Notes of chapter: Electric Current and its Effects| Chemical Effects of Electric Current are presented below. Indepth notes along with worksheets and NCERT Solutions for Class 8.
Electric circuit is a circuit in which electric current flows when battery is at on position. Battery, bulb and wire are the parts of the electric circuit.
(2) Some features of electric circuit-
(i) The key or switch can be placed anywhere in the circuit.
(ii) 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.
(iii) 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.
A combination of two or more cells where the positive terminal of one cell is connected to the negative terminal of the next cell is called a battery. We need a cell holder to use batteries for our activities. It can be prepared at home or we can buy from market.
Uses:- Electric cells are used in
(ii) TV remotes
A wire is a long thin metal like thread, covered with rubber which carries electric current in to electric circuit.
A metal strip or thick wire connects the negative terminal of one cell to positive terminal to another cell. ‘ +’ and ‘ –‘ symbols are always printed on the batteries.
The cells can be connected one after another as shown in figure below-
The cells can also connected to place them side by side as shown in figure below:
(5) We use symbols for every electric components when we draw a circuit. Some symbols are giving in table.
A filament of a bulb is a thin wire which glows when an electric current passes through it. When the bulb gets fused, its filament is broken.
When the filament of a bulb is broken the circuit will not be complete. So, the bulb will not glow.
(7) When electric current passes through wire, it gets hot, this effect is known as heating effect of the electric current.
A coil of wire in which heat is produced by an electric current is called element. It is used in every electric appliance to heat those appliances.
Take an electric cell, a bulb, a switch and connecting wires. Make a circuit of OFF position. Now, touch the bulb. Now, move the electric switch to the ON position. Bulb will glow. Now touch the bulb. Bulb will be hotter than previous position.
Make a circuit as shown in diagram given below. Take about 10cm long piece of nichrome wire and tie it between the nails. Touch the wire. It will be cool. Now switch on the current in the circuit by moving the switch to the ON position. Now touch the bulb, it will hot.
(8)The amount of heat produced in a wire depends on its material, length and thickness. The wires of different materials and different lengths and thickness are used for different requirements.
Electric fuse is a device which prevent electric component from over current of an electric circuit.
(i) Wires which are made of some special materials melt quickly and break when large electric current are passed through them.
Make a circuit same as in previous activity. Replace cell with a battery of four cells. Also in place of nichrome wire, tie a thin strand of steel wool. Switch off the fans in the room. Now, pass the current through the circuit for sometime. When we pass high current the wire melted.
(ii) Fuses are inserted in all electrical circuits. These can flow a maximum limit of current through the circuit. If current exceeds the safe limit, a fuse will blow off and break the circuit. A fuse is a safety device which prevents damages to electrical circuits and possible fires.
(iii) Fuses of different kinds are used for different purposes. Figure given below shows fuses used in our houses.
Electromagnetic effect is an effect when an electric current flowing in a wire produces a magnetic field around it. Electromagnetic effect is also known as magnetic effect of electric effect.
Magnetic effect of the electric current was first noticed by scientist Hans Christian Oersted.
Take the cardboard tray from inside a discarded matchbox. Wrap an electric wire a few times around the cardboard tray. Place a small compass needle inside it. Now connect the free ends of this wire to an electric cell through a switch as shown in fig given below.
Bring a bar magnet near the compass needle. It deflected because needle of compass is a tiny magnet. Now move the switch to ON position, needle again deflected. It means, when the electric current passes through the wire, it behaves like a magnet. This is the magnetic effect of the electric current.
The current is passes through a coil which is surrounded on a metal turn it to a magnet, is called electromagnet.
Take around 75 cm long piece of insulated flexible wire and an iron nail of 6cm long. Wind the wire around the nail in a form of coil. Connect the free ends of the wire to the terminals of a cell through a switch as shown in fig given below.
Place some pins near the end of nail. Now, switch on the current. Nail will pull the pins. Pins will cling to the end of the nail. After switching off the current pins will lose the end of nail. It happens because of the electromagnets.
(12) The electromagnets can be made very strong and can lift very heavy loads.
(i) The end of a crane has a strong electromagnet attached to it to pull heavy loads.
(ii)These are also used to separate magnetic material from the junk.
(iii)Doctors also use tiny electromagnets to take out small pieces of magnetic material from the eye.
(iv) Many toys also have electromagnets inside them.
(v) Electric bell has an electromagnet in it.
(13) Good conductor-
The materials which allow electricity to pass through them are known as good conductor of electricity.
Eg:- Metals, tap water, caustic sauda etc.
(14) Poor conductor-
The materials which do not allow electricity to pass through them are known as poor conductor of electricity.
Eg:- Rubber, plastic, wood etc.
(15) Liquids can be good or poor conductors of the electricity.
Pour few drops of lemon juice in the rubber or plastic cap. Now put ends of the tester in to the lemon juice. Bulb will glow. It shows lemon juice is a good conductor of electricity.
If bulb does not glow it shows that liquid is a poor conductor of electricity. Sometime, liquid is conducting but bulb is not glowing. It happens because of less heating effect of the current which is not able to heat filament of the bulb at high temperature. In this condition we will have to use LED bulb. LED bulb can glow in weak current.
(16) Repeat above activity with the tester using magnet effect of the electric current.
Now, compass deflates. It shows that lemon juice is a good conductor of the electricity.
Good or poor conducting liquids are tabulated below:-
|SN||Material||Compass Needle shows deflation (yes/No)||Good conductor/Poor conductor|
|1.||Lemon juice||Yes||Good conductor|
|3.||Tap wear||Yes||Good conductor|
|4.||Vegetable oil||Yes||Good conductor|
|7.||Distilled water||No||Poor conductor|
(17) Most liquids that conduct electricity are solutions of acids, bases and salts.
Take four rubber caps filled with distilled water. Now pour few drops of lemon juice or dilute hydrochloric acid, caustic soda, sugar and salt in separate caps. Now test these solutions by passing electric current.
The compass of circuit will deflate for solutions of lemon juice, caustic soda and salt but compass will not deflate for sugar solutions. Therefore, it is clear that liquids that contain acid, bases and salt are good conductor of electricity.
An electrode is a solid electrical conductor that carries electric current into nonmetallic part of a circuit.
(18) Chemical effects of electric current-
(i) Bubbles of a gas can appear on the electrodes.
(a)Preparation of electrodes-
Clean metal caps of carbon rods taken out from discarded cells. Wrap copper wire around metal caps of the carbon rods and join them to a battery. These carbon rods are known as electrodes.
(b) Pour some water in to glass bowl. Add some salt or few drops of lemon juice in to the water to make it more conducting. Immerse the electrodes in the solution and keep metal cap out of the water. After 3 – 4 minutes some air bubbles appears near the electrodes. It happens because of electric current passes through a conducting solution causes chemical reactions.
(ii) Colour of liquid or solution can change.
Insert the copper wire of a tester into the potato. After half an hour a greenish blue colour spot find out around the wire as a result of the chemical effect in the potato produced by current. But no such spot appears around other wire. The wire which has greenish blue spot around it, is a positive terminal. It could be effective method to find out positive terminal.
The reaction would depend on what solution and electrodes are used.
The process of depositing a layer of any desired metal on another material by means of electricity is called electroplating.
Take two copper plate and copper sulphate. Pour distilled water in the glass bowl. Add copper sulphate in it. Now add few drop of dilute sulphuric acid to copper sulphate solution to make it more conducting. Clean copper plates (electrodes). Connect them with terminals and immerse them in the solution. Pass the electric current about 15 minutes.
When electric current passes in to the solution copper and sulphate dissociates from the copper sulphate solution. The dissociate copper deposited to the electrodes at negative terminal. Copper dissolves from the electrodes at positive terminal. Therefore, copper is restored in the solution.
Eg:- Electroplating is used in coating of different metals for different purposes.
(i) Chromium is an expensive metal. So it is used only for coating other metals. It does not corrode and resists scratches. Therefore, chromium plating is done on many objects as bath taps, kitchen gas burners, car parts, furniture etc.
(ii) Less expensive ornaments also shines after electroplating with gold or silver. These ornaments look like original but in real these are not real.
(iii)The tin cans used for the food storage are also electroplating of tin on iron. Tin is less reactive than iron . Therefore, it is useful for food storage.
(iv) Iron is protected by applying zinc coating on it. Then, zinc coated iron is used in making bridges and automobiles.