Electric Current | Class VII
Electric Current: The flow of electric charge is called electric current. In an electric circuit, the charge is often carried by moving electrons. The SI unit of electric current is Ampere (A). The SI unit of electric charge is Coulomb.
Electric Circuit: The path through which electric charge moves is called electric circuit.
Symbols in Electric Circuit: Different components of electric circuit are shown by certain symbols. Use of symbol makes it easy to represent an electric circuit. Use of standard symbol makes it easy to understand for anybody. For example; a TV mechanic can identify different components of a TV by looking at its circuit diagram.
Electric Cell: It is a source of electric current to run a number of devices in everyday life. The cell which is used in a torch is called dry cell. The wet cells are used in car batteries. A normal dry cell gives an output of 1.5 Volt (V).
Battery: A group of cells is called battery. More than one cell is used in most of the devices; because the device needs more than 1.5 V of power.
Closed Circuit: When the circuit is complete, it is called closed circuit. Current flows only in a closed circuit.
Open Circuit: When the circuit is incomplete, it is called open circuit. Current does not flow in an open circuit.
Question 1: What is the SI unit of electric current?
Question 2: What is an electric cell?
Answer: It is a source of electric current to run a number of devices in everyday life.
Heating Effect of Electric Current
When electric current flows through a conducting wire, the temperature of wire increases. This is called heating effect of electric current.
Devices which work on Heating Effect of Electric Current: When an electric current passes through a high resistance wire (like nichrome wire), the electrical energy is converted into heat energy which heats up the resistance wire. This is called the heating effect of current. The amount of heat produced in a wire due to the heating effect of current depends on two factors-
a) Resistance of material and
b) Magnitude of current passed through the wire.
For e.g.-Electric iron, water heater, geyser, toaster, etc. are some other devices which work on heating effect of electric current.
Elements of Heating Appliances: A heating appliance has a highly coiled wire or a metallic rod. This is called the element of the heating appliance. The highly coiled structure increases the surface area and thus provides more heat. The elements are usually made of constantan which is a metal with very high melting point.
Electric Fuse: Electric fuse is a safety device which is used in household wirings and in many appliances. Electric fuse has body made of ceramic and two points for attaching the fuse wire. The fuse wire melts whenever there is overload in the wiring. This breaks the circuit and helps in preventing damage to costly appliances and to the wiring. In electrical devices a glass fuse is often used. This is a small glass tube, in which lies the fuse wire.
MCBs (Miniature Circuit Breaker): MCBs have been replacing electric fuse from wirings at most of the places. The electric fuse has a big practical problem. Whenever the wire fuses, one needs to replace the wire to resume electric supply. More often than not, this proves to be a cumbersome task. Miniature circuit breakers break the circuit automatically. One just needs to switch it on to resume the electric supply. Many models of MCBs have a built in mechanism by which the electric supply is automatically resumed.
Question 1: The electric bulb works on which property of electric current?
Answer: Heating effect of electric current.
Question 2: What is the full form of MCB?
Answer: Miniature Circuit Breaker
Magnetic Effect of Electric Current:
Hans Christian Oersted (1777 – 1851) was the first to show that electric current also produces magnetic effect. He kept a magnetic compass near a current carrying conductor. It was observed that the magnetic compass shows deflection, when the current passes through the conductor.
Electromagnet: Magnetic effect of electric current has been used in making powerful electromagnets. For this, wire is wrapped around an iron rod in many turns and electric current is supplied to the wire. As long as the current is supplied the iron rod behaves like a magnet. More number of turns makes more powerful magnet. Electromagnet is used in electric bells and also in powerful cranes.
Electric Bell: Electric bell is composed of two rods of cast iron. Coil is made around the iron rods. A metallic strip is placed parallel to the coils. The metallic strip is fitted with a hammer at one end. Another end of the strip is connected to the circuit. A gong is placed in a position so that it can be hit by the hammer. When current flows in the circuit, the cast iron rods become electromagnet and attract the metallic strip. The metallic hammer hits the gong because of that. Once the metallic strip is pulled towards the electromagnet, it gets disconnected from the point and there is a break in the circuit. A spring pulls the metallic strip back because no magnetism is left in the electromagnet. This cycle continues and the electric bell produces a ringing sound.
Question 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.
Question 2: Draw the circuit diagram to represent the circuit shown in figure. (See textbook for question figure)
Question 3: The given figure (in textbook) 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.
Question 4: The bulb in the circuit shown in given figure (textbook) does not glow. Can you identify the problem? Make necessary changes in the circuit to make the bulb glow.
Answer: The positive terminals of both the cells are together and that is why the bulb is not glowing. The cells should be kept in a way that the positive terminal of one cell is near the negative terminal of another cell; to make the bulb glow.
Question 5: Name any two effects of electric current.
Answer: The two effects of electric current are; heating effect and magnetic effect.
Question 6: When the current is switched on through a wire, a compass needle kept nearby gets deflected from its north-south position. Explain.
Answer: This happens because when electric current passes through a conducting wire, a magnetic field is created around the wire. The magnetic field causes deflection in the magnetic compass.
Question 7: Will the compass needle show deflection when the switch in the circuit shown is closed?
NCERT Solution Part 2
Question 8: Fill in the blanks:
- Longer line in the symbol for a cell represents its …………..terminal.
- The combination of two or more cells is called a ……………
- When current is switched ‘on’ in a room heater, it ……………….
- The safety device based on the heating effect of electric current is called a ……………
Answer: (a) Positive, (b) Battery, (c) heats up, (d) electric fuse
Question 9: Mark ‘T’ if the statement is true and ‘F’ if it is false:
- To make a battery of two cells, the negative terminal of one cell is connected to the negative terminal of the other cell.
- When the electric current through the fuse exceeds a certain limit, the fuse wire melts and breaks.
- An electromagnet does not attract a piece of iron.
- An electric bell has an electromagnet.
Answer: (a) F, (b) T, (c) F, (d) T
Question 10: Do you think an electromagnet can be used for separating plastic bags from a garbage heap? Explain.
Answer: Since plastic is a non-magnetic material, so electromagnet cannot be used to separate plastic bags from a garbage heap.
Question 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.
Answer: A special wire with low melting point should be used for the fuse wire and any other wire would not serve the purpose. Hence, I would not agree with the electrician.
Question 12: Zubeda made an electric circuit using a cell holder shown in the given figure (textbook), 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.
Answer: There can be following possibilities:
Question 13: In the circuit shown in the figure
(a) Would any of the bulb glow when the switch is in the ‘OFF’ position?
(b) What will be the order in which the bulbs A, B and C will glow when the switch is moved to the ‘ON’ position?
a) None of the bulbs would glow.
b) All the bulbs would glow at once.