Thursday, September 13, 2012

CBSE Class 6 - Science - CH 12 - Electricity and Circuits

Electricity and Circuits

NCERT Chapter Solutions and Q & A

Q1. Fill in the blanks :
(a) A device that is used to break an electric circuit is called _______________.
(b) An electric cell has _______________ terminals.
(c) Electric cell is a device which converts energy of chemicals into ________________energy.
(d) The electric energy which is supplied in our home comes from electric  _______________houses.
(e) An electric ______________ is a continuous path along which the current flows.
(f) If the filament of a bulb breaks, it is said to be ______________.
(g) Rubber is a good example of electric ______________.
(a) switch
(b) two terminals
(c) electrical
(d) poles nearby
(e) circuit
(f) fused
(g) insulator

Q2: Mark 'True' or 'False' for following statements:

(a) Electric current can flow through metals. (✓ True)
(b) Instead of metal wires, a jute string can be used to make a circuit. (✗ False)
(c) Electric current can pass through a sheet of thermocol. (✗ False)
(d) When current flows through a circuit, the circuit is called open circuit. (✗ False)
(e) An electric bulb has one terminal (✗ False)

Q3: What is an electric cell?

Answer: An electric cell is a device which converts chemical
energy into electric energy. It has two different metal plates:
  • one is the positive terminal
  • the other is the negative terminal
These plates are kept inside a chemical called electrolyte, which stores the chemical energy.

Q4: Explain why the bulb would not glow in the arrangement shown in Fig. below:

Answer: The handle of the screw driver acts as an insulator. It does not allow current to flow and circuit is still open. Therefore the bulb does not glow in the arrangement.

Q5: Complete the drawing shown in Fig  below to indicate where the free ends of the two wires
should be joined to make the bulb glow. 

Answer: The bulb will glow when circuit is complete i.e. one end of the wire from clip is connected to positive terminal. Second end of the wire from the clip is connected to the bulb.

Q6: Would the bulb glow after completing the circuit shown in Fig. above (Q5) if instead of safety pin we use an eraser?

Answer: No. An eraser is made of rubber which is a poor conductor of electricity. Therefore, bulb would not glow.

Q7: What is the purpose of using an electric switch? Name some electrical gadgets that have switches built into them. 

Answer: An electric switch is a simple device which is used to break or complete an electric circuit. Switches are widely used in different electrical or electronic gadgets. A few are:
  • Toggle switches used in houses.
  • Pushbutton switches used in microwaves, water pump, mixer etc.
  • Joystick switches used in Video Games Controllers, Toy Cars etc.
Q8: Do all materials allow electricity to pass through them?

Answer: No.

Q9: Would the bulb glow in the circuit shown in Fig. below?
Answer: The electric bulb has two terminals, both the wires are connected to the one terminal of the bulb. Current will not flow from the bulb and it will not glow. To make glow, the two wires should be connected to each terminal of the bulb, as shown below.

Q10: What are the components of an electric circuit?

Answer: In general a circuit has the following components:
  1. A cell or battery : source of electricity.
  2. Connecting wires
  3. Bulb or electric device
  4. Key or switch.

Q11: How many terminals are there in an electric cell?

Answer:An electric cell has two terminals; positive and negative. The metal cap is the positive terminal of the cell. The metal disc is the negative terminal.

Q12: Using the "conduction tester" on an object it was found that the bulb begins to
glow. Is that object a conductor or an insulator? Explain.

Answer:Since the object allows current to flow and completes the circuit. Due to this bulb begins to glow. It means the object is a conductor of electricity.

Q13: What are conductors? Give examples.

Answer:  The materials which allow electric current to pass through them are called conductors.
Examples are: All metals (copper, silver, iron) are good conductors. Water usually contains impurities and traces of salt is also a good conductor. Human body is also a conductor.

Q14: What are insulators? Give examples.

Answer: Materials that do not allow electricity to pass through them are called insulators. Insulators oppose electric current and so they are used as protection from the dangerous effects of electricity.
Examples of  insulators are glass, air, plastic, cotton, thermocol, wood and rubber.
Never touch electric circuit boards
(image credits: Phillip Martin)

Q15: Why should an electrician use rubber gloves while repairing an electric switch at your home? Explain

Answer: Electricians need to touch copper wires or they may accidentally come in contact with live wires (wires carrying current). Since human body is also a good conductor, electricians may get shock. Rubber is good insulator. To prevent themselves from electric shock or flow of current, the electricians use rubber gloves while repairing.

Q16: The handles of the tools like screwdrivers and pliers used by electricians for repair work usually have plastic or rubber covers on them. Can you explain why?

Answer: Without the help of insulators, the use of electrical tools like pliers and screwdrivers is impossible. Wood and plastics are insulators and help in avoiding direct contact with electric current.  When electricians touch these live electric wires with the tools covered with plastic and wood, current does not flow in their body avoid any accident.

Q17(MCQ): In an electric bell, the electric energy is transformed into ____________energy.
(a)Mechanical energy
(b) Magnetic energy
(c) Sound energy
(d) Light energy

Answer:  (b) Magnetic energy

Q18(MCQ): The tiny coiled metal wire present inside the bulb is called____________.
(a) Element
(b) Conductor
(c) Filament
(d) None of these

Answer: (c) Filament

Q19(MCQ): Which of the following is NOT a good conductor of electricity?

(a) Mercury
(b) Copper
(c) Plastic
(d) Aluminum foil

Answer: (c) Plastic

Q20(MCQ): Which of the following is a good conductor of electricity?

(a) Wood
(b) Paper
(c) Glass
(d) Graphite

Answer:  (d) Graphite

Q21: What will happen if you join the two terminals of a cell without connecting them through a switch or a bulb?

Answer: By doing so, the chemicals in the electric cell get used up very fast and the cell will no longer produce electric current. It will stop working.

Q22: Why do an electric bulb and an electric cell have two terminals?

Answer: Because the two terminals (positive and negative) can be connected to form an electric circuit.

Q23: Is Distilled water a conductor of electricity?

Answer: No. The distilled water is the purest water and it acts as an insulator. Therefore, it used in batteries as an insulator. The tap water, sea water contains some traces of salts and impurities, due to which it becomes good conductor of electricity.