Living Science 2019 2020 Solutions for Class 7 Science Chapter 17 Electric Current And Its Effects are provided here with simple step-by-step explanations. These solutions for Electric Current And Its Effects are extremely popular among class 7 students for Science Electric Current And Its Effects Solutions come handy for quickly completing your homework and preparing for exams. All questions and answers from the Living Science 2019 2020 Book of class 7 Science Chapter 17 are provided here for you for free. You will also love the ad-free experience on Meritnation’s Living Science 2019 2020 Solutions. All Living Science 2019 2020 Solutions for class 7 Science are prepared by experts and are 100% accurate.
Page No 204:
Question 1:
In a simple electric circuit consisting of a cell, conducting wires, a bulb, and a switch, the switch must be connected very close to the cell; otherwise, it will not work properly. Do you agree? Give reasons.
Answer:
No, the position of the switch does not affect its functionality. A switch can be connected anywhere in the circuit.
Page No 204:
Question 2:
Which insulator is used in an electric switch to cut off the supply of current–plastic, wood or air?
Answer:
Air is used as an insulator in an electric switch to cut off the supply of current.
Page No 204:
Question 3:
What kind of wire will you use for the following–one offering low resistance or one offering high resistance?
a. connecting wire
b. filament of a bulb
Answer:
(a) I will use a wire of low resistance as a connecting wire because it will produce less heat in the circuit.
(b) For the filament of a bulb, I would use a wire of high resistance because it would produce more heat and would glow brighter.
Page No 204:
Question 4:
Which terminal of a cell is at higher potential–positive or negative? How does this affect the direction of flow of electric current?
Answer:
The positive terminal of a cell is at a higher potential. Current flows from higher potential to lower potential in any circuit. So, which terminal of the battery is at higher potential, decides the direction of flow of current in the circuit.
Page No 208:
Question 1:
The wire used in an electric fuse should conduct electricity. What other property must it have?
Answer:
Along with the good conductivity, a fuse wire must have a low melting point, so that it can melt and break the circuit when an unusual amount of current flows in the circuit.
Page No 208:
Question 2:
A coil of wire wound around a soft iron bar is called an electromagnet. When does an electromagnet act like a magnet?
Answer:
An electromagnet acts like a magnet only when a current is passed through the coil wound around the soft iron bar.
Page No 208:
Question 3:
A straight wire carrying current acts like a magnet. Then why is a coil of wire used in an electromagnet instead of just a straight wire?
Answer:
The magnetic field produced by a straight wire is circular in shape and weak in strength. That's why it is of no use. But the magnetic field produced by a coil of many turns is similar in shape and stronger in strength than the field produced by a bar magnet. This magnetic field can be used for various purposes as well. Therefore, a coil of wire is used in an electromagnet instead of just a straight wire.
Page No 209:
Question 1:
Which of these is not a circuit element?
(a) potential difference
(b) voltmeter
(c) resistor
(d) battery
Answer:
(a) potential difference
It is the difference between the potentials of two terminals of a battery or any other current source connected to a circuit.
Page No 209:
Question 2:
The necessary requirement for a fuse wire is
(a) conductor with low melting point
(b) conductor with high melting point
(c) insulator with low melting point
(d) insulator with high melting point
Answer:
(a) conductor with low melting point
A fuse is a safety device made up of a conducting material having low melting point. It works on the principle of heating effect of current.
Page No 209:
Question 3:
The strength of a magnetic field due to a solenoid can be increased by
(a) increasing the number of turns in the solenoid
(b) increasing the current in the solenoid
(c) winding the solenoid around a magnetic material
(d) all of these
Answer:
(d) all of these
The strength of a magnetic field due to a solenoid can be increased by increasing the number of turns in the solenoid. It can also be increased by increasing the current and winding the solenoid around a magnetic material.
Page No 209:
Question 4:
Which of these appliances does not use an electromagnet?
(a) washing machine
(b) refrigerator
(c) room heater
(d) electric bell
Answer:
(c) room heater
A room heater does not use an electromagnet. The heating effect of current is used in an electric heater.
Page No 209:
Question 5:
Which of these is absolutely necessary in an electric circuit?
(a) bulb
(b) switch
(c) voltmeter
(d) connecting wires
Answer:
(d) connecting wires
All the given options play an important role in an electric circuit but connecting wires are absolutely necessary to complete the circuit.
Page No 209:
Question 6:
The same current is passed through the following. Which will show the strongest magnetic strength?
(a) straight conducting wire
(b) coil of wire
(c) solenoid wound on a wooden core
(d) solenoid wound on an iron core
Answer:
(d) solenoid wound on an iron core
It will show the strongest magnetic strength. It is strong enough to attract metal paper clips as well as big iron nails.
Page No 209:
Question 1:
Which insulator does a switch use to cut off the current in a circuit?
Answer:
Air is used as an insulator in a switch to cut off the current in a circuit.
Page No 209:
Question 2:
Metals are conductors of electricity. True or false?
Answer:
True.
Metals are conductors of electricity. The free and mobile electrons present in the metals are responsible for their high thermal and electrical conductivity.
Page No 209:
Question 3:
Every conductor offers resistance to the flow of electric current. True of false?
Answer:
True; all conductors offer some resistance to the flow of electric current. While some metals like copper offer very little resistance, others such as tungsten offer high resistance.
Page No 210:
Question 4:
We use symbols to draw a circuit __________
Answer:
We use symbols to draw a circuit element.
Page No 210:
Question 5:
A point A in a circuit is at a lower potential than point B. In which direction will the current flow in the circuit?
Answer:
In an electric circuit, electrons flow from low to high potential. In contrary, current flows from high to low potential.
Thus, current will flow from B to A in the given circuit.
Page No 210:
Question 6:
In circuit, a conductor offering low resistacne will heat up more than conductor offering high resistance. True of false?
Answer:
False; a conductor offering low resistance will heat up less than the conductor offering high resistance.
The greater the resistance offered by the conductor to the flow of current, the greater would be the heat generated.
Page No 210:
Question 7:
Which effect of current does the filament in eletric bulb utilize?
Answer:
Heating effect of current is used in the filament of an electric bulb.
Page No 210:
Question 8:
The core of an electromagnet can be made up of any metal. True of false?
Answer:
False; the core of an electromagnet must be a metal that can allow current to flow and can produce necessary magnetic field.
Page No 210:
Question 9:
An ____________ behaves like a magnet when an electric current passes through it.
Answer:
An electromagnet behaves like a magnet when an electric current passes through it.
Page No 210:
Question 10:
An electric bell has an arrangement to supply intermittent current to the electromagnet in it. True or False?
Answer:
True; an electric bell has an arrangement to supply intermittent current to the electromagnet in it.
Page No 210:
Question 1:
What is an electric circuit?
Answer:
An electric circuit can be defined as a closed path through which electric current can flow. Various components such as battery, resistor, wire, key, bulb etc. can be connected in different ways to construct an electric circuit.
Page No 210:
Question 2:
What is the relationship between potential difference and direction of flow of current in an electric circuit?
Answer:
Potential difference is the difference in the potential between two terminals of a battery or any other current source connected to an electric circuit. In the circuit, current flows from high to low potential because electrons flow from low potential to high potential.
Page No 210:
Question 3:
Does every conductor heat up when an electric current is passed through it? What does the amount of heat depend on? Name the conductor normally used in heating appliances.
Answer:
Yes, every conductor heats up when electric current is passed through it. Heating is caused by the resistance provided by the conductor to the current flowing through it. The amount of heat depends on the length, thickness and nature of the material of conductor.
Nichrome is used in heating appliances as it offers high resistance to the flow of current.
Page No 210:
Question 4:
A domestic circuit develops a short circuit. What will be the effect on a fuse wire connected in the circuit?
Answer:
A fuse wire is made up of an alloy with a low melting point. The heat produced due to short circuit melts the fuse wire and breaks the circuit.
Page No 210:
Question 5:
What is a miniature circuit breaker?
Answer:
Miniature circuit breaker is a switch that automatically stops the current in a circuit if the current in it exceeds the specified maximum limit.
Page No 210:
Question 6:
Draw the symbols used in a circuit for-a battery of four cells, a switch, a lamp.
Answer:
A battery of four cells:
Switch in the ‘ON’ position:
Switch in the ‘OFF’ position:
Lamp:
Page No 210:
Question 1:
Draw a circuit diagram of a dry cell connected to a bulb through a switch. Mark the positive and negative terminals of the cell, and the direction of flow of current in the diagram.
Answer:
Page No 210:
Question 2:
How can you increase the magnetic strength of a solenoid?
Answer:
Magnetic strength of a solenoid can be increased by:
(i) Increasing the number of turns in the solenoid
(ii) Increasing the current in the solenoid
(iii) Winding the solenoid around a magnetic material
Page No 210:
Question 3:
What is an electromagnet? Give three uses of electromagnets.
Answer:
An electromagnet is a coil of insulated wire wound around a piece of a magnetic substance such as soft iron. The magnetic substance acts as a magnet as long as the current flows in the wire.
Uses of electromagnet:
(i) Electromagnets are used in a number of electric appliances such as electric bells and electric buzzers.
(ii) They are used in electric motors, which find wide application in fans, washing machines etc.
(iii) Very strong electromagnets are used in factories and cranes for lifting heavy iron pieces.
Page No 210:
Question 4:
Explain the working of a simple electric bell.
Answer:
An electric bell works with the help of an electromagnet. It is made up of a coil of wire wound on an iron piece, which acts as an electromagnet. On passing the current, magnetic field is produced by the electromagnet.
The hammer of the bell is attracted towards the electromagnet when the circuit is complete. Further, the circuit breaks after the hammer moves towards the electromagnet. When the circuit breaks, the electromagnet doesn’t attract the hammer. The hammer comes back to its original position due to spring effect. As the hammer comes back to its original position, it touches the metal connector and the circuit is complete again. This cycle continues till the key is kept ON.
Page No 210:
Question 1:
Fuse wires of miniature circuit breakers are not used in a circuit containing electric cells. Why?
Answer:
MCBs are most commonly used in place of fuses because it is easier to turn on an MCB when it gets tripped.
In a circuit where electric cells are used to supply low values of voltage and current, no fuses are required because there is very little risk of short circuit. To avoid any accident, these fuses are used in circuits where high voltages and currents are delivered.
Page No 210:
Question 2:
Can you use a copper wire to make the filament of an electric bulb? Why?
Answer:
Filament of an electric bulb is made up of conductors that offer high resistance to electric current. Copper wire cannot be used in filaments because it offers very little resistance. ​Tungsten is used for making the filaments as it offers high resistance.
Page No 210:
Question 3:
Mohit used a fuse wire with a specification of 5 amperes in a circuit connected to an air conditioner which draws a current of 10 amperes. What will happen when the air conditioner is switched on? What will you advise him to do?
Answer:
When the air conditioner is switched on, the fuse wire will melt due to extra current.
The fuse wire must be selected keeping in mind the minimum current that will melt it. Its specification should be slightly more than the normal operating current of the electrical appliance.
Mohit should be advised to use a fuse wire with a specification of 15 amperes.
Page No 210:
Question 4:
Rita used a fuse wire with a 100 amperes specification in a circuit at home which draws a maximum current of 20 amperes when all devices in the circuit are switched on. Do you think she used the correct fuse wire? What will you advise her to do?
Answer:
No, Rita did not use the correct fuse wire. The ampere rating of a fuse should not exceed the current carrying capacity of the circuit except in some cases.
In the given circuit, she should use a fuse wire with 20 amperes specification.
Page No 211:
Question 5:
You have seen that the heating effect of current can be put to good use. Can you think of situations where this effect results in wastage of energy?
Answer:
Yes, when electric current passes through long conductors, a portion of energy is wasted in the form of heat. Also, in case of an incandescent lamp, only a part of electricity is converted into light and the rest is wasted as heat, which is unwanted.
Page No 211:
Question 6:
A coil with no current passing through it is suspended freely. It points in the east-west direction. Explain what will happen if a current is passed thourgh the coil.
Answer:
Current flowing through a wire gives rise to a magnetic field around it. Therefore, if current is passed though the coil, it will become a magnet and will align along the north-south direction.
Page No 211:
Question 1:
In which of these circuits will the lamp glow when switch is on? Explain the reason in each case.
Answer:
The lamp will glow only in the circuit C.
In circuit A, when the switch is closed, the ends of the lamp gets short-circuited and no current flows through the lamp. In circuit B, when the switch is on, the potential difference across the lamp becomes zero because both the batteries are connected with opposite polarities. So, no current flows in the circuit B as well. In the circuit C, upon closing the switch the batteries will send current in the circuit together as they are connected in series with similar polarities. Hence, the lamp will glow only in the circuit C.
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