ELECTRIC CURRENT AND IT’S EFFECTS Class VII Chapter 14

Objectives–

Introduction; Symbols of Electric components in a circuit; Heating effect of current; Magnetic effect of current; Electromagnet; Electric bell

Introduction–

When we rub our dried hairs with a plastic ruler scale or comb and then bring this comb immediately near some small bits of paper, it starts attracting them. Why this happens so? It is because of developing of some static charges on the plastic ruler scale or comb as well as on the dried hairs due to rubbing. Such electricity is called static electricity. We study static electricity under electrostatics. The small bits of papers are attracted toward the charged owing to developing of opposite charge than plastic ruler scale or comb. This force is called as electrostatic force of attraction.

Two Simple electric circuits

Attraction of bits of paper comb due to static charge

 

 

 

 

 

 

 

The bits of papers get attracted owing to electrostatic induction. On the other hand, the charges may move through a conducting wire like copper wire forming current. Current is also called as electricity and current electricity. Current is the flow of charges (or electrons) through a conductor in an electric circuit. In the above figure, a simple electric circuit has been shown, showing glowing of bulb. In this chapter we will try to understand various components of electric circuit and effects of current. Lastly, we will learn about applications of current in daily life. 

Symbols of components in an electric circuit-Symbols are representation of cumbersome components in an electric circuit. By the help of symbols one can easily draw the simple electric circuit. A simple electric circuit has mainly four components. These are- 

1. A Source of current like a dry cell or battery– The symbol of an electric cell is a long line and a short cum thicker line parallel to it. The long line represents positive (+) terminal of the cell while short line represents negative (-) terminal of the cell.

A battery is a combination of two or more cells. In a battery, the positive terminal of one cell remains connected with the negative terminal of other cell either directly as in torch or by wire in other forms of battery. Battery provides more power i.e. current than a single cell.

When switch is ‘ON’ the electric circuit gets completed and current flows from positive terminal to the negative terminal of the cell or battery. The flow of electrons starts.

2. The bulb– It is the resistance or load in the circuit which consumes current. It  is shown with a special symbol.

3. The switch– It controls the supply of current in the circuit. In ‘ON’ position the supply of current occurs but in ‘OFF’ position supply of current stops.

4. Connecting wire-They connect the different components of the circuit. Metal wire is represented by a simple line.

Q. If the voltage (or Potential difference) of a dry cell is 1.5 V (Volt) , what will be the voltage of a battery having 4 cells connected?

Ans– Given that, Voltage of one cell is 1.5 V

         So, Combined voltage of the battery is 1.5 V x 4= 6 V

Heating Effect of Electric Current–

In the above electric circuit when switch is ON the bulb starts glowing because of supply of current. The coil or element of bulb first warms and then starts producing heat and light when temperature becomes sufficiently high. The production of heat by electric bulb and appliances is called heating effect of current. e.g.- C.F.L. (Compact fluorescent lamp), Water heater, Immersion rod, Electric press, Water Geysers etc.

In such appliances the coil or element is prepared by special material like Tungsten, Nichrome (an alloy of nickel and chromium) etc. They have high resistance for flowing current. So, their temperature increases and starts producing heat or/ and light. The amount of heat to be produced by electric appliance depends on three factors-

 (a) The nature of material- Good conductors produce less or negligible heat. Poor conductors and insulators produce more heat. e. g. Silver wire being best conductor of current produces less heat while Tungsten produces more heat.

(b) The length of material- More is the length, more is the resistance in the flow of current. Similarly, less is the length, less is the resistance and current flows easily.

(c) The thickness of material- More is the thickness, less is the resistance and easy is the flow of current. Similarly, less is the thickness; more is the resistance for current flow.

Therefore, wires of different (type) material, length and thickness are used for different purposes.

Electric current is used to generate heat by ‘heating element’ based electric appliances like- heater, electric bulb, hair dryer, immersion rod for heating water, toaster, electric press, glue gun, hair straighter, electric tea kettles, electric girdle, clothes dryers, fridge etc. All these appliances work on principle of heating effect of current. The amount of heat produced by an electric appliance depend upon-amount of current, resistance of conductor or poor conductor and duration of current flow.

      H (Heat Produced) = I².R.t

                    Here, I- represents current

                              R- represents resistance

                    and   t –represents time

 In honor of James Prescott Joule, the S.I unit of energy, heat and work is called Joule (J).

S.N.	HEATING ELEMENT	CONCERNED ELECTRIC APPLIANCE
1	Tungsten metal	Electric Bulb
2	Nichrome wire (Alloy of 80 % Nickel and 20 % Chromium)	Electric heater
3	Nichrome wire	Electric press
4	Mica Sheet	Hair dryer, Electric press and toaster

Electric appliances based on heating effect of current–

1. Incandescent Bulb– It has a filament of tungsten that work as a heating element. It produces light by getting sufficiently hot as well as heat. The thin filament is excessively coiled, so, more resistance for current is caused.  The temperature of filament goes up-to 3300 degree Celsius.

2. Fluorescent tube lights– The bulbs produce heat which is wastage of current. This demerit of bulbs was corrected almost by fluorescent tube lights. The long cylindrical glass rods remain filled with gas. It produces more light and less heat. It works on fluorescence principal. It consumes less current and lasts longer than bulbs.

3. C.F.L. (Compact Fluorescent Lamp) – The bulbs produce heat which is wastage of current. This demerit of bulbs was corrected almost by C.F.Ls. They are concise and compact than fluorescent tube lights. They produce very less heat and more light.

4. Electric Press– It is used for pressing the clothes. It has a heating element that converts current into heat. In some electric press, amount of heat production can be controlled by controlling amount of current flow in it.

Electric safety devices-

When the flow of current in the electric circuit is very high, the electric appliances may get damaged. The high voltage current in the circuit may be due to- short circuit. In short circuit, the positive and negative wires touch each other. So, the amount of current in circuit of houses increases manifold. The person touching may get shock and unfortunate things may occur. To have safety ceramic fuses, electric fuses or M.C.Bs (Miniature Circuit Breakers) are used in the electric circuits

Ceramic Fuse

M.C.B.

Fuse used in Electric Appliances

MAGNETIC EFFECT OF CURRENT– The needle of a magnetic compass is a tiny magnet. When any other magnet is brought near it, the needle gets deflected. This deflection was also seen when the magnetic compass was brought near a current carrying wire. So, from the two events it was found that a magnet is similar to current carrying wire. This effect was called magnetic effect of current. This phenomenon was first described by Hans Christian Oersted.

Hans Christian Oersted (1777-1851)

ELECTROMAGNET-The highly coiled conducting wire having a soft iron that behaves like a temporary magnet on passing the electric current is called electromagnet. When current passed through the wire the soft iron becomes magnetized in the magnetic field. So, the Alpins get adhered. But, as soon as current is switched off the magnetism is lost. Electromagnets are powerful magnets.

Uses-  

1. In cranes for uplifting the heavy goods.

2. For separation of magnetic and non-magnetic materials.

3. In many types of toys.

4. To remove the small pieces of magnetic substances like iron accidentally fallen in the eyes.

ELECTRIC BELL-Electric bell consist of a of  of wire wound on the iron piece. The coil acts as electromagnet on passing current. An iron strip having a hammer at one end is kept close to the electromagnet.  A contact screw is found near the iron strip. The coil becomes electromagnet when the screw is in contact of the iron strip. So, the iron strip is pulled. Due to it the hammer strikes the metallic gong and sound is produced by the bell. When current stops, the coil is no longer an electromagnet. The iron strip return back to its original state. The hammer strikes the gong on passing the current. This process is repeated in quick succession. This is the working of an electric bell.

EXERCISE QUESTIONS –

Q.1 Draw in your notebook the symbols to represent the following components of electric circuits: connecting wires, switch in the ‘OFF’ position, bulb, cell, switch in ‘ON’ position, and battery.

 Ans-

Q.2 Draw the circuit diagram to represent the circuit shown in the fig.14.21

Ans- The Circuit diagram representing the electric circuit in the question is shown as below-

Q.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-

Q.4 The bulb in the circuit shown in figure 14.23 does not glow. Can you identify the problem? Make necessary changes in the circuit to make the bulb glow.

Q.5 Name any two effects of electric current.

Ans- The two effects of electric current are-

(a) Heating effect of current

(b) Magnetic effect of current

Q.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, the current passes through it and a magnetic field is created around it. So, the compass kept near it gets deflected from its north-south direction.

Q.7 Will the compass needle show deflection when the switch in the circuit shown by Fig. 14.24 is closed?

Ans- No, there will not be any deflection in the needle of compass because no current passes through the circuit. There is no source of current in this circuit. There will be no magnetic field in absence of current. Therefore, the magnetic compass will not show any deflection.

Q.8 Fill in the blanks:

(a) Longer line in the symbol for a cell represents it’s ……………..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) becomes hot        (d) electric fuse

Q.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

Q.10  Do you think an electromagnet can be used for separating plastic bags from a garbage heap? Explain.

Ans- No, an electromagnet can’t be used to separate the plastic bags from a garbage heap. It is because the plastic bags are non-magnetic substances. An electromagnet can’t attract them.

Q.11 An electrician is carrying out some repairs in your house. He wants to replace a fuse wire by a piece of wire. Would you agree? Give reasons for your response.

Ans-  The fuse wire has a low melting point. It is an alloy of tin and lead. When excess current passes in the circuit due to short circuit or overload, the fuse wire get heated and melts. Thus, the circuit remains protected.

When the fuse wire will be replaced by a copper wire, the copper wire will not melt when excess current flows through the circuit. As a result the circuit will burn out.

Q.12 Zubeda made an electric circuit using a cell holder shown in figure 14.4, a switch and a bulb. When she put the switch in the ‘ON’ position, the bulb does not glow. Help Zubeda in identifying the possible defects in the circuit.

Ans- The possible reasons for not glowing of the bulb may be-

(a) The correct sequence of the sides of the cells in the cell holder was not followed.

(b) The bulb may be fused.

(c) The cells may be discharged.

Q.13 In the circuit shown in figure 14.25

(i) Would any of the bulbs 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 when switch is in the off position.

(ii) All the bulbs will glow at once.