Direct Current Electricity

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 currentThe flow of charge in a definite direction•Measure of electric currenttime rate of flow of charge through any cross section of a conductor Electric current = Total charge flowing / Time

takenI = q / T1 Ampere = 1 Coulomb / 1 sec

Direction of currentDirection of flow of positive charge gives the direction of current (conventional current)Direction of flow of electrons gives the direction of electronic currentDirection of conventional current is opposite to electronic current

Electric current

Factors essential for production of electric current are1. Potential difference2. Conducting pathway between the points of

potential difference

Electrons flow only for as long as the potential difference & pathway exist

Electric potentialCondition of a body when compared to neutral

potential of earthUnit is: volt

Potential gradientRate of change of potential with respect to distanceDirected from an area of low potential to an area of high potentialIt is a vector quantity

W = V/dW- potential gradientV- potential of that pointD- distance

Current carriersCharged particles whose flow in a definite direction constitute the electric current are called current carriers

Current carriers in solid conductorsIn solid conductors like metals valence electrons of the atom do not remain attached to individual atoms, but are free to moveUnder an effect of an external electric field they move in a definite direction causing an electric current

Current carriers in liquidsIn an electrolyte like CuSO4, NaCl etc. there are positively & negatively charged ionsThese are forced to move in definite direction under the effect of an external electric field

Current carriers in gasesGases are insulators of electricityBut, 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 bodyif 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

Obstruction to the flow of electrons in a conductorThe 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

Resistances

Factors affecting resistance 1. Material of the conductorCopper for example has a single electron in its outer shellAt room temperature kinetic energy of atoms displaces some of these electronsThey are free to act as conduction electronsThey carry electric charge from one end of conductor to the other2. Length of the pathwayAt normal temperatures eve good conductors offer some resistance to electron flowThe longer the pathway the greater is the electrical resistance3. Cross sectional area of the conductorWhen cross sectional area is greater there is more room for the electrons to passTherefore resistance is lowerIf high resistance is required thin wire is used4. TemperatureAs temperature increases kinetic movement of molecule increasesIncreased movement disturbs the passage of electronsSo resistance increases

Ohm’s LawThe 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 = IRR is resistance

R = V/ISo 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 law1. Temperature of the conductor should remain constant2. The conducting body should not be deformed3. It takes place in metallic conductors only

Resistance in SeriesIf components of an electrical circuit are connected in a seriesthere is only one possible pathway for a currentas 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 differenceFrom Ohm’s lawV1 = IR1V2 = IR2V3 – IR3If potential difference of whole circuit is VV = V1 + V2 + V3V = IR1 + IR2 + IR3V = I ( R1+R2+R3)R = R1 + R2 + R3

Resistance in ParallelIn 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 resistancesBy applying Ohm’s lawWe 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 rheostat1. Series rheostat

rheostat is wired in series with apparatusIf all the wires are included in the circuit resistance is maximum & current is loweste.g.- in wax bath

2. Shunt rheostatwired across a source of potential differenceany other circuit has to be taken parallel to itthis apparatus has a current regulating mechanismcurrent is applied directly to patientcurrent intensity can be gradually increased from zero to maximumalso known as potentiometer rheostat

Fundamental Electric ChargesThe least charge found on any body is equal to the

charge of electron or protone = 1.6 X 10-19 coulombsCharge on any body can only be the integral multiple of the charge of electron

q = +/- ne

n is integer 1,2,3,….

Electric FieldElectric 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 asA 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 curvesThey start from a positively charged body & end at a negatively charged bodyNo electric lines of force exist inside a charged bodyTangent to the line of force at any point gives the direction of electric intensity at that pointNo two electric lines of force can intersect each otherThe 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 conductorq ∝ Vq = CVC 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 chargeIn its simplest form it consists of two metal plates separated by an insulator called the dielectricIf 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 capacitorCapacitor can be charged using electrostatic inductionStatic electric charge is allowed to build up on the plates of the capacitorOrBy 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 capacitorMost commonly used capacitorConsists of two thin conducting plates of areaHeld parallel to each otherSuitable distance d apartPlates 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

Q

Variable capacitor Consists of two sets of plates interleaving with one anotherConstructed in such a way that one set of plates can be moved relative to another, varying the surface area of the plates facing each otherWhen all the surfaces of both sets of plates are fully interleaved the capacitance is maximumFound in radio set & short wave diathermy machines

Grouping of capacitors

Two types

1. Capacitors in series2. 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 capacitorSingle 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

If capacitors are in parallelTotal charge developed on them is the sum of charges on each of them

Q = Q1 + Q2 + Q3 + Q4C = Q1/V + Q2/V + Q3/V Q4/VC = C1 + C2 + C3 + C4

Capacitors in parallel

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