Electricity 2 Flashcards
state ohm’s law
The potential difference, V, across the ends of
a given metallic wire in an electric circuit is directly
proportional to the current flowing through it, provided its
temperature remains the same.
V∝ I
V=IR
R is constant for every ratio of Voltage to current
where I is the current through the conductor in units of amperes, V is the potential difference measured across the conductor in units of volts, and R is the resistance of the conductor in units of ohms. More specifically, Ohm’s law states that the R in this relation is constant, independent of the current.
define resistance
It is the property of a conductor to oppose the flow of charges through it
(OR)
It also defined as the ratio of the potential dofference across two ends of a conductor to the current flowing through the conductor.
Its SI unit is ohm represented by omega.
Define 1 ohm
Resistance of a conductor is a said to be 1 ohm when the potential difference of 1 volt across the ends of the conductor makes 1 ampere of current to flow through it.
why is a v-i graph always a straight line
It is because voltage is linearly dependent on current.
give the limitations of ohm’s law
Ohm’s law is invalid when
i) the temperature is not constant. Resistance is directly proprtional to temp.
ii) Non-ohmic materials. They do not follow ohm’s law. Forexample electrolytes. In their case, when temperature increases, the formation of ions increases and hence conduction increases.
what are ohmic materials
design an experiment to prove ohm’s law
learn from tb
what are the factors that resistance depends upon?
r is directly proprtional to temperature,
r is directly proportional to legnth of resistor
r is directly proportional to thickness of resistor
r is directly proportional to resisitivity of resistor.
derive the equation representing factors affecting resistance
R ∝ l (12.8)
and R ∝ 1/A (12.9)
Combining Eqs. (12.8) and (12.9) we get
R ∝ l/A
or
R=ρx l/A
where rho is the constant of proportionality
what is rho or resistivity.
specific resistance or resistivity refers to the resistance offered by a resistor of length 1m and area of cross section 1 m^2.The SI unit of resistivity is
Ω m. It is a characteristic property of the material.
the connecting cord of an electric heater does not glow but heating element does. why
The heat produced by electric current is directly proportional to the resistance.Now the heater elementis made of nichrome alloy, which has very high resistance so a lot of heat is produced in the heater element due to which it becomes very hot and glow.On the other hand,the cord of an elecric heater is made of pure metal Cu has very low resistance due to which very little heat is produced in it and so the cord becomes slightly warm but doesn ‘t glow.
what is the significance of resistivity?
the resistivity of a material decides the conductivity of the metal. If resistivity of the meaterial is low, it allows current to pass through iteasily. If resistance of a material is low, it does not allow electric currentto pass easily. conductors allow current to pass easier than in alloys. The insulators have very hgih resistivity and hence electric current cannotpass through the insulators.
list reasons: why nichrome wire is used for making heating element
i) nichrome is an ally made of nickel, chromium,manganese and iron.
ii) it has high resistivity, and therefore produces more heat due to flow of current
iii) It has high melting point and does melt easily on heating
iv) it does not get oxidisedeven at high temps.
what is the diff between resistance and resistivity
Resistance:
i) it refers to the property of a material due to which it reists the flow of current through it.
ii)It depends on the legnth and thickness of the conductor.
iii)R=V/I
vi) S.I unit of resistance is ohm/.
vii)not constant
Resistivity;
i)It rfers to the resistance offered by a conductor of legnth 1m and area of corss-section 1m2
ii) it does not depend on the legnth of thickness of conductor but it is a charcateristic of the conductor.
iii)ρ= R* A/l
iv) it SI unit is ohm metere.
vii)constant
