VAIRAMS MATRIC.HR.SEC.SCHOOL-PUDUKKOTTAI CLASS: VII LESSON-14 SUB: Physical Science

ELECTRIC CURRENT AND ITS EFFECTS

I. Multiple choice Questions: A) Select the correct option. 1. A coil refers to ______a) an electric wire b) a fuse c) a current carrying conductor d) a wire twisted in the form of a circle

2. The effect of current on which the working of a fuse is based is ______a) magnetic effect b) chemical effect c) heating effect d) induction effect

3. A filament of low melting point metal/alloy is used in ______a) electric bulb b) electric iron c) heating effect d) electric fuse

4. A circuit diagram is ______a) a picture of a circuit b) a drawing of a circuit with standard pictures for the different electrical components. c) a drawing of an electrical circuit with standard symbols for the different electrical components d) a difficult representation of an electric circuit.

5. In an electric bell, we have ______a) an b) an interrupter c) a hammer d) all of these 6. An electric fuse is ______a) a safety device b) an appliance c) used to produce electricity d) used to heat a room 7. An electromagnet acts like a magnet ______a) When a current is passed through the coil. b) all the time c) because it has a d) only if a current does not pass through the coil. 8. An electric fuse wire melts if the amount of current flowing through it is a) more than a minimum amount b) less than a minimum amount c) more than a maximum amount d) less than a maximum amount B. Give one word answer to the following questions. 1. The effect produced in a current carrying wire – Heating effect. 2. The principle on which the working of an electric bell depends – Magnetic effect. 3. The unbroken path through which an flows – closed electric circuit. 4. A safety device which prevents excessive current from flowing in a circuit – Electric fuse. 5. The effect of electric current, which electric irons, room heater and water heaters use. – Heating effect. 6. A plece of wire twisted (or wrapped) to form a circle – coil. 7. A drawing of an electrical circuit with standard symbols – Circuit diagram. 8. The effect of electric current when an electric current passes through a conductor and it behaves as a magnet – Magnetic effect. 9. An automatically operated electric switch that prevents and electric circuit during an overload or short circuit – Miniature circuit Breaker (MCB) 10. A combination of two or more cells. - Battery

C. Fill in the blanks: 1. When current is switched ‘on’ in a room – heater, it becomes hot 2. The safety device based on the heating effect of electric current is called electric fuse. 3. Longer line in the symbol for a cell represents its positive terminal. 4. The combination of two or more cells is called a battery. 5. The filament of a bulb is made from an element having high melting point. 6. If a live wire of an electric circuit happens to come in contact with neutral wire, it is called short circuit. 7. The overheating of electrical wires in any circuit due to the flow of large current is called over loading. 8. All electrical heating appliances are fitted with a heating coil made from nichrome wire. 9. Nichrome has high resistance.

D. State True (T) or False (F) against each of the following statements. 1. An electric bell has an electromagnet. T 2. To make a battery of two cells, the negative – terminal of one cell is connected to the negative terminal of the other cell. F 3. An electromagnet does not attract a piece of iron. F 4. When the electric current flowing through a fuse exceeds a certain limit, the fuse wire melts and breaks. T 5. Electrical fuse is based on the magnetic effect of current. F 6. All electrical circuits always contain at least one dry cell. T 7. Excessive overloading of an electrical circuit for a long period may lead to fire. T 8. Greater is the current in the wire, hotter it will be. T 9. Circuit diagram is a drawing of an electrical circuit with standard symbols. T E. Match the following: Column – A Column – B 1. Melts easily - a) Fuse wire 1 2. Circuit diagram - b) Fuse 3 3. Safety device - c) A drawing of an electrical circuit with standard symbols. 2

4. Short circuit - d) The overheating of electrical wiring in any circuit due to the flow of a large current.5

5. Over loading. - e) The circuit in which electrical contact at any point is broken. 7

6. Miniature circuit Breaker - f) A sudden flow of very large current due to direct contact of a live and neutral wire. 4

7. Open circuit - g) In an open circuit. 8 8. No current flows - h) Trips and the supply of electricity is cut off 6

Short answers: 1. Differentiate between Electrical circuit and circuit diagram.

Electrical circuit Circuit diagram An arrangement for A diagram in which a obtaining electric current in a complete electric circuit is conductor by connecting it to shown by the use of standard a cell or battery is known as symbols is known as circuit circuit. diagram

2. are temporary magnets – Give reason. An electromagnet is a coil of wire wound on an iron core. It works like a magnet only when the electric current passes through it. When the electric current stops, it becomes a simple iron core. Thus it is only a temporary magnet. 3. Define an electrical circuit. The path along which an electric current flows is called an electric circuit. 4. List two uses of an electromagnet. i) Electromagnets are used in the hospitals for treating eye-injuries in which iron splinters are removed from the eyes. ii) An electromagnet is used in a loudspeaker. 5. Name three appliances that use the heating effect of electric current. Appliances that use the heating effect of electric current. i) Room heater ii) Electric iron iii) Glowing bulb 6. Define open circuit. If the electric path which starts from the positive terminal of a cell or battery is broken at some point, it is called an open electric circuit. 7. State the reason: a) The handle of electricians screw driver is made of plastic. b) Copper wires are usually covered with rubber or plastic. a) The handle of electricians screw driver is made of plastic because plastic is an insulator of electricity. Thus current cannot flow through plastic. b) Copper wires are usually covered with rubber or plastic because the bare wires will come in contact with each other. This will cause electric sparking and hence an electric fire. 8. The filament of bulb is made from a very thin high resistance tungsten wire – Give reason. The filament of a bulb is made from a very thin high resistance tungsten wire because it has high melting point so it cannot melt at high temperature as required for emitting yellowish white light. Long answer: 9. Describe the working, construction, basic principle of an electric bell. Electric bell: An electric bell has different parts as – electromagnet, , contact spring, contact screw adjustment, a hammer and gong. Principle of electric bell: An electric bell works on the principle of magnetic effect of current. Working: When the switch of the bell is pushed, the electric current starts to flow toward the electromagnet. Thus the magnet attracts the soft iron armature. the hammer attached to the strip then hits the gong, causing a ring. When iron strip gets attracted towards the electromagnet, it no longer touches the screw and hence, the circuit is broken. The hammer stops touching the gong. Thus the same process goes on regularly.

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