Chapter 14 Current Electricity (Long Questions)
14.1. Define and explain the term electric current.
14.2. What is the difference between electronic current and conventional current?
14.3. What do we mean by the term e.m.f? Is it really a force? Explain.
14.4. How can we differentiate between e.m.f. and potential difference?
14.5. Explain Ohm’s law. What are its limitations?
14.6. Define resistance and its units.
14.7. What is the difference between conductors and insulators?
14.8. Explain the energy dissipation in a resistance. What is Joule’s law?
14.9. What is difference between D.C and A.C?
14.10. Discuss the main features of parallel combination of resistors.
14.11. Determine the equivalent resistance of series combination of resistors.
14.12. Describe briefly the hazards of household electricity.
14.13. Describe four safety measures that should be taken in connection with the household circuit.
14.14. Design a circuit diagram for a study room that needs the following equipment’s in parallel:
(a) One 100 W lamp operated by one switch.
(b) One reading lamp fitted with a 40 W bulb which can be switched ON and OFF from two points.
(c) What is the advantage of connecting the equipment’s in parallel instead of series?
CONCEPTUAL QUESTIONS
14.1. Why in conductors charge is transferred by free electrons rather than by positive charges?
14.2. What is the difference between a cell and a battery?
14.3. Can current flow in a circuit without potential difference?
14.4. Two points on an object are at different electric potentials. Does charge necessarily flow between them?
14.5. In order to measure current in a circuit why ammeter is always connected in series?
14.6. In order to measure voltage in a circuit voltmeter is always connected in parallel. Discuss.
14.7. How many watt-hours are there in 1000 joules?
14.8. From your experience in watching cars on the roads at night, are automobile headlamps connected in series or in parallel.
14.9. A certain flash-light can use a 10 ohm bulb or a 5 ohm bulb. Which bulb should be used to get the brighter light? Which bulb will discharge the battery first?
14.10. It is impracticable to connect an electric bulb and an electric heater in series. Why?
14.11. Does a fuse in a circuit control the potential difference or the current?
14.1. Define and explain the term electric current.
Ans. Statement: “The rate of flow of electric charges through cross-section area of a conductor is called electric current”.
The flowing electric charge may be positive or negative. In case of conducting metal wires the electric current is due to flow of free electrons which carry negative charge.
Formula: If “Q” denotes the amount of charges flow, through any cross section, in time “t” then value of current “I” is given by I = Q/t (Ampere)
14.2. What is the difference between electronic current and conventional current?
Ans. Electronic current: The current due to negatively charged particles (free electrons) that is from lower potential to higher potential is called electronic current. Conventional current: The current due to positively charged particles that is from higher potential to lower potential is called conventional current.
Current in metals is electronic current but we take conventional current.
14.3. What do we mean by the term e.m.f? Is it really a force? Explain.
Ans. e.m.f. stands for electromotive force. Basically it is the potential energy provided by the source which causes to accelerate the charges (electrons)
to move that is current. There is not force but energy.
14.4. How can we differentiate between e.m.f. and potential difference?
Ans. e.m.f. is the total amount of energy supplied by the battery or the cell in moving in a one coulomb of positive charge from the negative to positive terminal of the battery, The current passes’ through the external resistor(bulb, appliance) due to which potential drops across it. By Ohm’s law V = IR It is called the potential difference’ across the resistor. e.m.f. is greater than potential difference.
14.5. Explain Ohm’s Law. What are its limitations?
Ans. Statement: It states that potential drop (voltage) across a resistance is directly proportional to value of electric current passing through it provided the temperature and the physical state of the conductor does not change.
Consider V is the potential difference between the two ends of a conductor and I is the current is flowing through it. By ohm’s law we can write:
Mathematically:
V ∝ I
V = (Constant) I
The constant of proportionally is denoted by R called electrical resistance of the conductor, so that
V = IR Mathematical form of Ohm’s law.
Limitations:
- The temperature of the conductor does not change.
2. The physical state of the conductor does not change.
14.6. Define resistance and its units.
Ans. Electrical Resistance :(R)
The property of a substance which offers opposition to the flow of current through it is called its resistance,
Unit of Resistance: ( Ohm Ω)
The SI unit of resistance is ohm, its symbol is Ω.
By Ohm’s law V=IR ⇒ R = V/I
The resistance is said to be one ohm if a potential difference of one volt across the conductor produces an electric current of one ampere through the conductor.
14.7. What is the difference between conductors and insulators?
Ans. Conductors: Materials offer low resistance to pass current through them are called conductors. For example: AI! metals are good conductors of electricity. Metals have excess of free electrons which are responsible for the flow of current in metals.
Insulators: Materials, offer very high resistance and negligible current passes through them, are called insulators.
For example: Materials like rubber, glass, wood etc. have not free electrons to move.
14.8. Explain the energy dissipation in a resistance. What is Joule’s law?
Ans. As a charge q moves through a resistor, it loses a potential energy qV where V is the potential drop across the resistor. This energy goes into heat, much like the way a ball of putty that falls off a cliff converts its potential energy to heat when it hits the ground. We refer to this conversion of potential energy into heat as dissipation.
The power dissipated in a resistor is the energy dissipated per time. If an amount of charge Dt moves through the resistor in a time Dt, the power loss is
where I is the current through the resistor and V is the voltage drop across it.
The formula P = I V also gives the power generated by a battery if I is the current coming from the battery and V is its voltage.
Joule’s Law:
This law governs the heating effect of current as heat energy released by a conductor when current passes through it. If the conductor is having resistance R and current I passes through it for time t. The mathematical expression of Joule’s law is as explained below.
Q = I2 R T
Where,
- Q indicates the amount of heat
- I show electric current
- R is the amount of electric resistance in the conductor
- T denotes time
14.9. What is difference between D.C and A.C?
Ans. Direct Current:(D.C)
The current which does not change its direction of flow is known as direct” current or D.C. The positive and negative terminals of D.C battery have fixed polarity so the voltage of D.C remains constant with time.
Alternating Current: (A. C) The current which changes its direction of flow after regular intervals of time is known as alternating current or A.C.
14.10. Discuss the main features of parallel combination of resistors.
Ans. Characteristics:
1. If there are more than one path for the flow of current in a circuit then the combination of resistances is called Parallel Combination.
2. In parallel combination current through each resistor is different.
3. Potential difference across each resistor is constant.
4. Equivalent resistance of circuit is always less than either of the resistances included in the circuit.
ADVANTAGE:
In parallel combination of resistors, if one component of circuit (resistor) is damaged then rest of the component of the circuit will perform their work without any disturbance. It is due to the presence of more than paths for the flow of electric current.
EQUIVALENT RESISTANCE IN PARALLEL COMBINATION:
| Consider three resistances R1 , R2 & R3 connected in parallel combination with a power supply of voltage V. | |
| Now | |
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I = I1 + I2 + I3
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| According to Ohm’s law | |
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V/R = I
Therefore, V/Re = V/R1 + V/R2 + V/R3 V/Re = V(1/R1 + 1/R2 + 1/R3) V/ReV = 1/R1 + 1/R2 + 1/R3 OR |
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14.11. Determine the equivalent resistance of series combination of resistors.
Ans.
14.12. Describe briefly the hazards of household electricity.
Ans. The major hazards associated with electricity are electrical shock and fire. Electrical shock occurs when the body becomes part of the electric circuit, either when an individual comes in contact with both wires of an electrical circuit, one wire of an energized circuit and the ground, or a metallic part that has become energized by contact with an electrical conductor.
Examples:
Electrical hazards come in a variety of forms, but all have the potential to cause serious injury. Common types of electrical hazard include:
- Contact with live wires resulting in electric shock and burns,
- Fires due to faulty wiring,
- Exposed electrical parts,
- Ignition of fires or explosions due to electrical contact with potentially flammable or explosive materials,
- Inadequate wiring,
- Improper grounding, sometimes caused by workers deliberately removing the ground pin on an electric plug to fit a two-prong extension cord,
- Interaction with overhead power lines,
- Damaged wire insulation, causing electrical conductors to make contact with each other, tools, or a worker’s body,
- Overloaded circuits,
- Wet conditions.
14.13. Describe four safety measures that should be taken in connection with the household circuit.
Ans. (i) To avoid shocks from electrical appliances use proper-earthing arrangement.
(ii) Replace old worn out and damaged wires with new set
(iii) Put the main switch off when removing any fault in electric circuits.
(iv) Wear rubber shoes and gloves while dealing with replacement of any appliance.
(v) Work with electric circuits in proper light.
(vi) Do not put your hand inside water being heated with immersion rod, when the rod is inside and on.
14.14. Design a circuit diagram for a study room that needs the following equipment’s in parallel:
(a) One 100 W lamp operated by one switch.
(b) One reading lamp fitted with a 40 W bulb which can be switched ON and OFF from two points.
(c) What is the advantage of connecting the equipment’s in parallel instead of series?
Ans. (a) Draw a simple circuit with one switch.
(b)
(c) There are two advantages of connecting the equipment’s in parallel instead of series combination. All the appliances get the same input voltage and if one fuses or faulty then the other appliances can work properly.
14.1. Why in conductors charge is transferred by free electrons rather than by positive charges?
Ans. Conductor means the material having large number of free electrons. In all materials positive charges are bound to their respective nuclei. When potential difference is applied across a conductor the free electrons are mobile charges move under the influence of electric field but positive charges are bound.
14.2. What is the difference between a cell and a battery?
Ans. Both are the sources of emf (means provide electrical potential energy) to flow the charges. 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 potential energy.
14.3. Can current flow in a circuit without potential difference?
Ans. A conductor has large number of free electrons. They move randomly like gas particles. They constitute current of very small value but the net current is zero. It means net current is zero without potential difference.
14.4. Two points on an object are at different electric potentials. Does charge necessarily flow between them?
Ans. The charges will flow even if there is potential difference is present between two points. Unless these two points are connected with some conducting material having free charged particles.
14.5. In order to measure current in a circuit why ammeter is always connected in series?
Ans. In order to measure/detect current ammeter / galvanometer is always connected in series because the whole current which is to be measured should be passed through the ammeter.
14.6. In order to measure voltage in a circuit voltmeter is always connected in parallel. Discuss.
Ans. Voltmeter is a high resistance galvanometer. The voltmeter is connected in parallel across the resistor whose potential difference is to be measured.
The voltmeter will draw very small amount of current for measuring purpose without changing the current in the circuit.
14.7. How many watt-hours are there in 1000 joules?
Ans. As we know I watt-hour = 3600 joules. As 1 h = 3600 sec.
So E = 1000/3600 = 0.28 watt-hour
14.8. From your experience in watching cars on the roads at night, are automobile headlamps connected in series or in parallel.
Ans. It is common experience, sometime one head light of the car is working. It means they are in parallel combination. If one fuses or faulty the other can work properly.
14.9. A certain flash-light can use a 10 ohm bulb or a 5 ohm bulb. Which bulb should be used to get the brighter light? Which bulb will discharge the battery first?
Ans. A bulb of 5 ohm gives brighter light because low resistance gives rise high current according to the equation
V = IR
Formula for power is P = I2R .
High current and low resistance of 5 ohm bulb will discharge the battery first.
14.10. It is impracticable to connect an electric bulb and an electric heater in series. Why?
Ans. There are two reasons due to which it is impracticable to connect an electric bulb and heater is series. Heater and bulb having different resistances and the potential will drop according to the resistance. If one appliance fuses the other will not run.
14.11. Does a fuse in a circuit control the potential difference or the current?
Ans. Fuse in a circuit controls the current. If the current increases from a certain limit produces large amount of heat energy which melts the wire of fuse.
Due to which circuit breaks and the supply of the current goes off.