Electric current

What is electric current?

1) The flow of charge in a definite direction constitutes the electric current.
2) The electric current in a circuit is defined as the rate of flow of charge through it. i.e. electric current = charge flowing(q)/time taken(t).
3) S.I unit of current is ampere or coulomb/second.
4) The arrow head marked in circuit represents the direction of conventional current i.e the direction of flow of positive charge, whereas the direction of flow of electrons gives the direction of electronic current which is opposite to that of conventional current.
5) The current flowing through a conductor depends upon the nature and dimension of the conductor.
6) Current is a scalar quantity.
7) 1 ampere = eighteen times of 6.25 electrons/second.

electric current

An alternating current

It is which changes continuously in magnitude and periodically in direction. It can be represented by a sine curve or a cosine curve.

Transient currents

The electronic currents which vary for small finite time, while growing from zero to maximum value or while decaying from maximum value to zero value to called transient currents.

Dielectrics

Dielectrics are of two types non polar and polar. The non polar dielectrics (like nitrogen, oxygen, benzene, methane) etc. are made up of non polar atoms/molecules, in which the centre of mass of positive charge coincide with the centre of mass of negative charge of the atom/molecule. The polar dielectrics (like Water, carbon dioxide, ammonia) etc. are made up of polar atoms/molecules, in which the centre of mass of positive charge does not coincide with the centre of mass of negative charge of the atom/molecule.

Current carriers

1) The charged particles whose flow in definite direction constitutes the electric current are called current carriers.
2) Examples of current carriers are electrons in conductors, ions in electrolytes, electrons and holes in semiconductors.
3) The current carriers can have positive or negative charge.

Electromotive (E.M.F.) of a cell

1) The electromotive force is associated with an arrangement or mechanism which can supply energy or does work to move the electric charge from lower potential to higher potential energy.
2) E.M.F. of a cell is defined as the total work done in carrying a unit positive charge once around the complete circuit including the cell.
3) If W is the work done in carrying a charge q once around the complete circuit, then e.m.f. of the cell is e= W/q.
4) E.M.F. of a cell is also defined as the maximum potential different between two electrodes of a cell when the cell is in the open circuit.
5) E.M.F. of a cell depends upon nature of electrodes, nature and concentration of electrolyte used in the cell and the temperature of the cell.
6) E.M.F. of a cell does not depend upon size of the cell, quantity of electrolyte, distance between the electrodes, surface area of the electrodes in contact with electrolyte.
7) Inside the cell the direction of current is always from negative electrode to positive electrode.
8) When current is drawn from the cell, then the potential difference (V) across the two electrodes of the cell is less than its electromotive force (e) i.e. V < e.
9) When no current is drawn from the cell, then e = V.
10) When the cell is charged, then the potential difference across the two electrodes of a cell is greater than e.m.f. of the cell.
11) E.M.F. of (e) a cell is equal to the ratio of power (P) generated by the cell to the current (I) flowing through the circuit i.e. e = P/I.
12) The unit of E.M.F. is jule per coulomb or volt or watt/ampere.

Drift Velocity

1) It is defined as the average velocity with which free electrons get drifted towards the positive end of the conductor under the influence of an external electric field.
2) Draft velocity of electrons depends upon (a) the nature of conductor (b) electric field applied across the conductor.
3) Relaxation time = mean free path/ r.m.s. velocity of electrons.

Electric power

1) It is defined as the rate at which work is done by the source of e.m.f. in maintaining the current in the electric circuit.
2) Unit of electric power is watt or ampere-volt. The practical unit of power is kilowatt and horse power; where 1 kilowatt = 13 watt and 1 H.P = 746 watt.
3) In series combination of resistance, the potential difference and power consumed will be more in larger resistance.
4) In parallel combination of resistance the current and power consumed will be more in smaller resistance.
5) In series grouping of bulbs across a given source of voltage, the bulb of greater wattage will give less brightness and will have lesser resistance and potential difference across it but same current.
6) In parallel grouping of bulbs across a given source of voltage, the bulb of greater wattage will give more brightness and will allow more current to pass through it, but will have lesser resistance and same potential difference across it.

Electric energy

1) Electric energy is defined as the total work done or energy supplied by the source of e.m.f. in maintaining the current in an electric circuit for a given time.
2) Electric energy = electric power * time = P*t.
3) Expression for electric energy = Pt = VIt
4) S.I. unit of electric unit is joule (denoted by J).
5) The number of units of electricity consumed is n= (total wattage*time in hour)/103.
6) Bill of electricity i.e. the cost of consumption of electricity in a house = no. of units of electricity consumed*amount for one unit of electricity.

Electric energy

Mean or average value of alternating current

Mean or average value of alternating current over any half cycle is that value of steady current, which would send the same amount of charge through a circuit in the time of half cycle (i.e. T/2) as is sent by A.C. through the same circuit in the same time.

The root mean square (r.m.s.) value

The root mean square (r.m.s.) value of alternating current is defined as that value of steady current, which would generate the same amount of heat in a given resistance in a given time, as is done by the alternating current, when passed through the same resistance for the same time. The r.m.s. value of a.c. is also called effective value or virtual value of a.c. It is represented by Ims, Ieff orI v .

Power factor

Power factor of an a.c. circuit is the ratio of true power to apparent power of the circuit. It is equal to cosine of the phase angle between alternating voltage and alternating current, which is equal to the ratio of resistance to the impedance of a.c. circuit. Power factor = True power/ Apparent power.

Effective current

The component of alternating current, which remains in phase with the alternating e.m.f. is defined as the effective current. The peak value of effective current is I.cos.

Watt less current

The component of alternating current, which does not contribute to any power loss, is defined as the wattles current. Such a current leads the alternating e.m.f. by a phase angle of 90 degree.

Conduction current

1) It is a current in the electronic circuit which arises due to the flow of electrons in the connecting wires of the circuit, in a definite closed path.
2) When a capacitor is connected to the battery, it starts storing the charge, due to conduction current. When the capacitor gets fully charged, the conduction current becomes zero in the circuit.
3) Conduction current exists even if the flow of electrons is t uniform rate

Displacement current

1) This law states that the line integral of magnetic field over a closed path in vacuum is equal to µ₀ times the some of the conduction current (I) and displacement current (I_D)
2) The some of the conduction current and displacement current has the property of continuity along any closed path although individually they may not be continuous.
3) The conduction current and displacement current are equal at an instant.
4) Like conduction current, the displacement current is also a sure of magnetic field.