CBSE Class 7 Science Electric Current And Its Effects Worksheet

ELECTRIC CELL
An electric cell is a device which converts chemical energy into electric energy.
The cell has two different metal plates – one is the positive terminal and the other is the negative terminal.
These plates are kept inside a chemical called electrolyte. The cell is a source of electric current. Electric current is the flow of electrons or charge.

ELECTRIC CIRCUIT
An electric circuit is the closed path along which electric current flows from the positive terminal to the negative terminal of the cell.
A circuit generally has:
(a) a source of electric current - a cell or battery.
(b) connecting wires for carrying current.
(c) a device which uses the electricity - a bulb etc.
(d) a key or a switch – This may be connected anywhere along the circuit to stop or allow the flow of current.

(A) Battery : The positive terminal of one cell is connected to the negative terminal of the next cell. Such a combination of two or more cells is called a battery. Many devices such as torches, transistors, toys, TV remote controls, use batteries. 

Connecting two cells together to make a battery

(B) Bulb :
In the bulb there is a thin wire, called the filament, which glows when an electric current passes through it.
When the bulb gets fused, its filament is broken. 

(C) Electric switch :
An electric switch is a device that opens or closes an electric circuit.
When the switch (key) K is closed, the circuit is complete; current flows through the circuit and the bulb glows.
When the switch (key) K is open, the circuit is not complete; current does not flow through the circuit and the bulb does not glow. 

Some common electric components can be represented by symbols. 

CONDUCTORS AND INSULATORS
CONDUCTORS : The materials which allow electric current to pass through them.
Ex. All metals like Copper, Iron, Silver and Human body.
INSULATORS : The materials which do not allow electric current to pass through them.
Ex. Plastic, Wood, Rubber and Glass.

EFFECT OF ELECTRIC CURRENT

(A) HEATING EFFECT OF CURRENT :
The wire which are not very good conductors gets hot when an electric current passes through it. This is the heating effect of the electric current. 

The amount of heat produced in a wire depends on its material, length and thickness. Thus, for different requirements, the wires of different materials and different lengths and thickness are used.

(iii) Electric bulb :
The filament of an electric bulb gets heated to such a high temperature that it starts glowing.

(B) CHEMICAL EFFECT OF CURRENT (ELECTROPLATING)
The method of plating one type of over another by means of electricity is called electroplating.
Ex. To electroplate a stainless steel razor blade with copper.
Materials required : Glass beaker, copper sulphate solution, two pieces of connecting wire (50 cm long), a cell, stainless steel razor blade.

Method : Remove the insulation from the ends of both wires. Tie one end of a wire to the stainless steel razor blade and tape the other bare end to the negative terminal of the cell. Tape one end of the other wire to the positive terminal of the cell and dip its other end into the copper scrub the razor blade clean and dip it in the solution.
After electricity passes through the circuit for some time you will find a reddish-brown deposit on the blade.
This is the coating of copper.

(C) MAGNETIC EFFECT OF CURRENT :
The needle of a compass is a tiny magnet, which points in north-south direction. When we bring a magnet close to it, the needle gets deflected. Similarly we can also seen that compass needle gets deflected when the current flows in a nearby wire. Hans Christian Oersted was the first person who noticed the deflection of compass needle every time the current was passed through the wire. So, when electric current passes through a wire, it behaves like a magnet. This is the magnetic effect of the electric current. In fact, an
electric current can be used to make magnets.

Factors which effect the strength of magnetic effect by current :
(i) Magnetic effect of current depends on number of turns.
(ii) Magnetic effect of current depends the value of current.
(iii) Magnetic effect of current depends on the nature of core inside the coil.

APPLICATION OF HEATING & MAGNETIC EFFECT OF CURRENT

Electric Fuse :
Wires made from some special materials melt quickly and break when large electric currents are passed through them. These wires are used for making electric fuses,(figure). 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. 

MCB :
These days Miniature circuit breakers (MCBs) are increasingly being used in place of fuses. These are switches which automatically turn off when current in a circuit exceeds the safe limit. You turn them on and the circuit is once again complete. 

Electromagnet :
Wind the wire tightly around the nail in the form of a coil. Connect the free ends of the wire to the terminals of a cell through a switch as shown in Fig. 

The coil behaves like a magnet when electric current flows through it. When the electric current is switched off, the coil generally loses its magnetism. Such coils are called electromagnets. The electromagnets can be made very strong and can lift very heavy loads. The electromagnets are also used to separate magnetic material from the junk. Doctors use tiny electromagnets to take out small pieces of magnetic material that have accidentally fallen in the eye. Many toys also have electromagnets inside them.

Electric bell :
It consists of a coil of wire wound on an iron piece. The coil acts as an electromagnet. An iron strip with a hammer at one end is kept close to the electromagnet. There is a contact screw near the iron strip.
When the iron strip is in contact with the screw, the current flows through the coil which becomes an electromagnet. It, then,

pulls the iron strip. In the process, the hammer at the end of the strip strikes the gong of the bell to produce a sound. However, when the electromagnet pulls the iron strip, it also breaks the circuit & the current through the coil stops flowing. The coil is no longer an electromagnet & it no longer attracts the iron strip.
The iron strip comes back to its original position and touches the contact screw again. This completes the circuit. The current flows in the coil and the hammer strikes the gong again.
This process is repeated in quick succession. The hammer strikes the gong every time the circuit is completed. This is how the bell rings.

Question. The process by which chemical change takes place in a substance when electric current is passed through it is called -
(A) electrolysis
(B) electroplating
(C) electrodes
(D) thermionic conduction

Answer: A

Question. Electric bell works on the principle of -
(A) chemical effect of current
(B) magnetic effect of current
(C) heating effect of current
(D) all of the above

Answer: B

Question. An electrolyte is -
(A) a light electric cell
(B) a liquid that conducts electricity
(C) a metal
(D) non of the above

Answer: B

Question. To make a battery of 9 volts. how many 1.5 V cells are needed?
(A) 6
(B) 5
(C) 4
(D) 3

Answer: A

Question. Cathode is -
(A) positively charged electrode
(B) negatively charged electrode
(C) a positively charged ion formed in the electrolyte
(D) a negatively charged ion formed in the electrolyte.

Answer: B

Question. Which is the false statement ?
(A) Fuse wire has low resistance and melting point
(B) Heater wire has high specific resistance and melting point
(C) In these day M.C.B. is used in place of fuse wire
(D) Current does not flow in close circuit.

Answer: D

Question. An electric current produces -
(A) Magnetic effect
(B) Chemical effect
(C) Heating effect
(D) All of the above

Answer: D

Question. An electric bell when ringing-
(A) carries no electric current
(B) carries continuous current
(C) carries intermittent current
(D) has a permanent magnet to make it work

Answer: C

Question. A moving charge produces -
(A) neither electric field nor magnetic field
(B) electrostatic field only
(C) magnetic field only
(D) both magnetic and electrostatic field

Answer: D

Question. Which is the best conductor ?
(A) carbon
(B) Copper
(C) Iron
(D) Aluminium

Answer: B

Question. Nichrome is used for making
(A) the filaments of bulbs
(B) fuse wires
(C) heater elements
(D) coils for electromagnets

Answer: A

Question. Strength of an electromagnet increases by -
(A) increasing the number of turns of the coil
(B) increasing the current flowing through the coil
(C) using soft iron core for the coil
(D) all of the above

Answer: D

Question. The magnitude of a current flowing through a device depends
(A) only on the voltage across it
(B) only on its resistance
(C) on its resistance and the voltage across it
(D) none of these

Answer: C

ACTIVITY- 1 : To make an electrc circuit. ` 

Q2. Draw the symbols for the following:
a) A cell            d) Switch in ON position
b) A battery      e) Switch in OFF position
c) A bulb           f) Connecting wires

Q3. Draw a circuit diagram to show how you will connect a battery of three cells toan electric bulb through a switch.

Q4. Choose the correct option:

a) If a circuit is open:
i) the bulb glows
ii) the bulb does not glow
iii) the bulb flickers
iv) none of these

b) Electric bulb converts electrical energy into:
i) sound energy
ii) magnetic energy
iii) light energy
iv) chemical energy

c) Which of the following appliances is based on the magnetic effect of current:
i) electric kettle
ii) electric bell
iii) electric iron
iv) electric oven

d) In an electric bell, which of these gets attracted to the electromagnet?
i) the hammer
ii) the soft iron strip
iii) the screw
iv) none of these

Q5. What happens during a short circuit ? Explain how it can be prevented.

Q6. Anu and her friends visited their neighbourhood and encouraged them to use CFL instead of bulbs. What values do they want to be inculcated?

Q7.Describe an experiment to show:
a) The effect of current on a compass needle.
b) How you can make an electromagnet.

Q8. List any four uses of electromagnets.

Q9. Rohanmade four electromagnets A, B, C, and D with 20, 40, 60 and 80 turns.
He connected them one by one to a battery of two cells and brought them near a box of all-pins. Which electromagnet will attract the maximum pins and why?

Q10. Describe the construction and working of an electric bell.

I Fill in the blanks :

1. A thin wire in the bulb that glows when an electric current passes through it is called the ____________________.

2. When the bulb gets _______________, its filament is broken.

3. The coil of wire in an electric heater is known as ________________.

4. The amount of heat produced in a wire depends on its, ______________, ________________ and __________________.

5. When an electric current flows through a wire, it behaves like a _________________.

6. The ___________of an electric bell acts as an electromagnet in its working.

II State whether the following statements are True or False :

1. When the electric current is switched off, the coil generally loses its magnetism.

2. The wires used for making electric circuits do not normally become hot.

3. The filament of an electric bulb never gets heated.

4. In a battery, the electric cells are always placed one after the other.

5. Connecting many devices in a single socket does not affect the flow of current in a circuit.

6. Wear and tear of insulated wires can cause short circuits.

7. CFLs consume more electricity than ordinary bulbs.

III Name the following :

1. The name of the scientist who first noticed the magnetic effect of current.

2. The switches being used in place of fuses.

3. A diagram made using symbols of electric components.

4. The mark of safety necessary on electrical appliances.

IV Define :

1. Battery

2. Electromagnet

3. Fuse