Electricity Flashcards
Electric current
It is defined as the rate of flow of charges in a conductor.
I=Q/t
SI unit of electric current
one ampere, when a charge of one coulomb flows across any cross-section of a conductor, in one second. Hence,
1 ampere = 1 coulomb / 1 second
A charge of 12 coulomb flows through a bulb in 5 second. What is the current through the bulb?
Solution:
Charge Q = 12 C, Time t = 5 s. Therefore, current I = Q /t
12 /5 =I
2.4 A
Used to fix the magnitude of the current through a circuit
Resistor
Used to select the magnitude of the current through a circuit.
Variable resistor or Rheostat
Used to detect the current and its direction
Galvanometer
Used to provide protection to the electrical components. It also serves as a reference point to measure the electric potential.
Ground connection
The electric potential
at a point is defined as the amount of work done in moving a unit positive charge from infinity to that point against the electric force.
The electric potential difference
The electric potential difference between two points is defined as the amount of work done in moving a unit positive charge from one point to another point against the electric force.
V=W/Q
The SI unit of electric potential or potential difference is
volt (V).
1 volt
The potential difference between two points is one volt, if one joule of work is done in moving one coulomb of charge from one point to another against the electric force.
I volt = 1 joule / 1 coulomb
Solved Problem-2
The work done in moving a charge of 10 C across two points in a circuit is 100 J. What is the potential difference between the points?
V=w/q
100/10
V=10
A German physicist, who established the relation between the potential difference and current, which is known as Ohm’s Law.
Georg Simon Ohm
Ohm’s law
Ohm’s law, at a constant temperature, the steady current ‘I’ flowing through a conductor is directly proportional to the potential difference ‘V’ between the two ends of the conductor.
I~v
Resistance
Resistance of a material is its property to oppose the flow of charges and hence the passage of current through it. It is different for different materials.
From Ohm’s Law, v/i= R.
The SI unit of resistance is
ohm
and it is represented by the symbol Ω
Resistance of a conductor
is said to be one ohm if a current of one ampere flows through it when a potential difference of one volt is maintained across its ends
1 R= 1 volt / 1ampere
Solved Problem-3
Calculate the resistance of a conductor through which a current of 2 A passes, when the potential difference between its ends is 30 V.
15 Ω
the resistance of any conductor ‘R’ is directly proportional to the ______ and is inversely proportional to its_______
length of the conductor ‘L’
area of cross section ‘A’.
the electrical resistivity of a material
the electrical resistivity of a material is defined as the resistance of a conductor of unit length and unit area of cross . Its unit is ohm metre.
¶ rho = RA/L
of a conductor is a measure of the resisting power of a specified material to the passage of an electric current. It is a constant for a given material.
Electrical resistivity
Nichrome is a conductor with highest resistivity equal to ______ Hence, it is used in making heating elements.
1.5 x 10^-⁶ Ωm.
Conductance
Conductance
of a material is the property of a material to aid the flow of charges and hence, the passage of current in it.
G=1/R
defined as the reciprocal of its resistance
Electrical conductivity of a conductor
Electrical conductivity of a conductor
is a measure of its ability to pass the current through it
σ=1/¶(rho)
The reciprocal of electrical resistivity of a material is called its electrical conductivity.
Unit of conductance
ohm-¹.
Unit of electrical conductivity
ohm-¹ metre-¹. Or
mho meter-¹
Resistivity of copper
1.62*10-⁸Ωm
Resistivity of nickel
6.84*10-⁸ Ωm
Resistivity of chromium
12.9*10-⁸
Resistivity of glass
10¹⁰ to 10¹⁴
Resistivity of rubber
10¹¹ to 10¹⁶
The resistance of a wire of length 10 m is 2 ohm. If the area of cross section of the The wire is 2*10-⁷ m². determine its (i) resistivity call (ii) conductance and (iii) conductivity
¶ (rho)=RA/L
Resistivity= 410-⁸Ωm
Conductance G= 1/R
0.5 mho
Conductivity (Sigma)
0.2510⁸ mho m-¹
The two basic methods of joining resistors together are:
a) Resistors connected in series, and b) Resistors connected in parallel.
Series circuits are commonly used in devices such as
flashlights
The equivalent resistance in a series combination is ______ than the highest of the individual resistances.
greater
Three resistors of resistances 5 ohm, 3 ohm and 2 ohm are connected in series with 10 V battery. Calculate their effective resistance and the current flowing through the circuit.
effective resistance =10Ω
Current=1A
The equivalent resistance in a parallel combination is _____ the lowest of the ass individual resistances.
less than
heating effect of current
the passage of electric current through a wire, results in the production of heat. This phenomenon is called heating effect of current
H=I²Rt
Joule’s law of heating
. Joule’s law of heating states that the heat produced in any resistor is:
H=I²Rt
directly proportional to the square of the current passing through the resistor.
directly proportional to the resistance of the resistor
directly proportional to the time for which the current is passing through the resistor.
Nichrome, which is an alloy of Nickel and Chromium is used as the heating element. Why? Because:
(i) it has high resistivity, (ii) it has a high melting point, (iii) it is not easily oxidized.
Application of heating effect
Electric heating device
Fuse wire
Filament in bulb
Problem-6
An electric heater of resistance 5 2 is connected to an electric source. If a current of 6 A flows through the heater, then find the amount of heat produced in 5 minutes.
H=I²Rt,
H=54000J
power
is defined as the rate of doing work or rate of spending energy. Similarly,
the electric power is defined as the rate of consumption of electrical energy
Electric power
electric power is the product of s the electric current and the potential difference due to which the current passes in a circuit
P=vi
The SI unit of electric power
The SI unit of electric power is watt.
When a current of 1 ampere passes across the ends of a conductor, which is at a potential difference of 1 volt, then the electric power is
P= 1 volt x 1 ampere = 1 watt
The horse power (hp)
The horse power (hp) is a unit in the foot-pound-second (fps) or English system, sometimes used to express the electric power. It is equal to 746 watt.
Consumption of electricity is based on
Consumption of electricity is based on two factors: (i) Amount of electric power and (ii) Duration of usage.
Electrical energy
Electrical energy consumed is taken as the product of electric power and time of usage
Consumption of electrical energy is measured and expressed in watt hour, though its SI unit is
watt second
One kilowatt hour is otherwise known as
one unit of electrical energy.
One kilowatt hour means that an electric power of
1000 watt has been utilized for an hour.
1watt second
3.6*10⁶ J
is used to record the consumption of electrical energy
The meter
The function of the L fuse wire or a MCB is to protect the house hold electrical appliances from
overloading due to excess current.
An MCB is a switching device, which can be activated automatically as well as manually. It has a spring attached to the switch, which is attracted by an electromagnet when an excess current passes through the circuit. Hence,
the circuit is broken and the protection of the appliance is ensured
One more advantage of the parallel connection of circuits is that each electric appliance gets an
equal voltage.
The fuse wire or MCB will disconnect the circuit in the event of an
overloading and short circuiting.
Over loading happens when a large number of appliances are connected in _______ to the same source of electric power. This leads to a flow of excess current in the electric circuit.
series
overloading
When the amount of current passing through a wire exceeds the maximum permissible limit, the wires get heated to such an extent that a fire may be caused. This is known as Bat overloading.
short circuit.
When a live wire comes in contact Wire with a neutral wire, it causes a ‘short circuit. This happens when the insulation of the wires get damaged due to temperature changes or some external force. Due to a short circuit, the effective resistance in the circuit becomes very ever small, which leads to the flow of a large current through the wires
With the help of the chemical compounds like ___________, the manufacturer can produce LED bulbs that radiates red, green, yellow and orange colours
Gallium Arsenide and Gallium Phosphide
Merits of a LED bulb
- As there is no filament, there is no loss of energy in the form of heat. It is cooler than the incandescent bulb.
- In comparison with the fluorescent light, the LED bulbs have significantly low power requirement.
- It is not harmful to the environment.
- A wide range of colours is possible here.
- It is cost-efficient and energy efficient.
- Mercury and other toxic materials are not required
The first LED television screen was developed by
James P. Mitchell in 1977
______ introduced the first commercial LED Television.
2009, SONY
Advantages of LED television
It has brighter picture quality.
It is thinner in size.
• It uses less power and consumes very less energy.
Its life span is more.
It is more reliable.
Calculate the current and the resistance of a 100 W, 200 V electric bulb in an electric circuit
P=vi=I=p/v
R=v/i
0.5 A
400 Ω
Which of the following is correct?
a) Rate of change of charge is electrical power.
b) Rate of change of charge is current.
c) Rate of change of energy is current.
d) Rate of change of current is charge.
b) Rate of change of charge is current.
Three resistors of 1 Ω, 2 2 and 4 2 are connected in parallel in a circuit. If a 1 Ω resistor draws a current of 1 A, find the current through the other two resistors
The potential difference across the 1 Ω resistor
=I1*R1=1/1=1 V
Since, the resistors are connected in parallel in the circuit, the same potential difference will exist across the other resistors also.
So, the current in the 2 Ω resistor,
V/R2=1/2=. 0.5A
Similarly, the current in the 4 Ω resistor,
V/R3=1/4=. 0.25A
In a simple circuit, why does the bulb glow when you close the switch?
a) The switch produces electricity.
b) Closing the switch completes the circuit.
c) Closing the switch breaks the circuit.
d) The bulb is getting charged..
Closing the switch completes the circuit.
Kilowatt hour is the unit of
a) resistivity
c) electrical energy
b) conductivity
d) electrical power
electrical energy
Assertion: Electric appliances with a metallic body have three wire connections.
Reason: Three pin connections in reduce heating of the connecting wires
if the assertion is true, but the reason is false.
