Electricity and Magnetism Questions

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Very Short Answers Questions [1 Mark Questions]

Ques. A moving electric charge generates a magnetic field.

1. True
2. False

Ans. The correct answer is a. True

Explanation: A moving charge is a source of magnetic and electric fields, while a charge at rest is only the source of the electric field.

Ques. The positive charge is known as

1. Neutron
2. Electron
3. Proton
4. None of the above

Ans. The correct answer is c. Proton

Explanation: A proton is a positively charged subatomic particle that is stable. Every atom has one or more protons in its nucleus.

Ques. Which type of electricity is used to power up electronic devices?

1. Static electricity
2. Current Electricity

Ans. The correct answer is b. Current Electricity

Explanation: Current electricity allows current to move from one location to another. In current electricity, there is a steady flow of charged particles such as electrons or ions. Current electricity acts as the basis for powering any electrical or electronic equipment.

Ques. Static electricity is the electric field developed due to stationary charges.

1. True
2. False

Ans. The correct answer is a. True

Explanation: Static electricity is caused by electric charges that accumulate on the surface of surfaces or substances.

Ques. Ceramic is a _____

1. Semiconductor
2. Conductor
3. Insulator
4. None of the above

Ans. The correct answer is c. Insulator

Explanation: Ceramics, since they are fired in a kiln, may be formed into a broad range of forms with exceptional heat resistance and durability. Ceramics have traditionally been employed as insulators for these reasons.

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Short Answers Questions [2 Marks Questions]

Ques. Define electromagnetism.

Ans. Electromagnetism is a field of physics that studies electromagnetic force, which is a type of physical interaction that occurs between electrically charged particles. This electromagnetic force is carried by electromagnetic fields, which are made up of electric and magnetic fields. Furthermore, it emits electromagnetic radiation in the same way that light does.

Ques. What is a magnet?

Ans. A magnet is a substance or object that generates a magnetic field. This magnetic field is invisible, but it is responsible for a magnet's most remarkable property: a force that attracts or repels other ferromagnetic elements such as iron, steel, nickel, cobalt, and so on.

Ques. What is electricity?

Ans. Electricity is a collection of physical phenomena related to the presence and motion of matter having an electric charge. Both electricity and magnetism are connected to the phenomena of electromagnetic, as defined by Maxwell's equations.

Ques. What is magnetic force?

Ans. The magnetic force is a result of the electromagnetic force, one of nature's four fundamental forces, and is produced by charge motion. A magnetic attraction force exists between two objects carrying charge and moving in the same direction. Objects with opposing charges traveling in opposite directions have a repulsive force between them.

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Long Answers Questions [3 Marks Questions]

Ques. What are the differences between an electric field and a magnetic field?

Ans. The difference between electric and magnetic fields are

• The electric field is the region surrounding a charge where another charge experiences electric force.
• The magnetic field is the area surrounding a magnetic pole where magnetic force is experienced by a magnetic substance.
• A magnetic field can be produced by a changing electric field.
• An electric field can be produced by a changing magnetic field.
• An electric field can influence an electric charge but not a magnetic charge.
• A magnetic field can influence a magnetic charge but not an electric charge.

Ques. What are the properties of magnetic field lines?

Ans. The following are the properties of magnetic field lines

• They form closed loops.
• They never cross each other.
• When the magnetic field is strong, the magnetic field lines gather towards the pole and spread out when the field is weak.
• Within a magnet, they flow from the south pole to the north pole, and from the north pole to the south pole outside.

Ques. What are the properties of electric field lines?

Ans. The following are the properties of electric field lines

• Field lines never cross each other.
• The field lines are perpendicular to the charge's surface.
• The magnitude of charge and the number of field lines are both proportional.
• The field lines begin at the positive charge and stop at the negative charge.
• A single charge must be utilized for the field lines to begin or finish at infinity.

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Very Long Answers Questions [5 Marks Questions]

Ques. A wire of length 20 cm and 50 mg lies in a direction 30° east to north. The earth’s magnetic field at this site is horizontal and has a magnitude of 8.0 x 10^-3 T. What current must be passed through the wire so that it may float in the air? (Take g = 10 m/s²)

Ans. Given

• Length of the wire, L = 20 cm = 20 x 10^-2 m
• Mass of the wire, m = 50 mg = 50 x 10^-3 kg
• Magnetic field strength, B = 8 x 10^-3 T
• The angle between the direction of the current and magnetic field, θ = 30°

Let I be the current flowing through the wire, then the magnetic force acting on the wire is given by

F_m = BIL sinθ

This force acts in a vertically upward direction.

The gravitational force acting on the wire or the weight of the wire is given by

F_g = m_g

This force acts in a vertically downward direction.

The wire will float in the air if the force on the wire due to the magnetic field is balanced by the weight of the wire. i.e. 

F_m = F_g

⇒ BIL sinθ = mg

⇒ I = (mg)/(BL sinθ)

On substituting the values, we get

I = (50 x 10^-3 x 10)/(50 x 10^-3 x 20 x 10^-2 x sin 30°)

⇒ I = 0.63 A

Hence, the current must be passed through the wire so that it may float in the air is 0.63 A.

Ques. Copper has 8 x 10²⁸ conduction electrons per cubic meter. A copper wire of length 1 m and cross-sectional area 8 x 10^-6 m² carrying a current and lying at right angles to a magnetic field of strength 5 x 10^-3 T experiences a force of 8 x 10^-2 N. Calculate the drift velocity of the free electrons in the wire.

Ans. Given

• The number density of free electrons, n = 8 x 10²⁸
• Length of the copper wire, L = 1 m
• The cross-sectional area of the wire, A = 8 x 10^-6 m² 
• Strength of magnetic field, B = 5 x 10^-3 T
• Magnetic force experienced by the wire, F = 8 x 10^-2 N
• The angle between the current carrying wire and the magnetic field, θ = 90°

The magnetic force experienced by the current-carrying wire placed in a magnetic field is given by

F = BIL sinθ

Therefore, the amount of current passing through the wire is given by

I = F/BL sinθ

On substituting the values, we get

I = (8 x 10^-2)/(5 x 10^-3 x 1 x sin 90°)

⇒ I = 16 A

The current flowing through the wire in terms of the drift velocity of free electrons is given by

I = neAv_d

Where e is the charge on an electron and vd is the drift velocity of the free electron.

⇒ v_d = I/neA

On substituting the values, we get

v_d = 16/(8 x 10²⁸ x 1.6 x 10^-19 x 8 x 10^-6)

⇒ v_d = 1.56 x 10^-4 m/s

Ques. A magnetic field 5.0 x 10^-4 T just balances a perpendicular electric field of 15 kV m^-1 in their effect on an electron beam passing through the two fields in a direction perpendicular to both of them. What is the speed of the electrons?

Ans. Given

• The magnitude of the magnetic field, B = 5.0 x 10^-4 T
• The magnitude of the electric field, E = 15 kV m^-1 = 15 x 10³ V m^-1
• The angle between the velocity of electrons and the magnetic field, θ = 90°

The electric force acting on the electrons is given by

F_e = qE

The magnetic force acting on the electrons is given by

F_m = qvB sinθ

Since the force due to the magnetic field balances the force due to the electric field, therefore

F_e = F_m

⇒ qE = qvB sinθ

⇒ v = E/B sinθ

On substituting the values, we get

v = (15 x 10³)/(5 x 10^-4 x sin 90°)

⇒ v = 3 x 10⁷ m/s

Hence the speed of the electron is 3 x 10⁷ m/s.

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