Solved examples for class 7 science : Chapter 14
Electric Current and Its Effects
I. Multiple Choice Questions (MCQs).
Q.1. The flow of electric charge through a conductor is known as
a) electric current
b) electric potential
c) electric energy
d) all of these
Answer: a) electric current
Q.2. The source of electric current in a circuit, is
a) electric cell
b) electric battery
c) both (a) and (b)
d) none of these
Answer: c) both (a) and (b)
Q.3. A device used to measure electric current is
d) all of these
Answer: (b) ammeter
Q.4. The thin longer line of the symbol of an electric cell is
a) positive terminal
b) negative terminal
c) either (a) or (b)
d) none of these
Answer: (a) positive terminal
Q.5. The safety device used in electric appliances is
d) all of these
Answer: (a) fuse
II. Fill in the blanks by selecting appropriate words from the given list : List : ON, ISI, Short circuit, open, components
Q.1. A bulb, a key and a dry cell are called electric ______________ of a circuit.
Q.2. In an electric circuit, bulb glows only when the switch is in the ____________ position.
Q.3. The circuit in which the conducting path is broken is called an ____________ circuit.
Q.4. Use only proper fuse which has been specified for particular appliances and carries ______________ mark.
Q.5. ______________ circuit is the main cause of excessive current in the circuit.
5- Short circuit
III. Match the items of Column A with those of Column B.
Answer: 1- (e ), 2-(a) , 3- (d), 4-(b), 5-(c)
IV. Write ‘T’ for the true and ‘F’ for the false statements.
Q.1. A vehicle battery can be recharged by an alternator which is driven by an engine.
Q.2. Resistor increases electric current in an electric circuit.
Q.3. The nichrome coil gets hot when electric current passes through it.
Q.4. The strength of electromagnet can be decreased by increasing the number of turns of wire.
Q.5. Nowadays, electric bell is replaced by an electric buzzer, which works on the principle different from that of electric bell.
Answer: 1-T, 2-F, 3-T, 4-F, 5-T
V. Very Short Answer Questions.
Q.1. What does the thicker but smaller line represent in the symbol of an electric cell? Answer: Thicker but smaller line in the symbol of an electric cell represents the negative terminal.
Q.2. When does an electric bulb glow in an electric circuit?
Answer: A bulb glows only when circuit is closed.
Q.3. Why has the glowing electric bulb become warm?
Answer: When a current flows through a conductor, heat energy generates in the conductor. So, When current flows through the filament, it gets heated up and becomes hot.
Q.4. Which material is the coil of an electric heating device made of?
Answer: The coil of an electric heating device made of nichrome.
Q.5. Which material is the filament of an electric bulb made of?
VI. Short Answer Questions.
Q.1. Why are the electric bulbs being replaced with CFL these days?
Answer: Compared to bulbs, CFLs use less power and have a longer rated life and give the same amount of visible light. So, electric bulbs being replaced with CFL these days.
Q.2. The wires used for making electric circuits do not normally become hot. Why?
Answer. Generally, copper wires are used as connecting wires. Copper is a good conductor of electricity and does not offer resistance in the path of electricity. Therefore, they do not become hot.
Q.3. Define miniature circuit breaker in your own words.
Answer: These days circuit breakers are being used mostly these days in place of fuses. When current in circuit crosses the safe limit this appliance automatically gets switched off. When its switch is turned on, the circuit is once again complete.
Q.4. What do you mean by electric circuit? Describe its kinds with proper examples.
Answer: Path along which electric current flows is known as electric circuit. There are two types of circuit
Open circuit: The circuit in which electrical contact at any point is broken is called an open circuit. No current flows through an open circuit.
Closed circuit: The circuit in which electric current flows from one terminal of a battery to the other is called a closed circuit.
Q.5. Why are the symbols of components of electric circuit needed? Describe some of the symbols and their uses also.
Answer: A standard method of drawing an electrical circuit is called circuit diagram, we use symbols for sketching electric circuit diagrams of various components like cells, wires, switches, bulbs, etc.
The following table shows some common electrical symbol used for electric circuit.
Q.6. What do you understand by heating effect of electric current? Describe some appliances based on it.
Answer: The production of heat in a conductor when electricity flows through it is called heating effect of current. The heat produced in a wire depends on :
(i) Material (ii) Length and (iii) Thickness of wire.
The heating effect of current is used in a large number of industrial and domestic appliances, such as immersion heaters, hot plates, irons, geysers, electric kettles, hair dryers, etc.
VII. Long Answer Questions.
Q.1. What do you mean by an electric fuse? Describe its principle, construction and uses briefly.
Answer: A fuse is a safety device used in an electric circuit.
Fuse works on the principle of heating effect of electric current.
An electric fuse consists of a thin wire made up of an alloy which has a low melting point and is usually placed inside a small glass cylindrical tube or ceramic container.
It is designed such that only a certain maximum amount of current can flow through it.
If a very high current suddenly passes through it, the heat produced due to the high current melts the fuse wire and breaks the circuit. Thus, the flow of current stops and damage of appliances is avoided.
Q.2. What is a magnetic effect of electric current? Describe construction and uses of electromagnet briefly.
Answer: When an electric current passes through a coil, it behaves like a magnet. This is the magnetic effect of electric current.
A coil wound around an iron core that acts as a magnet as long as an electric current flows through the coil is known as an electromagnet.
Making an electromagnet: For this we need a small iron rod and wind an insulated copper wire around it. Connect the ends of the wire to a battery, through a switch. Now switch on the circuit and bring some steel pins near the iron nail. You will find that the pins are attracted to the rod as long as electric current flows through the coil. When the circuit is switch off, the pins, falls off. This is because the iron rod no longer remains magnet. In this way you can make your own electromagnet.
Uses of electromagnet:
(a) Electromagnets are used in the hospitals for treating eye-injuries in which iron splinters are removed from the eyes with it.
(b)It is used in the cranes in steelworks and scrapyards for Lifting heavy Loads in the iron containers.
(c ) In Loudspeakers- An electromagnet is used in a Loudspeaker for simplifying a signal.
(d) In telephone – A typical telephone handset consists of two parts – the receiver and the microphone. The receiver consists of an electromagnet.
(e) In relays- A relay is an electrically operated switch. Some relays use electromagnets to control the operation of the switch. Some of the first computers used relays.
(f) In electric motors: Electric motors use electromagnets, which find wide application in electric fans, washing machines, refrigerators etc.
Q.3. With a well labelled diagram, explain the construction, basic principle and uses of an electric bell.
Answer: An electric bell has an electromagnet in it and works on the principle of magnetic effect of current. Circuit diagram of an electric bell is shown.
The steps of working of an electric bell are given here.
When the switch of the bell is pushed, the electric current flows to the electromagnet. The electromagnet attracts the soft iron armature (strip). The hammer attached to the strip then hits the gong, causing a ring. When the soft iron strip gets attracted to the electromagnet, it no longer touches the screw [interrupter] and hence the circuit is broken. It is much Like a switch being turned off. This turns off the electromagnets and it can no Longer attract the soft iron strip. The soft iron strip returns to its initial position, touching the screw. This results in the circuit to complete and the current flows again. This is how we hear a continuous ring of the bell. Nowadays, electric bell is replaced by an electric buzzer, which works on the principle different from that of electric bell.