ICSE Living Science Physics for Class 7 Solutions Chapter 7 Electricity and Magnetism
Check Your Progress (Page 90)
A. Write true or false. Correct the false statements.
Question 1.
Materials that are attracted by magnets are called magnetic materials.
Answer:
True.
Question 2.
The poles of a magnet can exist independently.
Answer:
False. The poles of a magnet cannot exist independently.
Question 3.
Attraction is the surest test of magnetism.
Answer:
False. Repulsion is the surest test of magnetism.
Question 4.
An electric bell works on the principle of electromagnetism.
Answer:
True.
B. Answer the following questions.
Question 1.
What do you understand by the term directive property of agents?
Answer:
A freely suspended magnet always rests in the north-south direction. This property is called the directive property of magnets. The end of the magnet that points towards the geographical North Pole is called the north pole (N) of the magnet, whereas the end pointing towards the geographical South Pole is called the south pole (S) of the magnet.
Question 2.
What is meant by electromagnetism?
Answer:
When an electric current is passed through a coil of insulated wire wound around a soft iron core, the core behaves like a magnet. This phenomenon is called electromagnetism.
Question 3.
What is the function of the contact adjustment screw in an electric bell?
Answer:
The contact adjustment screw can alter the frequency of the ringing of the bell and also helps to prevent sparking.
Think And Answer (Page 96)
Question 1.
Why are metals good conductors of electricity?
Answer:
Metals contain loosely bound electrons that can participate in the conduction of electricity. Rubber does not have free electrons.
A. Tick the most appropriate answer.
Question 1.
Which of the following is not true for a magnet?
a. Like poles attract each other
b. Like poles repel each other
c. Unlike poles attract each other
d. both b and c
Answer:
a. Like poles attract each other
Question 2.
A freely suspended magnet always points in the –
a. easiest direction.
b. north-south direction.
c. east-south direction.
d. north-west direction.
Answer:
b. north-south direction.
Question 3.
Moving charges constitute –
a. electrode.
b. electric current.
c. static electricity.
d. magnetism.
Answer:
b. electric current.
Question 4.
The SI unit of electric current is –
a. ampere.
b. joule.
c. coulomb.
d. metre.
Answer:
a. ampere.
Question 5.
A dry cell converts chemical energy into –
a. mechanical energy
b. electrical energy.
c. magnetic energy.
d. kinetic energy.
Answer:
b. electrical energy.
Question 6.
Which of the following cannot be recharged ?
a. A primary cell
b. A secondary cell
c. A mobile battery
d. All of these
Answer:
a. A primary cell
Question 7.
Accumulators are –
a. primary cells.
b. secondary cells.
c. positive cells.
d. neutral cells.
Answer:
b. secondary cells.
Question 8.
Electrical devices installed in houses, factories and offices are connected in –
a. open circuits
b. series circuits
c. parallel circuits
d. none of these
Answer:
c. parallel circuits
Question 9.
Materials that allow electricity to pass through them easily are called –
a. conductors.
b. resistors.
c. insulators.
d. electromagnets.
Answer:
a. conductors.
Question 10.
Which of the following substances offer very low resistance?
a. nichrome
b. water
c. graphite
d. steel
Answer:
a. nichrome
B. Fill in the blanks.
Question 1.
An electromagnet is a ………………… magnet.
Answer:
An electromagnet is a temporary magnet.
Question 2.
An electric bell works on the principle of ……………….
Answer:
An electric bell works on the principle of electromagnetism.
Question 3.
The SI unit of charge is ………………..
Answer:
The SI unit of charge is coulomb.
Question 4.
When the path of current is broken, the circuit is called an ………….. circuit.
Answer:
When the path of current is broken, the circuit is called an open circuit.
Question 5.
………….. is a non-metal, yet a good conductor of electricity.
Answer:
Graphite is a non-metal, yet a good conductor of electricity.
C. Write true or false. Correct the false statements.
Question 1.
Electric current is the flow of electrons through a material.
Answer:
True.
Question 2.
Dry cells have completely dry chemicals inside it.
Answer:
False. Dry cells have thick paste of chemicals inside it.
Question 3.
A circuit diagram represents the arrangement of electrical components in an electric circuit.
Answer:
True.
Question 4.
If any one of the appliances is out of order in a series circuit, all the other appliances continue working.
Answer:
False. If any one of the appliances is out of order in a series circuit, all the other appliances continue working.
Question 5.
Materials that regulate the flow of current through them are known as resistors.
Answer:
True.
D. Answer the following in one or two words or a sentence.
Question 1.
Name two appliances that use electromagnets.
Answer:
Electric bell, telephone.
Question 2.
Why are free electrons called so ?
Answer:
Atoms of most metals contain very loosely bound negatively charged electrons in their outermost orbits. These electrons can be knocked out easily and hence are called free electrons as these free electrons move randomly in all directions in a material.
Question 3.
Define one ampere.
Answer:
One ampere current is said to be flowing across a point if one coulomb of charge flows through it in one second.
Question 4.
Name any two conductors.
Answer:
Aluminium and copper.
Question 5.
Define the term electrical resistance.
Answer:
The property of a material due to which it opposes the flow of current through it is known as electrical resistance.
E. Answer the following in short.
Question 1.
Why are electromagnets also called temporary magnets?
Answer:
The magnetic behaviour of an electromagnets lasts for as long as the current flows through it. The magnetic effect disappears as soon as the supply of electric current is stopped. Hence, electromagnets are also called temporary magnets.
Question 2.
What do you understand by primary cells ?
Answer:
The cells that can be used only once are called primary cells. Example : Simple dry cells, button cells, voltaic cells.
Question 3.
Why are dry cells called so?
Answer:
Dry cells are called so because the chemicals stored in them are not in liquid form, but in the form of a thick paste.
Question 4.
What happens to a glowing torch bulb when the circuit is a. closed, b. open?
Answer;
When the circuit is open the torch bulb stops glowing. When the circuit is closed the bulb remains glowing.
Question 5.
Why is nichrome used for making heating elements of heaters and toasters?
Answer:
Nichrome offer appreciable resistance to the flow of electric current through it. This is why nichrome is used for making heating elements of heaters and toasters.
F. Answer the following in detail.
Question 1.
Write the properties of magnets.
Answer:
Properties of a magnet:
(a) A magnet is able to attract magnetic materials.
(b) Like poles of a magnet repel each other and unlike poles attract each other.
(c) A freely suspended magnet always rests in the north- south direction.
(d) The poles of a magnet cannot exist independently, they always occur together.
Question 2.
Explain the construction and working of an electric bell with a labelled diagram.
Answer:
The cells that can be used only once are called primary cells. When the chemicals are used up completely, the cell becomes dead, and cannot be reused. The most common primary electric , cell is the simple dry cell that is used in torches, transistors, radios, clocks, wrist watches, calculators and TV remotes.
Question 3.
Why is it possible to recharge a car battery.
Answer:
It is possible to recharge a car battery because car batteries consists of secondary cells connected together. Each cell has a plate of lead and another of lead oxide dipped in the acid. Secondary cells can be recharged by passing an electric current through them. Because the reaction in these cells are reversible.
Question 4.
Compare the behaviour of appliances connected in a parallel circuit and in a series circuit.
Answer:
When connected in series :
- All appliances work simultaneously when switch is closed (turned on) and they stop working when switch is open (off).
- If any one of the appliances is out of order, all others stop working.
- Appliances do not work to their full capacity.
When connected in parallel :
- Each appliance will receive same voltage.
- Each appliance works independently.
- If one is out of order, other continue working.
Question 5.
Write the differences between conductors and insulators.
Answer:
Materials that allow electric current to flow through them easily are called conductors. Such materials offer very low resistance to the flow of electric current through them. Materials that do not allow electric current to flow through them are called bad conductors of electricity or insulators. Such materials offer high resistance to the flow of electric current through them.
G. Draw diagrams of the following.
Question 1.
A neatly labelled inner structure of a dry cell.
Answer:
Question 2.
A neatly labelled simple circuit using electrical symbols. Ans.
Answer:
Question 3.
A simple circuit diagram for two bulbs connected in series to an electric cell and a switch.
Answer:
H. Write the names of the following electrical symbols.
Answer:
I. Look at the given figure and answer the following questions. –
Question 1.
Which type of circuit is shown here?
Answer:
Open circuit.
Question 2.
Why is the bulb shown in the figure not glowing?
Answer:
The bulb shown in the given figure is not glowing because the circuit is not closed.
Question 3.
Write the differences between an open and a closed circuit.
Answer:
When the path of current starts from one terminal of the cell and ends at the other terminal of the cell without any break, the circuit is called a closed circuit.
When the path of current is broken, the circuit is called an open circuit.
Think And Answer
Question 1.
An atom gains electrons from another atom. What will be the nature of the charge on the gainer atom?
Answer:
The atom will get negatively charged.
Question 2.
Why do electricians wear rubber gloves while working?
Answer:
As rubber is an insulator, it prevents the electric current to pass through it. So, electricians wear rubber gloves while working to prevent electric shocks.
Question 3.
What happens when the heating element made of nichrome in an electric iron is replaced by copper?
Answer:
When the heating element made of nichrome in an electric iron is replaced by copper, the electric current will flow through it and the person will get the shock.
Living Science Physics Class 7 ICSE Solutions