Class 10 Electric Current Exercise 11.1 Solutions | Science and Technology Curriculum Development Centre

Most ceiling fans are designed to operate on AC (alternating current) power, while solar panels usually provide DC power. 

If the fan is not connected to an inverter that converts the DC current to AC current, it will not function correctly. 

Hence, a ceiling fan if connected to the circuit of the solar panel does not rotate.

A magnetic compass consists of a small magnet (the needle) that is free to rotate. 

The compass needle aligns itself with the Earth's magnetic field under normal circumstances.

 However, when placed near a circuit which has electric current flowing in it, the magnetic field created by the electric current exerts a force on the compass needle. 

This causes the needle to deflect from its original alignment.

An electromagnet is used in an electric bell due to its ability to produce a magnetic field when an electric current flows through it, which is essential for the bell's operation.

In the case of an electric bell, when the switch is pressed, it completes the circuit, allowing current to flow through the coil of wire wrapped around a soft iron core, creating a magnetic field due to electromagnetic induction.

The magnetic field generated by the electromagnet attracts a nearby armature (a metal strip), which is connected to a hammer. This hammer strikes the bell or gong, producing sound.

Once the hammer strikes the bell, it breaks the circuit, stopping the current flow and causing the magnetic field to collapse. A spring mechanism then returns the hammer to its original position, allowing the circuit to reconnect if the button is still pressed, thus repeating the process rapidly and creating a continuous ringing sound.

The difference in the number of windings is what allows the transformer to either increase or decrease the voltage based on the needs of the electrical circuit. If the Primary and secondary coil in transformer get equal then same amount of voltage get transfer from the source to circuit which can cause damage in the object. To maintain the need of voltage to the circuit and save it, the coils in transformer are unevenly tie up. 

The core of a transformer is laminated to reduce eddy currents. Eddy currents are circular electric currents that flow within the core and can cause energy loss and heating. Laminating the core increases its resistance to these currents, thereby reducing energy loss and improving the efficiency of the transformer.

Mobile chargers comprises  step down transformer that changes the high voltage coming from source to a low voltage. This help mobile phone form getting damage and overall help in saving people life. Thus, transformer make the mobile charging safe. 

Frequency refers to how many times the current or voltage alternates (or cycles) in a second. It is measured in Hertz (Hz), where 1 Hz equals one cycle per second.

In an Alternating current (AC) system, the current flows back and forth, constantly changing direction. The frequency tells us how many times this change happens each second.

The frequency of a.c. in our country is 50 Hz means that the current reverses its direction 50 times per second. So, for every second, the AC voltage completes 50 cycles of moving in one direction and then reversing back in the other.

Direct Current (D.C.):

• The graph of direct current is represented as a straight horizontal line above the time axis, indicating a constant value. The current flows in only one direction (does not change polarity).
• The value of the current remains constant over time, making it suitable for devices that require a stable voltage supply, such as batteries or electronic circuits.

Alternating current (A.C.):

• The graph of alternating current is represented as a wave that oscillates above and below the time axis, indicating that the current periodically reverses direction( changes polarity).
• The typical representation is a sine wave, indicating that both the magnitude and direction of the current vary over time.

The magnetic field lines around the current-carrying straight wire is shown as below:

The direction of magnetic field lines can be determined using Right-Hand Thumb Rule.

Solenoid is a type of electromagnet formed by wrapping a insulated wire coil around a soft iron.

A picture showing the magnetic field developed around a solenoid:

If direction of current in a solenoid is known, the magnetic field developed around it can be determined by the use of Maxwell's right hand thumb rule as well.

The Maxwell's right hand thumb rule states that “ When the conductor is held in your right hand, such that the direction of the thumb points the direction of the current and the curled finger gives the direction of the magnetic field. ”

For example, if the current flows upwards through a vertical wire, and you position your right hand with your thumb pointing up (in the direction of the current), your fingers will curl around the wire in a counterclockwise direction. This indicates that the magnetic field lines are oriented in a counterclockwise manner around the wire.

Hence, Maxwell's right-hand thumb rule shows the direction of the magnetic field produced when an electric current flows through a straight wire.

Maxwell's right-hand grip rule states that “if a solenoid is gripped by the right hand in such a way that the fingers are in the direction of the current flowing in the wire, the thumb will point to its north pole.”

The opposite pole will be the south pole. 

The direction of lines of force outside the magnet is from the north pole to south pole of a magnet.

Hence, Maxwell's right-hand grip rule is used to find the direction of magnetic field lines of force around a solenoid.

When an electric current is passed through a conducting wire, it produces a magnetic field around it.

This phenomenon is known as magnetic effect of current.

Hans Christian Oersted first discovered magnetic effect of current in 1819AD.

Magnetic flux through the surfaces is defined as the magnetic lines of force passing through the surface held perpendicular to these lines of force.

 Magnetic flux is a measurement of the total magnetic field which passes through a given area.

A conducting straight wire produces a magnetic field around it when current is passed through it due to magnetic effect of current.

To demonstrate the magnetic field produced around such conducting wire by using iron dust, cardboard the following procedures should be followed:

• Keep the conducting straight wire with current passed through it at the middle of the cardboard.
• Lay a thin sheet of paper over the cardboard, directly above the conducting wire. This will act as a surface to hold the iron dust and prevent it from scattering too much.
• Evenly sprinkle a small amount of iron dust over the paper.
• Once the current is flowing, gently tap or shake the paper. 

The iron dust will begin to align itself in a particular manner. The iron dust shows the magnetic field of the conducting straight wire produced by current in the wire.

In this way, magnetic field produced around straight current carrying conducting wire be demonstrated by using iron dust and cardboard.

The Maxwell's right hand thumb rule states that “ When the conductor is held in your right hand, such that the direction of the thumb points the direction of the current and the curled finger gives the direction of the magnetic field. ”

So this rule can be used to demonstrate the magnetic field developed around a straight current carrying wire. 

The above diagram shows the magnetic field developed around a straight current-
carrying wire ( given by the curled fingers).

Solenoid is a type of electromagnet formed by wrapping a insulated wire coil around a soft iron.

A picture showing the magnetic field developed around a solenoid :

If direction of current in a solenoid is known, the magnetic field developed around it can be determined by the use of Maxwell's right hand thumb rule.

Solenoid is a type of electromagnet formed by wrapping a insulated wire coil around a soft iron.

Any two uses of the solenoid are:

1. Electric bells :Solenoids are used in electric bells to produce sound. When current flows through the solenoid, it creates a magnetic field that attracts a metal hammer towards the bell. This action causes the hammer to strike the bell, producing a ringing sound. The solenoid can be turned on and off quickly, allowing for repeated ringing.
2. Locking Mechanisms :Solenoids are commonly found in locking mechanisms for electronic door locks and security systems. When energized, the solenoid moves a plunger or bolt to engage or disengage the lock, providing secure access control.

Electromagnetic induction is the process of generating an electromotive force (EMF) in a conductor due to a changing magnetic field. 

This occurs when a conductor moves through a magnetic field or when the magnetic field around a stationary conductor changes, inducing an electric current. This principle is fundamental in devices like generators and transformers.

Faraday's law of electromagnetic induction states that:

1. Whenever magnetic flux linked with a closed coil changes, an emf (electromotive force) is induced in the coil.
2. The induced emf lasts as long as there is change in magnetic flux.
3. The magnitude of the induced emf is directly proportional to the rate of change of the magnetic flux linked with the closed coil.

The observations regarding the bulb connected to a bicycle dynamo—where it glows brightly, dims, and eventually turns off when the bicycle comes to rest—can be explained by the following points based on the working principle of a dynamo:

1. Dependence on Wheel Rotation: The dynamo generates electricity through electromagnetic induction, which occurs when a magnet rotates within a coil of wire.     The faster the wheel spins, the more electricity is produced. When the bicycle is in motion, the dynamo generates enough power to keep the bulb bright.                                                   However, as the speed decreases, less electricity is produced, causing the bulb to dim.
2. Loss of Power at Rest: When the bicycle comes to a complete stop, the dynamo ceases to generate electricity because there is no longer any rotation of the magnet within the coil. As a result, the bulb will turn off completely since it relies on the electrical energy generated by the dynamo.             

These factors illustrate how the performance of a dynamo is closely tied to the mechanical motion of the bicycle, affecting the brightness of the connected bulb.

A dynamo is an electrical generator that converts mechanical energy into direct current electricity using a commutator. It operates on the principle of electromagnetic induction. 

Any two ways of increasing the current produced by a dynamo are:

1. Increasing the number of coils in the dynamo.
2. Increasing the rate of rotation of wheel of bicycle in which dynamo is used. This increases the rate of change in magnetic flux hence produced current will get increased.

Nepal is endowed with significant natural resources for electricity generation, primarily through hydropower and solar power. 

One of the hydropower station in Nepal is Upper Trishuli III A.

Upper Trishuli III A :

• Capacity: 60 MW
• Type of Electricity Produced: The Upper Trishuli III A Hydropower Station in Nepal produces alternating current (AC) electricity. Renewable energy is generated from flowing water.
• Transmission: The electricity generated is transmitted through a network of high-voltage transmission lines to distribution centers across the country.  It is done through transformers.

One of the solar power plant in Nepal is Nuwakot Solar Power Station.

Nuwakot Solar Power Station :

• Capacity: 25 MW (largest operational solar plant in Nepal)
• Type of Electricity Produced: Solar photovoltaic (PV) energy is converted from sunlight into electricity. The Nuwakot Solar Power Station produces direct current (DC) electricity initially, which is then converted to alternating current (AC) for integration into the national grid.
• Transmission: The generated electricity is connected to the 66 kV sub-station at Devighat Hydropower Station. This integration allows for effective management of energy supply and storage. After conversion to AC, the electricity is transmitted through transformers.

Both hydropower and solar power are crucial for Nepal's energy landscape.

A transformer is an electrical device that works on the principle of electromagnetic induction, transferring voltages either by stepping up or stepping down from one electric circuit to another.

1. Step-Up Transformer

A step-up transformer increases the voltage from the primary winding to the secondary winding. It has more turns in the secondary coil than in the primary coil, which allows it to step up the voltage while stepping down the current.

Two uses of step-up transformers are: 

• Power Transmission: Used in power plants to increase voltage for efficient long-distance transmission of electricity, reducing energy loss due to resistance in wires.
• Induction Heating: Employed in induction heating applications where high voltages are necessary for heating metals.           

   2.Step-Down Transformer

A step-down transformer decreases the voltage from the primary winding to the secondary winding. It has more turns in the primary coil than in the secondary coil, which allows it to step down the voltage while stepping up the current.

Two uses of step-down transformers are:

• Power Adapters: Commonly found in power adapters for electronic devices to convert high-voltage electricity from outlets to lower voltages suitable for devices.
• Lighting Systems: Used in lighting systems, such as street lights and outdoor lighting, where lower voltages are required for safety and efficiency.

Here, 

Given:

Number of primary windings ( \(\rm N_p\) )= \(\rm 550\) 

Number of secondary windings ( \(\rm N_s\))=  \(\rm ?\) 

Primary Voltage( \(\rm V_p\) )=  \(\rm 220V\) 

Secondary Voltage( \(\rm V_s\) )=   \(\rm 20V\) 

To find the secondary windings of the charger, we can use the transformer voltage formula:

$\frac{ N_p}{N_s}$ = $\frac{ V_p}{V_s}$

\(\rm or, \) $\frac{550 }{N_s}$ = $\frac{220}{20}$

\(\rm or, \) $\frac{ 550}{N_s}$ = $\frac{11 }{1}$

\(\rm or, \) $\frac{ 550}{11}$ = \(\rm N_s \)  

\(\rm or, \) \(\rm N_s \)   = \(\rm 50 \) 

The number of secondary windings of the charger is 50 turns.

Here,

Let the number of windings in primary coil be ‘x’.

Then the number of windings in secondary coil will be ‘10x’.

Given:

Number of primary windings ( \(\rm N_p\) )= \(\rm x\) 

Number of secondary coils ( \(\rm N_s\))=  \(\rm 10x\) 

Primary Voltage( \(\rm V_p\) )=  \(\rm 220V\) 

Secondary Voltage( \(\rm V_s\) )= ?

To find the secondary voltage obtained from the transformer, we can use the transformer voltage formula:

$\frac{ N_p}{N_s}$ = $\frac{ V_p}{V_s}$

\(\rm or, \) $\frac{ x}{10x}$ = $\frac{220 }{V_s}$

\(\rm or, \) $\frac{ 1}{10}$ = $\frac{220 }{V_s}$

\(\rm or, \) \(\rm V_s \)   = \(\rm 220 . 10\) 

\(\rm or, \) \(\rm V_s \)   = \(\rm 2220 \) 

Thus, the secondary voltage obtained from the transformer is 2220 V.

Here,

Given:

The ratio of primary windings to secondary windings is 22:1. This means:

\(\rm N_p:N_s\)= \(\rm 22:1\)

\(\rm or, \)$\frac{ N_p}{N_s}$ = $\frac{ 22}{1}$

Primary Voltage( \(\rm V_p\) )=  \(\rm 220V\)  

Secondary Voltage( \(\rm V_s\) )= ?

To find the output voltage obtained from the transformer, we can use the transformer voltage formula:

$\frac{ N_p}{N_s}$ = $\frac{ V_p}{V_s}$

\(\rm or, \) $\frac{ 22}{1}$ = $\frac{220 }{V_s}$

\(\rm or, \) \(\rm V_s \)   = $\frac{220 }{22}$

\(\rm or, \) \(\rm V_s \)   = \(\rm 10\) 

Thus, the secondary voltage obtained from the transformer is 10 V.