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Properties of magnet - class-VII

Description: properties of magnet
Number of Questions: 22
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Tags: physics properties of magnet magnetism
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A DMM is arragned at the magnetic pole of earth in $\tan{A}$ position. If a bar magnet is placed at some distance from the needle, deflection is

  1. ${0}^{o}$

  2. ${90}^{o}$

  3. ${45}^{o}$

  4. ${180}^{o}$


Correct Option: A

The needle of a dip circle shows an apparent dip of $45^o$ at a place and $60^o$ when the circle is rotated through $90^o$. The true dip is

  1. $cos^{-1} \left (\cfrac 1{\sqrt 3}\right)$

  2. $cos^{-1} \left (\cfrac {\sqrt 3}{2}\right)$

  3. $cos^{-1} \left (\cfrac {1}{2}\right)$

  4. $cos^{-1} \left (\cfrac { 1}{\sqrt2}\right)$


Correct Option: A

If earth's magnetic field $B _H$, if the frequency of oscillation of a magnetic needle is n, then -

  1. $ n \propto B _H$

  2. $ n^2 \propto B _H$

  3. $ n \propto B _H^2$

  4. $n^2 \propto \frac{1}{B _H}$


Correct Option: B

An aeroplane, in which the distance between the tips of the wings is $50 m$, is flying horizontally with a speed of $360$ km/hour, over a place where the vertical component of earth's magnetic field is $2.0 \times 10^{-4} \ tesla$ . The potential difference between the tips of the wings would be

  1. $0.1 V$

  2. $0.5 V$

  3. $0.2 V$

  4. $1.0 V$


Correct Option: D

In an uniform field the magnetic needle completes 10 oscillations in 92seconds. When a small magnet is placed in the magnetic meridian 10cm due north of needle with north pole towards south completes 15 oscillations in 69seconds. The magnetic moment of magnet ($B _H =0.3 \ G$) is

  1. $4.5 \ A m^2$

  2. $0.45 \ A m^2$

  3. $0.75 \ A m^2$

  4. $0.225 \ A m^2$


Correct Option: A

A magnetic needle is kept in a nonuniform magnetic field. It

  1. may experience a force but certainly not a torque

  2. may experience a torque but certainly not a force

  3. may experience both a torque as well as force

  4. will certainly experience neither a force not a torque


Correct Option: C
Explanation:

If magnet is placed parallel to the magnetic field then two poles of magnetic will experience different forces because of non uniform magnetic field. The forces will be in opposite direction therefore, there will be a net force also because forces are not equal. 
Now , when magnetic is not parallel to the magnetic field it will experience torque its a dipole placed in a field. Hence, (C)

The needle of a dip circle when place at a geomagnetic pole stays along

  1. south-north direction only

  2. east-west direction only

  3. vertical direction

  4. horizontal direction


Correct Option: C
Explanation:

Dip is the angle between the Earth's magnetic field and horizontal plane at that point. The needle of a dip circle when place at a geomagnetic poles stays along vertical direction.

Which of the following helps detecting the presence of magnetic field at a point ?

  1. a strong magnet

  2. a solenoid

  3. a compass needle

  4. a current carrying wire


Correct Option: C
Explanation:

A compass consists of a magnetized needle which tends to align in presence of a magnetic field, thus causing a deflection in the needle.

What kind of magnet  is used in a magnetic compass ?

  1. electromagnet

  2. permanent magnet

  3. temporary magnet

  4. none of the above


Correct Option: B
Explanation:

Magnetic compass is used for navigation and its magnetic needle is made of permanent magnet which is always aligned according to the earth's magnetic field. 

What is the circular enclosure that the magnetic compass is placed in ?

  1. iron case

  2. stainless steel case

  3. aluminium case

  4. nickel case


Correct Option: B
Explanation:

The case of a magnetic compass should be made of a non-magnetic material. 

Stainless steel serves the purpose. 

A device used by the sailors to find the direction in which their ship is moving is called .................

  1. horse shoe

  2. curve

  3. magnetic field

  4. magnetic needle


Correct Option: D
Explanation:

A device used by the sailors to find the direction in which their ship is moving is called magnetic needle.
A compass contains a magnetic needle pivoted in a horizontal plane that indicates the direction of magnetic north at points on the earth's surface. It is an instrument containing a freely suspended magnetic element which displays the direction of the horizontal component of the Earth's magnetic field at the point of observation.
It is the oldest instrument for navigation and has been a vital tool for navigators at sea for centuries.

State whether true or false:

A tracing needle is used by the sailor for navigation.  

  1. True

  2. False


Correct Option: B
Explanation:

Magnetic compass is used for navigation while tracing needle is used for tracing magnetic field lines. 

The presence of magnetic field at a point can be detected by

  1. ammeter 

  2. galvanometer

  3. a compass needle

  4. strong magnet


Correct Option: C
Explanation:

A compass can be used to detect presence of magnetic field as it will deflect from its N-S direction to direction if magnetic field at that point.

The magnetic compass is not useful for navigation near the magnetic poles, since

  1. $R=0$

  2. $V=0$

  3. $H=0$

  4. $\theta={0}^{o}$


Correct Option: C
Explanation:

Near magnetic poles, $H=0$, therefore, magnetic compass will not work.

Two parallel conductors are separated by 5cm. They carry 6A and 2A in the same direction. Between them the location from first at which magnetic needle lies along $B _H$ is

  1. 1.25 cm

  2. 3.75 cm

  3. 2.5 cm

  4. no point exist


Correct Option: B

A magnetic needle oscillates in a horizontal plane with a period $T$ at a place where the angle of dip is $60^0$. When the same needle is made to oscillate in a vertical plane coinciding with the magnetic meridian, its period will be 

  1. $\frac{T}{\sqrt 2}$

  2. T

  3. 2T

  4. 6T


Correct Option: A

When a current of $1$ ampere is passed in a coil lying in the magnetic meridian then a magnetic needle at its centre gives some deflection. If the current in the coil is increased to $\sqrt8$ ampere then at what distance from the centre of the coil will the deflection of needle remains underchanged?

  1. $2R$

  2. $4R$

  3. $8R$

  4. $R$


Correct Option: B

The magnetic fields of atoms in a material are grouped together and aligned to form _________.

  1. Strong magnetic fields

  2. An area of magnetic field

  3. A region called magnetic domain

  4. A bar magnet


Correct Option: C
Explanation:

A magnetic domain is a region within a magnetic material in which the magnetization is in a uniform direction. This means that the individual magnetic moments of the atoms are aligned with one another and they point in the same direction.

The magnetic fields of atoms in a material are grouped together and aligned to form  region called magnetic domain

The smallest magnet available is ___________.

  1. a bar magnet with an area of $1 mm^2$

  2. a spinning electron

  3. a magnetic dipole

  4. a molecule or atom with a magnetic dipole


Correct Option: B
Explanation:

An electron is not a spinning ball of charge and the intrinsic spin of particles cannot be understood in such terms. a value of the ratio between the magnetic moment and the angular momentum that is a factor 2 too small.

The smallest magnet available is a spining electron.

The magnetic domains of an unmagnetised object are found ___________.

  1. Parallel to each other

  2. Perpendicular to each other

  3. Pointing towards south

  4. Pointing in different direction


Correct Option: D
Explanation:

A magnetic domain is the region in which the magnetic fields of atoms are grouped together and aligned. In an unmagnetized object the magnetic domains are pointing at different directions.

The primary origin of magnetism lies in the

  1. atomic current and intrinsic spin of electrons.

  2. polar and non polar nature of molecules.

  3. pauli exclusion principle.

  4. electronegative nature of materials


Correct Option: A
Explanation:

Atomic currents and intrinsic spins of electrons are responsible for origin of magnetism.

If we go inside a mine and drop a $104 lb$  iron ball and $1$ lb aluminum ball from the top of a high platform, neglecting air resistance.

  1. Both will reach the floor at the same time

  2. $1$ lb weight will reach the floor first

  3. $10$ lb weight will reach the floor first

  4. It is not possible to indicate which of the two will reach the floor first without further data


Correct Option: A
Explanation:
In real life the one having less volume and more weight will reach first

but in ideal situation where viscosity of air is neglected both will hit ground at same time irrespective of their masses if dropped from the same height 

this is because the acceleration produced due to gravity does not depend on the body.


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