CBSE Class 7 Science Electric Circuits and Heating Effect of Current Worksheet Set 07

Answer: An MCB (miniature circuit breaker) switches off the circuit automatically when there is overload or a fault, protecting wires and appliances.

Teacher's Note:
1. It is used in place of a fuse in modern wiring.
2. It protects people and property from fires and damage.

Answer: It works on the heating effect of electric current. When the current becomes more than the safe value, a metal strip inside heats up and bends, which opens the contacts and cuts off the supply. After the fault is removed, the switch can be reset by hand.

Teacher's Note:
1. Unlike a fuse, it does not have to be replaced after tripping.
2. It is also more sensitive to excess current.

Answer: (b) filament. The thin coil inside a bulb glows when current passes through it and is called the filament.

Teacher's Note:
1. In heaters and irons the heating coil is called the element.
2. In a bulb it is called the filament.

Answer: When an electric current flows through a conductor, the conductor becomes hot. This is the heating effect of current.

Teacher's Note:
1. Heaters, irons and toasters use this effect.
2. It also makes wires warm when too much current flows.

Answer: Fuse and MCB (miniature circuit breaker).

Teacher's Note:
1. A fuse wire melts when the current is too large.
2. An MCB trips for the same reason and can be reset.

Answer: Components joined end to end in a single line, so the same current flows through each of them one after the other.

Teacher's Note:
1. If one part breaks, the whole circuit stops working.
2. Cells in a torch are usually connected in series.

Answer: All the positive terminals of the cells are joined together and all the negative terminals are joined together.

Teacher's Note:
1. Each cell supplies part of the current, and the voltage stays the same as that of one cell.
2. Parallel connection lasts longer.

Answer: Series: cells placed one after another in a torch (or a string of decorative lights). Parallel: the electrical appliances in a house connected to the mains.

Teacher's Note:
1. In a parallel connection each appliance works on its own.
2. In a series connection all stop together.

Answer: (a) Both A and R are true and R is the correct explanation of A. Heat produced depends on resistance, and the filament has much higher resistance than the line wires.

Teacher's Note:
1. The same current produces more heat in a part with higher resistance.
2. This is why the filament glows.

Answer: (a) Both A and R are true and R is the correct explanation of A. Low resistance means little heat loss, so copper is a good wire material.

Teacher's Note:
1. Copper conducts current very well.
2. It is also easily drawn into wires.

Answer: The longer line represents the positive terminal and the shorter, thicker line the negative terminal.

Teacher's Note:
1. Circuit diagrams use symbols for quick drawing.
2. Learning them helps in reading circuits correctly.

Answer: In a closed circuit the path is complete and current flows. In an open circuit there is a gap (for example, the switch is off) and no current flows.

Teacher's Note:
1. A switch opens or closes the circuit.
2. The bulb glows only in a closed circuit.

Answer: (b) a low melting point. It must melt quickly when the current is more than the safe value, to break the circuit.

Teacher's Note:
1. If it did not melt easily, it would not protect the circuit.
2. Pure copper is unsuitable for this reason.

Answer: Copper has a high melting point and a thick wire has low resistance, so it would not melt when the current is too large.

Teacher's Note:
1. A fuse must be thin and easy to melt.
2. Fuse wire is usually an alloy such as tin and lead.

Answer: All the other bulbs go off, because the circuit is broken and no current can flow in a series connection.

Teacher's Note:
1. In a series connection there is only one path.
2. In a parallel connection the other bulbs would keep glowing.

Answer: (b) nichrome. Nichrome has high resistance and a high melting point, so it becomes red hot without melting.

Teacher's Note:
1. Copper would stay cool because of its low resistance.
2. Nichrome is an alloy of nickel and chromium.

Answer: A switch is used to open or close the circuit, thereby stopping or allowing the flow of current.

Teacher's Note:
1. It lets us control an appliance safely.
2. It is placed in the live wire.