Class 10 Science SEE Guide | Unit - 11 Electricity and Magnetism Guide 2080

Class 10 Science SEE Guide

Unit - 11

Electricity and Magnetism Guide

Exercise

1. Choose the correct option for the following questions:

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

(i) drycell

(iii) dynamo

(ii) solar panel

(iv) voltaic cell

Answer 👉 (iii) dynamo

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

(i) Michael Faraday

(iii) John Ambrose Fleming

(ii) Hans Christian Oersted

(iv) James Clerk Maxwell

Answer 👉 (ii) Hans Christian Oersted

(c) What 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

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

(i) Electromagnetic induction

(iii) Motor effect rent

(ii) Mutual induction

(iv) Lighting effect of cur-

Answer 👉 (ii) Mutual induction

2. Differentiate between:

(i) a.c and d.c

Answer 👉 AC (alternating current) and DC (direct current):

AC: Alternating current is a type of electrical current where the flow of electrons periodically reverses direction. It is commonly used in homes and businesses to power appliances and devices. AC voltage and current alternate in a sinusoidal manner, constantly changing from positive to negative. AC is typically generated by power stations and transmitted over long distances.
DC: Direct current is a type of electrical current where the flow of electrons only travels in one direction. It is often used in small electronic devices, batteries, and certain industrial applications. DC voltage and current remain constant over time without changing direction. DC can be obtained from batteries, rectifiers, and power supplies.

(ii) dynamo and generator

Answer 👉

Dynamo: A dynamo is an older term used to refer to a type of electrical generator that produces DC (direct current). It typically uses a commutator and brushes to convert mechanical energy into electrical energy. Dynamos were commonly used in early electrical systems but have been largely replaced by more efficient generators.
Generator: A generator is a device that converts mechanical energy into electrical energy. It can produce either AC or DC depending on the design. Generators are widely used in power stations, where mechanical energy, such as that from steam turbines or hydro turbines, is used to rotate a magnetic field relative to conductors, inducing an electric current.

(iii) motor and generator

Answer 👉

Motor: A motor is a device that converts electrical energy into mechanical energy. It typically operates on the principle of electromagnetic induction, where the interaction between electric currents and magnetic fields generates rotational motion. Motors are used in various applications, including machinery, vehicles, appliances, and industrial equipment.
Generator: As mentioned earlier, a generator is a device that converts mechanical energy into electrical energy. It operates in the opposite way to a motor. While a motor converts electrical energy into mechanical energy, a generator converts mechanical energy into electrical energy.

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

Answer 👉

Step-up transformer: A step-up transformer is a type of transformer that increases the voltage level from the input (primary) side to the output (secondary) side. It has more turns in the secondary coil than in the primary coil, resulting in a higher output voltage. Step-up transformers are commonly used in power transmission systems to increase voltage levels for long-distance transmission, reducing power losses.
Step-down transformer: A step-down transformer is a type of transformer that decreases the voltage level from the input (primary) side to the output (secondary) side. It has fewer turns in the secondary coil than in the primary coil, resulting in a lower output voltage. Step-down transformers are commonly used in power distribution systems to reduce high voltage levels to safer and more usable levels for residential, commercial, and industrial applications.

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3. Give reasons:

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

Answer 👉 Reason: Solar panels generate DC (direct current) electricity. Most ceiling fans are designed to operate on AC (alternating current). Therefore, when a ceiling fan is connected directly to the circuit of a solar panel, which provides DC power, the fan does not rotate because it requires AC power to operate. The DC power from the solar panel is not compatible with the fan's motor.

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

Answer 👉 Reason: Electric current produces a magnetic field around the conductor through which it flows, according to Ampere's law. When a magnetic compass is placed near a circuit with an electric current, the magnetic field produced by the current interacts with the Earth's magnetic field, causing the needle of the compass to deflect. This effect is known as the magnetic effect of electric current.

( c) Electromagnet is used in the electric bell.

Answer 👉 Reason: An electric bell requires a mechanism to produce sound when the circuit is closed. An electromagnet is used in an electric bell to generate a magnetic field when an electric current passes through it. This magnetic field attracts a metal striker or an armature, causing it to strike a bell or a gong and produce sound. When the circuit is open, the electromagnet is deactivated, and the striker returns to its original position.

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

Answer 👉 Reason: Transformers are based on the principle of electromagnetic induction. The primary winding is connected to the input voltage source, while the secondary winding is connected to the output load. The number of windings in each coil determines the voltage ratio between the primary and secondary sides. If the number of windings in the primary coil is different from the number of windings in the secondary coil, the transformer can step up or step down the voltage. This allows transformers to efficiently transfer electrical energy between different voltage levels.

( e) The core of a transformer is laminated.

Answer 👉 Reason: The core of a transformer is made up of a laminated stack of thin metal sheets, typically made of silicon steel. The core is laminated to reduce the energy losses due to eddy currents. When the transformer operates, alternating magnetic fields are generated in the core, which can induce circulating currents called eddy currents. These eddy currents can result in significant power losses and heating. By laminating the core, the individual layers are electrically insulated from each other, effectively reducing the eddy current losses and improving the efficiency of the transformer.

(f) Transformers are used in mobile chargers.

Answer 👉 Reason: Mobile chargers typically require a lower voltage output than the standard electrical grid or power source. Transformers are used in mobile chargers to step down the voltage from the main power source to a lower voltage suitable for charging mobile devices. This ensures that the mobile devices receive the appropriate voltage level for safe and efficient charging. Transformers also help isolate the charging circuit from the main power source, providing additional safety and protection against voltage fluctuations.

Class 10 Science SEE Guide Unit - 11 Electricity and Magnetism Guide

4. Answers the following questions:

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

Answer 👉 The frequency of AC (alternating current) in our country being 50 Hz means that the direction of the current changes 50 times per second. In other words, the current alternates its direction back and forth 50 times in one second.

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

Answer 👉 Here are the time graphs of direct current (DC) and alternating current (AC):

Direct Current (DC):

|_________

The graph represents a constant flow of current in one direction without any changes.

Alternating Current (AC):

\ / \ / \ / \ /

/ / / /

The graph represents the alternating nature of the current, where it continuously changes its direction in a periodic manner.

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

Answer 👉

Magnetic field lines around a current-carrying straight wire: The magnetic field lines form concentric circles around the wire, with their direction determined by the right-hand thumb rule. The magnetic field lines form closed loops around the wire.
Magnetic field lines around a solenoid: The magnetic field lines inside a solenoid are parallel and uniform, running from one end to the other. Outside the solenoid, the magnetic field lines are similar to that of a bar magnet, forming closed loops.

( d) Explain the following rules.

(i) Maxwell's right-hand thumb ruleto show the direction

of the magnetic field produced when an electric cur- rent flows through a straight wire.

(ii) Maxwell's right-hand grip rule to find the direction of magnetic field lines of force around a solenoid.

Answer 👉

(i) Maxwell's right-hand thumb rule: This rule is used to determine the direction of the magnetic field produced by an electric current flowing through a straight wire. According to the rule, if you point your right thumb in the direction of the current, the curling of your fingers represents the direction of the magnetic field lines around the wire.
(ii) Maxwell's right-hand grip rule: This rule is used to determine the direction of the magnetic field lines around a solenoid. If you wrap your right hand around the solenoid with your fingers in the direction of the current, your thumb points in the direction of the magnetic field lines.

( e) What is the magnetic effect of current?

Answer 👉 The magnetic effect of current refers to the phenomenon in which an electric current flowing through a conductor generates a magnetic field around the conductor. This magnetic field can exert forces on other magnets or magnetic materials and can be used in various applications such as electric motors, transformers, and generators.

(f) Define magnetic flux.

Answer 👉 Magnetic flux is a measure of the total magnetic field passing through a given area. It is represented by the symbol Φ (phi) and is defined as the product of the magnetic field strength (B) and the area (A) perpendicular to the magnetic field. Mathematically, magnetic flux (Φ) = B * A * cos(θ), where θ is the angle between the magnetic field lines and the normal to the area.

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

Answer 👉 To demonstrate the magnetic field produced around a straight current-carrying wire using iron dust, cardboard, and a conducting straight wire, you can follow these steps:

Sprinkle iron dust on a piece of cardboard.
Place the cardboard over the conducting straight wire.
When current flows through the wire, the magnetic field produced around the wire causes the iron dust to align along the magnetic field lines, creating a pattern that represents the magnetic field.

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

Answer 👉 The magnetic field developed around a straight current-carrying wire forms concentric circles around the wire. The direction of the magnetic field lines can be determined using the right-hand thumb rule.

(i) What is Solenoid? Drawa picture showing the magnetic field developed around a solenoid

Answer 👉 A solenoid is a coil of wire wound in a cylindrical shape. It produces a magnetic field when an electric current passes through it. The magnetic field lines inside a solenoid are parallel and uniformly spaced, resembling the magnetic field of a bar magnet. Here is a simplified representation of the magnetic field developed around a solenoid:

----> Current direction ---->

|----------------------------|

| |

| Magnetic Field |

(j) Write two uses of the solenoid.

Answer 👉 Two uses of a solenoid are:

Electromagnets: When a ferromagnetic core is inserted into the solenoid, it becomes an electromagnet. Electromagnets are used in various applications, including electric locks, relays, speakers, and MRI (Magnetic Resonance Imaging) machines.
Inductor in electrical circuits: Solenoids can be used as inductors in electrical circuits. They store energy in the form of a magnetic field and are employed in devices such as power supplies, filters, and electronic ballasts.

(k) Which effects are demonstrated in the given figures? Figures

Answer 👉 Do yourself

(m) What is electromagnetic induction?

Answer 👉 Electromagnetic induction is the process of generating an electric current in a conductor by varying the magnetic field passing through it. When a magnetic field changes in strength or when a conductor moves relative to a magnetic field, an induced electric current is produced in the conductor. This phenomenon is the basis for generating electricity in generators and is utilized in various applications such as transformers and induction coils.

( o) State Faraday's law of electromagnetic production.

Answer 👉 Faraday's law of electromagnetic induction states that the magnitude of the induced electromotive force (EMF) in a circuit is directly proportional to the rate of change of magnetic flux passing through the circuit. Mathematically, it can be expressed as EMF = -dΦ/dt, where EMF is the induced electromotive force, dΦ/dt represents the rate of change of magnetic flux, and the negative sign indicates the direction of the induced current opposes the change in magnetic flux.

(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 observations mentioned can be explained based on the working principle of a dynamo:

When the bicycle is in motion, the dynamo generates electricity due to the relative motion between the magnet and the coil. The bulb glows with a steady brightness as the dynamo provides a continuous supply of electric current.
When the bicycle comes to rest, there is no relative motion between the magnet and the coil. Therefore, the dynamo does not generate electricity, causing the bulb to dim or turn off completely.

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

Answer 👉 Two ways to increase the magnitude of current produced by a dynamo are:

Increase the speed of rotation: The faster the rotation of the dynamo, the greater the rate of change of magnetic flux, resulting in a higher induced electromotive force and thus a larger current.
Increase the strength of the magnetic field: By increasing the strength of the magnetic field in the dynamo, either by using stronger magnets or increasing the current through the electromagnets, the rate of change of magnetic flux is enhanced, leading to a higher induced electromotive force and increased current.

(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 transmis- Sion.

Answer 👉 Hydroelectric Power Station:

Capacity: Nepal has a significant potential for hydroelectric power generation due to its mountainous terrain and abundant water resources. The installed capacity of hydroelectric power stations in Nepal is around X megawatts (MW).
Type of Electricity Produced: Hydroelectric power stations generate electricity by harnessing the potential energy of flowing water. The kinetic energy of the water is converted into electrical energy using turbines and generators. The electricity produced is typically AC (alternating current).
Transmission: The generated electricity is transmitted through a network of transmission lines to distribution substations. From the substations, the electricity is distributed to consumers through distribution lines.

Solar Power Plant:

Capacity: Nepal has been increasingly investing in solar power plants to harness solar energy. The installed capacity of solar power plants in Nepal is around Y megawatts (MW).
Type of Electricity Produced: Solar power plants generate electricity using photovoltaic (PV) cells, which convert sunlight directly into electrical energy. The electricity produced is typically DC (direct current), which is then converted into AC for transmission and distribution.
Transmission: Similar to hydroelectric power, the electricity generated from solar power plants is transmitted through the existing grid infrastructure to distribution substations. It is then distributed to consumers through distribution lines.

( s) What is a transformer?

Answer 👉 A transformer is a static electrical device used for transferring electrical energy between two or more circuits through electromagnetic induction. It consists of two or more coils of insulated wire, known as windings, and a magnetic core. The primary winding is connected to the input voltage source, while the secondary winding is connected to the load or the output circuit. Transformers are designed to step up or step down the voltage level, depending on the number of windings in each coil, to efficiently transfer electrical power from one circuit to another. They are widely used in power transmission and distribution systems, electrical appliances, and industrial applications.

5. Solve the following mathematical problems:

(a) To charge a laptop of 20V, a charger with 550 primary turns is connected to an a.c. source of 220V. Calculate the number of secondary windings of the charger. [Answer: 50]

Answer 👉 Using the transformer equation, we can find the ratio of primary turns to secondary turns:

Primary voltage / Secondary voltage = Primary turns / Secondary turns

220V / 20V = 550 / Secondary turns

11 = 550 / Secondary turns

Secondary turns = 550 / 11

Secondary turns = 50

Therefore, the number of secondary windings of the charger is 50.

(b) The number of secondary windings of the coil of a trans- former used in a microwave oven is 10 times the number of windings in the primary coil. If it is connected to a source of 220V, what is the secondary voltage obtained from the transformer?

Answer 👉 f the number of secondary windings is 10 times the number of windings in the primary coil, we can use the transformer equation to find the secondary voltage:

Primary voltage / Secondary voltage = Primary turns / Secondary turns

220V / Secondary voltage = 1 / 10

Secondary voltage = 220V / (1/10)

Secondary voltage = 220V * 10

Secondary voltage = 2200V

Therefore, the secondary voltage obtained from the transformer is 2200V.

(c) The ratio of the number of the primary winding to the number of secondary windings of a transformer is 22:1. If an adopter with that transformer is connected to an electric circuit having a potential difference of 220V, calculate the output voltage so obtained. while the same charge flows in the opposite direction. The direction of a current of this type changes at a certain time interval. In the same way, the magnitude of the current also gradually changes from the minimum value to the maximum value. Thus, the current in which magnitude and polarity change continuously at a fixed interval of time is called alternating current (a.c.). Dynamo, a.c. generator, etc. are sources of a.c.

Answer 👉 The ratio of primary windings to secondary windings is given as 22:1. Using the transformer equation:

Primary voltage / Secondary voltage = Primary turns / Secondary turns

220V / Secondary voltage = 22 / 1

Secondary voltage = 220V / 22

Secondary voltage = 10V

Therefore, the output voltage obtained from the transformer is 10V.