Electricity And Magnetism
Electric power:
Electric power of the device is defined as the
rate at which electrical energy is converted into
other form of energy.
The electrical power of device is the product of
the current (I) and voltage (V) across.
P = VI
P = V²/R from ohm’s law V = IR
P = V /R
P =RI
Fuse:
Fuse is a safety device to protect a circuit from
excessive heating. A fuse wire is a short metal
wire having low melting point made of an alloy
of tin and lead. It is connected in series with a
circuit.
Short –circuiting:
The overflow of the current due to the low
reactance path in the circuit is called short-
circuiting.
Transformer:
A transformer is a device which is used to
convert low ac voltage into the high a.c
voltage or vice –versa.
A step -up transformer is used to convert low
a-c voltage to high ac voltage
A step –down transformer is used to convert
high a-c voltage to low ac voltage
Principle of transformer:
When an alternating emf is applied to the
primary coil, a changing current flowing in it
produces an alternating magnetic flux in it. This
causes to change the magnetic flux linked with
the secondary coil. An alternating emf is then
induced in the secondary coil. It is called the
principle of mutual inductance on which
transformers are based.
Uses :
Transformers are used in computer, television,
air condition, doorbells etc.
Lighting effect of current:
Some electricaldevices converts electrical
energy into the light energy this is called lighting
effect of electric current
Lighting effect of current is used in:
Florescent lamp:
A florescent lamp consists glass tube filled the
mercury vapor and coated with florescent
powder. It converts 30% of the electrical energy
to the light energy and very useful for the
household purposes.
Filament lamp:
Filament lamp has tungsten filament with inert
gases filled inside the bulb. These inert gases
prevent from the oxidation and evaporation of
tungsten metal at high temperature. It converts
10% of the electrical energy into light energy.
Fleming right hand rule :
Fleming’s right hand rule states that “If the first
three fingers of the right hand are held mutually
perpendicular to each other, with the index
finger in the direction of the magnetic flux and
the thumb in the direction of motion of coil, the
middle finger points to the direction of the
induced current.”
Fleming’s right hand rule is used to determine
the direction of the induced current in a
generator.
Ac and Dc circuit:
Alternating current is one which changes its
magnitude continuously and reverses its
direction periodically and if the polarity of an
electrical source does not change with time it is
called direct current.
Electromagnetic induction
The phenomenon of inducing the emf in a
closed circuit whenever there is change in the
magnetic flux is called electromagnetic
induction. The resulting current is called induced
current.
Faraday laws of electromagnetic induction
Faradays laws of electromagnetic induction are:
1. Whenever magnetic flux linked with a closed
circuit changes, an emf is induced in the circuit.
2. The induced emf lasts as long as the change
in the magnetic flux is taking place.
3. The magnitude of the induced emf is directly
proportional to the rate of change of the
magnetic flux.
One kilowatt-hour
One Kilowatt-hour is the energy supplied when
one kilowatt power is used for one hour. One
kilowatt hour (KWh) is also called one unit.
Energy = Power/time
1kWh = 3.6J
1kWh = (1kW) (1h)
=1000W×3600s
Total cost of electricity = total electrical energy
in kWh × unit cost rate.
# Direct Current:
If the polarity of electrical sources does not
change with time, it is called direct current
(DC).A dry cell and DC generators produce this
current.
# Alternating Current:
If the polarity of electrical sources changes
continuously with the time, it is called
alternating current (AC).AC generators
produces it.
# Differences between AC and DC current
AC
1. The polarity changes continuously with time
in it.
2. It is produced generators or dynamos.
3. Its frequency is 50 Hz (in Nepal)
DC
1. The polarity does not change with time
in DC, i.e., it always l flows in one direction.
2. It is produced by dry cells or batteries and DC
generators.
3. It has no frequency.
# Fuse:
A fuse is a safety device having short length and
thin wire of low melting point used for
protecting electric circuit. It works s on heating
effect of current. A fuse wire is connected in
series in the circuit. The maximum amount of
current which can pass through a fuse without
breaking the circuit called fuse rating.
Q] The fuse is connected to live wire, why?
Ans] The fuse is connected to live wire. When
large current flows through the circuit, the fuse
wire gets heated because of its low melting
point. As a result of this the circuits is broken,
and hence no current flows through the
electrical device and it remains safe.
Q] A copper wire cannot be used as a fuse, why?
Ans] A copper wire cannot be used as a fuse
because it has high melting point. Due to this
reason it does not melt easily when current
overflows in it.
The fuses normally available for household
purposes are 3A, 5A, 10A, 13A, 15A, etc.
Q] What is the size of fuse for an electric device
of power 1 KW on 220V?
Solution
Here, Power (P) = 1 KW= 1000W
Voltage (V) =220V
Current (I) =?
We know, P = VI
Or,
The current is 4.54A.
Therefore, the capacity of the fuse should be 5A.
# Domestic Wiring:
Electricity generated form a power station is fed
into house by two insulated wires, the live wire
(L) and the neutral wire (N). The live wire ‘L’ is
at high voltage 220V and the neutral wire (N) is
at the ground of potential IV. The wire enters the
box which contains (M) main fuse in the live
wire and meter. The two wires form the meter
box enters. Main switch box is that connected to
the distribution board through a main switch
box. Appliances like electric bulb, fans, and
three pin sockets are connected from the
distribution box as shown in the figure.
The fuse in the meter box is called electricity
authority’s main fuse and the fuse after the main
switch box is called consumer’s fuse. The live
terminal of each socket is connected to the live
wire, earth wire. A switch is kept in the live
wire of three pin socket.
# Some important rules for household wiring
are:
⊗The fuse and all the switches should be
connected in live wire in order to break the
power supply in the circuit when the fuse is
broken or the switch is off.
⊗The fuse wire should be of proper rating and
proper material.
⊗There should be separate fuses in each room or floor.
⊗There should be separate circuits. They are light circuit and power circuit. In case fuse of power circuit is blown, the light circuit still works.
⊗Wiring should not be done in a damp area or in
the region where water leaks on the wire. It
prevents leakage of electricity and the chance of
electric shocks.
# Hazards or dangers of electricity are:
⊗If a person touches a live wire, he/she gets a
severe electric shock.
⊗Short-circuit due to damaged wiring or
overloading of the circuit may cause electrical
fires in a building.
⊗The defects in the household wiring like loose
connections and defective switches, sockets and
plugs can cause sparkling and leads to fires.
⊗The phase wire is a live wire. In the live wire
the indicator of the line tester glows, while in
other wires the line tester does not glow.
⊗The colour codes used in household wiring is
given:
Wire Colour codes
Phase/live: Red, Brown
Neutral: Blue, Black
Earthing: Green, Green with yellow strip
# Grounding /Earthing
Grounding /Earthing is the process of connecting earth- wire to the metal body or frame of electrical appliance. It is a safety device for the consumers, which protect them from electric shocks.
Q] A person wearing shoes with rubber sole does not get electric shock even when he touches the live wire, why?
Ans] A person wearing shoes with rubber sole
does not get electric shock even when he touches
the live wire, because the current will not pass
through the body due to insulation between the
body and the ground. But if the person is on the
ground (earth) or touching the neutral line, the
person gets and electric shock because the earth
and our body both are conductors of electricity.
# Effects of Electricity
Current electricity has following effects
⊕Heat effect
⊕Light effect
⊕Magnetic effect
⊕Chemical effect
1. Heating Effect:
When electric current is passed through a
conductor, it gets heated up. This is called
heating effect of electric current.
Nichrome, a heating element, is an alloy of
nickel and chromium. Its resistance is very high
and it does not oxidize even at very high
temperature, therefore it is used as a heating
element. Its melting point is also very high.
2. Lighting effect:
When electric current is passed through a
conductor, it becomes very hot due t which it
emits light. This is called lighting effect of
electric current, e.g., electric bulbs, fluorescent
lamps, other lighting devices, etc.
a. Filament lamp:
The structure of a filament lamp is as shown in
the diagram. When electric current is passed in
it, due to more resistance the tungsten filament
gets heated to white and emits light. Its working
capacity is about 1000 hours. It converts about
10% of electrical energy into light energy. The
rest changes into heat energy.
b. Fluorescent lamp:
The structure of fluorescent lamp is shown in
the diagram. When electric current is passed in
it, the choke magnifies the voltage and the
starter provides the complete circuit before the
mercury is ionized. After the ionization of
mercury, the starter disconnects the circuit and
the ionized mercury completes the circuit. It also
produces ultraviolet rays. The rays make the
fluorescent powder bright, which emits light. It
changes more parts of electrical energy into high
energy and its working capacity is about 3,000
hours only.
Q] Tungsten filament is used in an electric bulb,
why?
Ans] Tungsten filament is used in an electric
bulb because of its high resistance. When
electric current is passed through the filament of
t tungsten, it attains high temperature and emits
light. Its melting point is also very high (3400°C).
Q] Electric bulbs (or filament lamps) are filled
with inert gases like nitrogen or argon, why?
Ans] Electric bulbs (or filament lamps) are filled
with inert gases like nitrogen or argon in order
to prevent oxidation at high temperature due to
which the filament remains safe.
Q] Fluorescent lamps are filled with mercury
vapour, why?
Ans] Fluorescent lamps are filled with mercury
vapour in order to produce ultraviolet rays,
which are emitted due to the flow of electrons
through the mercury vapour. The fluorescent
powder absorbs the ultraviolet rays and emits
visible light.
Q] When electric current is passed in a filament
lamp, the filament is heated to bright but not the
other wires, why?
Ans] When electric current is passed in a
filament lamp, the filament is heated to bright
but not the other wires because the filament is
very thin and has high resistance. Due to high
resistance the filament converts electrical energy
into heat and light but not the other wires.
# Differences between filament lamps and
fluorescent lamp
Filament lamps
1. It is an electric lamp made up of a glass bulb,
inert gas, filament, etc.
2. It converts 10% light and 90% heat of total
electrical energy.
3. Its average life is shorter, i.e. about 1000 hours
only.
4. It is less efficient and cheap
5. Tungsten filament gets heated and emits light.
Fluorescent lamps
1. It is an electric lamp made up of fluorescent powder,mercury vapour,glass tube,electrodes,etc.
2. It can convert 30% light of total electrical energy.
3. Its average life is longer, i.e. about 3000 hours
4. It is more efficient and expensive
5. Fluorescent powder absorbs UV rays obtained
from Hg and emits light.
3. Magnetic Effect:
When an electric current is passed through a
conductor, it behaves like a magnet. This is
called magnetic effect of current.
Magnetic effect of electricity was discovered by
Hans Christian Oesrted in 1819 AD
Magnetic field around a straight wire is found in
the form of rings. The direction of lines of force
is determined by using magnetic compass or by
Maxwell’s screw rule or by right hand thumb
rule.
# Electromagnet:
When an iron bar is wrapped with an insulated
wire and the current is passed through it, the
iron bar changes into a magnet. The temporary
magnet so formed is called an electromagnet.
The strength of an electromagnet can be
increased by
#Increasing the number of turns in the coil.
#Increasing the amount of current passing
through the wire.
#Decreasing the length of air gap between the
poles of a ‘U’ shaped magnet.
Electromagnets are used in electric bells, electric
motors, loudspeakers, the receivers of telephones
etc.
Advantages of an electromagnet over a
permanent magnet are
1)) It is temporary.
2)) Its strength can be altered.
3)) It can be modified into any shape.
# Electric Bell:
When electric current is passed in an electric
bell the electromagnet pulls the hammer towards
the gong and the bell rings. On this condition,
the circuit gets disconnected at point of contact
‘X’ and the electromagnet gets demagnetized.
The soft magnetic plate touches at ‘X’ again and
the circuit becomes closed. The process repeats
again and again till the switch is on. It causes
the bell to ring.
4. Chemical Effect:
When current is passed through a liquid and it
under goes chemical, the effect is called the
chemical effect of current.
# Electromagnetic Induction:
The phenomenon by which the electric current
is produced in a closed coil due to relative
motion between the coil and a magnet is called
electromagnetic induction. The electric current
so produced is called induced current.
# Faraday’s Laws of Electromagnetic induction:
Whenever the magnetic flux linked with a closed
circuit changes, an induced emf is produced in
the circuit.
~The induced emf lasts as long as the change in
magnetic flux continues in the circuit.
~The induced emf is directly proportional to the
rate of change of magnetic flux in the circuit.
~Induced emf directly proportional ti the rate of change of magnetic flux linked with the circuit.
# Magnetic flux:
Magnetic flux through a surface is the number of
magnetic lines of force crossing through the
surface of coil placed perpendicular to the line of
forces.
# Fleming’s right hand rule:
When a conductor moves across a magnetic
field, it cuts the lines of force and an emf is
induced current is given by Fleming’s right hand
rule. The rule states that if the first three fingers
of the right hand are held mutually
perpendicular to each other, the fore finger, the
thumb and the central finger give the direction
of the magnetic field, motion of the coil and the
induced current respectively.
# Electrical generator or dynamo:
The electrical device to produce electric current
or electricity is known as electrical generator or
dynamo. It converts mechanical energy into
electrical energy.
A device which generated small amount of AC is
known as dynamo and the device, which
generates large amount of AC, is known as
generator.
# Ways of increasing emf or voltage in a
generator or dynamo:
The induced emf (or voltage) of generator or
dynamo can be increased by:
∅Increasing the number of turns in the coil.
∅Increasing strength of magnetic field.
∅Wounding coil on a soft iron core.
∅Increasing speed of rotation of the magnet near the coil
∅Decreasing the distance between the coil and the magnet.
# Electric motor:
An electric motor is a device which converts
electrical energy to mechanical energy. It is
based on the principle of motor effect. When a
current carrying conductor is kept in a magnetic
field, the conductor moves. This is called motor
effect.
# Differences between electric motor and
generator
Electric Motor
≌It converts electrical energy into mechanical
energy.
≌It is based on the principle motor effect.
≌Fleming’s left hand rule gives the direction of
motions of the conduction.
It is used in fans, cars etc.
Generator
≌It converts mechanical energy into electrical energy.
≌It is based on the principle of electromagnetic
induction Fleming’s right hand rule gives the direction of induced current through the conductor.
≌It is used in bicycles, power plants etc.
# Fleming’s left hand rule:
The motion of an electric motor is given by
Fleming’s left hand rule. The rule states that if
the first three fingers are mutually
perpendicular to each other, the index finger,
the middle finger and the thumbs represent the
direction of magnetic field, current and the
motion of the conductor respectively.
# Transformer:
A transformer is a device which converts low
alternating voltage at high current into high
alternating voltage at low current and vice
versa. In practice, it is used to increase or
decrease alternating emf.
The transformer which changes high voltage
alternating current to low voltage alternative
current is called step down transformer. They
are used at power substations and in streets to
lower the supplied AC voltage.
The transformer which changes low voltage
alternative current to high voltage alternative
current is called step up transformer. They are
used at power stations.
A transformer works on the principle of mutual
induction i.e., when a change in current or
magnetic flux takes place in one coil, induced
emf is produced in the other coil.
The coil connected to the a.c., source is called
primary coil and the coil connected to the output
is called secondary coil.
The current in the primary coil is called input
current and the voltage across the primary coil
is called the input voltage or primary voltage VP.
The current in the secondary coil is called output
current and the voltage across the secondary coil is called the output voltage or secondary voltage VS.
Q] The core of a transformer is laminated, why?
Ans] The core of a transformer is laminated in
order to reduce the eddy-current loss to prevent
heating of transformer due to which out put is
increased.
# Eddy current:
Eddy current is the induced current set up in a
conductor by a changing magnetic field.
# Differences between step up & step-down
transformer
Step up Transformer
⊗It changes low voltage ac into high voltage.
⊗The number of turns in the secondary coil is
more than that in the primary coil.
⊗It is used between electric tower and power
generating station
Step down transformer
⊗It changes high voltage ac into low voltage.
⊗The number of turns in the secondary coil is less than that in the primary coil.
⊗It is used between the high
power transmission line