Chapter 17. MAGNETIC EFFECT OF ELECTRIC CURRENT
INTEXT QUESITONS 17.1
1. Define magnet and list its properties?
Answer: A magnet is an object or material that produces a magnetic field and attracts magnetic substances such as iron, nickel, cobalt, and some of their alloys.
The properties of magnet are :
(i) Attracts iron towards itself.
(ii) Freely suspended magnet always rests at the north-south direction.
(iii) Like poles repel while unlike poles attract.
(iv) If iron pieces are brought near a strong magnet they also start behaving as magnets.
(v) The poles of a magnet cannot be separated.
2. What happen, with the properties of magnet when it is broken into two pieces?
Answer: When a magnet is broken into two pieces, each piece becomes a new magnet with its own north pole and south pole. The magnetic properties remain; separate magnetic poles cannot be obtained by breaking a magnet.
3. Name the part of telephone where magnet is used?
Answer: The magnet is used in the speaker of the telephone handset.
4. Hang the bar magnet with the string, it will always rest in which directions
(i) East-west (ii) West-south (iii) North-south (iv) North-east
Answer: (iii) North–south .
[ When a bar magnet is hung freely with a string, it always comes to rest in the north–south direction.]
5. Do magnetic field exist throughout space?
Answer: Yes, but their strength depends on where you are
6. The north pole, magnetic needle points towards earth’s
(i) North pole (ii) South pole
(iii) Centre (iv) None of the above
Answer: (ii) South pole .
[ The north pole of a magnetic needle points towards the Earth’s magnetic south pole, which is near the geographic North Pole.]
7. What are magnetic poles?
Answer: Magnetic poles are the surfaces from which the invisible line of magnetic field emanate and connect on return to the magnet.
INTEXT QUESITONS 17.2
1. Presence of magnetic field around an electric wire can be proved by which an behaviour of iron filings
(i) They form a circular patterns as soon as the electricity is turned on.
(ii) Try filing fly off the card when the current is on
(iii) They do not prove anything, because it is magic trick.
(iv) None of the above
Answer: (i) They form a circular patterns as soon as the electricity is turned on.
[ Iron filings arrange themselves in circular patterns around the wire, showing the presence of a magnetic field around an electric wire carrying current.]
2. Among these which property is not belong to electromagnet?
(i) It is permanent magnet
(ii) Its magnetic strength can not be decreased or increased at as well
(iii) Its polarity can be remove by reversing flow of electric current.
(iv) It produce strong magnetic field.
Answer: (i) It is permanent magnet .
[ An electromagnet is not a permanent magnet because it shows magnetic properties only when electric current flows through it.]
3. To findout the direction of force in the electric motor which rules is used?
(i) Flemings right hand rule
(ii) Flemings left hand rule
(iii) Right plam rule of right hand
(iv) Left palm rule of left hand
Answer: (ii) Fleming’s left hand rule.
[ Fleming’s left hand rule is used to find the direction of force (motion) in an electric motor.]
4. Why does an iron core increase the magnetic field of a coil of wire
(i) The iron atom line up to add the magnetic field
(ii) Iron attract things includes magnetic fields
(iii) The iron core actually decrease the field, allowing it to be turned off
Answer: (i) The iron atoms line up to add the magnetic field .
[ When an iron core is placed inside a coil of wire, the iron atoms align and strengthen the magnetic field produced by the coil.]
5. List the factors affecting the strength of an electromagnet?
Answer: The factors affecting the strength of an electromagnet are:
(i) Number of turns
(ii) Current flowing in the coil
(iii) Length between the poles.
6. What is the role of solenoids in making electromagnet?
Answer: A solenoid is used for making electromagnets. The use of soft iron rod as core in a solenoid produces strong magnetism.
INTEXT QUESTIONS 17.3
1. The work of an electric generator is to:
(i) Change chemical energy to electrical energy.
(ii) Change mechanical energy to electrical energy.
(iii) Change electrical energy to mechanical.
(iv) Change electrical energy to chemical energy.
Answer: (ii) Change mechanical energy to electrical energy.
[ An electric generator converts mechanical energy into electrical energy.]
2. Appliance that works on the principle of electromagnetic induction.
(i) Electric kettle (ii) Electric bell
(iii) Electric lamp (iv) Electric generator.
Answer: (iv) Electric generator .
[ An electric generator works on the principle of electromagnetic induction.]
3. Electric fuse should have the following combination of melting point and resistance:
(i) high resistance and low melting point
(ii) low resistance and high melting point
(iii) high resistance and high melting point
(iv) Low resistance and low melting point
Answer: (i) High resistance and low melting point .
[ An electric fuse should have high resistance and low melting point so that it melts quickly and breaks the circuit when excess current flows.]
4. Which principle states that by changing magnetic field, current produced:
(i) Coulombs law (ii) Magnetic behaviour of a solenoid
(iii) Electromagnetic induction (iv) Ohms law
Answer: (iii) Electromagnetic induction .
[ Electromagnetic induction states that when the magnetic field changes, an electric current is produced in a conductor.]
5. Appliance that converts high voltage to low voltage is:
(i) Step up transformer (ii) Step down transformer
(iii) Rectifier (iv) Amplifier
Answer: (ii) Step down transformer .
[ A step-down transformer converts high voltage into low voltage.]
6. Fuse wire is made up of
(i) Silicon and Tin alloy (ii) Tin coated with zinc
(iii) tin coated with nickel (iv) Tin coated with aluminium
Answer: (i) Silicon and Tin alloy .
Fuse wire is made of an alloy with a low melting point so it melts and breaks the circuit when excess current flows.
7. According to Flemmings left hand rule the force acting on the current carrying conductor is:
(i) Parallel to magnetic field and current flow.
(ii) Perpendicular to magnetic field and current flow.
(iii) Parallel to magnetic field but perpendicular to the direction of current flow.
(iv) Perpendicular to magnetic field but parallel to direction of current flow.
Answer: (ii) Perpendicular to magnetic field and current flow .
[ According to Fleming’s left-hand rule, the force (motion) acting on a current-carrying conductor is perpendicular to both the magnetic field and the direction of current flow.]
8. Which wire saves appliances from damage among those that come into our homes?
(i) Phase (ii) Neutral
(iii) Earth (iv) None of the above.
Answer: (iii) Earth .
[ The earth wire protects electrical appliances and users from damage and electric shock by providing a safe path for excess current to flow into the ground.]
9. Give the three reason of accident caused by electricity.
Answer: Three reasons for accidents caused by electricity are:
(i) Leakage of current
(ii) Short circuit
(iii) Overloading
10. Name one of the tool used to check the current in wire and commonly used in home and industry.
Answer: One tool used to check the current in a wire and commonly used in homes and industries is an electrical tester.
11. Sometimes while touching the electrical appliance we have a shocked. What will the commonly answer for this?
Answer: Sometimes we get an electric shock while touching an appliance because proper earthing is not there, causing leakage current to pass through the body.
12. Two coils A and B are placed close to each other. If the current in the coil A is changed, will some current be induced in the coil B? Give reason.
Answer: When the current in coil A is changed the magnetic field associated will also changes. As a result magnetic field around coil B also changes. This changes in magnetic field lines around coil B induced an electric current. This is called electromagnetic induction.
TERMINAL EXERCISE
1. Why does a compass needle get deflected when brought near a bar magnet?
Answer: A compass needle gets deflected when brought near a bar magnet because the magnetic field of the bar magnet exerts a force on the magnetic needle, causing it to change direction.
2. Explain magnetic field using the concept for magnetic line of forces.
Answer: A magnetic field is the region around a magnet where its magnetic effect can be felt. It is represented by magnetic lines of force, which originate from the north pole and enter the south pole outside the magnet. These lines show the direction and strength of the magnetic field.
3. Write down the properties of magnetic lines of force.
Answer: The properties of magnetic lines of force:
(i) Magnetic line of forces always start from north pole and end at south pole of tthe magnet.
(ii) These line of forces never intersect each other.
(iii) Near the poles magnetic lines are very near to each other which shows that magnetic field at the poles is stronger as compare to other parts.
4. Explain the force acting on current carrying conductor in a magnetic field.
Answer: When a current-carrying conductor is placed in a magnetic field, a force acts on the conductor and it starts moving. The direction of this force depends on the direction of the current and the magnetic field and can be found using Fleming’s Left-Hand Rule.
5. How is an electromagnet made from a solenoid? Explain. Write down the differences between bar magnet and electromagnet.
Answer: An electromagnet is made by placing a soft iron core inside a solenoid (coil of wire) and passing electric current through the coil. The iron core becomes magnetised and acts as an electromagnet.
The differences between bar magnet and electromagnet:
|
Bar Magnet |
Electromagnet |
|
This is a permanent magnet. Its magnetic field remains constant. |
This is a temporary magnet. Its magnetism remains till current flows through it. |
|
Its magnetic strength cannot be reduced or increased. |
Its magnetic strength may be changed at will by changing the amount of current flow. |
|
This is a weak magnet. |
This is a strong magnet. Strength of magnetic field can be controlled. |
|
The poles do not change. |
By mere change in the direction of flow of electric current the poles may be altered. |
6. What is electromagnetic induction. Explain in detail the functioning of any one appliance based on this principle.
Answer: Electromagnetic induction is the production of an electric current across a conductor moving through a magnetic field.
Appliance based on this principle – Electric Generator:
An electric generator works on the principle of electromagnetic induction. In a generator, a coil of wire rotates between the poles of a magnet. Due to the changing magnetic field around the coil, electric current is induced in the coil. This generated current is then supplied through wires for use. Thus, a generator converts mechanical energy into electrical energy.
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7. Describe the advantages of AC over DC.
Answer: dvantages of AC (Alternating Current) over DC (Direct Current):
(i) AC can be transmitted over long distances with less loss of energy.
(ii) Voltage of AC can be increased or decreased easily using transformers.
(iii) AC power generation is more economical.
(iv) AC is widely used in homes and industries.
8. What is the function of an earth wire? Why is it necessary to earth electrical appliances.
Answer: The function of an earth wire is to provide a safe path for excess or leakage current to flow into the ground.
It is necessary to earth electrical appliances because it prevents electric shock and protects appliances from damage.
9. Make such a household circuit that shows current coming from a pole to the room and at least a fan and a bulb can be lit. Explain the use of socket, switch and fuse as well.
Answer:
10. Name some devices in which electric motors are used.
Answer: Some devices in which electric motors are used are:
(i) Electric fan
(ii) Mixer grinder
(iii) Washing machine
(iv) Water pump
(v) Electric drill
(vi) Vacuum cleaner
11. How does current reach from electricity production centre to the houses?
Answer: Electric current reaches houses from the electricity production centre through transmission lines, substations, and distribution lines to homes.
12. Explain the structure and functions of AC generator.
Answer: Structure of AC Generator: An AC generator consists of a rectangular coil of insulated copper wire placed between the poles of a strong magnet. The coil is connected to slip rings and carbon brushes. The coil is rotated using mechanical energy.
Functions of AC Generator:
(i) Converts mechanical energy into electrical energy.
(ii) Produces alternating current (AC).
(iii) Supplies electricity to homes, industries, and other places through power stations.
13. Explain the magnetic effects of electric current and on the basis of this explain the functioning of the electric bell.
Answer: Magnetic effect of electric current:
When electric current flows through a wire, it produces a magnetic field around the wire. This is called the magnetic effect of electric current. A current-carrying coil behaves like a magnet.
Functioning of an electric bell:
An electric bell works on the principle of the magnetic effect of electric current. When the switch is pressed, current flows through the coil and makes it an electromagnet. The electromagnet attracts the iron armature, causing the hammer to strike the bell and produce sound. As the contact breaks, the electromagnet loses magnetism and the armature returns. This process repeats rapidly and the bell rings continuously.
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14. What is Flemings left hand rule?
Answer: Fleming’s left-hand rule: If the thumb, forefinger and middle finger of the left hand are held mutually perpendicular, then the forefinger shows the magnetic field, the middle finger shows the current, and the thumb shows the direction of force (motion).
15. When does an electric short circuit occur?
Answer: An electric short circuit occurs when the live wire and neutral wire come into direct contact, causing a sudden flow of large current.
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