Chapter 12: Electricity and Circuits

    I. Multiple Choice Questions (MCQs).
    Q.1. Electricity in motion is called
    a) electric current
    b) electric energy
    c) electrostatics
    d) none of these
    Answer: a) electric current
    Q.2. A combination of two or more cells forms a
    a) electric source
    b) series of cells
    c) battery
    d) none of these
    Answer: c) battery
    Q.3. The metal cap at one end of the cell acts as a
    a) negative terminal
    b) positive terminal
    c) either (a) or (b)
    d) none of these
    Answer: (b) positive terminal
    Q.4. The filament of an electric bulb is made of
    a) Tungsten
    b) copper
    c) nichrome
    d) manganin
    Answer: (a) tungsten
    Q.5. The material that allows electricity to pass through it, is called
    a) insulator
    b) electric path
    c) conductor
    d) none of these
    Answer: (c) conductor

    II. Fill in the blanks by selecting appropriate words from the given list :
    List : Cells, path, power, chemicals, filament

    Q.1. The electric current that is supplied to our homes come from _______________ stations.

    Q.2. _______________ are the common source of electric current in transistor, radio, torch, alarm clocks, wristwatches and other many devices.

    Q.3. An electric cell produces electricity from the _____________ stored inside it.

    Q.4. In an electric bulb, electric current enters ____________ through one terminal and passes out through other terminal.
    Q.5. The _________ made for electric current to flow is known as the electric circuit.

    Answer: 1- power,
    2- Cells,
    3- chemicals,
    4- filament,
    5- path
    III. Match the items of Column A with those of Column B.

    Answer: 1-(c), 2-( d), 3- (b), 4-(e), 5-(a)

    IV. Write ‘T’ for the true and ‘F’ for the false statements.
    Q.1. Electricity is very useful and helpful, but it can be very dangerous if not handled properly.
    Q.2. Never connect the both terminals of a cell without connecting an appliance in a circuit, this will destroy the cell.
    Q.3. In open circuit, current can flow and the bulb glows.
    Q.4. An electric bulb is filled with hydrogen gas.
    Q.5. The non-metal graphite is good conductor of electricity.

    Answer: 1-T, 2-T, 3-F, 4-F, 5-T

    V. Very Short Answer Questions.
    Q.1. Define battery.
    Answer: A combination of two or more cells is known as battery.

    Q.2. How are the positive and negative terminals of a cell defined?

    Answer: The metal cap at one end of the cell acts as positive terminal, whereas flat metal base acts as negative terminal.

    Q.3. What is a source of electricity in a torch?

    Answer: The cells inside the torch act as source of electricity.

    Q.4. Write two conductors of electricity.
    Answer: Copper and Silver
    Q.5. Write two insulators of electricity.
    Answer: Glass and rubber

    VI. Short Answer Questions.

    Q.1. Write the principle on which a torch works.
    Answer: The principle on which a torch works is very simple. Cells inside it are connected
    through a switch to a small bulb. When you push the switch to the ‘on’ position, the bulb glows.

    Q.2. What materials are needed to make an electric bulb glow?
    Answer: To make an electric bulb glow, you need two wires, cell and a torch bulb.

    Q.3. Define closed and open electric circuits.

    Answer: When both wires of the cell are connected to the bulb in a proper way, in other words, circuit is complete. Such a circuit is called a closed circuit. In closed circuit, current flows and, as a result, the bulb glows.

    When in a both the wires are not connected in a proper way the circuit is not complete, and it is called an open circuit. In open circuit, current cannot flow and the bulb does not glow.

    Q.4. Describe construction and principle of an electric cell.
    Answer: Cell is the common source of electric current in transistor, radio, torch, alarm clocks, wristwatches and other many devices. A cell has a positive (+) sign close to one end and a negative (–) sign close to the other. They are the positive and negative terminals respectively. The metal cap at one end of the cell acts as positive terminal, whereas flat metal base acts as negative terminal.
    An electric cell produces electricity from the chemicals stored inside it.

    Q.5. Describe construction and working of an electric bulb.

    Answer: An electric bulb is made up of a spherical glass case fixed on a metallic base. If you look closely, you will find a thin coiled wire inside the bulb. This wire is called filament and it is made up of tungsten. It is this filament which glows when a bulb is switch on. The filament is supported by two thick wires on two sides. Out of these wires, one is connected to a metal piece at the tip of the base and the other is connected to the metallic base. These form the two terminals of the bulb. Electricity passes through one terminal, flows through the filament and passes out through the other terminal. When electricity flows through the tungsten wire, it gets heated up and glows.

    Q.6. Discuss possible closed circuits in which bulb glows and also possible open circuits in which bulb does not glow. Give your opinion based on facts.

    Answer:

    The bulb will glow only when its terminals and the terminals of the cell form a complete loop or path. It does not matter which terminal of the bulb is connected to the negative terminal of the cell and which to the positive terminal. The bulb does not glow either the terminals of the bulb are connected to the same terminal of cell or there is a break in the path.

    VII. Long Answer Questions.
    Q.1. (a) What is an electric circuit? Explain its kinds with suitable examples.
    (b) What is an electric switch? Discuss how to make a simple electric switch.

    Answer(a) The ‘path’ made for electric current to flow is known as the electric circuit.

    There are two type of electric circuit close circuit and open circuit.

    Close circuit: In figure below, both wires of the cell are connected to the bulb in a proper way. In other words, circuit is complete. Such a circuit is called a closed circuit. In closed circuit, current flows and, as a result, the bulb glows.

    Open circuit: In figure below one wire of the cell is connected to the bulb, while the other wire is not. In the other words, circuit is not complete. Such a circuit is called an open circuit. In open circuit, current cannot flow and the bulb does not glow.

    b) A switch is an ON/OFF device helps controlling the flow of electricity. It works by making or breaking a circuit.

    To make a simple switch, following things are required.
    Two drawing pins, a safety pin, a small sheet of thermocol and three pieces of wire.
    Procedure : Take a piece of thermocol and place the safety pin on it. Remove plastic covering from ends of two wires. Connect the one end of each wires to two drawing pins. Push the drawing pin through the end loop of the safety pin into the thermocol. Make sure that the safety pin can be rotated freely.
    Push the second drawing pin into the cardboard about 1 cm apart from the first pin. Ensure that the safety pin held in place by first drawing pin can be turned to touch the other pin. Now switch is ready for use.
    Working of the switch : Connect the free end of one wire to one terminal of the cell and connect the free end of other wire to one terminal of the bulb. Now connect the cell and the bulb with the help of third wire.

    Rotate the safety pin such that its free end touches the other drawing pin. Bulb glows with a bright light. This position of switch is called ON position.

    Now move the safety pin in such a way that it was not in touch with the other drawing pin. The bulb does not glow. This is because in this condition the circuit is not complete and there is a gap between two pins. This position of switch is called ‘OFF’ position.

    Q.2. What are electric conductors and insulators? Explain with an experiment that how you can identify conductors and insulators from a given set of materials.

    Answer: The materials that allow electricity to pass through them, are called conductors.
    The materials that do not allow electricity to pass through them, are called insulators.

    Experiment to identify conductors and insulators from a given set of materials.

    Things Required: Coin, cork, rubber, glass, key, pins, plastic scale, wooden block, pencil lead, aluminium foil, candle, paper, etc., three pieces of wire, a small torch bulb, cardboard, cellotape and cell.
    Procedure : Connect and make the circuit to test the materials such as key as shown in figure. Touch the free ends of wires to the ends of various objects. If the bulb glows, it is a conductor. If it does not, it is an insulator.

    Observations are tabulated in table below:

    From the table, it is clear that key, coin, pins, pencil lead and aluminium foil are conductors and other are insulators.

    Q.3. What do you mean by electric torch? Explain with an experiment that how you can find out source of electricity in a torch.

    Answer: A torch is an electric device which use electric cells to produce light at home in emergency.
    Experiment to find out the source of light in a torch. Requirements: An electric torch containing cells.

    Procedure: Take an electric torch containing cells. Now push the button to the ‘on’ position. You observe that the bulb glows. Push the button to the ‘off’ position, Open the case and remove the cells. Again, push the button to the ‘on’ position. The bulb does not glow. Push the button to the ‘off’ position and insert cell again. By closing the case, push the button to the ‘on’ position. Bulb glows again. This activity clearly proves that cells are the main source of light in a torch.

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