Class 7 Science Chapter 14 Important Questions
Class 7 Science Chapter 14 Important Questions Answers of Electric Current and Its Effect with extra practice questions for academic session 2020-2021. All the answers are taken from latest NCERT Textbook for CBSE 2020-21.Practice here with Extra practice questions and NCERT Textbook Solutions to prepare the chapter for exams. You can also practice the chapter with MCQ with answers available on Tiwari Academy website and app.
Class 7 Science Chapter 14 Important Questions for 2020-2021
|Chapter: 14||Electric Current and Its Effect|
|Contents:||Important Extra Question Answers|
Class 7 Science Chapter 14 Important Questions Set – 1
What is electricity?
Electricity is a form of energy. The flow of electricity through a conductor (such as metal wire) is called electric current. Generally, electric current is actually a flow of charged particles called “electrons” in a conductor which contains energy.
How does electricity important for us? Discuss.
Electricity is produced at power stations or power plants. Electricity or current is useful because it can run many appliances or machines in our day-to-day life which make our work easier. The electric bulb, tube lights, radio, television, refrigerator, washing machine, coolers, air conditions, computers, electric iron, electric kettle, room heater, electric oven, mixer and grinder, geyser, trains etc., all work with electricity or electric current.
Explain, how electricity is dangerous for us.
We know that electricity is produced at power stations or power plants. The big generator at power stations produce electricity. Electricity is very useful but it can also be very dangerous. If electric wires and electrical appliances are handled carelessly, then a person can get an electric shock which may cause severe burns and even death. This is because electricity from electric mains or power supply line, portable generators as well as from inverters is at a very high voltage of 220 volts. Human beings cannot tolerate electrical energy for long time. Improper handling of electricity can also cause fires. The electricity, produced by portable electric generators and inverters used in our homes, is equally dangerous.
What do you mean by circuit?
A cell or battery is a source of electric current, but this current cannot flow out on its own. There must be conducting materials (like wires) between the two terminals of the cell through which electric current can flow. A continuous conducting path or consisting of wires, bulb, switch, etc., between the two terminals of a cell or battery along which an electric current flow, is called a circuit.
Can you make any simple electric circuit in your home?
For making a simple electric circuit we have to take a cell having a positive terminal and negative terminal. The positive terminal of the cell is connected to one end of the switch with a piece of copper wire. The other end of switch is connected to one end of bulb holder with another piece of copper wire. When the terminals of the bulb are connected with that of the electric cell by wires, the current passes through the filament of the bulb. This makes the bulb glow.
In an electric circuit, the direction of current is taken to be from the positive to the negative terminal of the electric cell.
Class 7 Science Chapter 14 Important Questions Set – 2
Why do you need to drawing the circuit symbols?
To draw the electric circuits by making the actual sketches for drawing of the cell, battery, switch, bulb or lamp and connecting wires is a difficult job and takes a lot of time. So, the scientists have devised symbol for “Electrical components” like the cell, battery, switch, electric bulb and connecting wires etc. which are easy to draw. Such drawings provide a quicker mental picture of the actual circuit. And these symbols are used to draw the electric circuit diagram easily and clearly.
How electrical components are represented by circuit symbols? Explain.
A single cell or other power source is represented by a long and a short parallel line. A collection of cells or battery is represented by a collection of long and short parallel lines. In both cases, the long line is representative of the positive terminal of the energy source and the short line represents the negative terminal. A straight line is used to represent a connecting wire between any two components of the circuit. The symbol of electric bulb consists of a circle having a semi-circle inside it.
An electrical device that offers resistance to the flow of charge is generically referred to as a resistor and is represented by a zigzag line. An open switch is generally represented by providing a break in a straight line by lifting a portion of the line upward at a diagonal. These circuit symbols will be frequently used as electric circuits which are represented by schematic diagrams. It will be important to either memorize these symbols or to refer to this short listing frequently until you become accustomed to their use.
Why do we cover plug pinholes which are within the reach of children with cello-tape or a plastic cover when not in use?
We do cover plug pinholes which are within the reach of children with cello-tape or plastic cover to avoid electric shocks. If unconsciously, a child puts his finger into the electric socket, he or she may get shock which could be fatal.
Write three important points which we keep in mind during the joining of batteries.
- When joining two or more cells to make a battery, the positive terminal of one cell is always kept in contact with the negative terminal of the other cell.
- If we combine two cells by keeping the positive terminal of one cell is in contact with the positive terminal of another cell. Similarly, negative terminal of one cell in contact with the negative terminal of the other cell, then the battery obtained will not work.
- Use proper cells to glow a torch bulb because a single electric cell gives 1.5 volts of electricity. Many devices do not work properly with just one cell because they require higher voltage for working.
- Never join the two terminals of the electric cell without connecting them through a switch and a device like a bulb. If you do so, the chemicals in the electric cell get used up very fast and the cell stops working.
“Greater the resistance of a wire, greater will be the heat produced in it by a given current”. Explain.
Copper has a very low resistance, so if electric current is passed through a copper wire then the heat produced in it will be extremely small or little. On the other hand, nichrome has a very high resistance, so if the same electric current is passed through a nichrome wire, then the heat produced in it will be too much.
(Note: A short wire has less resistance whereas a long wire has more resistance and also a thick wire has less resistance whereas a thin wire has more resistance.)
Class 7 Science Chapter 14 Important Questions Set – 3
“Greater the magnitude of current passed through a given wire, greater will be the heat produced in it.” Comment.
When a small current passing in a wire produces less heat but a large current passing through the same wire produces much more heat. As an example, when normal amount of current flows through the copper wires of household electric wiring, the wires do not become much hot but when a large current flows in the wires of household wiring accidentally, then the wires become extremely hot and a fire may be started.
“The electric heating appliances work on the heating effect of current”. Justify your answer.
All the electric heating appliances contain coils of high resistance wire made of nichrome alloy. The coil of high resistance wire or coil of nichrome wire present in an electric heating appliance is called the “heating element” of the appliance.
For example, the electric room heater has an element made of a coil of nichrome wire. When current is switched on in an electric room heater. The element of heater becomes red hot and gives out heat. The element becomes red hot because a lot of heat is produced in it, on passing current through the nichrome coil. We know that because of the high resistance of nichrome coil, it looks red and give out heat.
What do you mean by closed circuit and open circuit?
When the switch is in the ‘ON’ position, the circuit from the positive terminal of the battery to the negative terminal is complete. The circuit is then said to be closed and the current flows throughout the circuit instantly.
When the switch is in the ‘OFF’ position, the circuit is incomplete. It is said to be open. No current flows through any part of the circuit.
Explain the working of electric bell.
Electric bell consists of an iron hammer around which a wire is wound. When current is passed by pressing the button, the wire turns into a temporary magnet. The hammer is attracted towards going to make sound. When this strike, the current is disconnected automatically. It happens due to shift arrangement in it. The magnetic property of wire is lost and it comes back to its original position through a spring. In this way, we get repeated hammering, resulting in ringing of an electric bell.
What is an electromagnet? Write its uses.
When current is passed through a coil of wire then the coil behaves like a magnet. One end of coil acts as north pole and other end acts as south pole. When the electric current is switched off, the coil generally loses its magnetism. Such coils are called “electromagnets”. The electromagnets are very strong and their power can be increased so, they are very important.
Two important applications or uses of electromagnets are:
- (i) They are used in electric bells, telegraphs, telephones and speakers etc.
- (ii) They are used in cranes to lift heavy loads to scrap iron or iron sheets.
Class 7 Science Chapter 14 Important Questions Set – 4
Why should we not touch the lighted bulb?
Never touch a lighted electric bulb connected to the mains. It may be very hot and your hand may get burnt badly. In the same way, we should not experiment with the electric supply from the mains or a generator or an inverter. It may get an electric shock, which may be dangerous.
When the current is switched on through a wire, a compass needle kept nearby gets deflected from its north-south position. Explain.
When current is passed through the wire, it deflects the compass near it from its north south position like a magnet. This is called magnetic effect of the current. As we know, the middle of the compass is made up of a thin magnet. When this needle comes in contact with another magnet then the like poles of the magnet repel each other. So, the deflection is seen in the needle. In this case, the wire behaves like a magnet and causes deflection in the needle of the compass.
What are electric fuses? Write its importance.
Fuse is a safety device which protects our houses, appliances and electric circuits electrical hazards at the time of the short circuiting or overloading. The fuse works on the basis of heating effect of the electric current.
Wires made from some special materials melt quickly and break when large electric currents are passed through them. In all buildings, fuses are inserted in all electrical circuits. There is a maximum limit on the current which can safely flow through a circuit. If by accident the current exceeds this safe limit, the wires may become overheated and may cause fire. If a proper fuse is there in the circuit, it will blow off and break the circuit. A fuse is thus a safety device which prevents damages to electrical circuits and possible fires. In this way, it helps in prevention of short circuits.
State the conclusion of H.C. Oersted from his experiments on the deflection of a compass needle when current was passed through the wires around it.
When current was passed through a wire around a compass needle, H.C. Oersted conclude that:
- When an electric current is passed through a wire it behaves like a magnet.
- The magnitude of magnetic field increases with the intensity of current.
- The induced magnetic field is at right angle to the direction of the flow of electric current.
What do you understand by electric circuit?
Electric circuit: The path along which an electric current can flow is called electric circuit. The components of an electric circuit are:
- (i) The cell or battery – The source of electrical energy,
- (ii) The connecting wires or conductors,
- (iii) The bulb – that uses the current,
- (iv) Switch – to close or open the circuit.
Class 7 Science Chapter 14 Important Questions Set –
Distinguish between a bar magnet and an electromagnet.
Bar magnet is a permanent magnet whereas electromagnet is a temporary magnet. The strength of bar magnet cannot be changed whereas the strength of electromagnet can be changed. The uses of bar magnet are limited whereas the uses of electromagnet are more wider.
Can we use the same fuse in a geyser and a television set? Explain.
No, we cannot use same fuse in a geyser and in a television set because the fuse used in every appliance has some limit to withstand the current flowing through it. So, different appliances have fuses of different ratings.
What are CFLs? Why are CFLs preferred over electric bulbs?
An electric bulb is used for light but it also gives heat. This is not desirable. This result in the wastage of electricity can be reduced by using fluorescent tube lights (CFLs) in place of the bulbs. CFLs are also used to reduce wastage of electricity. There are compact fluorescent lamps. These can be fixed in ordinary bulb holders.
Are you familiar with the word MCB?
MCB stands for Miniature Circuit Breaker. These days MCBs are being used increasingly in household electric wiring in place of traditional fuse.
MCB is a special kind of electrical switch which automatically turns off when the current in the circuit becomes too large and cuts off the electricity supply. MCB does not work on the heating effect of current. MCB works on the magnetic effect of current. MCB can be reset in the “on” position when the fault in the wiring has been set right. MCB can be reset manually to restore electricity supply. When the MCB is re-set the circuit becomes complete an electric current starts flowing in the household wiring again.
Why fluorescent tube light and compact fluorescent lamps (CFLs) are expensive?
A fluorescent tube light a lamp contains mercury vapor. The inside of glass tube is coated with an opaque, white material called “phosphor”. When a fluorescent tube light is switch on and current passes through it, the mercury vapor emits invisible ultraviolet radiations. The phosphor absorbs this ultraviolet radiations and glows to emit white light. Though, fluorescent tube lights and CFLs are initially more expensive to buy but there a running cost is much less because they consume much less electricity and they also last much longer.