Chapter 12 Effects of Currents:- Electric current, potential difference and electric current. Ohm’s law;Resistance, Resistivity, Factors on which the resistance of a conductor depends. Series combination of resistors, Parallel combination of resistors and its applications in daily life. Heating effect of electric Current and its applications in daily life. Electric power, Interrelation between P,V,I and R. Magnetic effects of current : Magnetic field, field lines, field due to a current carrying conductor, field due to current carrying coil or solenoid; Force on current carrying conductor, Fleming’s Left Hand Rule, Electric Motor, Electromagnetic induction. Induced potential difference, Induced current. Fleming’s Right Hand Rule, Electric Generator, Direct current. Alternating current: frequency of AC. Advantage of AC over DC. Domestic electric circuits. Electricity is an important source of energy in the modern times. Electricity is used in our homes, in industry and in transport. For example: electricity is used in our homes for lightning, operating fans and heating purpose etc. Charge is a characteristic unit of matter by means of which matter experiences electric forces. It can be either negative (electron) or positive (proton) “Coulomb” is the SI unit of charge, represented by C. Net charge (Q) – Total charge 1C Net charge is equivalent to the charge contained in nearly electrons electrons. e= 1.6×10-19C (charge on electron is negative) There are two types of charges: Opposite charges (or unlike charges) attract each other. Similar charges (or Like charges) repel to each other.
Conductor And Insulator
Conductor: Those substances through which electricity can flow are called conductors. Example: Silver,coppor and aluminium etc. Insulator: Those substances through which electricity cannot flow are called insulator. Example: mica, cotton, rubber and dry wood etc. The electric potential at a point in an electric field is defined as the work done in moving a unit positive charge from infinity to that point. Potential is denoted by symbol V. The difference in electric potential between two points is known as potential difference. The potential difference between two points in an electric circuit is defined as the amount of work done in moving a unit charge from one point to the another point. Potential difference= Work done / Quantity of charge moved Q = quantity of charged moved The S.I. unit of potential difference is volt (V). The potential difference between two points is said to be 1 volt if 1 joule of work is done in moving 1 coulomb of electric charge from one point to the other point. The potential difference is measured by means of an instrument called voltmeter. Electric current is defined the continuous flow of electron in an electric circuit is called an electric current. Electric current is denoted by I. The S.I. unit of current is coulomb/sec or amperes (A). Current is measures by an instrument called Ammeter. Ohm’s law gives a relationship between current and potential difference. At constant temperature ,the current flowing through a conductor is directly proportional to the potential difference across its end. I ∝ V (At constant temp.) This can be also written as V ∝ I Where, R is resistance of conductor and I is current.
Resistance of a Conductor
The property of a conductor due to which it opposes the flow of current it is called resistance.
Resistance = Potential difference / Current
or R = V/I
The resistance of conductor depends on length , thickness, nature of material and temperature
of the conductor.
The S.I. unit of resistance is ohm which is denoted by the symbol of omega. The unit of resistance ohm, can be defined by using ohm’s law.
According to Ohm’s law
Potential difference / Current = Resistance ( Constant)
That is, V / I = R
So, Resistance, R = V / I
Good Conductors, Resistors and Insulators
- Those substances which have very low electrical resistance are called good conductors.
- A good conductor allows the electricity to flow through it easily.
- Silver metal is the best Conductor of electricity. Copper and aluminium metals are also good conductor.
- Those substance which have comparatively high electrical resistance, are called resistors.
- The alloy like nichrome, manganin and constantan (or eureka) all have quite high resistances, so they are called resistors.
- Those substance which have infinitely high electrical resistance are called insulator.
- An insulator does not allow electricity to flow it. For example rubber, wood, plastic etc.
Factors affecting the resistance of a Conductor
- length of the conductor
- area of cross-section of the conductor ( or thickness of the conductor).
- nature of the material of the conductor,
- temperature of the conductor
Effect of Length of the conductor
The resistance of a conductor is directly proportional to its length.
(where l is the length of conductor)
When the length of wire is doubled , its resistance also gets doubled ; if the length of wire is halved, then its resistance also gets halved.
(i) The resistance of a given conductor is directly proportional to its length
R ∝ l ………(1)
(ii) The resistance of a given conductor is inversely proportional to its area of cross section.That is:
R ∝ 1 / A…….(2)
By combining the relations (1) and (2), we get
R ∝ l /A
or R = ρ x l / A ….. (3)
where, ρ (rho) is a constant known as resistivity of material of the conductor. Resistivity is also known as specific resitance.
Combination of Resistances (or Registors)
The resistances can be combined in two ways:
(i) in Series
(ii) in Parallel
- When two (or more) resistances are connected end to end consecutively, they are said to be connected in series.
- When two or more resistances are connected between the same two points, they are said to be connected in parallel.
Resistances in Series
The combined resistance of any number of resistances connected in series is equal to the sum of individual resistances.
R = R1 + R2 + R3