## Saturday, June 27, 2009

### 3. EFFECTS OF ELECTRIC CURRENT

3. EFFECTS OF ELECTRIC CURRENT

1.STATE JOULE’S LAW OF HEATING EFFECT.
Joule’s law: H = VIt . For a resistance R, H = I2Rt . and H =(V2/R) t .
The heat produced is (i) directly proportional to the square of the
current for a given R (ii) directly proportional to resistance R for a given
I and (iii) directly proportional to the time of passage of current. Also
the heat produced is inversely proportional to resistance R for a given V.
2.What is Nichrome?Why is it used as heating element?
Nichrome which is an alloy of nickel and chromium is used as the heating element for the following reasons. (1) It has high specific resistance
(2) It has high melting point (3) It is not easily oxidized
3.Write note on fuse wire.
Fuse wire is an alloy of lead 37% and tin 63%. It is connected in series in an electric circuit. It has high resistance and low melting point. When large current flows through a circuit due to short circuiting, the fuse wire melts due to heating and hence the circuit becomes open. Therefore, the electric appliances are saved from damage.
4.State Seebeck effect
In 1821, German Physicist Thomas Johann Seebeck discovered that in a circuit consisting of two dissimilar metals like iron and copper, an emf is developed when the junctions are maintained at different temperatures.
Two dissimilar metals connected to form two junctions is called
thermocouple. The emf developed in the circuit is thermo electric emf.
The current through the circuit is called thermoelectric current. This
effect is called thermoelectric effect or Seebeck effect
5.Define Neutral and Inversion temperature
Keeping the temperature of the cold junction constant, the temperature of the hot junction is gradually increased ,the thermo emf rises to a maximum at a temperature (θn) called neutral temperature.
Beyond neutral temperature if we increase the temperature of hot junction the thermo emf gradually decreases and eventually becomes zero at a particular temperature (θi) called temperature of inversion
6. State Peltier effect
In 1834, a French scientist Peltier discovered that when electric
current is passed through a circuit consisting of two dissimilar metals,
heat is evolved at one junction and absorbed at the other junction. This
is called Peltier effect.

7.Define Peltier Co-efficient .
The amount of heat energy absorbed or evolved at one of the
junctions of a thermocouple when one ampere current flows for one
second (one coulomb) is called Peltier coefficient. It is denoted by π. Its
unit is volt. If H is the quantity of heat absorbed or evolved at one
junction then H = π It
8.Define :Thomson effect.
Thomson suggested that when a current flows through unequally
heated conductors, heat energy is absorbed or evolved throughout the
body of the metal.
9.Define :Thomson coefficient .
The amount of heat energy absorbed or evolved when one ampere
current flows for one second (one coulomb) in a metal between two
points which differ in temperature by 1oC is called Thomson coefficient.
It is denoted by σ. Its unit is volt per oC.
10.State Maxwells’s right hand cork screw rule
If a right handed cork screw is rotated to advance along the
direction of the current through a conductor, then the direction of
rotation of the screw gives the direction of the magnetic lines of force
around the conductor.
11.State Biot – Savart Law.
Biot and Savart conducted many experiments to determine the
factors on which the magnetic field due to current in a conductor
depends. i) directly proportional to the current (I)
(ii) directly proportional to the length of the element (dl )
(iii) directly proportional to the sine of the angle between dl and
the line joining element dl and the point P (sin θ)
(iv) inversely proportional to the square of the distance of the
point from the element ( 1/r2)
dB =(μ/4π) (Idlsin θ/r2)
12.State the principle of tangent galvanometer.
Tangent galvanometer is a device used for measuring current. It works on the principle of tangent law. A magnetic needle suspended at a point where there are two crossed fields at right angles to each other will come to rest in the direction of the resultant of the two fields.
13.State Ampere’s Circuital Law
∫BL = μoIo . ∫B.dl = μoIo ...
The line integral ∫B.dl for a closed curve is equal to μo times
the net current Io through the area bounded by the curve.
14. State Right hand palm rule
The coil is held in the right hand so that the fingers point in the direction of the current in the windings. The extended thumb, points in the direction
of the magnetic field.

15.State End rule:
When looked from one end, if the current through the solenoid is along
clockwise direction the nearer end corresponds to south pole
and the other end is north pole.
When looked from one end, if the current through the solenoid is
along anti-clock wise direction, the nearer end corresponds to north
pole and the other end is south pole .
16. Define Magnetic Lorentz force
The force experienced by a moving charge placed in uniform magnetic field depends (i)the force is proportional to the magnitude of the charge (q),
(ii) the force is proportional to the magnetic induction (B), (iii) the force is proportional to the speed of the charge (v) : F = Bqv sin θ
17.State the principle of Cyclotron.
Cyclotron works on the principle that a charged particle moving
normal to a magnetic field experiences magnetic lorentz force due to
which the particle moves in a circular path.

18.State the Limitations of cyclotron.
(i) Maintaining a uniform magnetic field over a large area of the
Dees is difficult. (ii) At high velocities, relativistic variation of mass of the particle upsets the resonance condition. (iii) At high frequencies, relativistic variation of mass of the electron is appreciable and hence electrons cannot be accelerated by cyclotron.
19.State Fleming’s Left Hand Rule.
The forefinger, the middle finger and the thumb of the left hand are stretched in mutually perpendicular directions. If the forefinger points in the direction of the magnetic field, the middle finger points in the direction of the current, then the thumb points in the direction of the force on the conductor.
20.Define one ampere.
Ampere is defined as that constant current which when flowing through two parallel infinitely long straight conductors of negligible cross section and placed in air or vacuum at a distance of one metre apart, experience a force of
2 × 10-7 newton per unit length of the conductor.
21.Define current sensitivity of a galvanometer.
The current sensitivity of a galvanometer is defined as the
deflection produced when unit current passes through the
galvanometer. A galvanometer is said to be sensitive if it produces large
deflection for a small current. In a galvanometer, I =(C/nBA)/θ
Current sensitivity (θ/ I) =(nBA /C)
22.How will you increase current sensitivity of a galvanometer.?
The current sensitivity of a galvanometer can be increased by
(i) increasing the number of turns (ii) increasing the magnetic induction
(iii) increasing the area of the coil (iv) decreasing the couple per unit twist of the suspension wire.

23.Define: Voltage sensitivity of a galvanometer
The voltage sensitivity of a galvanometer is defined as the
deflection per unit voltage. Voltage sensitivity(θ/V) = (θ/IG) = (nBA/CG)
where G is the galvanometer resistance.
24. Increasing the current sensitivity does not necessarily, increase the voltage sensitivity . Why?
When the number of turns (n) is doubled, current sensitivity is also doubled. But increasing the number of turns correspondingly increases the resistance (G). Hence voltage sensitivity remains unchanged.
25.How will you convert galvanometer into an ammeter & voltmeter?
Agalvanometer is converted into an ammeter by connecting a low resistance in parallel with it.
A galvanometer can be converted into a voltmeter by connecting a high resistance in series with it.
26.Define the magnetic moment of a current loop
The magnetic moment of a current loop is defined as the product
of the current and the loop area. Its direction is perpendicular to the
plane of the loop. M = IA