Class 10 Science Lesson 11

Exercise

1. Choose the correct options for the following questions.

(a) Which of the following is the source of a.c.?

(i) dry cell
(ii) solar panel
(iii) dynamo
(iv) voltaic cell

Answer: (iii) dynamo

(b) Which scientist discovered the magnetic effect of electric current?

(i) Michael Faraday
(ii) Hans Christian Oersted
(iii) John Ambrose Fleming
(iv) James Clerk Maxwell

Answer: (ii) Hans Christian Oersted

(c) Which is the direction of the magnetic field when current is flowing upwards through a conducting straight wire?

(i) anticlockwise direction
(ii) clockwise direction
(iii) perpendicular to the direction of electric current
(iv) opposite to the direction of the electric current

Answer: (i) anticlockwise direction

(d) Which of the following statements is true for the current source shown in the figure?

(i) The value of electric current produced by A is constant.
(ii) The frequency of electric current produced by B is constant.
(iii) The brightness of the lamp fluctuates if the current produced by A is used.
(iv) The direction of the electric current produced by B changes constantly.

Answer: (iv) The brightness of the lamp fluctuates if the current produced by A is used.
[ Hint: emf produced depends upon the speed of the bicycle and brightness depends upon emf ]

(e) On which of the following principles is the working of a transformer based?

(i) Electromagnetic induction
(ii) Mutual induction
(iii) Motor effect
(iv) Lighting effect of current

Answer: (ii) Mutual induction

(f) Which is the transformer’s formula?

(i) Vp/Vs = Ns/Np
(ii) Vs/Np = Ns/Vp
(iii) Vs/Vp = Ns/Np
(iv) Vs/NP= Vp/Vs

Answer: (i) Vs/Vp = Ns/Np

2. Differentiate between:

(i) a.c and d.c

(ii) dynamo and generator

(iii) motor and generator

(iv) step-up transformer and step-down transformer

3. Give reasons:

(a) When a ceiling fan is connected to the circuit of the solar panel, the fan does not rotate.

Ans: This happens because solar panels typically generate direct current (d.c.), but most ceiling fans are designed to operate on alternating current (a.c.). The motor inside the fan requires a changing current direction to create the rotating magnetic field necessary for rotation. Additionally, the solar panel may not supply enough voltage or current to start the fan motor, as ceiling fans require a higher and steady power input that small solar panels often cannot provide.

(b) When a magnetic compass is placed near a circuit in which an electric current is flowing, its needle deflects.

Ans: Electric current flowing through a wire produces a magnetic field around the wire, as discovered by Hans Christian Oersted. When a compass is brought near the current-carrying conductor, the magnetic field generated interacts with the magnetic needle of the compass, causing it to change direction or deflect. This deflection indicates the presence and direction of the magnetic field caused by the electric current.

(c) Electromagnet is used in the electric bell.

Ans: An electromagnet can be turned on or off by controlling the electric current flowing through its coil. When current flows, the electromagnet produces a strong magnetic field, which attracts a metal hammer to strike the bell, producing sound. When the current is interrupted, the magnetic field collapses, and a spring returns the hammer to its original position, ready to strike again. This on-off operation of the electromagnet allows the electric bell to ring repeatedly.

(d) The number of primary windings and secondary windings of a transformer are not the same.

Ans: Transformers work on the principle of mutual induction, where a changing current in the primary coil induces a voltage in the secondary coil. To increase or decrease voltage, the number of turns in the primary and secondary coils are adjusted accordingly. If the secondary coil has more turns, the transformer steps up the voltage; if it has fewer turns, the transformer steps down the voltage. This difference in the number of windings controls the voltage transformation ratio.

(e) The core of a transformer is laminated.

Ans: The transformer core is made of thin sheets of iron insulated from each other (laminations) to reduce eddy currents. Eddy currents are loops of electric current induced inside the core due to changing magnetic fields, which cause energy loss in the form of heat. Laminating the core increases the electrical resistance and breaks the path of eddy currents, significantly reducing these losses and improving transformer efficiency.

(f) Transformers are used in mobile chargers.

Ans: Transformers in mobile chargers step down the high voltage alternating current (usually 220 V) from the electrical outlet to a low voltage alternating current (e.g., 5 V or 12 V) suitable for the charger circuit. This reduced voltage is then converted to direct current required to safely charge the mobile device’s battery. Using a transformer makes charging safe, efficient, and compatible with household power supplies.

4. Answer the following questions:

(a) The frequency of a.c. in our country is 50 Hz, what does it mean?

Answer: The frequency of alternating current (a.c.) being 50 Hz means that the current changes its direction 50 times every second. In other words, the current completes 50 full cycles of changing from positive to negative and back to positive every second. This is the standard frequency for the electrical power supply in many countries, including ours, ensuring the smooth operation of electrical appliances designed for this frequency.

(b) Draw the time graph of direct current and alternating current.

Answer:

(c) Draw the magnetic field lines around the current-carrying straight wire and solenoid.

Answer:

Magnetic field lines around the current-carrying straight wire

Magnetic field lines around the current-carrying Solenoid

(d) Explain the following rules:

(i) Maxwell’s right-hand thumb rule:

Answer: This rule helps to find the direction of the magnetic field produced around a straight current-carrying wire. When you hold the wire with your right hand such that your thumb points in the direction of the electric current, the curled fingers around the wire show the direction of the magnetic field lines.

(ii) Maxwell’s right-hand grip rule:

Answer: This rule applies to solenoids or coils. When you curl the fingers of your right hand in the direction of the current flowing through the coils, your thumb points towards the north pole of the solenoid, showing the direction of the magnetic field inside the solenoid.

(e) What is the magnetic effect of current?

Answer: The magnetic effect of current refers to the phenomenon where an electric current flowing through a conductor produces a magnetic field around it. This was first discovered by Hans Christian Oersted, and it forms the basis of electromagnetism, which is the principle behind devices like electromagnets, electric motors, and generators.

(f) Define magnetic flux.

Answer: Magnetic flux is the measure of the total magnetic field passing through a given area. It is defined as the product of the magnetic field strength and the area perpendicular to the field through which the lines of magnetic force pass. Magnetic flux is measured in Weber (Wb).

(g) How can the magnetic field produced around a straight current-carrying wire be demonstrated by using iron dust, cardboard, and conducting straight wire? Explain it.

Answer:

Topic: To demonstrate the magnetic field around the current carrying straight conductor 

Materials required: current carrying conductor, cardboard and iron dust 

Procedure: 

1) We should take a cardboard and pass a straight conducting wire through its centre. 

2) We should sprinkle iron filling on the cardboard 

3) We should pass current through the wire with a battery. 

4) We should shake the cardboard a bit.

Observation: The iron dust will be arranged in a circle around the wire. 

Result and conclusion: The magnetic field around the straight current-carrying conductor is circular.

(h) Draw the magnetic field developed around a straight current-carrying wire.

Answer:

(i) What is a solenoid? Draw a picture showing the magnetic field developed around a solenoid.

Answer: A solenoid is a long coil of insulated wire wound in the shape of a helix. When an electric current passes through the solenoid, it creates a magnetic field similar to that of a bar magnet, with a distinct north and south pole. The magnetic field lines inside the solenoid are nearly parallel and closely spaced, indicating a strong uniform magnetic field.

(j) Write two uses of the solenoid.

Answer:

1. Electromagnets: Solenoids are used to create electromagnets which are used in electric bells, relays, and cranes for lifting heavy magnetic materials.
2. Valves and switches: Solenoids are used in controlling valves and switches in various machines, allowing remote control of mechanical systems using electric current.

(k) Which effects are demonstrated in the given figure? 

Answer:

Effect Demonstrated:

Motor Effect: When current flows through a conductor placed in a magnetic field, the conductor experiences a force. This force is perpendicular to both the direction of current and the magnetic field, causing motion.
The figure demonstrates the motor effect, where a current-carrying conductor placed in a magnetic field experiences a force.

(l) Explain the working process of the simple electric motor based on motor effect.

Answer:

Working Process:
A simple electric motor works on the motor effect, which states that a current-carrying coil placed in a magnetic field experiences a force and starts to rotate.

Step-by-Step Explanation:

1. Current Supply:

• When the dry cell is connected, electric current flows through the coil.

2. Magnetic Field:

• The coil is placed between the poles of a permanent magnet, creating a strong magnetic field.

3. Force on Coil:

• According to Fleming’s Left-Hand Rule, the current in the coil interacts with the magnetic field, generating forces on the sides of the coil in opposite directions. One side moves up, and the other side moves down.

4. Rotation:

• This creates a turning effect (torque) that makes the coil rotate.

5. Continuous Motion:

• As the coil rotates, the contacts (paper clips) reverse the current direction every half turn, ensuring the coil continues spinning in the same direction.

(m) What is electromagnetic induction?

Answer: Electromagnetic induction is the process of generating an electric current in a conductor by changing the magnetic field around it. This phenomenon occurs when a conductor moves through a magnetic field or when the magnetic field around a stationary conductor changes with time. It is the fundamental principle behind the working of devices such as generators, transformers, and inductors.

(n) Study the given picture and write what happens in the following situations:

(i) As the bar magnet is slowly introduced into the solenoid:

Ans: A small induced current is generated in the solenoid as the magnetic flux through it changes slowly.

(ii) While introducing the bar magnet rapidly into the solenoid:

Ans: A larger induced current is produced because the magnetic flux changes more rapidly, causing a stronger induced electromotive force (emf).

(iii) Holding the bar magnet stationary inside the solenoid:

Ans: No current is induced because there is no change in the magnetic flux; the magnetic field inside the solenoid remains constant.

(iv) On pulling the bar magnet quickly out of the solenoid:

Ans: A significant induced current flows in the opposite direction compared to when the magnet was introduced because the magnetic flux is decreasing rapidly.

(o) State Faraday’s law of electromagnetic induction.

Answer:

Faraday’s laws are:

1. Whenever magnetic flux linking a coil or a conductor changes , an emf is induced in it.
2. The magnitude of induced emf is directly proportional to the rate of change of magnetic flux.
3. The induced emf in the coil circuit lasts as long as the change of magnetic flux linking the coil continues.

(p) A bulb connected to a dynamo attached to the tire of a bicycle is not found to be glowing with steady brightness. It was found that the bulb was bright, dimmed, and also turned off when the cycle came to rest. Mention the reasons for such observations based on the working principle of dynamo.

Answer: The brightness of the bulb depends on the speed at which the dynamo rotates. When the bicycle moves fast, the dynamo rotates quickly, inducing a large current, which makes the bulb glow brightly. When the bicycle slows down, the dynamo spins slower, inducing less current and causing the bulb to dim. If the bicycle stops, the dynamo stops rotating, no current is induced, and the bulb turns off. This happens because the dynamo works on electromagnetic induction, which requires motion to change magnetic flux and generate current.

(q) What can be done to increase the magnitude of current produced by a dynamo? Write any two ways.

Answer:

1. Increase the speed of rotation: Rotating the dynamo faster increases the rate of change of magnetic flux, producing a higher induced emf and hence more current.
2. Increase the number of turns in the coil: More coil turns mean more magnetic flux linkage, which increases the induced emf for the same rate of flux change.

(r) Prepare a research report on any two sources of electricity in Nepal (Hydro power station, solar power plant) including their capacity, type of electricity produced, and transmission.

Answer: Nepal primarily generates electricity through hydropower, utilizing the vast river systems and mountainous terrain. For example, the Upper Tamakoshi Hydropower Station has a capacity of 456 MW and produces alternating current (a.c.) electricity, which is transmitted via high voltage transmission lines to different parts of the country. Additionally, solar power plants are emerging as a clean energy source. The Butwal Solar Park in Nepal produces around 4 MW of power using photovoltaic cells, also generating a.c. power through inverters. Solar electricity is mainly used in remote areas and connected to local grids via transformers and transmission lines.

(s) What is a transformer?

Answer: A transformer is an electrical device used to change the voltage level of alternating current (a.c.) electricity. It consists of two coils, called the primary and secondary coils, wound around a common iron core. When an alternating voltage is applied to the primary coil, it produces a changing magnetic field that induces a voltage in the secondary coil. Depending on the number of turns in each coil, the transformer can either increase (step-up transformer) or decrease (step-down transformer) the voltage while maintaining the same frequency.

(t) Write the type of transformers J and K shown in the figure.

Answer: The transformer J is step-up transformer. And the transformer K is a step-down transformer.

(u) Draw the block diagrams of the step-up transformer and step-down transformer and write two uses of each.

Answer:

Step-up Transformer:

Uses:

1. Used in power stations to increase voltage for efficient transmission over long distances, reducing power loss.
2. Used in electrical appliances like cathode ray oscilloscopes to generate high voltages.

Step-down Transformer:

Uses:

1. Used near homes and industries to reduce high transmission voltage to safe levels for use.
2. Used in battery chargers and adapters to step down the voltage from the mains supply.