Goa Board Class 7 Solutions for Science – Electric Current and Its Effects (English Medium)
Page No. 170:
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.
Solution :
Symbols for various components of electrical circuits are:
Question 2:
Draw the circuit diagram to represent the circuit shown in the figure.
Solution :
In the given circuit, one terminal of the bulb is connected to one terminal of the cell and the other terminals of the bulb and the cell are connected to a safety pin. However, the safety pin is not connected with one of the drawing pins. Thus, the circuit is not complete. Hence, safety pin represents a switch in ‘OFF’ position. The circuit diagram of the given figure is shown in the following figure.
Concept insight: For current to flow it is necessary that circuit should be complete. As circuit is not complete here so bulb does not glow.
Question 3:
Figure 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
Solution :
To make a battery, the negative terminal of one cell must be connected to the positive terminal of the next cell, as shown in the following figure.
Question 4:
The bulb in the circuit shown in Figure does not glow. Can you identify the problem? Make necessary changes in the circuit to make the bulb glow.
Solution :
The bulb in the circuit is not glowing because the two cells are not connected properly i.e., positive terminal of one cell is connected to the positive terminal of another cell. Thus, to make the bulb glow, the negative terminal of one cell should be connected to the positive terminal of the other cell, as shown in the given figure.
Question 5:
Name any two effects of electric current.
Solution :
The two effects of electric current are
(i) Heating effect of electric current
(ii) Magnetic effect of electric current
Concept insight: Heating effect of electric current: When an electric current is passed through a wire, the wire becomes hot and produces heat. This is called heating effect of electric current.
Magnetic effect of electric current: When electric current passes through a wire, it behaves like a magnet. This is called magnetic effect of electric current.
Page No. 171:
Question 6:
When the current is switched on through a wire, a compass needle kept nearby gets deflected from its north-south position. Explain.
Solution :
The current flowing in the wire gives rise to a magnetic field around it which exerts a force on the compass needle (which itself is a tiny magnet) kept nearby and deflects it from its North-South position.
Concept insight: A current-carrying wire behaves like a magnet.
Question 7:
Will the compass needle show deflection when the switch in the circuit shown by figure is closed?
Solution :
No.
Concept insight: The given circuit does not have any battery or cell (current source). In the absence of current, the wire does not behave as a magnet and hence, the compass needle will not show any deflection.
Question 8:
Fill in the blanks:
(a) Longer line in the symbol for cell represents its_________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 _________.
Solution :
(a) Positive
(b) Battery
Concept insight: To make a battery, the negative terminal of one cell is connected to the positive terminal of the next cell and so on.
(c) Produces heat
Concept insight: An electric heater works on the principle of heating effect of electric current. Its heating filament has high resistance; so when electricity passes through it, it produces large amount of heat.
(d) Fuse
Concept insight: Working of fuse is based on heating effect of electric current. So when large amount of current passes through a fuse, it melts and breaks the circuit.
Question 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)
Solution :
(a) False
Concept insight: To make a battery of two cells, the negative terminal of one cell must be connected to the positive terminal of the other cell.
(b) True
Concept insight: The wire of the fuse is made of lead which has a low melting point. When large current passes through a fuse, it melts quickly and breaks the circuit.
(c) False
Concept insight: Working of an electromagnet is based on the magnetic effect of electric current. An electromagnet behaves as a magnet when current passes through it. Hence, it attracts a piece of iron.
(d) True
Concept insight: An electric bell has a piece of iron wound by a coil. When current passes through this coil, it behaves as an electromagnet.
Question 10:
Do you think an electromagnet can be used for separating plastic bags from a garbage heap? Explain.
Solution :
No, because plastic is a non magnetic substance so an electromagnet cannot attract and separate plastic bags from garbage heap.
Concept insight: Important application based question.
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.
Solution :
No, because fuse is made of lead wire which has a very low melting point. A fuse wire melts quickly and breaks when a large amount of current passes through the circuit. If the electrician replaces a fuse by a piece of ordinary wire (which has high melting point), it increases the risk of overheating due to flow of excessive current. This may lead to short-circuit and fire.
Question 12:
Zubeda made an electric circuit using a cell holder shown in Figure, 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.
Solution :
The possible defects in the circuit are:
(i) The rubber band used in the cell holder may not be tight enough to keep the two cells in contact with each other. If the cells are not in proper contact with each other, then the circuit will not be complete and current will not flow through the circuit. Hence, the bulb will not glow.
(ii) The two cells may not be connected properly. The negative terminal of one cell must be connected to the positive terminal of the other cell.
(iii) The bulb may be fused.
(iv) The cells may be used up i.e. the chemical inside the cells has been exhausted.
(v) The circuit connections may be loose.
Page No. 172:
Question 13:
In the circuit shown in figure.
(i) Would any of the bulb glow when the switch is in ‘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.
Solution :
(i) No.
Concept insight: When the switch is in ‘OFF’ position, the current does not flow through the circuit. Hence, none of the bulbs will glow.
(ii) Bulbs will glow simultaneously.
Concept insight: When the switch is moved to the ‘ON’ position, then all the bulbs will glow at once. This is because all the electrons around the circuit move instantaneously at the same time.