Class-6th SCIENCE STUDY NOTES

& WORKSHEET

Electricity and Circuits

ELECTRICITY and CIRCUITS

1. ELECTRICITY

Electricity is a very clean form of energy. Electricity is useful because it can be converted easily into various other forms of energy such as heat energy, light energy, mechanical energy, sound energy and magnetism, etc. We use electricity to run many appliances in our day to day life which make our work easier. For example, at the touch of a switch, we can make an electric bulb glow and produce light. Thus, electricity is used for lighting purposes. Electricity makes it possible to light up our homes, offices, markets, roads and factories after sunset when it gets dark. Electricity is produced at power stations. The big electric generators at power stations produce electricity. From the power station, electricity is brought to our homes through thick wires fixed atop tall electric poles. Underground electric wires are also used to bring electricity into our homes. Though 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 electric shock which may cause severe burns and even death. We should never use electricity from power points (sockets) in our homes or school for performing science experiments because it is very unsafe and dangerous to do so. We can use an electric cell to get electricity that is safe for performing experiments.

2. ELECTRIC CELL

An electric cell is a device which produces a small amount of electricity. Thus, electric cell is a source of electricity. In fact, electric cell is a source of safe electricity as well as portable electricity. The electric cell which we use in torch, etc., is commonly known as ‘dry cell’ or just ‘cell’. It is called dry cell because it does not contain any liquid chemical. The electric cell is a cylindrical device having a small metal cap on one side and a metal disc on the other side. The electric cell has two terminals: a positive terminal and a negative terminal. The metal cap is positive terminal of the electric cell whereas metal disc is the negative terminal of the electric cell. The positive terminal of electric cell is marked ‘plus’ (+) whereas the negative terminal of the electric cell is marked ‘minus’ ().

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We should never join the two terminals of a cell directly by a wire. This is because if we join the two terminals of a cell directly by a wire only, then the chemicals present in the cell get used up very fast, the cell gets damaged quickly and hence stops working.

An Electric Cell 3. TORCH BULB

The small electric bulb which produces light when a torch is switched on is called a ‘torch bulb’.

(a) Torch bulb and (b) its inside view

The torch bulb consists of a small glass bulb fixed on a metal case. The thin wire inside the bulb is called filament of the bulb. Filament is usually made of a very thin tungsten wire. It is the filament of the bulb which glows when electricity from a cell is passed through it. The ‘metal tip’ at the base of the bulb and the ‘metal case’ of the bulb are the two terminals of the torch bulb. When electricity from the cell passes through the filament of the bulb, the filament gets heated too much. The filament becomes white hot and starts giving light. So, it is the heating effect produced by electricity from the cell which makes a torch bulb work and produce light. Thus, a torch bulb converts electricity into light.

4. ELECTRIC CIRCUIT

An electric circuit is a path along which electricity can flow. We can make a simple electric circuit by using a cell, a torch bulb and two pieces of wire. The electric circuit provides a complete path for electricity to pass between the two terminals of an electric cell. The bulb glows only when electricity passes through the circuit.

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Electric Circuit

The electric circuit in which there is no gap in the connection between the terminals of the cell, wires and bulb, etc., is called a complete circuit or closed circuit. In an electric circuit, the direction of flow of electricity is taken to be from the positive terminal of the cell to the negative terminal of the cell. The electric circuit, in which there is a gap in the connections between the terminals of the cell, wires and the bulb, etc., is called an open circuit. Electricity does not flow through an open circuit because there is a gap in its path.

Closed Circuit Open Circuit

4.1 The case of Fused Bulb Sometimes an electric bulb does not glow even if it is connected to a cell by wires without any gap in the circuit. This can happen if the bulb has got ‘fused’. When the filament of a bulb gets broken, the bulb is said to be fused. Electricity can only flow if there is a complete circuit made of conductors.

Glowing filament of an electric bulb

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Symbols of some commonly used components in circuit diagrams

Sl. Components SymbolsNo.

1 An electric cell

2 A battery or a combination of cells

3 Plug key or switch (open)

4 Plug key or switch (closed)

5 A wire joint

6 Wires crossing without joining

7 Electric bulb or

8 A resistor of resistance R

9 Variable resistance or rheostat or

10 Ammeter

11 Voltmeter

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4.2 Switch

The switch is a simple device which is used to ‘open’ or ‘close’ a circuit. An open circuit means that there is a break in the circuit whereas a closed circuit means that it is a complete circuit. So, we can also say that a switch is a simple device which breaks a circuit or completes a circuit, as desired by us. The purpose of using a switch is to ‘turn on’ or ‘turn off’ an electrical appliance by closing the circuit or opening the circuit respectively.

Some of the electrical appliances which have inbuilt switches are: table fan, room heater, room cooler, mixer and grinder, table lamp, washing machine, hair drier, drilling machine, torch, television, radio set and microwave oven. 4.3 Torch

A torch is a portable electric lamp which uses two cells to light a small bulb. A torch is used to provide light when going out during a dark night or when electricity supply fails at home. The torch has a bulb that lights up when it is switched on.

Inside view of a torch

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5. ELECTRIC CONDUCTORS and INSULATORS

5.1 Conductors

Those materials which allow electric current to pass through them are called conductors. All the metals are conductors of electricity. Silver metal is the best conductor of electricity. Copper and aluminium metals are also good conductors. Electric wires are made of copper and aluminium because they are conductors of electricity. All the objects made of metals are also conductors. A non-metal called carbon is also a conductor of electricity. The thin, black core of our pencil is called pencil lead. Pencil lead is made of graphite. The human body conducts electricity to a small extent. So, the human body is a poor conductor of electricity. A person gets electric shock on touching an uncovered electric wire carrying current because some electric current passes through his body. Water also conducts electricity to a small extent. So, water is also a poor conductor of electricity.

(a) A conduction tester (b) Testing whether the bulb glows when the tester is in contact with a key 5.2 Insulators

Those materials which do not allow electric current to pass through them are called insulators. Insulators are actually the non-conductors of electricity. Rubber, plastics, PVC and Bakelite, ceramics, porcelain, asbestos, wood, glass, mica, cotton, jute, thermocol, wax, paper, air and most of the gases are insulators. All the objects made of insulator materials are also insulators. For example, rubber is an insulator; therefore, an eraser and balloon are also insulators. The outer covering of electric wires is made of plastic because plastic is an insulator. Plastic covering does not allow electric current to pass through it. Air is also an insulator. Rubber,

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plastics and ceramics are often used to protect us from the dangers of electricity because they are good insulators. For example: electricians wear rubber hand gloves while handling naked electric wires because rubber is a good insulator and protects them from electric shocks.

6. EFFECTS OF ELECTRIC CURRENT

An electric current can produce three effects: (a) Heating effect (b) Magnetic effect (c) Chemical effect

The degree to which a material opposes the passage of current through itself is called its resistance. Copper and aluminium metals have very low electrical resistance whereas alloys such as nichrome have very high electrical resistance (Nichrome is an alloy of nickel, chromium, iron and manganese). A wire made of material having high electrical resistance is called a resistance wire. Thus, nichrome wire is a resistance wire.

6.1 Heating Effect of Electric Current

When an electric current is passed through a high resistance wire (like nichrome wire), the resistance wire becomes hot and produces heat. This is called the heating effect of current. Actually, when an electric current passes through a high resistance wire, the electric energy gets converted into heat energy. This heat energy heats up the resistance wire.

The amount of heat produced in a wire due to heating effect of current depends on two factors:

(i) Resistance of wire (This depends on material of wire, length of wire and thickness of wire). Greater the resistance of a wire, greater will be the heat produced in it by a given current.

(ii) Magnitude of current passed through the wire. Greater the magnitude of current passed through a given wire, greater will be the heat produced in it.

Applications of the Heating Effect of Current:

Some of the applications of heating effect of electric current in our daily life are given below:

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(A) Electric Heating Appliances

The electric heating appliances (such as electric room heater, electric iron, etc.) work on the heating effect of current. The coil of high resistance wire present in an electric heating appliance is called the ‘heating element’ (or just ‘element’) of the appliance. A lot of heat is produced in the heating element because it is made of high resistance nichrome wire. A negligible heat is produced in the connecting wires of copper because copper has very low resistance. The heating elements of an electric room heater and the electric heater used for cooking food can be seen by us from outside.

(B) Electric Bulb

The electric bulb which we use for producing light works on the heating effect of electric current. The electric bulb has a very thin filament of tungsten metal. The filament of an electric bulb has a high resistance. A major part of the electricity consumed by the filament of an electric bulb is converted into heat (due to which the bulb becomes hot), only a small part of electricity is converted into light. The wastage of electricity can be reduced by using fluorescent tube lights, compact fluorescent lamps (CFLs) and LEDs for producing light (in place of filament-type electric bulbs).

(C) Electric Fuse

A fuse is a safety device which prevents electric fires and damage to electrical appliances due to excessive flow of current. The fuse for protecting our household electric wiring is fitted just near the main switch on the switch board. A fuse works on the heating effect of current.

Fuse used in buildings Fuses used in electrical appliances

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A fuse consists of a short length of a thin, tin-planted copper wire having low melting point. The thin fuse wire has a much greater resistance than the rest of the electric wiring in the house. If due to some reason, the current in household wiring suddenly becomes too large (and exceeds the safe limit), then the fuse will blow off (its wire will get heated too much, melt and break the circuit). When the fuse wire breaks, electricity supply in the household wiring is automatically cut off before any damage can be done to the rest of wiring or to the electrical appliances being used. If, however, there were no fuse in the household wiring, then the large current flowing through the wiring could heat up the wires to very high temperatures and cause electric fires. The large current flowing through electrical appliances such as TV and refrigerator could also damage them. When a fuse gets blown, a new fuse has to be fitted in its place to restore the electricity supply in the household circuit. We use a thin wire in a fuse because it has a much greater resistance than the rest of connecting wires. We should not use a thick wire as a fuse wire because it will have a low resistance and hence it will not get heated to its melting point easily.

6.2 The Cause of Large Current Flow: Short-Circuit and Overloading

An extremely large current can flow in the household electric wiring circuits under two circumstances:

(i) Short Circuit. In household electric wiring, electric current is supplied through two insulated wires which run together and reach each and every electrical appliance. One insulated wire is called live wire and the other insulated wire is called neutral wire. The touching of live wire and neutral wire directly is known as short circuit. When the live wire and neutral wire touch each other directly, then a large current flows through the household wiring.

(ii) Overloading. When too many electrical appliances are connected to a single socket, they draw an extremely large current from the household circuit. This is known as overloading.

Use of MCB in Place of Fuse

MCB stands for Miniature Circuit Breaker. MCB does not work on the heating effect of current. MCB works on the magnetic effect of current. MCB is a special kind of electrical switch which automatically turns off

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when the current in the circuit becomes too large, and cuts off the electricity supply. MCB can be re-set in the ‘on’ position when the fault in the wiring (which caused a large current to flow in the circuit) has been set right.

Miniature circuit breaker (MCB)

7. MAGNETIC EFFECTS

of ELECTRIC CURRENT

When electric current passes through a wire, the current-carrying wire behaves like a magnet. This is called the magnetic effect of current. The magnetic effect of current is used to make a large number of devices which we use in our daily life.

Effect of current on a compass needle

The magnetic effect of current was discovered by a scientist named Hans Christian Oersted. Since a current-carrying wire was able to deflect a compass needle, it was concluded that the electric current flowing in a wire gives rise to a magnetic field around it. The magnetic effect is increased if we use a long coil of wire. The magnetic effect is even further increased if the coil of wire is wound around an iron rod and then current is passed through it.

7.1 Electromagnets

An electric current can be used to make magnets. The magnet made by using electric current is called an electromagnet. An electromagnet works on the magnetic effect of current. The magnetism of an electromagnet remains as long as the current is flowing in its coil. If we

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switch off the current in the coil, all the magnetism of the iron rod disappears and it no longer behaves as a magnet. An electromagnet is a magnet consisting of a coil of insulated wire wrapped around a piece of iron that is magnetised only when electric current is passed through the coil.

Advantages of Electromagnets over Permanent Magnets:

(a) The magnetism of an electromagnet can be switched on or switched off as desired. This is not possible with a permanent magnet. (b) An electromagnet can be made very strong (i) by increasing the number of turns in the coil, and (ii) by increasing the current passing through the coil. On the other hand, a permanent magnet cannot be made so strong.

Uses of Electromagnets

Some of the important uses of electromagnets are given below:

1. Electromagnets are used in the construction of a large number of devices like electric bells, loudspeakers, electric motors, electric fans, toys and telephone instruments, etc. 2. Electromagnets are used to lift heavy loads like big machines, steel girders and scrap iron objects for loading and unloading purposes. The electromagnets which are used for loading and unloading heavy iron and steel objects are fitted on cranes. 3. Electromagnets are used to separate magnetic materials like iron and steel object from a heap of metal scrap. Electromagnets, however, cannot be used to separate non-magnetic materials such as wood, plastic, copper and aluminium objects. For example, we cannot use an electromagnet for separating plastic bags from a garbage heap. This is because plastic bags do not stick to the electromagnet. 4. Electromagnets are used by doctors to remove tiny iron particles from the eyes of a person (which may have fallen into the eyes accidently).

7.2 Electric Bell

The electric bell works on the magnetic effect of current. It has an electromagnet in it. The electric bell uses an electromagnet to pull the clapper on to a metal ball. The clapper of electric bell is also known as hammer. An electric bell uses a U-shaped electromagnet for its working.

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The U-shaped electromagnet is made by using U-shaped iron piece. In the U-shaped electromagnet, there are two coils of the same insulated copper wire wound on each side of the U-shaped iron piece.

Circuit of an electric bell

8. CHEMICAL EFFECTS OF ELECTRIC CURRENT

Electric current can bring about chemical changes, so it is said to have a chemical effect. For example, when electric current is passed through acidified water by using carbon electrodes, then a chemical reaction takes place to form hydrogen gas and oxygen gas. This chemical reaction can be written as:

Electric CurrentWater Hydrogen + Oxygen The chemical decomposition produced by passing an electric current through a conducing liquid is called electrolysis. The decomposition of acidified water into hydrogen and oxygen by passing an electric current is an example of electrolysis. The chemical effect produced by an electric current depends on the nature of conducting solution, and on the nature of electrodes used for passing the electric current. The fresh fruits and vegetables conduct electricity to some extent due to the presence of various salt solutions in them. Some fruits and vegetables also contain acid solutions in them.

When an electric current flows through a conducting solution, it causes a chemical reaction. The chemical reactions brought about by an electric current may produce one of the following effects:

(i) Bubbles of a gas may be formed on the electrodes. (ii) Deposits of metals may form on electrodes. (iii) Changes in colour of solutions may occur.

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If electric current is passed through acidified water then bubbles of oxygen gas and hydrogen gas are produced at the two electrodes immersed in it.

(i) Oxygen gas is formed at the positive electrode which is connected to the positive terminal of battery, and

(ii) Hydrogen gas is formed at the negative electrode which is connected to the negative terminal of the battery.

Passing current through water

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CLASS-6th (Science) Topic: ELECTRICITY and CIRCUITS

WORKSHEET-1

1. Name a safe source of electricity for performing science experiments in the school laboratory. 2. What are (a) conductors, and (b) insulators ? Give two examples of conductors and two of insulators. 3. Why are electric wires made of copper ? 4. An electric circuit containing a cell and a torch bulb is completed by using the following objects turn by turn:

Matchstick, Iron nail, Eraser, Glass bangle, Key Which of the above will make the bulb glow and which will not ? Give reasons for your answer. 5. What is a switch ? Explain the working of a switch with the help of diagrams. 6. Explain why, we should never join the two terminals of a cell directly by a wire only. 7. Name the portable electric lamp which works with cells. 8. Define torch. Draw the labelled sketch of a torch. Where does a torch get electricity from ? 9. Name any five gadgets (or devices) which use electricity from cells or batteries for their working. 10. Draw the symbols for following electric components: (i) Cell (ii) Bulb (iii) Battery (iv) Open Key 11. Fill in the blanks:

(a) A device that is used to break an electric circuit is called ______. (b) An electric cell has _______ terminals. 12. Mark 'True' or 'False' for following statements:

(a) Electric current can flow through metals. (b) Instead of metal wires, a jute string can be used to make a circuit. (c) Electric current can pass through a sheet of thermocol. 13. Explain why the bulb would not glow in the arrangement as shown in figure on next page.

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14. (a) Complete the drawing shown in figure (1) to indicate where the free ends of the two wires should be joined to make the bulb glow. (b)Would the bulb glow after completing the circuit shown in figure (1) if instead of safety pin we use an eraser ?

Figure – (1)

15. What is the purpose of using an electric switch ? Name some electrical gadgets that have switches built into them. 16. Would the bulb glow in the circuit shown in figure ?

17. 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. 18. Why should an electrician use rubber gloves while repairing an electric switch at your home ? Explain. 19. 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 ?

HOTS

20. Define resistance. What is its SI unit ? 21. What is rheostat ? Draw its symbol. 22. What is an ammeter ? How is it connected in a circuit ? 23. What is voltmeter? 24. What is conductance ?

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CLASS-6th (Science) Topic: ELECTRICITY and CIRCUITS

WORKSHEET- 2

1. State the three effects produced by electric current. 2. What is the name of a safety device which cuts off the electricity supply during a short circuit:

(a) by turning off a switch ? (b) by breaking a thin wire ?

3. Which important discovery was made by Oersted ? 4. Write the full form of following: (a) MCB (b) CFL 5. What is an electric circuit? Explain with the help of a diagram. 6. What is an electromagnet ? Mention two advantages of an electromagnet over a permanent magnet. 7. What is meant by the heating effect of current ? Name two appliances which work on the heating effect of current. 8. Can we use a U-shaped permanent magnet in making an electric bell? 9. Define the following terms:

(i) Short-circuit (ii) Over loading 10. Fill in the blanks: (a) Longer line in the symbol for a cell represents its_______ terminal. (b) The combination of two or more cells is called a _______. (c) An electric current flowing in a wire produces a _________effect. (d) When current is switched on in a room heater, its element becomes _______. (e) The conducting path through the bulb, wires, switch and battery is called_________. (f) The safety device based on the heating effect of electric current is called a________. 11. 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 12. Draw the circuit diagram to represent the circuit shown in above figure.

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13. Given 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.

14. 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.

15. Name any two effects of electric current. 16. When the current is switched on through a wire, a compass needle kept nearby gets deflected from its north-south position. Explain. 17. Will the compass needle show deflection when the switch in the circuit shown by figure is closed ?

18. 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) 19. Do you think an electromagnet can be used for separating plastic bags from a garbage heap ? Explain. 20. 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.

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21. 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.

22. In the circuit shown in figure:

(i) Would any of the bulb glow when the switch is in the ‘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 ?

HOTS

23. What effect does an electric current produce when flowing through a conducting liquid ? 24. Explain why, distilled water does not conduct electricity but tap water conducts some electricity ? 25. What is electrolysis ? 26. Explain the three effects produced by electric current.

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