The DAV Class 7 Science Book Solutions and DAV Class 7 Science Chapter 17 Question Answer – Electric Current and Its Effects are essential study tools for DAV public school students in Class 7.
DAV Class 7 Science Ch 17 Question Answer – Electric Current and Its Effects
DAV Class 7 Science Ch 17 Solutions – Electric Current and Its Effects
Something To Know
A. Fill in the blanks.
Question 1.
When current is switched on, an electric fan converts ________ energy (mainly) into ________ energy.
Answer:
Electrical, Mechanical
Question 2.
When current is switched on in a room heater , it converts ________ energy into ________ energy.
Answer:
Electrical, Heat
Question 3.
We prefer a wire of ________ for making the heating element in domestic appliances like electric iron, electric heater, electric toaster, etc.
Answer:
Nichrome
Question 4.
A safety device, based on the heating effect of electric current is the ________
Answer:
Electric fuse
Question 5.
Electromagnet should have a rapid ________ and ________ action like electric switches.
Answer:
ON, OFF
B. Write True or False for the following statements.
1. An electric current can produce a heating effect but not a magnetic effect.
2. The heating coils/elements of different electrical appliances are usually made from copper or aluminium wires.
3. When the current through a fuse wire exceeds its specified value, the fuse wire melts and breaks.
4. Connecting many devices in a single socket is not likely to cause any problem in a circuit.
5. The strength of an electromagnet can be increased by decreasing the number of turns of its coil.
Answer:
1. An electric current can produce a heating effect but not a magnetic effect. (True)
2. The heating coils/elements of different electrical appliances are usually made from copper or aluminium wires. (False)
3. When the current through a fuse wire exceeds its specified value, the fuse wire melts and breaks. (False)
4. Connecting many devices in a single socket is not likely to cause any problem in a circuit. (False)
5. The strength of an electromagnet can be increased by decreasing the number of turns of its coil. (False)
C. Tick (✓) the correct option.
1. When electric current flows through a filament of electric bulb, the filament gives out
(a) Heat energy only
(b) Light energy only
(c) Both heat and light energy
(d) Chemical energy
Answer:
(c) Both heat and light energy
2. A fuse wire is a wire of
(a) High resistivity and low melting point
(b) High resistivity and high melting point
(c) Low resistivity and low melting point
(d) Low resistivity and high melting point
Answer:
(a) High resistivity and low melting point
3. We are more likely to observe a strong magnetic effect with a
(a) Current carrying straight wire
(b) Current carrying circular coil of many turns wound on a soft iron core
(c) Current carrying coil of many circular turns
(d) Current carrying circular coil of single turn
Answer:
(a) Current carrying straight wire
4. A common household device that uses an electromagnet in its working, is
(a) The electric bell
(b) A room heater
(c) A geyser
(d) The tube light
Answer:
(a) The electric bell
5. A good electromagnet would
(a) Act like a magnet when the current is ‘ON’ but would not do so when the current is ‘OFF’.
(b) Not act like a magnet when the current is ‘ON’ but acts like one when the current is ‘OFF’. .
(c) Act like a magnet both when the current is ‘ON’ or is ‘OFF’.
(d) Not act like a magnet both when the current is ‘ON’ or is ‘OFF’.
Answer:
(a) Act like a magnet when the current is ‘ON’ but would not do so when the current is ‘OFF’.
D. Answer the following questions in brief.
Question 1.
The bulb in the circuit shown in figure does not glow. Why? Redraw the circuit diagram by making the necessary changes in it, so that the bulb starts glowing.
Answer:
In this circuit diagram, the cells in the battery have been wrongly placed. Negative terminals of both the batteries are facing each other. Due to this, the bulb is not glowing.
Question 2.
Name three practical devices based on the heating effect of current.
Answer:
Electric iron, toaster, geyser, sandwich maker, electric kettle
Question 3.
Write two special characteristics of the wire needed for making the ‘element’ of an electric heater.
Answer:
The wire should have low resistivity and high melting point.
Question 4.
Give the meaning of the terms – ‘short circuiting’ and ‘overloading’ in an electric circuit.
Answer:
Short Circuit: When a positive wire touches a negative wire, it results in electric spark and sometimes can lead to accidental fire. This is called short circuit. Overloading: When more than one appliance is attached to a socket, it leads to overload in the circuit.
Question 5.
Do you think an electromagnet can be used for separating plastic bags from a garbage heap? Explain.
Answer:
Electromagnet can attract only magnetic materials. Plastic is a non-magnetic material and the garbage may contain many other non-magnetic materials. Hence, an electromagnet cannot be used for separating plastic bags from a garbage heap.
Question 6.
State any three applications of an electromagnet.
Answer:
Three applications of electromagnet are as follows:
- In electric bell
- In electric motor
- In powerful cranes.
E. Answer the following questions.
Question 1.
The same amount of current flows through the connecting wires and the bulb filament. However, it is only the filament that glows. Why?
Answer:
The filament of the bulb is made of tungsten which has a very high melting point and very lew resistivity. Due to this, the filament of the bulb glows; which is not the case with the connecting wires.
Question 2.
The brightness of light emitted by a bulb is observed to decrease when an electric heater is connected in series with it. Suggest the likely reason for this observation.
Answer:
When an electric heater is connected in series with an electric bulb, some of the current is utilised by the heater and as a result, the bulb gets lesser amount of current. Due to this, the brightness of light emitted by the bulb is observed to decrease.
Question 3.
Suggest an experimental set up to illustrate the action of an electric fuse.
Answer:
For this, use a battery, connecting wires and a fuse wire. Connect everything to make a circuit. Increase the number of cells in the battery one by one. After adding a particular number of cells, you will observe that the fuse wire melts. This experiment shows the action of electric fuse.
Question 4.
A novice electrician, while carrying out some repairs in Sumit’s house, tried to pull a piece of tungsten wire in a ‘fuse’. Sumit’s father, on noticing this, stopped him from doing so. Give reasons as to why he did that.
Answer:
Tungsten has very high melting point. A fuse wire needs to have very low melting point so that it can break the circuit in case of overload or short circuit. This purpose cannot be served by tungsten wire. Due to this, the electrician was stopped by Sumit’s father.
Question 5.
Draw a labelled diagram of an electric bell and explain its working.
Answer:
Working of Electric Bell:
- When current flows through the coil, it becomes electromagnet.
- The electromagnet attracts the soft iron armature. With this, the iron hammer moves and hits the gong to produce sound.
- When the iron hammer and armature move towards the gong, the electric circuit breaks and electromagnet loses its magnetic property.
- The iron armature returns to its original position which helps in completing the circuit again. The process keeps on repeating and the bell produces a continuous ringing sound.
Value Based Question
The school principal, in her morning address, urged the students to work in a hard, dedicated and focused way. She also advised the teachers, seniors and parents to go to a ‘little out of their way’ to guide those who need a Tittle extra help’. Mohan, on returning home, discussed the principal’s comments with his grand-father. The grandfather told him that the principal’s suggestions were very good. These suggestions, if followed, can increase the student’s strength, and capability, in the same way as appropriate adjustments can help increase the strength of an electromagnet.
Question 1.
State any two of the values that the principal urged her students to develop in themselves.
Answer:
Helping attitude, making extra effort to help someone
Question 2.
Write two factors that can be adjusted to increase the strength of and electromagnet.
Answer:
Strength of an electromagnet can be increased by (i) increasing the number of
turns in the coil and (ii) by increasing the strength of electric current.
Question 3.
Share, with your friends and classmates, any incidence in which you went ‘a little out of the way’ to help/guide someone.
Answer:
Once one of my classmates could not attend school for 10 days as he was sick. He needed my help in covering up the lessons which he missed during this period. One day, I sat for extra two hours with him and helped in noting down all lessons which were taught during this period.
Something To Do
Question 1.
Collect data about the maximum current rating of domestic appliances like an electric mixer, television set, washing machine, toaster, electric oven, etc. With the help of your teacher/electrician, find out the type of fuse used for their safety.
Answer:
In India, domestic appliances come in two ratings, i.e. 5 ampere and 15 ampere. You can find the rating written on an appliance.
Question 2.
Take out the parts of an old on ‘out of use’ electric bell or a telephone or toy working with cells/battery. Is an electromagnet used in them.? Discuss and record your findings.
Answer:
Do it yourself.
Question 3.
Make an electric circuit consisting of an electric switch, two electric bulbs and a battery (three cells of 1.5 V each). Connect the switch between the two bulbs. Does any of the bulbs glows when the switch is in the OFF position? Is there an order, in which the bulbs glow when the switch is in the ON position?
Answer:
Do it yourself.
Question 4.
In the activity 2, given on page 167, use (i) nichrome wires of longer lengths but of same thickness, (ii) nichrome wires of same length but of different thickness. Switch on the circuit. Observe the change, if any. Try thinking of an explanation for your observations.
Answer:
Both length and thickness of the wire has a positive effect on heating effect of electric current. So, in both the cases, the nichrome wire would heat up more quickly.
DAV Class 7 Science Chapter 17 Solutions – Electric Current and Its Effects
I. Fill in the blanks.
Question 1.
An electric iron works on the principle of ________ effect of electric current.
Answer:
Heating
Question 2.
An electric bell works on the principle of ________ effect of electric current.
Answer:
Magnetic
Question 3.
Electric fuse has been replaced by________ in most of the households.
Answer:
MCBs
Question 4.
A group of electric cells is called ________
Answer:
Battery
Question 5.
There are ________ terminals in a battery.
Answer:
Two
Question 6.
________ happens when a positive wire touches a negative wire.
Answer:
Short circuit
II. Tick (✓) the correct option.
1. Which of these devices works on heating effect of electric current?
(a) Electric iron
(b) Toaster
(c) Electric bulb
(d) All of these
Answer:
(d) All of these
2. Which of these is a safety device?
(a) Electric heater
(b) Electric fuse
(c) Electric bulb
(d) Electric bell
Answer:
(b) Electric fuse
3. The filament of electric bulb is made of which material?
(a) Copper
(b) Nichrome
(c) Tungsten
(d) Aluminium
Answer:
(c) Tungsten
4. The element of an electric iron is made of which material?
(a) Copper
(b) Nichrome
(c) Tungsten
(d) Aluminium
Answer:
(b) Nichrome
5. Which of these works for the same purpose as an MCB?
(a) Toaster
(b) Geyser
(c) Electric fuse
(d) None of these
Answer:
(c) Electric fuse
III. Answer the following questions.
Question 1.
What do you understand by heating effect of electric current?
Answer:
When electric current flows through a conductor, it can produce heat. This is known as the Heating Effect of Electric Current.
Question 2.
The amount of heat produced in a conductor depends on which factors?
Answer:
According to Joule, the amount of heat produced in a conductor depends on following factors:
(a) Strength of the current through the wire
(b) Nature of the material of wire
(c) Length and thickness of wire
(d) Time for which electric current flows through the wire.
Question 3.
What are the disadvantages of heating effect of electric current?
Answer:
During transmission of electricity through wires, a major portion of electric current goes waste in the form of heat energy. The heating effect of electric current also breaks the insulation of wiring and various devices.
Question 4.
What are the various reasons for electric current crossing its limit?
Answer:
Short circuit, overload, insulation breakdown.
Question 5.
What is magnetic effect of electric current?
Answer:
When electric current flows through a conductor, a magnetic field is created around it. This is known as Magnetic Effect of Electric Current.
Question 6.
Make a list of some practical uses of magnetic effect of electric current.
Answer:
Practical Uses of Electromagnet:
- Electromagnets can be made very powerful. Such magnets are used for lifting heavy objects.
- Electromagnets are used in many devices; like microphone, speaker, telephone, etc.
- Electromagnets are used in automatic switches; like in MCBs.
- Electric bell works because of electromagnet.
Question 7.
Discuss the ways of increasing the strength of an electromagnet.
Answer:
Following means can increase the strength of an electromagnet:
(a) Wrapping the insulated coil around a soft iron core
(B) Increasing the number of turns in the coil
(c) Increasing the strength of electric current
IV. Projects
Question 1.
With the help of two cells, some connecting wires and an iron nail, make an electromagnet. Demonstrate this in your class.
Answer:
Do it yourself.
Question 2.
Make a well labelled diagram of an electric bulb.
Answer:
Do it yourself.