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Magnetism is the property displayed by magnets and is produced by the movement of electric charges. The chapter deals with the flow of the electric charge in a conductor is called electromagnetism. Some of the essential topics in the chapter are Oersted’s Experiment, Biot-Savart’s Law, Ampere’s, etc.

Magnetism and Moving Charges contain a weightage of about 6% in **NEET** and 3-4 questions are always expected from this topic in the exam. Read the article to get detailed information about various topics of magnetism and moving charges with solved questions and important formulas.

### Oersted’s Experiment:

- He noticed a noticeable deflection in a magnetic compass needle placed nearby a straight wire with current flowing in it.
- The deflection increases on increasing the current or bringing the needle closer to the wire. Thus, Oersted concluded that,

“Moving charges or currents produced a magnetic field in the surrounding space.”

Use of this property: Even a hidden live wire can be located because of this property of electric current.

### Magnetic Field

The space around the current carrying conductor within which its influence can be felt by the magnetic needle is called Magnetic Field of the current carrying conductor.

### Biot-Savart’s Law and its Applications

- Biot-Savart’s Law states that the magnitude of the magnetic field
*dB*at any point due to a small current element dl is given by,

where,

I is the magnitude of the current

dl is the length of the element

ϴ is the angle between the length of element and the line joining the element to the point of observation

r is the distance if the point from the element

### Application of Biot-Savart’s Law

- The magnitude if magnetic field B at any point at a perpendicular distance ‘a’ from a straight conductor carrying current
*‘I’*is given by

Where, ϴ1 and ϴ2 are the angles which the perpendicular from the point of observation to the conductor makes with the lines joining the two ends of the conductor to the point of observation as shown in the diagram.

For infinitely long straight conductor and point P lies near the end Y(or X), then

ϴ1 = 90 and ϴ2 = 0, Hence

- The magnitude of magnetic field B at any point on the axis of the circular coil at a distance ‘a’ from its centre is given

where n is the number of turns of the circular coil

I is the current flowing through the circular coil, and

a is the radius of the circular coil. For anti-clock wise current, the field will be outward along the axis.

If point P lies at the centre of the coil, then x = 0

Therefore,

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### Ampere’s Circuital Law

“The line integral of magnetic field around any closed path in vacuum is equal to times the total current threading the closed path,” i.e.,

#### Application of Ampere’s Circuital Law

- The magnetic field due to infinite long straight wire carrying current is calculated by applying Ampere’s Law i.e.,

The direction of is determined by Right-hand thumb rule.

- At a point inside the current carrying solenoid magnetic field is uniform and parallel to the length of solenoid and is given by

where n is the number of turns per unit length of the solenoid and I is the current flowing through the solenoid.

- The magnitude of magnetic field B at a point (P) inside the turns of a toroid is given by,

where n is the number of turns per unit length of toroid and I is the current flowing through the toroid.

#### Sample Questions on Biot- Savart’s Law and Ampere’s Circuital Law

- A length of wire carries a steady current I. It has bent first to form a circular plane coil of one turn. The same length is now bent more sharply to give a double loop of smaller radius. The magnetic field at the centre caused by the same current is
- A quarter of its value
- Unaltered
- Four times of its first value
- A half of its first value

Solution: C

- A vertical straight conductor carries a current upwards. A point P lies to the east of it at a small distance and another point Q lies to the west at the same distance. The magnetic field at P is
- Greater than at Q
- Same as at Q
- Less than at Q
- Greater or less than at Q depending upon the strength of the current

Solution: B. If distance is same field will be same

- If a copper rod carries a direct current, the magnetic field associated with the current will be
- Only inside the rod
- Only outside the rod
- Both inside and outside the rod
- Neither inside nor outside the rod

Solution: C. Magnetic field lies inside as well as outside the solid current carrying conductor.

### Force on a Moving charge in uniform magnetic and electric field

- The magnitude of the Force F on the charge ‘q’ moving with a velocity ‘v’ in a magnetic field ‘B’ making an angle ϴ between the direction of v and B is given by

The direction of F is perpendicular to v as well as B and it is perpendicular to the plane in which v and B lie.

- If charge is at rest v = 0: F = 0. It experiences no force.
- If charge is moving along the direction of B, ϴ =0; F=0.
- If charge is moving perpendicular to the direction of B, ϴ = 90

F = q v B (Maximum Force)

- The magnitude of the electrostatic force F on a charge q in an electric field E is given by

- The magnitude of total force experienced by a moving charged particle in both electric and magnetic field is called Lorentz Force which is given by

- Rate of work done by the magnetic field on the charge ‘q’ is zero

As power =

So, KE of a charge is never altered by the magnetic field.

- The path followed by a charge in a magnetic field,
- Is a circle if v parallel to B is equal to 0 and v perpendicular to B is not equal to zero.
- Is a helix if v parallel to B is constant and not equal to zero and v perpendicular to B is not equal to zero.

- The pitch of the helical path is given by,

### Sample Questions based on the above Topic

- A uniform electric field and a uniform magnetic field are acting along the same direction in a certain region. If an electron is projected in the region such that its velocity is pointed along the direction of fields, then the electron
- Will turn towards right of direction of motion
- Speed will decrease
- Speed will increase
- Will turn towards left of direction of motion

Solution: B

- When an electron enters perpendicularly in a magnetic field with velocity v, time period of its revolution is T. If it enters in the same magnetic field with a velocity 2v, then its time period will be:
- 2T
- 4T
- T/2
- T

Solution: D

### Cyclotron

The charged particle (positive charge or proton or alpha particle but not electrons and neutrons) is allowed into a cyclotron which accelerates the charged particle by suitable synchronization of magnetic and electric field. The cyclotron frequency is given by

(Independent of velocity and radius of circular path)

where T is the time period of alternating electric field.

#### Sample Questions

- The Angular Frequency of a cyclotron is independent of
- Speed
- Mass
- Magnetic Field
- Charge

Answer: A. Speed

### Torque experienced by a current loop in a uniform magnetic field-galvanometer, ammeter and voltmeter

The magnitude of the torque on a current carrying rectangular coil placed in an uniform magnetic field (B) is given by,

Where, N – number of turns in the rectangular coil

I – current flowing through the coil

A – area of the rectangular coil

ϴ – angle which the plane of the coil makes with the direction of the magnetic field.

If normal unit vector to the plane of the coil makes an angle with the direction of the magnetic field, then

Where M is the magnitude of magnetic moment of the current carrying loop and is equal to

M = N*I*A

Its SI unit is A-m2.

**Galvanometer:**A galvanometer is an instrument used for detection of small currents in electrical circuits. It is always connected in series in the circuit. It is based on the principle that when a current carrying conductor is placed in a magnetic field, it experiences a torque.**Ammeter:**An ammeter is an instrument used for measurement of current in electrical circuits. It has low resistance and is always connected in series in the circuit.**Voltmeter:**It is an instrument used for measurement of potential difference across various parts of electrical circuits. It has high resistance and is always connected parallel to the part across which potential difference is to be measured.**Current Sensitivity:**It is defined as the deflection produced in the galvanometer when unit current is passed through its coil.- A galvanometer can be converted into an ammeter of a given range of current (I) by connecting a low resistance in parallel to the galvanometer.

where, G – Resistance

S – Low resistance in parallel to galvanometer, called SHUNT

IG – Current flowing through the coil of the galvanometer for full scale deflection

- A galvanometer can be converted into a voltmeter of a given range of potential difference (V) by connecting a high resistance in series with the galvanometer.

#### Sample Question

- A milli voltmeter of 25 milli volt range is to be converted into an ammeter of 25 ampere range. The value in ohms of necessary shunt will be
- 0.001
- 0.01
- 1
- 0.05

Solution: A

### Magnetic Properties of Materials

Materials are classified into Dia, para or ferromagnetic based on their behavior in various conditions.

- Diamagnetism: Diamagnetic substances are those which have tendency to move from stronger to the weaker part of the external magnetic field.
- Paramagnetism: Paramagnetic substances are those which get weakly magnetized when placed in an external magnetic field. They get weakly attracted to a magnet.
- Ferromagnetism: Ferromagnetic substances are those which get strongly magnetized when placed in an external magnetic field. They got strongly attracted to a magnet.

#### Permanent Magnets and Electromagnets

- Substances which at room temperature retain their ferromagnetic property for a long period of time are called permanent magnets.
- Electromagnets are used in electric bells, loudspeakers and telephone diaphragms. Giant electromagnets are used in cranes to lift machinery, and bulk quantities of iron and steel.

#### Sample Questions

- Magnets cannot be made from which of the following substances?
- Iron
- Nickel
- Copper
- All of the above

Solution: C

- Which of the following is the most suitable for the core of electromagnets
- Soft Iron
- Steel
- Copper-nickel alloy
- Air

Solution: A. Soft Iron is highly ferromagnetic.

#### Previous Year Solved Sample Questions

- An arrangement of three parallel straight wires placed perpendicular to plane of paper carrying same current ‘I’ along the same direction as shown in fig. Magnitude of force per unit length on the middle wire ‘B’ is given by

Solution: C

- A long straight wire of radius a carries a steady current I. The current is uniformly distributed over its cross-section. The ratio of the magnetic fields B and B’, at radial distances a/2 and 2a respectively, from the axis of the wire is
- 1
- 4
- ¼
- ½

Solution: A

- An electron moving in a circular orbit of radius r makes n rotations per second. The magnetic field produced at the centre has magnitude

Solution: (b)

- Two identical long conducting wires AOB and COD are placed at right angle to each other, with one above other such that O is their common point for the two. The wires carry I1 and I2 currents, respectively. Point P is lying at distance d from O along a direction perpendicular to the plane containing the wires. The magnetic field at the point P will be

Solution: (d)

- A long straight wire carries a certain current and produces a magnetic field 2× 10-4 Wb m-2 at a perpendicular distance of 5 cm from the wire. An electron situated at 5 cm from the wire moves with a velocity 107 m/s towards the wire along perpendicular to it. The force experienced by the electron will be
- 3.2 N
- 3.2 × 10-16 N
- 1.6 × 10-16 N
- Zero

Solution: B

- A circular coil ABCD carrying a current ‘i’ is placed in a uniform magnetic field. If the magnetic force on the segment AB is F, the force on the remaining segment BCDA is

– F B.3F C.-3F D.F

Solution: A

- A galvanometer of resistance, G, is shunted by a resistance S ohm. To keep the main current in the circuit unchanged , the resistance to be put in series with the galvanometer is

Solution: (d)

**Download NEET Sample Questions**

#### Formula Used

- Biot-Savart’s Law

- Ampere’s Circuital Law

- Force F on the charge ‘q’ moving with a velocity ‘v’ in a magnetic field ‘B’ making an angle ϴ between the direction of v and B

- The magnitude of the electrostatic force F on a charge q in an electric field E is given by

- The magnitude of total force experienced by a moving charged particle in both electric and magnetic field is called Lorentz Force which is given by

- Cyclotron

- Galvanometer can be converted into an ammeter by connecting a low resistance in parallel to the galvanometer

- Galvanometer can be converted into a voltmeter by connecting a high resistance in series with the galvanometer

### PREPARATION PLAN & STRATEGY

Some tips to follow while preparing for **NEET 2021**:

- Practice about 90 questions daily.
- Check and analyze the mistakes and try not to repeat them.
- Build the basic concepts as most of the questions were asked from the basics.
- Prepare the weak topics again and again to score better in it.
- Keep a copy of syllabus with you while studying.
- Set small goals daily and try to achieve them.
- Give equal time to all the subjects.

**Check Mobile App and Online Resources for NEET Preparation**

### Monthly Plan for Physics

You should study whole chapter thoroughly in 6 days and revise it in the 7th day. After that you will not forget the data you are feeding in your mind.

Duration | Topics to be covered |
---|---|

1st Week (6 days) | Electrostatics |

2nd Week (6 days) | Current Electricity |

3rd Week (6 days) | Magnetic Effects of Current and Magnetism |

4th Week (6 days) | Electromagnetic Induction and Alternating Currents |

5th Week (6 days) | Electromagnetic Waves |

6th Week (6 days) | Optics |

7th Week (6 days) | Dual Nature of Matter and Radiation |

8th Week (6 days) | Atoms and Nuclei |

9th Week (6 days) | Electronic Devices |

The following strategy demonstrates the minimum time you would need to prepare for NEET.

Total no. of Chapters in Physics | 19 |

Total no. of Chapters in Chemistry | 31 |

Total no. of Chapters in Biology | 9 |

Number of Days left for the exam | 90 Days or 3 Months |

Days to revise each subject | 30 Days each |