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Notes for science --Electricity and magnetism

# Combination of Resistors


Series Combination    Parallel Combination       
In this combination, resistors are connected from end to end    In it, resistors are connected between two common points.       
In series combination, current through each resistor is the same but pd across each resistor is different.    In parallel combination, current through each resistor is different but pd across each resistor is the same.       
The effective resistance ( R) of the combination is given by  R = R1+R2+R3 
i.e., R is more than individual resistance.     The effective resistance (R) of the combination is given by 
i.e., R is less than individual resistance.       
Individual use of each resistor is not possible in this combination.    Individual use of each resistor is possible in this combination.       
In it, brightness of the bulb changes with change in the number of bulbs.    In it, brightness of the bulb does not change with change in the number of bulbs.       
Other resistors doe not work when one of them   fails to work.    Other resistors do their work when any one of them fails to work.    

 # Grouping of cells.









Series combination    Parallel Combination       
Negative terminal of one cell is connected to the positive terminal of another cell and so on.     Positive terminal of the cells are connected to one point and negative terminals to another point.       
Sum of potential difference across each cell is equal to the pd across the external resistance .i.e., pd across R (E) = 3V in the given figure.     Potential difference between two common points A and B is always constant. i.e., Pd across R = E in the given figure.       
Value of current increases by increasing the number of cells in it because in this combination, current (I) =  (internal resistance of the cell is neglected)    Value of current does not increase by increasing the number of cells in it because in this combination, (I) =E/R (internal resistance of the cell is neglected.       
In this grouping the cells lasts for short time    In this grouping, the cells last for a long time.    

Q] In parallel combination of electric bulbs, there is no difference in the brightness of the bulb even when their number is increased or decreased, why?
Ans] In parallel combination of electric bulbs, there is no difference in the brightness of the bulb even when their number is increased or decreased because; Pd across each bulb remains the same. Therefore for household electrical circuit, parallel circuit is used.

# Electric Power:
The rate of doing work by the electrical device is known as electric power. The electric power (P) of an electrical device is equal to the product of the current (I) flowing through the device and potential difference (V) across its two ends i.e.
Using Ohm’s laws it can be expressed as:


       
P = IV    
                          

P =     
    




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 = Powertime

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    DC       
1. The polarity changes continuously with time in it.    1. The polarity does not change with time in DC, i.e., it always l flows in one direction.       
2. It is produced generators or dynamos.    2. It is produced by dry cells or batteries and DC    generators.       
3. Its frequency is 50 Hz (in Nepal)    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. (Ans)

# 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 in the table:

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 (3400oC).
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    Fluorescent lamps       
It is an electric lamp made up of a glass bulb, inert gas, filament, etc.    It is an electric lamp made up of fluorescent powder, mercury vapour, glass tube, electrodes, etc.       
It converts 10% light and 90% heat of total electrical energy.    It can convert 30% light of total electrical energy.       
Its average life is shorter, i.e. about 1000 hours only.    Its average life is longer, i.e.  about 3000 hours       
It is less efficient and cheap    It is more efficient and expensive       
Tungsten filament gets heated and emits light.    Fluorescent powder absorbs UV rays obtained from Hg and emits light.     

# 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
It is temporary.
Its strength can be altered.
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.








# Chemical Effect:
When current is passed through a liquid and it under goes chemical, the effect is called the chemical effect of current.

Q] A few of sulphuric acid is added for the electrolysis of water, why?
 Ans] Pure water does not conduct electricity therefore, a few of sulphuric acid is added for the electrolysis of water.

# Electrolysis of water:
O2 gas collects at anode and H2O gas collects at cathode.
The volume of H2 is twice than the volume of O2.
The diagram for electrolysis of water is given below:
Water  is ionized  as follows:
H2OH++OH-    (Ionization)



Reaction at anode    Reaction at cathode       
OH- becomes neutral (OH) by losing electron which later forms molecules of water and oxygen.
OH- OH+e-
4OH2H2O +O2    H+ becomes neutral (H) by gaining electron  which later forms molecules of hydrogen
H+  e- H
4H –> 2H2    

Q] For electrolysis of water, copper anode should not be used, why?
Ans] For electrolysis of water, copper anode should not be used because nascent oxygen formed during the process reacts with copper to form copper oxide due to which the amount of oxygen will be reduced. Therefore, copper anode should be replaced by platinum or graphite anode.
# Note:
Hydrogen gas does not react with copper; therefore copper cathode may be used in electrolysis of water.
The diagram for electroplating of silver on spoon is given below:

# Electroplating:
The process of coating a conducting surface with metal by passing electric current in it is called electroplating.
Some tips for electroplating
The object to be electroplated to be electroplated should be placed at cathode.
 The electrolyte should be a water-soluble salt of the metal, which is to be polished, e.g., for  copper plating copper sulphate, for silver plating silver nitrate and for gold plating AuCl3 etc.
The metal by which we have to coat is made anode.

# Application of electrolysis: 
To obtain pure metals form the related ores.  
For electroplating of copper, silver, gold. etc.
For the extraction of sodium, calcium magnesium, etc.
In the manufacture of H2 and O2.
# 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 ∞ 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    Generator       
It converts electrical energy into mechanical energy.    It converts mechanical energy into electrical energy.       
It is based on the principle motor effect.    It is based on the principle of electromagnetic induction       
Fleming’s left hand rule gives the direction of motions of the conduction.    Fleming’s right hand rule gives the direction of induced current through the conductor.       
It is used in fans, cars etc.    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.
The relation between primary and secondary voltage in a transformer is given by












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    Step down transformer       
It changes low voltage ac into high voltage    It changes high voltage ac into low voltage       
The number of turns in the secondary coil is more than that in the primary coil    The number of turns in the secondary coil is less than that in the primary coil       
It is used between electric tower and power generating station    It is used between the high power transmission line houses    

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