrical image of the sound, i.e., they produce a voltage or a current which is proportional to the sound signal. The most common microphones for musical use are dynamic, ribbon, or condenser microphones.


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