Download 10th Class Physics Notes (Unit # 14) for Federal Board (FBISE) Islamabad

Download 10th Class Physics Notes (Unit # 14) for Federal Board (FBISE) Islamabad

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Download 10th Class Physics Notes (Unit # 14) for Federal Board (FBISE) Islamabad

Define electric current.
The rate of flow of electric charge through any cross-sectional area is called electric current. If the charge Q is passing through an area A in time t second, then the current flowing through it will be I,
Current      =      charge/time
I               =      q/t
The unit of current is ampere in SI system.
One ampere is the amount of electric current due to the flow of electric charge at the rate of one coulomb per second.

What is meant by conventional current?
A current produced due to flow of negative charges is equivalent to a current due to flow of an equal amount of positive charge in opposite direction. This equivalent current of positive charge is known as conventional current.

Which type of charge is responsible for the flow of current in metallic conductors?
In metals or metallic conductors, the current is due to the flow of free electrons i.e. negative charges. For example in a copper wire there are large numbers of free electrons which are in random motion. When we apply potential difference across the wire, these free electrons move through the wire.

In electrolyte which charge are responsible for the flow of current?
The molecules of the electrolyte are dissolved among positive and negative ions in a solution. Thus current in electrolytes is due to the flow of both positive and negative charges.

How energy is obtained due to flow of charges.
When a positive charge moves from a point of higher potential to the point of lower potential, it gains the energy from the electric field. During flow of electric current, positive charges flow continuously from a high potential to a low potential point. Thus the electric current becomes a continuous source of energy.

How a galvanometer is converted into voltmeter?
The galvanometer is converted into voltmeter by connecting suitable resistance in series with it. The value of the resistance depends upon the range of the voltmeter. Usually its value is several thousand ohms. Thus the resistance of a voltmeter is very high.

How a galvanometer is converted into ammeter?
Galvanometer can be converted into an ammeter by connecting a small resistance parallel to it. This small resistance is known as “shunt”. Shunt provides an alternative path for the current to flow. The major part of the current passes through the shunt and small fraction of it flows through the galvanometer.

Why resistance of the ammeter is kept low?
If the resistance of the ammeter is kept high, then high amount of current flows through the galvanometer. When high amount of current will flow through the galvanometer then galvanometer can be burnt. That is why resistance of the ammeter is kept low.

Why resistance of the voltmeter is kept high?
If the resistance of the voltmeter is comparatively low, it will draw more current from the circuit. Due to this the potential difference across the resistance for the measurement, of which the voltmeter was connected, would drop.

On what factor reliability of voltmeter depend?
Higher the resistance of the voltmeter, more reliable would be its readings. Therefore a good voltmeter should have such a high resistance so that no or very little current could pass through it.

Differentiate between electromotive force and potential difference.
Electromotive Force:
The electromotive force of a battery or cell is the total energy supplied in driving one coulomb charge round a complete circuit in which cell is connected.
Potential Difference:
The potential difference determines the energy between any two points of the circuit which is required in moving a charge from one point to another.

State and explain Ohm’s law. Write down its limitations.
The value of current I passing through a conductor is directly proportional to the potential difference V applied across its ends, provided the temperature and the physical state of the conductor does not change.
V      α      I
V      =      IR
Ohm’s law is applicable only in case of metallic conductors when their temperature and physical state do not change.

Define resistance and its unit.
The property of a substance which opposes the flow of current through it is called its resistance.
R      =      V/I
S.I unit of resistance is Ohm.
If a current of one ampere passes through it when a potential difference of one volt is applied across its ends then resistance would be one Ohm. Ohm is usually represented by the Greek letter (Ω).

What are the factors upon which the resistance of u conductor depends?
  • Length of the conductor (L)
  • Area of cross-section of the conductor (A)
  • Nature of the conductor
  • Temperature.

Why does the resistance of a conductor increase with the rise of its temperature?
When the temperature of the conductor rises, average speed of the random motion of the free electrons increases which enhances the rate of collision of electrons and atoms. This causes an increase in the resistance of the conductor.

Why do we always use metal wires for conduction of electricity?
Because, they are good conductors of electricity and offer less resistance to the flow of current.

What do you mean by insulators?
The substances through which almost no current flow are called insulators. For example glass, wood, plastic, fur, silk etc.

State Joule's Law.
The amount of heat generated in a resistance due to flow of charges is equal to the product of square of current I, resistance R and the time duration t.
W     =      I2Rt

Define electric power.
The amount of energy supplied by current in unit time is known as electric power.
                        Work                                 QV
Power        =      -------        =      P      =      ----   =      IV     =      I2R   
                        Time                                 t

Define kilowatt hour?
The amount of energy delivered by a power of one kilowatt in one hour is called kilowatt - hour. Mathematically:
One kilowatt - hour 1 kwh = 1000 w x 1 hour

Differentiate between A.C and D.C.
Alternating Current (A.C.):
The current which changes its direction again and again is called alternating current.
AC current can transfer electrical energy over the long distance.
The frequency of AC is 50Hz.
Direct Current (D.C.):
A current which always flows only in one direction is called direct current.
The DC cannot travel very far until it begins to lose energy.
The frequency of DC is zero.

What are live and neutral wires?
Neutral wire:
  • One wire is earthed at the power station, so it is at zero potential.
  • This wire is called neutral wire.
  • This wire provides the return path of current.
  • It is a black or blue in colour.
Live wire:
  • The other wire on power station is at some certain potential called the live wire.
  • The potential difference between both wires is 220.
  • It is red or brown in colour.

How electricity Is dangerous for us?
  • Our body is a good conductor of electricity through which current can easily pass.
  • Therefore if a person holds live wire, then because of the presence of voltage in it, current will start flowing to ground through the human body which may prove fatal for the person.

What is a cable?
An insulated covered wire is known as cable.

Define fuse and write down its principle.
A small wire connected in series with the live wire is known as fuse wire or fuse.
A specified amount of current can safely pass through it. When the current following through it exceeds this limit, it gets so hot that it melts.

What is Circuit Breaker? Also write down its principle.
It is a safety device which is used in place of fuse. Due to any fault when the current exceeds the safety limit, then the button of the circuit breaker moves upward. Due to which the circuit breaks and the flow of the current is stopped in it.
It works on the principal of electromagnet. As soon as the current exceeds the limit, the magnetic force of the electromagnet is so increased that it attracts the iron strip towards it. Hence the contact points are separated and the circuit breaks.

How earth wire is useful to us? Principal of earth wire.
Whenever the metal casing of the appliance, due to faulty insulation, gets connected with the live wire, the circuit shorts and a large current would immediately flow to ground through the earth wire and causes the fuse wire to melt or the circuit breaker breaks the circuit. Therefore, the person who is using the appliance is saved.

What is mean by electromotive force? Write its equation and explain its unit.
The electromotive force of a battery or cell is the total energy supplied in driving one coulomb charge round a complete circuit in which cell is connected.
In other words, it is the energy supplied by a battery to a unit charge when it flows through the closed circuit.
                        Energy                               W
e.m.f.        =      --------       =      E      =      ---
                        Charge                               Q

Unit of e.m.f.
SI units of energy and charge are Joule and coulomb, then the unit of emf will be JC-1

Explain the V-I characteristics of Ohmic and non Ohmic conductor.
Ohmic Conductor:
Ohmic conductors have a linear current-voltage relationship over a wide range of applied voltages.
The straight line shows a constant ratio between voltage and current, So Ohm’s law is obeyed.
For example most metals show Ohmic behavior.
Non-Ohmic Conductor:
Non Non-Ohmic materials have a non-linear current-voltage relationship.
For example, Filament lamp, Thermister, Filament lamp

Define and explain the term specific resistance. Discuss different factors which affect the resistance of conductors
The resistance of one meter cube of a substance is called its specific resistance.
A short pipe offers less resistance to water flow than a long pipe. A pipe with larger cross-sectional area offers less resistance than the pipe having smaller cross-sectional area.
Same is the case for resistance of wire that carry current the resistance of wire depends both on the cross-sectional area and length of the wire, current flow also depends upon the nature of the material of the wire.
a)   Length of Conductor    =      Longer wires have more resistance.
b)   Cross-Sectional Area    =      Thick wires have less resistance.
c)   Nature of Conductor     =      Copper wire has less resistance than steel wire.
d)   Temperature              


R      α      L
R      α      1/A
R      α      L/A
R      =      p L/A
Where (p) is the constant of proportionality and known as specific resistnace.

How are resistances connected in series? What are characteristics of this combination?
i)            In this method only one path is present for flow of current because resistances are connected end to end.
ii)           The magnitude of current through each resistor is same.
iii)         The sum of voltages across each resistor is equal to the voltage of battery.
V = V1  +  V2  +  V3
                If current is passing through resistors R1  ,R2  ,R3   then
                                V = IR1  +IR2  +IR3
                                                V = I(R1  +R2  +R3)
iv)     The equivalent resistance Re of a series combination is that resistance which when substituted in place of the combination the same current would flow through the circuit.
        R   = R1 + R2 + R3  +………….Rn

How are resistances are connected in parallel. What are the characteristics of this combination?
i)            In this combination potential drop across all the resistances is the same.
ii)           The sum of current flowing through the various resistances is equal to the total current.
I = I1  +I2  +I3
iii)         The equivalent resistance Re of the parallel combination is that resistance which when substituted in place of the parallel combination does not alter the total current of the circuit.
1/Re  = 1/R1  + 1/R2  + 1/R3
                1/Re  = 1/R1  + 1/R2  + 1/R3  …………….1/Rn (For n resistances)

Describe the Electric Energy and Joule’s Law?
i)            We know that during flow of current positive charges always flows towards negative. Thus electric current becomes a continuous source of energy.
ii)           Consider two points having a potential difference of V volts. If one coulomb of charge passes between these point, the amount of energy supplied by the charge would be V Joule. Hence when Q coulomb of charge flows between these two points then we will get QV joule energy. We can represent it as:
W     =      QV.
iii)         As current is rate of flow of charge so the energy gained during t second is: -
W     =      QV    =      I x t x V ……….(i)
By ohm’s law
V      =      IR
W     =      I2Rt Substituting the value of V in Equ (i)
iv)         This energy can be utilized for different functions e.g. fan convert this energy into mechanical energy.
Define Cycle.
The set of all the value of current during one period is known as one cycle. The number of cycles completed by alternating current in one second is called its frequency.

How Circuit become short?
A short circuit occurs when circuit with a very low resistance is formed. The low resistance causes the current to be very large.

How insulation damage?
  • Electrical current exceeds the rated current carrying capacity of the conductor it can produce excess current that can damage insulation due to overheating of cables.
  • Constant friction may also remove the insulation from the wire.
  • Too much moisture also damages the insulation because moisture decrease resistance and increase the rote of current.

The voltage chosen for the transmission of electrical power over large distances is many times greater than the voltage of the domestic supply. State two reasons why electrical power is transmitted at high voltage.
i)            Sending power at high voltage would reduce the power loss in the form of heat dissipation i.e. I2Rt. For same power, high voltage means lower current and hence less power loss.
ii)           Sending power at high voltage means low current. It means we need thinner wires and also less number of repeater step-up transformers to send power over long destinations.

Why is the voltage used for the domestic supply much lower than the voltage at which the power is transmitted?
Domestic appliances operate on low voltage because high voltage can damage these instruments. High voltage can also be dangerous for users as it can cause electric shock. It may also damage property and other valuables as a result of some serious electric shock.

Which metal is used as the filament of an electric bulb?
Tungsten is used as the filament due to high resistance.

A bird can sit harmlessly on high tension wire. But it must not reach and grab neighboring wire. Do you know why?
A bird can sit harmlessly on high tension wire as no current passes through its body, since the potential of the wire is constant. However, if the bird grabs the neighbouring wire, then due to potential difference of two wires, current will flow through the body of the bird and can be fatal

In order to measure voltage in a circuit, voltmeter is always connected in parallel. Discuss.
In this way, voltmeter does not disturb the current and hence the voltage of the circuit. Due to high resistance of voltmeter, no current passes through it and hence voltage of the circuit remains unaffected.

In order to measure current In a circuit, why ammeter Is always connected in series?
In order to measure current ammeter is always connected in series with the circuit so that all the current to be measured must flow through it (due to its low resistance).

What is difference between cell and battery?
·        Cell is the specific name of source of emf in which chemical energy is converted into electrical energy.
·        Battery is more general name of source of emf in which any kind of energy (heat, solar, chemical, mechanical) energy is converted into electrical energy.

From your experience in watching cars on the roads at night, are automobile
headlamps connected in series or in parallel?
Head lamps of automobiles are connected in parallel because of the following reasons:
·        The potential difference between headlamps remains same.
·        If one head-lamp is out of order the other lamps still glow. Also we can turn ON or
·        OFF any individual head lamp independently, which is only possible if they are connected in parallel.

It Is Impracticable to connect an electric bulb and an electric heater in series. Why?
·        When appliances are connected in series, total resistance of circuit increases. This decreases the current and hence the power through each appliance.
·        In order to avoid this loss of current and power, through bulb and heater, they are connected in parallel.

Does a fuse in a circuit control, the potential difference or the current?
·        Fuse in a circuit is used to control the current in the circuit.
·        When current exceeds the limited value as allowed by the fuse, it burns out, stops the current and beaks the circuit.




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Download Class Notes: Download 10th Class Physics Notes (Unit # 14) for Federal Board (FBISE) Islamabad
Download 10th Class Physics Notes (Unit # 14) for Federal Board (FBISE) Islamabad
Download 10th Class Physics Notes (Unit # 14) for Federal Board (FBISE) Islamabad
Download Class Notes
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