1. Electric Circuit-
Figure 01- Circuit
ü A continuous conducting path (that has wires, bulb, switch, etc.) between the two terminals of a cell or battery along with an electric current flows, is known as an electric circuit.
ü For example, take a cell having a positive terminal (+) and a negative terminal (-). Now try to connect the positive terminal of the cell to one end of the switch with a piece of copper wire and other ends of the switch to one end of bulb holder with another piece of copper wire.
ü The negative terminal of the cell is connected directly to the other end of the bulb holder with a wire (as shown in the figure), so this kind of setup is known as an electric circuit.
2. Circuit Diagram-
Figure 02- Components of circuit and their symbols
ü A circuit diagram tells us how the various components in an electric circuit have been connected by using the electrical symbols of the components.
ü When the bulb glows In an electric circuit when the switch is closed, then the switch is said to be in the ON position. And when the switch in a circuit is open, then the switch is said to be in the OFF position.
ü So, in an electric circuit, a bulb lights up only when the switch is in the ON position and at that time, we can say that the electric circuit is complete because the current flows throughout the circuit instantly (as shown in the figure) electric circuit.
ü When the bulb does not glow While checking the circuit notice that sometimes the bulb does not glow even when the switch is in the ON position. This condition can occur only if the bulb gets fused, i.e. its filament breaks. It is a difficult and time-consuming job to draw the electric circuit by making the actual drawings of cell, battery, switch, bulb, etc.
ü So, the scientists have devised a very easy way to represent the component of an electric circuit (cell, battery, switch, bulb, etc.) with the help of symbols, which are easy to draw.
3. Electric Cell and Battery-
ü The common source of electricity to run the number of devices, e.g. torches, radio, electric clocks and watches, toys, etc., is an electric cell, but sometimes a single cell is not sufficient to run many devices as they require high voltage to run all these appliances.
ü The cell provides much less electricity as compared to that provided by the electric supply line, e.g. in the case, a single electric cell which provides only 1.5 V of electricity, whereas electricity from the power station is supplied to our home at very high voltage of 220 V.
Figure 03- Combination of electric cells
ü Since the higher voltage can be obtained by combining a number of cells in series. So, when the positive terminal of one cell is joined with the negative terminal of the other cell, then the cells are said to be joined in series (as shown in figure given below).
ü So, a battery can be defined as the group of cells joined together in the series, e.g. in a torch, the cells are placed one after the other. But in many devices, cells are not placed one after the other, e.g. in a TV remote control, the two cells are placed side by side (or parallel to each other) instead of single one. So, two or more cells connected in side by side manner are said to be joined in parallel. This combination is also known as battery.
ü If we combine two cells by keeping the positive terminal of one cell in contact with the positive terminal of the other cell or negative terminal of one cell in contact with the negative terminal of the other cell, then the battery obtained will not work.
ü The batteries which are used in cars, buses, trucks, inverters, etc., are also made of cells. There is a special feature of car battery is that its cells can be recharged. Ordinary cells, however, cannot be recharged.
ü ‘+’ and ‘-‘ symbols are printed in the battery in order to have an exact placement of the cells in their respective battery compartment.
n The switch or key can be placed anywhere in the circuit.
n The circuit is complete and it is said to be closed only when the switch is ON.
n The circuit is incomplete and it is said to be open, only when the switch is OFF.
n There is a thin wire in the bulb, called filament which glows when an electric current passes through it. So, if the bulb gets fused, then its filament gets broken.
Note- Never touch a lighted electric bulb connected to the mains as it may be very hot and can damage your hands.
4. Heating Effect of Electric Current-
ü Production of heat in an electric device due to the flow of electric current is called the heating effect of electric current.
ü We have seen an electric heater used for cooking, an electric bulb or room heater. So, when these appliances are switched ON after connecting to the electric supply, then their elements become red hot and release the heat. This happens due to the heating effect of electric current.
ü The degree to which a material opposes the passage of current through itself is known as its resistance.
ü When an electric current passes through a high resistance wire, the electric energy gets converted into heat energy and this heat energy heats up the wire.
ü Factors on which the heating effect of current depends are-
n Resistance of wire- Greater the resistance of a wire, greater will be the heat produced in it by a given current, e.g. if we choose two wires, one of copper and other is nichrome of equal length and equal thickness and pass them the equal amount of current through them for the same duration, then we will notice that nichrome wire will become hotter in comparison to the copper wire. It is due to the reason that the resistance of nichrome wire is more than that of the copper wire. That is why the nichrome wire is used to make heating elements of electric heating appliances such as electric room heater, electric iron, etc. The resistance of a wire depends on the material of the wire, length of wire and thickness of the wire.
n Magnitude of current passed through a given wire- If the magnitude of current passed through a given wire is greater, then the heat produced in it will also be greater, e.g. if a normal amount of current flows through the copper wires of household electric wiring, then the wires do not become much hot but if a large current flows in the same wiring accidentally, then the wires become extremely hot and a fire may be started.
5. Applications of the Heating Effect of Current -
ü For the production of light, the heating effect of electric current is utilised in the electric bulbs.
ü For the working of electrical heating appliances such as water heater, electric room heater, electric iron, etc., the heating effect of electric current is utilised.
ü The heating effect of electric current is utilised in a safety device called ‘electric fuse’.
ü Electric Current and Its Effects Class 7 Notes Science Chapter 14 5
ü Compact Fluorescent Lamps (CFLs)
ü An electric bulb is basically used for producing light but it also releases the heat which is not desirable because a major part of the electricity consumed by the filament of a bulb is converted into heat and results in the wastage of electricity. So, this wastage can be decreased by using fluorescent tube light or CFLs (as shown in the figure) in place of the bulbs.
ü However, before purchasing bulbs, tubes orCFL’s we should look for the ISI marl? (ISI- Bureau of Indian standard). It is because the ISI mark ensures that the appliance is safe and wastage of energy is minimum.
6. Element -
ü All electrical heating devices consist of a coil of wire called an element.
ü When these appliances are switched ON after connecting to the electric supply, then their elements become red hot and release the heat.
ü There are some electric appliances such as immersion heaters, hotplates, irons, geysers, electric kettles, hair dryers, etc., which have elements inside them.
7. Electric Fuse -
Figure 04- Fuse
ü A safety device which works on the heating effect of current and prevents electric fires or damage to electrical appliances due to excessive flow of current is known as the fuse. This safety device consists of a short length of a thin wire of tin plated copper having a low melting point and this wire has a much greater resistance than the rest of the electric wiring in the house.
ü So, due to this, if the current in the electric wiring suddenly increases too much, then the fuse wire gets heated, then it melts and breaks the circuit which means that the current flowing in the household circuit will stop.
ü This prevents fire in the house and also the damage of various electrical appliances like refrigerator, fans, tube lights, TV, etc. When a fuse gets blown (breaks), a new fuse has to be fitted in its place in order to restore the electricity supply in the household circuit.
8. Overloading -
ü It is a situation when too many electrical appliances are connected to a single socket, they draw an extremely large amount of current from the household circuit.
ü The flow of large current due to overloading may heat the copper wires of household wiring to a very high temperature and fire may be started.
9. Short Circuit-
ü Electric current is supplied by household through two insulated wires which run together and reach each and every electrical appliances.
ü One insulated wire is called live wire and the other insulated wire is called neutral wire and both these wires are necessary for the working of an electrical appliance (say an electric iron).
10. Miniature Circuit Breakers (MCBs) -
Figure 05- MCB
ü This ore increasingly being used these days in place of fuses. MCB does not work on the heating effect of current as it works on the magnetic effect of current.
ü These switches automatically turn OFF when the current in a circuit exceeds the safe limit. We turn them ON and the circuit ¡s once again completed.
11. Magnetic Effect Of Electric Current -
Figure 06- Deflection of compass needle due to magnetic field
ü If the electric current passes through a wire, then the current carrying wire behaves like a magnet. This phenomenon is known as the magnetic effect of current.
ü It was discovered by a scientist Hans Christian Oersted who found that when an electric current is passed in a wire, then the compass needle placed near it got deflected from its usual North-South position.
ü A straight wire carrying an electric current produces a magnetic effect.
ü The magnetic effect is increased only if we use a long coil of wire instead of a straight wire.
ü Even further the magnetic effect is increased if the coil of wire is wound around an iron rod and then current is passed through it.
12. Electromagnets -
ü It is a magnet made by using electric current. An electromagnet works on the magnetic effect of current. An electromagnet consists of a coil of insulated wire wrapped around a piece of iron which is magnetized only when an electric current is passed through the coil.
ü This magnet consists of a long coil of insulated copper wire wound around an iron rod and when the two ends of the coil get connected to a cell, then a current passes through the coil and produces a magnetic effect. The magnetic effect magnetizes the iron rod. In this way, the iron rod becomes an electromagnet.
ü The magnetism of an electromagnet remains as long as the current is flowing in its coil. So, if we switch OFF the current in the coil, then all the magnetism of the iron rod disappear and it will no longer behave like a magnet.
ü There are two factors through which an electromagnet can be made stronger viz-
n By increasing the amount of current used in the coil.
n By increasing the number of turns forming the coil.
13. Applications Of Electromagnets-
ü These magnets are used in electrical appliances such as an electric bell, electric fan, electric motor.
ü These magnets have their utilization in electric generators where the very strong magnetic field is required.
ü For deflecting electron beam of the picture tube of TV electromagnets are used.
ü For the magnetic separation of iron ores from the earthly substances, electromagnets are used.
ü For preparing strong permanent magnets, electromagnets are used.
ü Advantages of Electromagnets over Permanent Magnets
ü An electromagnet is a temporary form of the magnet because its magnetism is only for the duration of current flowing in its coil. Actually, an electromagnet is better than a permanent magnet in many respects.
ü There are some of the advantages of the electromagnets over the permanent magnets which are stated as follows-
n The magnetism of an electromagnet can be switched ON or switched OFF as desired. While it is not possible with a permanent magnet.
n By increasing the number of turns in the coil and by increasing the current passing through the coil an electromagnet can be made very strong. On the other hand, a permanent magnet cannot be made so strong.
14. Electric Bell-
Figure 07- Bell
ü An electric bell works on the magnetic effect of current. It has an electromagnet in it. Let us study its construction and working as well.
ü Construction of electric bell -
n The electric bell has a U-shaped electromagnet.
n There is a small iron bar called armature which is held in front of the poles of the electromagnet.
n The lower end of the iron bar is attached to a flat spring and the flat spring is itself fixed to a metal bracket.
n The upper end of the iron bar has a clapper attach to it. A metal gong is fixed near the clapper.
ü Working of electric bell-
n In order to ring the bell, first of all, we press the push button switch in order to ring the bell.
n So, when we press the switch, then the electric circuit of the bell is completed and a current passes through the coil of the electromagnet and it gets magnetized.
n The electromagnet attracts the iron armature towards itself.
NCERT PRACTICE QUESTIONS
1. Draw in your notebook the symbols to represent the following components of electrical circuits: connecting wires, switch in the ‘OFF’ position, bulb, cell, switch in the ‘ON’ position, and battery
Ans.-
2. Draw the circuit diagram to represent the circuit shown in Fig.14.21.
Ans.-
3. Fig.14.22 shows four cells fixed on a board. Draw lines to indicate how you will connect their terminals with wires to make a battery of four cells.
Ans.-
4. The bulb in the circuit shown in Fig.14.23 does not glow. Can you identify the problem? Make necessary changes in the circuit to make the bulb glow.
Ans.- In the circuit above bulb is connected on either side.
5. Name any two effects of electric current.
Ans.- i) Heating effect of electric current ii) Magnetic effect of electric current
6. When the current is switched on through a wire, a compass needle kept nearby gets deflected from its north-south position. Explain.
Ans.- When the current is switched on through a wire, magnetic field is created around it hence we see deflection in the compass needle kept nearby.
7. Will the compass needle show deflection when the switch in the circuit shown by Fig.14.24 is closed?
Ans.- No , compass needle does not show deflection when the circuit is a closed, magnetic field is not created until current is flowing through the circuit.
8. Fill in the blanks:
(a) Longer line in the symbol for a cell represents its terminal.
(b) The combination of two or more cells is called a .
(c) When current is switched ‘on’ in a room heater, it .
(d) The safety device based on the heating effect of electric current is called a .
Ans.- (a) positive (b) battery. (c) produces heat . (d) fuse .
9. Mark ‘T’ if the statement is true and ‘F’ if it is false:
(a) To make a battery of two cells, the negative terminal of one cell is connected to the negative terminal of the other cell. (T/F)
(b) When the electric current through the fuse exceeds a certain limit, the fuse wire melts and breaks. (T/F)
(c) An electromagnet does not attract a piece of iron. (T/F)
(d) An electric bell has an electromagnet. (T/F)
Ans.- a) False b) True c) False d) True
10. Do you think an electromagnet can be used for separating plastic bags from a garbage heap? Explain.
Ans.- No, because plastic does not have magnetic property to get attracted to a magnet hence magnet cannot be used to separate plastic bags.
11. An electrician is carrying out some repairs in your house. He wants to replace a fuse by a piece of wire. Would you agree? Give reasons for your response.
Ans.- It is not a wise idea to replace fuse by a piece of wire, as it has very low melting point. In case of metal piece, melting point will be high and the circuit will be intact in case there is overload or overheat.
12. Zubeda made an electric circuit using a cell holder shown in Fig. 14.4, a switch and a bulb. When she put the switch in the ‘ON’ position, the bulb did not glow. Help Zubeda in identifying the possible defects in the circuit.
Ans.- i) The connecting wire may be loose ii) The electric cell may be used up iii) switch may not be functioning well iv) cell power has been exhausted
13. In the circuit shown in Fig. 14.25
(i) Would any of the bulb glow when the switch is in the ‘OFF’ position?
(ii) What will be the order in which the bulbs A, B and C will glow when the switch is moved to the ‘ON’ position?
Ans.- i) No, the bulb will not glow as the circuit is not complete when the switch is off ii) If the switch is On, all the bulbs glow simultaneously.
Junior Science Topic - Electric Current And Its Effects. Chapter notes, video lectures, power point presentation and NCERT exercise solution. (Class 7, CBSE syllabus, chapter 14)
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