Class 7 Science Chapter 10 Important Questions

Electricity is a form of energy. The flow of electricity through a conductor (such as metal wire) is called electric current. Generally, electric current is actually a flow of charged particles called “electrons” in a conductor which contains energy.

Electricity is produced at power stations or power plants. Electricity or current is useful because it can run many appliances or machines in our day-to-day life which make our work easier. The electric bulb, tube lights, radio, television, refrigerator, washing machine, coolers, air conditions, computers, electric iron, electric kettle, room heater, electric oven, mixer and grinder, geyser, trains etc., all work with electricity or electric current.

We know that electricity is produced at power stations or power plants. The big generator at power stations produce electricity. Electricity is very useful but it can also be very dangerous. If electric wires and electrical appliances are handled carelessly, then a person can get an electric shock which may cause severe burns and even death. This is because electricity from electric mains or power supply line, portable generators as well as from inverters is at a very high voltage of 220 volts. Human beings cannot tolerate electrical energy for long time. Improper handling of electricity can also cause fires. The electricity, produced by portable electric generators and inverters used in our homes, is equally dangerous.

To draw the electric circuits by making the actual sketches for drawing of the cell, battery, switch, bulb or lamp and connecting wires is a difficult job and takes a lot of time. So, the scientists have devised symbol for “Electrical components” like the cell, battery, switch, electric bulb and connecting wires etc. which are easy to draw. Such drawings provide a quicker mental picture of the actual circuit. And these symbols are used to draw the electric circuit diagram easily and clearly.

A single cell or other power source is represented by a long and a short parallel line. A collection of cells or battery is represented by a collection of long and short parallel lines. In both cases, the long line is representative of the positive terminal of the energy source and the short line represents the negative terminal. A straight line is used to represent a connecting wire between any two components of the circuit. The symbol of electric bulb consists of a circle having a semi-circle inside it.
An electrical device that offers resistance to the flow of charge is generically referred to as a resistor and is represented by a zigzag line. An open switch is generally represented by providing a break in a straight line by lifting a portion of the line upward at a diagonal. These circuit symbols will be frequently used as electric circuits which are represented by schematic diagrams. It will be important to either memorize these symbols or to refer to this short listing frequently until you become accustomed to their use.

We do cover plug pinholes which are within the reach of children with cello-tape or plastic cover to avoid electric shocks. If unconsciously, a child puts his finger into the electric socket, he or she may get shock which could be fatal.

When a small current passing in a wire produces less heat but a large current passing through the same wire produces much more heat. As an example, when normal amount of current flows through the copper wires of household electric wiring, the wires do not become much hot but when a large current flows in the wires of household wiring accidentally, then the wires become extremely hot and a fire may be started.

All the electric heating appliances contain coils of high resistance wire made of nichrome alloy. The coil of high resistance wire or coil of nichrome wire present in an electric heating appliance is called the “heating element” of the appliance.
For example, the electric room heater has an element made of a coil of nichrome wire. When current is switched on in an electric room heater. The element of heater becomes red hot and gives out heat. The element becomes red hot because a lot of heat is produced in it, on passing current through the nichrome coil. We know that because of the high resistance of nichrome coil, it looks red and give out heat.

When the switch is in the ‘ON’ position, the circuit from the positive terminal of the battery to the negative terminal is complete. The circuit is then said to be closed and the current flows throughout the circuit instantly.
When the switch is in the ‘OFF’ position, the circuit is incomplete. It is said to be open. No current flows through any part of the circuit.

Never touch a lighted electric bulb connected to the mains. It may be very hot and your hand may get burnt badly. In the same way, we should not experiment with the electric supply from the mains or a generator or an inverter. It may get an electric shock, which may be dangerous.

When current is passed through the wire, it deflects the compass near it from its north south position like a magnet. This is called magnetic effect of the current. As we know, the middle of the compass is made up of a thin magnet. When this needle comes in contact with another magnet then the like poles of the magnet repel each other. So, the deflection is seen in the needle. In this case, the wire behaves like a magnet and causes deflection in the needle of the compass.

Fuse is a safety device which protects our houses, appliances and electric circuits electrical hazards at the time of the short circuiting or overloading. The fuse works on the basis of heating effect of the electric current.
Wires made from some special materials melt quickly and break when large electric currents are passed through them. In all buildings, fuses are inserted in all electrical circuits. There is a maximum limit on the current which can safely flow through a circuit. If by accident the current exceeds this safe limit, the wires may become overheated and may cause fire. If a proper fuse is there in the circuit, it will blow off and break the circuit. A fuse is thus a safety device which prevents damages to electrical circuits and possible fires. In this way, it helps in prevention of short circuits.

Bar magnet is a permanent magnet whereas electromagnet is a temporary magnet. The strength of bar magnet cannot be changed whereas the strength of electromagnet can be changed. The uses of bar magnet are limited whereas the uses of electromagnet are more wider.

No, we cannot use same fuse in a geyser and in a television set because the fuse used in every appliance has some limit to withstand the current flowing through it. So, different appliances have fuses of different ratings.

An electric bulb is used for light but it also gives heat. This is not desirable. This result in the wastage of electricity can be reduced by using fluorescent tube lights (CFLs) in place of the bulbs. CFLs are also used to reduce wastage of electricity. There are compact fluorescent lamps. These can be fixed in ordinary bulb holders.