Download Measure of electric current

Modern concepts of electricity
•Electric current occurs when
there is a flow of charged
particles (generally electrons) on
a conductor
•Current flow from positive to
negative
•The reverse direction of actual
flow of electrons
Electric current
The flow of charge in a definite direction
•Measure of electric current
time rate of flow of charge through any cross section of a
conductor
Electric current = Total charge flowing / Time
taken
I=q/T
1 Ampere = 1 Coulomb / 1 sec
Direction of current
Direction of flow of positive charge gives the direction of
current (conventional current)
Direction of flow of electrons gives the direction of electronic
current
Direction of conventional current is opposite to electronic
current
Electric current
Factors essential for production of electric current are
1. Potential difference
2. Conducting pathway between the points of
potential difference
Electrons flow only for as long as the potential
difference & pathway exist
Electric potential
Condition of a body when compared to neutral
potential of earth
Unit is: volt
Potential gradient
Rate of change of potential with respect to distance
Directed from an area of low potential to an area of
high potential
It is a vector quantity
W = V/d
W- potential gradient
V- potential of that point
D- distance
Current carriers
Charged particles whose flow in a definite direction constitute the
electric current are called current carriers
Current carriers in solid conductors
In solid conductors like metals valence electrons of the atom do not
remain attached to individual atoms, but are free to move
Under an effect of an external electric field they move in a definite
direction causing an electric current
Current carriers in liquids
In an electrolyte like CuSO4, NaCl etc. there are positively & negatively
charged ions
These are forced to move in definite direction under the effect of an
external electric field
Current carriers in gases
Gases are insulators of electricity
But, can be ionized by applying a high potential difference at low
pressures
Ionized gas contains positive ions & electrons
Electromotive Force (EMF)
Force producing the flow of electrons from
more negative to less negative body
if similar bodies are charged with different
quantities of electricity
A volt is that EMF when applied to a conductor
with a resistance of one Ohm produces a
current of one Ampere
Electrons move as so long as potential
difference exists between the ends of the
pathway
Resistances
Obstruction to the flow of electrons in a conductor
The unit of electrical resistance is the ohm
Cause of resistance
Due to the collisions of free electrons with the ions or atoms of the
conductor
Depends on the arrangement of atoms of the conducting material
& length and thickness of conducting wire
Resistance directly proportional to the length, temperature
inversely proportional to area of cross section & number of
free electrons in a unit volume
Factors affecting resistance
1.
Material of the conductor
Copper for example has a single electron in its outer shell
At room temperature kinetic energy of atoms displaces some of these
electrons
They are free to act as conduction electrons
They carry electric charge from one end of conductor to the other
2. Length of the pathway
At normal temperatures eve good conductors offer some resistance to
electron flow
The longer the pathway the greater is the electrical resistance
3. Cross sectional area of the conductor
When cross sectional area is greater there is more room for the electrons
to pass
Therefore resistance is lower
If high resistance is required thin wire is used
4. Temperature
As temperature increases kinetic movement of molecule increases
Increased movement disturbs the passage of electrons
So resistance increases
Ohm’s Law
The current flowing through a metallic conductor is proportional to the
potential difference across its ends, provided that all physical
conditions remain constant.
If V is potential difference & I is current
V = IR
R is resistance
R = V/I
So 1 ohm is defined as
The resistance of a body such that 1 volt potential difference across
the body results in a current of 1 ampere through it.
Limitations of Ohm’s law
1. Temperature of the conductor should remain constant
2. The conducting body should not be deformed
3. It takes place in metallic conductors only
Resistance in Series
If components of an electrical circuit are connected in a
series
there is only one possible pathway for a current
as the current has to pass through each resistance, the total
resistance equals the sum of individual resistances.
If R1, R2 & R3 =resistances
V1, V2 & V3 = potential difference
From Ohm’s law
V1 = IR1
V2 = IR2
V3 – IR3
If potential difference of whole circuit is V
V = V1 + V2 + V3
V = IR1 + IR2 + IR3
V = I ( R1+R2+R3)
R = R1 + R2 + R3
Resistance in Parallel
In this situation the current is offered a number
of alternative routs
The proportion of current in such resistance
depends upon the relative magnitude of the
resistances
By applying Ohm’s law
We can find that the largest resistance carries
the smallest current & the smallest resistance
carries larges current.
1/R = 1/R1 + 1/R2 + 1/R3
Rheostat
A device used to regulate current by altering either the resistance of the
current or potential in the part of the circuit
It consist of a coil of high resistance wire wound in to an
insulating block with each turn insulated from adjacent turns
Rheostats have two connections, one to the fixed end of a
resistor and the other to a sliding contact on the resistor.
Turning the control moves the sliding contact away from or
toward the fixed end, increasing or decreasing the resistance.
Rheostats control resistance, thus controlling current flow.
Types of rheostat
1. Series rheostat
rheostat is wired in series with apparatus
If all the wires are included in the circuit resistance is
maximum & current is lowest
e.g.- in wax bath
2. Shunt rheostat
wired across a source of potential difference
any other circuit has to be taken parallel to it
this apparatus has a current regulating mechanism
current is applied directly to patient
current intensity can be gradually increased from zero
to maximum
also known as potentiometer rheostat
Fundamental Electric Charges
The least charge found on any body is equal to the
charge of electron or proton
e = 1.6 X 10-19 coulombs
Charge on any body can only be the integral
multiple of the charge of electron
q = +/- ne
n is integer 1,2,3,….
Electric Field
Electric field intensity due to group of charges
The electric field intensity at any point due to a
group of point charges is equal to the vector
sum of the electrical field intensities due to
individual charges at the same point.
E = E1 + E2 + E3……En
Electric Lines of Forces
Electric line of force is defined as
A path, straight or curved, such that
tangent to it at any point gives the
direction of electric field intensity at that
point.
It is the path along which a unit positive
charge actually moves in the electrostatic
field if free to do so.
Properties of electric lines of forces
Electric lines of forces are discontinuous curves
They start from a positively charged body & end at
a negatively charged body
No electric lines of force exist inside a charged body
Tangent to the line of force at any point gives the
direction of electric intensity at that point
No two electric lines of force can intersect each
other
The electric lines of force are always starting &
ending on the conductor. Therefore no component of
electric field intensity parallel to surface of conductor
Lines of force due to single positive point charge are
directed outwards.
The lines of force extend to infinity
Force due to single negative point charge are directed
radially inwards
Line of force due to a pair of equal &
opposite charges
When charges are unequal the neutral point is
closer to smaller charge
Capacitance
Ability of the body to hold an electric charge
Its units are farad
A farad is the capacity of an object which is charged to a potential of
1 volt by 1 coulomb of electricity
At any stage if q is the charge on the conductor & V is the potential
of the conductor
q∝V
q = CV
C is a constant of proportionality & is called capacity or
capacitance of the conductor.
Value of C depends on the shape & size of conductor & nature of
medium in which capacitance is located.
Factors affecting the capacity of a conductor
1. Area of conductor: it is inversely related to
capacity
2. Presence of any conductor nearby: in case,
potential decreases, so capacity increases
3. Medium around conductor: the capacity
increases when any other medium is placed
around the conductor
The capacitor (condenser)
A device for storing an electric charge
In its simplest form it consists of two
metal plates separated by an insulator
called the dielectric
If the plates are given opposite static
electric charges the electric lines of force
concentrate between the plate
The electric field
between the plates has
an effect on the atoms of
the dielectric, causing
their electron orbits to
distort as they are
attracted towards the
positive plate.
Atoms remain in state of
tension until the
potential difference
across the capacitor is
removed
Types of capacitors
Electric field of a capacitor
Electric field between the plates of a
charged capacitor consist of electric
lines of force
They tend to take the shortest possible
route between plates
Charging & discharging a capacitor
Capacitor can be charged using electrostatic
induction
Static electric charge is allowed to build up on
the plates of the capacitor
Or
By applying a potential difference across the
plates from either the mains or the battery
A capacitor discharges when the accumulated
charge is allowed to flow of the plates
If two plates of opposite charges are connected
electrons flow from negative to positive plate
until the charges are equal
Parallel plate capacitor
Most commonly used capacitor
Consists of two thin conducting plates of area
Held parallel to each other
Suitable distance d apart
Plates are separated by an insulating medium like
air, paper etc.
Spherical capacitor
Consists of a hollow conducting sphere
Surrounded by another concentric conducting
spherical shell
Variable capacitor
Consists of two sets of plates interleaving with one another
Constructed in such a way that one set of plates can be
moved relative to another, varying the surface area of the
plates facing each other
When all the surfaces of both sets of plates are fully
interleaved the capacitance is maximum
Found in radio set & short wave diathermy machines
Q
Grouping of capacitors
Two types
1. Capacitors in series
2. Capacitors in parallel
Capacitors in series
A voltage applied across four capacitors in series induce
charges of +Q and –Q on plates of each.
1/C = V/Q
The potential difference across the row is the sum of
potentials across each capacitor
Single capacitance C equivalent to three capacitors
1/C = (V1 + V2 + V3 + V4) / Q
= V1/Q + V2/Q + V3/Q + V4/Q
= 1/C1 + 1/C2 + 1/C3 + 1/C4
Capacitors in parallel
If capacitors are in parallel
Total charge developed on them is the sum of charges on
each of them
Q = Q1 + Q2 + Q3 + Q4
C = Q1/V + Q2/V + Q3/V Q4/V
C = C1 + C2 + C3 + C4
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