Electricity Flashcards
what is electric circuit and electric current? Write the equation for electric current
A continuous and closed path of an electric
current is called an electric circuit. Electric
current is expressed by its rate of flow of elec
tric charges.
I = Q/t
SI unit for electric charge
coulomb
The electric current is expressed by a unit
Ampere (A)
what is ammeter?
An instrument called ammeter measures elec
tric current in a circuit.
what is potential difference? write the equation to find potential difference
Difference of electric pressure- called the potential difference.
V = W/Q
What is voltmeter?
The potential difference is measured by means
of an instrument called the voltmeter. The volt
meter is always connected in parallel
What is Ohm’s law?
It gives a relationship between current I, flowing in a metallic wire and potential difference
V, across its terminals. According to this law,
the electric current flowing through a conductor is directly proportional to the potential difference applied across its ends, providing the
physical conditions (such as temperature) remain unchanged.
If V is the potential difference applied across
the ends of a conductor through which current I flows, then according to Ohm’s law,
V ∝ I (at constant temperature)
V/I is constant
V/I = R
V = IR
I = V/R
Where, R is the constant of proportionality
called resistance of the conductor at a given
temperature.
The conductor which obey Ohm’s law are
called ohmic conductors while the conductors
which do not obey Ohm’s law are called non
ohmic conductors.
What are ohmic and non- ohmic conductors
The conductor which obeys Ohm’s law are called ohmic conductors while the conductors
which do not obey Ohm’s law are called non-ohmic conductors.
What is resistance?
It is that property of a conductor by virtue of
which it oppose/resists the flow of charges/
current through it. Its SI unit is ohm and it is
represented by Ω .
Resistance of conductor is given by,
R = V/I
V = IR
I = V/R
Factors on which the Resistance of a Conductor Depends
The electrical resistance of a conductor de
pends on the following factors.
(i) Length of the Conductor: The resistance
of a conductor R is directly proportional
to its length l.
i.e.,
R ∝ l
Area of Cross-section of the Conductor.
The resistance of a conductor R is inverse
ly proportional to its area of cross section
A. i.e.,
(iii) Nature of the Material of the Conduc
tor: The resistance of a conductor de
pends on the nature of the material of
which it is made. Some materials have
low resistance, whereas others have high
resistance.
Therefore, from Eqn. (i) and (ii), we can
write
R ℓ/A or R = ρℓ/A
ρ = RA/ℓ
Where, ρ is the constant of proportionality and
is called electric resistivity or specific resistivity of the material of the conductor.
What is resistivity?
It is defined as the resistance of a conductor of
unit length and unit area of cross-section. Its SI
units is ohm-metre (Ω m)
- The resistivity of a material does not de
pend on its length or thickness but depends
on the nature of the substance and temper
ature. It is a characteristic property of the
material of the conductor and varies only,
if its temperature changes. - Insulators such as glass, rubber, ebonite,
etc., have a very high resistivity - 1012 Ωm to 1017 Ωm , while conductors
have a very low resistivity 10-8 Ωm to 10-6 Ωm. - Alloys have higher resistivity than that of their constituent metals. They do not oxidise easily at high temperatures, this is
why they are used to make heating elements of devices such as electric iron,
heaters, etc. - Tungsten is almost used exclusively for
filaments of electric bulbs, whereas copper
and aluminum are generally used for electrical transmission lines.
Why are coils of electric toasters and electric irons made of alloy rather than a pure metal?
Alloys have higher resistivity than that of
their constituent metals. They do not oxidize easily at high temperatures, this is
why they are used to make heating elements of devices such as electric iron, heaters, etc.
Combination of Resistors
There are two methods of joining the resistors together
(i) Series Combination:
When two or more resistors are connected end
to end other, then they are said to be connected in series.
R = R1 + R2 + R3
The equivalent resistance is thus greater than
the resistances of either resistor. This is also
known as maximum effective resistance.
The current through each resistor is same. The
potential difference across each resistor is different.
(ii) Parallel Combination:
When two or more resistors are connected simultaneously between two points to each other, Then they are said to be connected in parallel
combination.
The reciprocal of equivalent resistance is equal
to the sum of the reciprocal of individual resistances.
i.e.,
1/R = 1/R1 + 1/R2 + 1/R3
Joule’s law of heating.
When an electric current is passed through a
high resistance wire like nichrome wire, then
the wire becomes very hot and produces heat. In purely resistive circuits, the source of ener
gy continuously gets dissipated entirely in the
form of heat. This is called the heating effect
of current.
This is obtained by the transformation of elec
trical energy into heat energy, eg, electric heat
er, electric iron, etc.
It is known as Joule’s law of heating.
This law implies that heat produced in a resis
tor is
(i)
Directly proportional to the square of
current for a given resistance.
(ii) Directly proportional to the resistance
for a given current.
(iii) Directly proportional to the time for
which the current flows through the
resistor.
Heat produced is expressed as,
H = I^2 x R x t
H = V^2/R t
H = VIt
Practical Applications of Heating Effect of Electric Current
(i) Electric Bulb:
It has a filament made of tungsten. So,
most of the power consumed by this, is
dissipated in the form of heat and some
part is converted into light because it has
high resistivity and high melting point.
The filament is thermally isolated and the
bulb is filled with chemically inactive nitrogen and argon gas to prolong the life of filament.
(ii) Electric Fuse:
It is used as a safety device in household circuits. It protects the circuits, by stopping the flow of any unduly high electric
current. It is connected in series with the mains supply. It consists of an alloy of lead and tin Which has appropriate melting point.
When the current flowing through the circuit exceeds the safe limit, then the fuse
wire melts and breaks the circuit. This helps to protect the other circuit elements
from heavy current. Fuses are always rat
ed for different current values such as 1A, 2 A, 5 A, 10 A, 15 A, etc.
Electric Power
It is defined as the amount of electric energy
consumed in a circuit per unit time.
Electric power is expressed as, P = VI
P = V^2/R
P = I^2R
P = H/t
P = W/t
The SI unit of electric power is watt (W)
Power is said to be 1 watt, if 1 ampere current
flows through a circuit having 1 volt potential
difference.
1 watt = 1 volt × 1 ampere = 1 VA
What is a magnet
The black ore of iron (Fe3O4) called mag
netite, capable of attracting similar pieces
of iron is called lodestone. They are natu
rally existing magnets used by human to
find the directions.
Who the found the magnetic effect of electric current?
H.C. Oersted, a Danish physicist first noticed the magnetic effect of electric cur
rent. According to him, a needle kept near
the wire carrying current will deflect due
to the magnetic field produced. Any
change in the direction of current will
show variation in the deflection.
* Magnet is any substance that attracts iron
like substances.
Properties of magnet
Every magnet has two poles i.e, North and
South.
(ii) Like poles repel each other.
(iii) Unlike poles attract each other.
(iv) A freely suspended bar magnet aligns it
self in nearly north-south direction, with
its north pole towards north direction.
