Current: The
flow of electrons in conducting loop called a current. Current denote as (I)
and unit is ampere. Ampere is equal to the current that flow through a circuit
of one ohm when one volt is applied across it. As we know the electrons are negatively
charged and direction flow from negatively point to positively point relatively
and value is -1.6X10-19 coulomb(C). Coulomb(C) is equal to quantity
of electricity transferred in one second by a current of one ampere, It the
unit for measurement of electricity quantity.
Two type current in circuits DC and AC. The flow of Direct current same
at all points in time although an alternating current flow of charged electrons
reverse direction periodically. The current free flow when the cross sectional
area of the conductor more but less dense means current density inversely
proportional to the area of conductor. When more area of the conductors current
density less vice versa. We can define the current density the amount of
electric current flow per unit cross sectional area of material.
Current density is the ratio of the current (I) flowing in a conductor to the cross sectional area (A) of that conductor expressed as J=I/A unit is Ampere/meter2.
The value of current density at
high frequency higher the outer surface
of the conductor and lower at the center of the conductor called the skin effect. Due skin effect transmission losses increased in high
voltage transmission lines can be reduced by increase the voltage and reduce
the current.
Ammeter is an instrument to measure the same current through a
circuit. Ammeter is always connected series with the circuit if it is connected
to parallel due to low resistance (few milli ohm) of ammeter cause short
circuit and burn the ammeter.
Moving
coil ammeter a movable coil placed in permanent magnate coil moved where
current passing through it and pointer moved connected with moving coil used
for DC current measurements.Moving iron
ammeter a soft iron piece moves by the electromagnetic force of a fixed coil of
wire used both AC and DC current.
Moving
magnet type meter a permanent magnet moves the pointer and coil is mounted
on the meter. Electrodynamics type
meter an electromagnet is used instead of permanent magnate to move the
pointer. Hot wire type ammeter a current pass through a wire which expands as
it heat cause move pointer. For
measuring of AC high current cannot pass through ammeter directly used current
transformer (CT) or shunt.
Current
transformer has primary and secondary windings, the secondary winding of CT
always short circuited otherwise core saturation due to high voltage which can
destroy the CT will be induced. CT will
reduce the current for measurement given to meter.
Current can also measure by
using clamp meter in this meter no
any test lid needed. Such type of meter has a split soft iron core which placed
around the conductor by pressing a trigger system and the change in magnetic
flux which proportional value of current read directly on the digital scale.
Such type of meter used both AC and DC measurement. In case of shunt ammeter high resistance alloy made
up of manganin actually a milivoltmeter which is calibrated in ampere.
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| Current density |
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| Skin effect |
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| Ammeter |
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| Moving coil ammeter |
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| Moving iron ammeter |
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Current transformer
|
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| Clamp meter |
Resistance: The flow of electrons
resists by resistance it is different for different materials. The unit of
resistance is ohm (Ω) and denoted by (R). Ohm is equal to the resistance
offered to unvarying electric current by a column of mercury 106.3cm long
having a cross sectional area of one sq.mm at the temperature of melting ice.
We can also say an e m f of one volt will cause a current of one ampere through
a resistance of one ohm. In any circuit value of resistance higher than then
the voltage and power losses more. Conductance is opposite of resistance the
unit of conductance is mhos (ʊ) and denoted by (Y).The resistance of conductor
directly proportional to the length of the conductor and inversely proportional
to the area of conductor. We can say when the length of conductor increases
resistance should increase vice versa if area of conductor increases resistance
should be decreases vice versa. In mathematical resistance is R=ρL/AL-length of the conductor in cm, R-resistance of the conductor, A- area
of conductor sq. cm,ρ-resisitivity in cm cube. Conductance I=VY where I-current in ampere,
V-voltage in volt, Y-Conductance mhos. We can define the resistivity with
resistance formula ρ=R A / L i.e. resistance offered by any
substance to flow of current per unit area per unit length known as
resistivity. The symbol is rho ρ and unit ohms/cm. The resisitivity of aluminium conductor per sqr mm per meter length is 0.028 ohms and copper 0.018 ohms. Resistance can be measured by Ohmmeter, Comparison between two resistances,Wheatstone bridge,Post office Bridge, Voltmeter method etc.
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| Ohmmeter |
Ohmmeter is an instrument used for measuring resistance directly a dry cell fitted as an energy source and galvanometer number of range with a suitable selector switch.Galvanometer calibrated to register ohms and before using the instrument checked resistance should be zero by touching two test prongs together.Start the measurement of resistance high range to right range reached otherwise instrument will be damaged.
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| Comparison of two resistance |
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Wheatstone bridge
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| Voltmeter |
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