CHAPTER # 14
CURRENT ELECTRICITY

Multiple Choice Questions

 

                    i.            Free Electrons

                  ii.            18V

                iii.            Decreases

                iv.            To provide each appliance the same voltage as the power source

                  v.            Are the same terms

                vi.            Current

              vii.            Doubles

            viii.            30W

                ix.            2 ohm

REVIEW QUESTIONS 

14.1 Define and explain the term electric current.

The rate of flow of charges through any cross section is known as an electric current.

Its unit is ampere (A).

14.2 What is the difference between electronic current and conventional current?

Electronic Current	Conventional Current
The rate of flow of negative charges is known as electronic current.	The rate of flow of positive charges is known as conventional current.

14.3 What do you mean by the term e.m.f? is it really a force?

The work done required to move charge from lower potential to higher potential is known as electromotive force. This is not a force. Its unit is volt. 

14.4 How can we differentiate between e.m.f and potential difference?

E.M.F	Potential Difference
Electromotive force is the total voltage of battery and it is always greater than the potential difference.	Potential difference are the voltage difference across a load in a circuit.

14.5 Explain ohms law. What are its limitations?

Ohm’s law states that “the potential difference is directly proportional to the current when resistance is constant”.       V = IR

• Ohm’s law is valid for conductors.
• Physical condition of conductor should be remains same.

14.6 Define resistance and its unit.

• The opposition offers to the flow of charges is known as resistance.
• Unit of resistance is ohm. Resistance will be one ohm if the potential difference of one volt is applied across the ends of a conductor and one ampere current will flow through it.

14.7 What is the difference between conductor and insulators?

Conductor	Insulator
The substances that have free electrons are known as conductor. For instance; silver and gold	The substances that do not have free electrons are known as an insulator. For instance; plastic and wood

14.8 Explain the energy dissipation in a resistance. What is joules law?

Joules law states that “the amount of heat generated in a resistance due to the flow of charges is equal to the product of the square of current (I), resistance (R) and the time duration (t)”.

P = I²Rt

The energy dissipation in a resister is the electrical energy that is converted into heat energy.

14.9 What is the difference between D.C and A.C?

Direct Current	Alternating Current
The current that does not change its polarity is known as direct current. This is produced by a cell or battery.	The current that does change its polarity is known as an alternating current. This is produced by an A.C generator.

14.10 Discuss the main features of parallel combination of resistors.

• In parallel combination the potential should be same as of battery across all resistors.
• In parallel combination the current divided.

14.11 Determine the equivalent resistance of series combination of resistors.

14.12 Describe briefly the hazards of household electricity.

Insulation Damage	Damped Conditions
All electrical wires are insulated with plastic material for the prevention of short circuit and electrical shock.	If electrical wires expose to wet conditions. This will be hazardous.

14.13 Describe four safety measures that should be taken in connection with the household circuit.

• Avoid Damp Condition
• Avoid bends in wire for the prevention of insulation damage.
• Use circuit breakers.
• Use fuse.
• Use earth wire to avoid excess charge.

CONCEPTUAL QUESTION

14.1 Why in conductors charge is transferred by free electrons rather than by positive charges?

Positive charge in an atom of conductor is bound with nuclei. However, free electrons are at some distance from nuclei and are free to move. That’s why charge is transferred by free electrons rather than by positive charge.

14.2 What is the difference between a cell and a battery?

Cell	Battery
A cell consists of electrodes which are dipped into an electrolyte. It converts chemical energy into an electrical energy.	When we combined cells in a group, this will make a battery. It stores large amount of energy as compare to a single cell.

14.3 Can current flow in a circuit without potential difference?

No, the current cannot flow in a circuit of zero potential. According to ohms law potential difference will be required for the flow of charges.

14.4 Two points on an object are at different electric potentials. Does charge necessarily flow between them?

Potential difference is required for the flow of charges. If one point has higher potential and the other at lower potential then the charge will flow between them.

14.5 In order to measure current in a circuit why ammeter is always connected in series?

Ammeter has very low resistance and it is always connected in series so that the maximum current to be measured will passes through it.

14.6 In order to measure voltage in a circuit voltmeter is always connected in parallel. Discuss.

Voltmeter has very high resistance so that current cannot pass through it. It is always connected in parallel to avoid the disturbance of current and to measure the unaffected voltages.

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.

Headlamps of automobile are connected in parallel because the brightness of both lamps are same and if one lamp is down then the other one will still glow.

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?

5-ohm bulb will be used to get brighter light and it will discharge the battery first as compared to 10-ohm bulb because lower the resistance more will be the current.

14.10 It is impracticable to connect an electric bulb and an electric heater in series. Why?

To avoid the loss of current an electric bulb and a heater is connected in parallel because when appliances are connected in series, the resistance of circuit increases and causes the loss of power.

14.11 Does a fuse in a circuit control the potential difference or the current?

The fuse is used to control the current in the circuit because if the current exceeds the fuse's limit, the circuit will be broken.

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