Chapter 11: Electricity and Magnetism
Class 10 Science Notes
Introduction to Electricity and Magnetism
The pictures show the alternative arrangements made by people during load shedding (electrical power cuts).
- Emergency Lights: People use rechargeable emergency lamps. The battery in such devices is charged when there is electricity in the main line.
- Solar Panels: People use solar panels. By connecting an inverter with this, different electrical devices can be operated.
- Inverters: When there is a main power supply, a battery can be charged, and many electrical devices can be operated by connecting the battery with an inverter.
- Generators: Electricity can be generated by using petrol/diesel generators of different capacities whenever there is a need.
Topics related to the various information mentioned above are included in this unit. Types of current, the construction of generators, and transformers and their working principles, etc., will also be discussed here.
Direct Current and Alternating Current
Figure 11.1 shows the two poles of a dry cell. It also shows that the electric charges in the conducting wire are flowing in a certain direction (from the negative terminal of the cell to its positive terminal) when the cell is connected to the electric circuit and the light is turned on. Similarly, Figure 11.2 shows the flow of current through a conducting wire in a particular direction when a bulb is lit by the electric current from a solar panel.
A glowing bulb run by the electricity produced by a dynamo is shown in Figure 11.3. In the picture, the negative and positive terminals are not separated as they are in the dry cell and solar panel. In Figure 11.4, the direction of the current generated when the magnet is rotated between the coils in the dynamo is presented in a simplified manner.
In the same figure, the blue arrow shows that the charge on the conducting wire moves forward, and the red arrow shows that after a 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.
Current Time Graph
The fact that the magnitude of the current remains constant with respect to time in direct current can be demonstrated in a time-current graph as shown in Figure 11.5. Similarly, the variation of magnitude and direction of alternating current with respect to time can be presented as shown in Figure 11.6.
Figure 11.5: Time-current graph of direct current
Horizontal line showing constant current of 4A
Figure 11.6: Time-current graph of alternating current
Sine wave showing current varying from +5A to -5A
In the time-current graph of direct current, it is shown that the magnitude of the current is constant at 4A even though the time is increasing. Similarly, in the time-current graph of the alternating current, it is shown that the magnitude of the current first increases from zero to 5A and then decreases to zero. After that, the value of the current increases again but in the exact opposite direction (i.e., goes to -5A) and then again decreases to zero. Thus, a cycle is completed.
The frequency of a.c. used in our country is 50 Hz and the average voltage ranges from 220 V to 240 V. This means that the direction of a.c. used in our country changes 100 times every second. Since the direction of the current in d.c. does not change with time, its frequency is zero.
Among the different electrical appliances used in our daily life, some are run on a.c, some on d.c., and some on both a.c and d.c. Electrical devices such as fans, motors, refrigerators, etc., run on a.c., whereas d.c. is used in the internal circuits of electrical appliances such as mobile phones, computers, etc. Electric heaters, filament lamps, etc., can operate on a.c as well as d.c. We can use a rectifier to convert alternating current into direct current.
Activity 11.1: Differences Between Direct and Alternating Current
| Feature | Direct Current (d.c.) | Alternating Current (a.c.) |
|---|---|---|
| Direction | Flows in one direction only | Direction changes periodically |
| Magnitude | Generally constant | Varies continuously |
| Frequency | Zero | 50 Hz (in Nepal) |
| Sources | Batteries, solar cells | Generators, power stations |
| Transmission | Inefficient for long distances | Efficient for long distances |
Magnetic Effect of Electric Current
In the year 1820, Hans Christian Oersted, a Danish physicist and science teacher, discovered the fact that electricity and magnetism are related to each other. He observed the needle of a magnetic compass deflecting near an electric circuit while performing an experiment involving electric current. After further study, it was found that the direction of the deflection of the magnetic needle changes along with the change in the direction of current flow in the circuit. Oersted concluded that the deflection of the compass needle is due to the magnetic effect produced by the electric current.
Magnetic field around a current-carrying straight wire
Activity 11.2
(a) Take a square piece of cardboard. Pierce it with a straight wire and connect it to an electric circuit as shown in Figure 11.9. Place plotting compasses on the cardboard and position them around the wire as shown in the figure. Turn on the switch to pass the current in the wire and then observe the needles of the compasses. Observe whether the needles of the magnetic compasses around the wire rotate in a certain direction when the switch is turned on. Because there is no proper load in the circuit, the switch should be turned off immediately. Otherwise, the wire heats up if the switch is left on for a long time. Swap the poles of the cell, turn the switch on again, and observe if the direction of the needles is opposite to the previous one.
(b) Pierce the cardboard with a straight wire as mentioned above and connect it to an electric circuit. Sprinkle very fine iron powder over the cardboard as shown in Figure 11.10. Turn the circuit switch on and then tap the cardboard gently with your fingers. Observe whether the iron dust settles in a circular geometric pattern or not.
The magnetic field formed around a straight wire due to the electric current in it can be observed by drawing lines of magnetic force as shown in Figure 11.11.
When magnetic compasses are placed around the wire in the above activity, the needles of the compasses point in a certain direction and indicate a circular pattern. This effect is caused by the magnetic field formed in a circular pattern around the wire when there is a current in it. The direction of the magnetic field thus formed depends on the direction of the electric current flowing in the wire. As shown in Figure 11.12, if the electric current is going upwards through the wire, the direction of the magnetic field is anticlockwise. On the contrary, if the current flows downwards from the wire, the direction of the magnetic field is clockwise. In the above activity, the direction of the magnetic field can also be determined by observing the direction of the pointer of the compass needle.
The Magnetic Field Around a Solenoid
When the insulation of the two ends of the wire used to make the solenoid is removed and connected to a battery, a current flows through the wire and a magnetic field develops in and around the solenoid. The magnetic field outside the solenoid is similar to the magnetic field around a bar magnet, i.e., the magnetic field of a solenoid is strong on both sides and weak in the middle portion. There is a uniform magnetic field inside the solenoid. Generally, when an electric current flows in the solenoid, one end of it becomes the North Pole and the other the South Pole. If the electric current flows in the opposite direction, the poles of the magnetic field formed around it are also reversed.
The direction of the magnetic field formed around the current-flowing solenoid can be found by using Maxwell’s right-hand grip rule. According to this rule, if a solenoid is held in the right hand such that the fingers indicate the direction of the current through the solenoid, the thumb points to the north pole of the magnetic field developed in the solenoid. The strength of the magnetic field around a solenoid depends on the following factors:
- The magnitude of the current in the solenoid.
- The number of turns in the coil of the solenoid.
- A material placed inside the solenoid (core). A material such as a soft iron cylinder increases the strength of the magnetic field.
Since the magnetic field created by the solenoid is temporary, it is used to make an electromagnet.
Magnetic Flux
Figure 11.16 shows the magnetic lines of force around a current-carrying straight wire, a wire loop, a solenoid, a bar magnet, and the Earth. Depending on the source of the magnetic field, the shape of the lines of magnetic force varies. As shown in Figure 11.17, the number of magnetic lines of force on an object placed in a magnetic field depends on its location.
To obtain information about the effect of a magnet at different locations within a magnetic field, one can simply look at the number of magnetic lines of force on the surface of an object. The total number of magnetic lines of force passing through the surface area within the magnetic field represents the magnetic flux.
The density of magnetic lines of force indicates the magnitude of the magnetic flux. The area with denser magnetic lines of force means a strong magnetic flux, whereas that of less density means a weak magnetic flux. On observing the density of magnetic lines of force of the bar magnet in Figure 11.17, we can tell that the magnetic flux is stronger at the poles than in the middle portion.
Motor Effect
In Figure 11.19, the direction of the magnetic field of a bar magnet and the circular magnetic field developed around a wire when an electric current flows in it are shown. The force of attraction and repulsion between the two magnetic fields produces motion in the wire.
A coil of a motor is formed by winding an insulated wire around a core. The coil is placed between two opposite magnetic poles as shown in Figure 11.19. Then current is passed through the coil. When a current is passed through the coil, a magnetic field develops around it. Because of the alternating current, the direction of the magnetic field in the coil changes.
Increasing Motor Efficiency
The speed of rotation of a motor’s coil can be increased by:
- Increasing the number of turns in the coil.
- Increasing the surface area of the coil.
- Using a more powerful magnet.
- Using a soft iron core.
Question to think: Is it possible to generate an electric current from a conducting wire placed in a magnetic field, just the way a magnetic field is developed around the wire when an electric current passes through it?
Electromagnetic Induction
Around the beginning of the nineteenth century (1800 AD), the voltaic cell was the main source of current. In 1831, Michael Faraday discovered that when the magnetic lines of force are cut perpendicularly by a conducting wire, a voltage is generated in the wire. An electric current flows when the two ends of the wire are connected in a circuit. This discovery brought a remarkable change in the field of electricity generation.
Activity 11.5
Connect a galvanometer to the two ends of a solenoid. Pass a bar magnet in and out of the hollow part of the solenoid. Keeping the bar magnet fixed, observe the galvanometer needle when the north and south poles of the magnet enter the solenoid.
| Poles of the bar magnet | The process by which the poles enter or exit the solenoid | The direction of deflection of the needle of the galvanometer |
|---|---|---|
| North pole | In | Left/right |
| South pole | Out | Left/right |
Observation Question: What difference is observed when the bar magnet is moved in and out quickly versus slowly?
When a bar magnet is moved inside a coil of conducting wire, the number of magnetic lines of force on its surface (magnetic flux) changes continuously. This induces a voltage in the coil. Similarly, if a coil is rotated in a magnetic field, the magnetic flux linked with the coil changes, and a voltage is induced. In both cases, mechanical energy is converted into electrical energy.
Faraday’s Law of Electromagnetic Induction
The voltage produced in a coil depends on the strength of the magnetic field and the number of turns in the coil. The greater the strength and the more turns, the more voltage is produced. The magnitude of the voltage also depends on how quickly the magnetic field lines link the coil. Moving the magnet faster induces more voltage.
This is the working principle of a generator. The induced e.m.f. lasts as long as the magnetic flux continues to change. The magnitude of the induced e.m.f. increases if you:
- Increase the number of turns of the coil.
- Increase the strength of the magnet.
- Increase the speed of rotation of the coil.
Dynamo and A.C. Generator
A dynamo is used to induce current on a small scale (e.g., for bicycle lights), while a generator is used to generate current on a large scale (e.g., for homes).
In a bicycle dynamo, a magnet is rotated. A coil is kept close to the magnet. As the bicycle tire turns, it causes the dynamo’s cap to rotate the magnet. This changes the magnetic flux linked with the coil, inducing a voltage. The magnitude of the induced voltage depends on the number of turns in the coil, the strength of the magnetic field, and the speed of rotation.
Large-Scale Sources of Electricity
Large quantities of electricity are produced by rotating a coil at very high speeds within a strong magnetic field in a generator.
- Hydropower: Water stored in a dam flows at high pressure through a tunnel to spin the turbines of a generator.
- Thermal Plants: Fossil fuels (coal, diesel) are burned to produce heat. In a coal plant, this heat boils water to create high-pressure steam, which rotates a turbine. In a diesel plant, a diesel engine directly rotates the turbine.
- Windmills: Wind energy is used to rotate the turbine of a generator.
- Nuclear Power Plants: Similar to coal plants, but the heat to boil water is generated from the controlled fission of radioactive elements like uranium.
Electricity in Nepal
(as of 2022 AD data from Nepal Electricity Authority):
- Hydroelectric potential: 2200 MW.
- Thermal plant production: ~487 MW (in Duhabi and Hetauda).
- Under construction: ~487 MW from hydroelectric projects.
- Proposed: Additional 3219 MW from projects like Upper Arun, Uttar Ganga, and Dudh Koshi.
Alternating Current (A.C.) Generator
In an A.C. generator, a rectangular coil of conducting wire is placed in a magnetic field. As the coil rotates, it cuts the magnetic field, and the magnetic flux associated with the coil changes. This produces an electromotive force (e.m.f.) in the coil. The magnitude of the e.m.f. is directly proportional to the rate of change of magnetic flux linkage.
Transformers
- Example 1: A mobile charger reduces 220V AC to 5.3V AC.
- Example 2: A microwave oven increases 220V AC to 2100V AC.
Activity 11.6: Voltage Ratings of Equipment
Collect the input and output voltage ratings for devices that use transformers.
| Device | Input voltage rating | Output voltage rating |
|---|---|---|
| Router adaptor | 220V | 12V |
| Laptop adaptor | 220V | 20V |
Electricity Transmission and Distribution
The use of A.C. electricity involves three main sections: production, transmission, and distribution.
- Production: Electricity is produced by a generator in a powerhouse.
- Transmission: The voltage is increased to a high value (e.g., 132 kV) using a transformer and sent through transmission lines.
- Distribution: Before being distributed to customers, the voltage is reduced to a lower value (e.g., 220V) at a substation.
Construction and Working Principle of a Transformer
Transformers work on the principle of mutual induction. They consist of two insulated copper wire coils that are not electrically connected.
- An alternating current is sent through the first coil (primary coil).
- This creates a periodically changing magnetic field around it.
- This changing magnetic field induces an e.m.f. in the nearby second coil (secondary coil).
- The output current is drawn from this secondary coil.
Transformers do not work with direct current (D.C.) because D.C. does not create a changing magnetic field.
The coils are wound on a core constructed from thin, insulated iron sheets (laminations) to prevent excessive heating. This is called core lamination.
- Primary Coil: The input coil where AC is fed. Its number of turns is denoted by \(N_p\). The input voltage is \(V_p\).
- Secondary Coil: The output coil where AC is induced. Its number of turns is denoted by \(N_s\). The output voltage is \(V_s\).
Types of Transformers
1. Step-Down Transformer:
- Reduces the voltage of an alternating current.
- The number of turns in the secondary coil (\(N_s\)) is less than the number of turns in the primary coil (\(N_p\)).
- Example: If \(N_s : N_p = 1 : 2\) and the input is 220V, the output will be 110V.
2. Step-Up Transformer:
- Increases the voltage of an alternating current.
- The number of turns in the secondary coil (\(N_s\)) is more than the number of turns in the primary coil (\(N_p\)).
- Example: If \(N_s : N_p = 2 : 1\) and the input is 110V, the output will be 220V.
Transformer Formula
The ratio of the number of turns in the coils is equal to the ratio of the voltages.
\[ \frac{\text{Primary turns }(N_p)}{\text{Secondary turns }(N_s)} = \frac{\text{Primary Voltage }(V_p)}{\text{Secondary Voltage }(V_s)} \]
Example 11.1
A router adapter is connected to a 220V power supply and provides an output of 12V. If the number of primary windings in its transformer is 500, calculate the number of secondary windings.
Solution: Given:
- Primary Voltage (\(V_p\)) = 220V
- Secondary Voltage (\(V_s\)) = 12V
- Primary turns (\(N_p\)) = 500
- Secondary turns (\(N_s\)) = ?
Using the transformer formula:
\[ \frac{500}{N_s} = \frac{220}{12} \]
\[ N_s = \frac{500 \times 12}{220} \]
\[ N_s = 27.27 \approx 28 \]
The number of secondary windings is approximately 28.
Example 11.2
The ratio of primary winding to secondary winding in a transformer is 1:10. Calculate the secondary voltage when the transformer is connected to a 220V power supply.
Solution: Given:
- Primary Voltage (\(V_p\)) = 220V
- \(\frac{N_p}{N_s} = \frac{1}{10}\)
- Secondary Voltage (\(V_s\)) = ?
Using the transformer formula:
\[ \frac{N_p}{N_s} = \frac{V_p}{V_s} \]
\[ \frac{1}{10} = \frac{220}{V_s} \]
\[ V_s = 220 \times 10 = 2200V \]
The secondary voltage is 2200V.
1. Choose the correct option for the following questions:
(a) Which of the following is the source of a.c.?
(b) Which scientist discovered the magnetic effect of electric current?
(c) What is the direction of the magnetic field when current is flowing upwards through a conducting straight wire?
(d) Which of the following statements is true for the current source shown in the figure? (Assuming Figure A shows a d.c. source and Figure B shows an a.c. source)
(e) On which of the following principles is the working of a transformer based?
(f) Which is the transformer’s formula?
2. Differentiate between:
(a) a.c and d.c
| Feature | Alternating Current (a.c.) | Direct Current (d.c.) |
|---|---|---|
| Direction of Flow | The direction of current flow reverses periodically. | The current flows in only one direction. |
| Magnitude | The magnitude of the current varies continuously in a sinusoidal pattern. | The magnitude is generally constant. |
| Frequency | Has a specific frequency (e.g., 50 Hz or 60 Hz). | Has zero frequency. |
| Source | Generated by a.c. generators (alternators) at power stations. | Supplied by batteries, solar cells, and d.c. generators (dynamos). |
| Transmission | Can be transmitted efficiently over long distances by stepping up the voltage. | Inefficient for long-distance transmission due to high power loss. |
(b) dynamo and generator
| Feature | Dynamo | Generator |
|---|---|---|
| Definition | A dynamo is a specific type of generator that produces direct current (d.c.). | A generator is a broad term for a machine that converts mechanical energy into electrical energy (either a.c. or d.c.). |
| Output Current | Exclusively produces direct current (d.c.). | Can produce either alternating current (a.c.) or direct current (d.c.). |
| Key Component | Uses a split-ring commutator to reverse the direction of the current and produce d.c. | An a.c. generator uses slip rings, while a d.c. generator uses a split-ring commutator. |
| Modern Usage | The term is often used historically; modern d.c. is usually produced by rectification of a.c. or batteries. | The term is widely used for all devices that generate electricity, especially a.c. generators (alternators). |
| Example | A simple d.c. generator on a bicycle light. | Power plant alternators, portable petrol/diesel generators. |
(c) motor and generator
| Feature | Electric Motor | Electric Generator |
|---|---|---|
| Energy Conversion | Converts electrical energy into mechanical energy (motion). | Converts mechanical energy (motion) into electrical energy. |
| Working Principle | Works on the motor effect: a current-carrying conductor in a magnetic field experiences a force. | Works on electromagnetic induction: a changing magnetic field induces a current in a conductor. |
| Guiding Rule | Fleming’s Left-Hand Rule is used to determine the direction of the force (motion). | Fleming’s Right-Hand Rule is used to determine the direction of the induced current. |
| Input | Requires an electrical current to operate. | Requires mechanical rotation (e.g., from a turbine) to operate. |
| Output | Produces torque and rotation. | Produces an electromotive force (voltage) and current. |
(d) step-up transformer and step-down transformer
| Feature | Step-up Transformer | Step-down Transformer |
|---|---|---|
| Function | Increases the alternating voltage from the primary to the secondary coil. | Decreases the alternating voltage from the primary to the secondary coil. |
| Turns Ratio | The number of turns in the secondary coil is greater than in the primary coil (\(N_s > N_p\)). | The number of turns in the primary coil is greater than in the secondary coil (\(N_p > N_s\)). |
| Voltage Relation | The secondary voltage is higher than the primary voltage (\(V_s > V_p\)). | The secondary voltage is lower than the primary voltage (\(V_s < V_p\)). |
| Current Relation | The secondary current is lower than the primary current (\(I_s < I_p\)). | The secondary current is higher than the primary current (\(I_s > I_p\)). |
| Application | Used at power plants to increase voltage for long-distance transmission. | Used at local substations and in device chargers (e.g., for phones, laptops) to reduce voltage for safe use. |
3. Give reasons:
(a) When a ceiling fan is connected to the circuit of the solar panel, the fan does not rotate.
A solar panel produces direct current (d.c.). Standard ceiling fans are designed with induction motors that require alternating current (a.c.) to create the rotating magnetic field necessary for their operation. Connecting a d.c. source to an a.c. motor will not make it rotate correctly and can damage the motor.
(b) When a magnetic compass is placed near a circuit in which an electric current is flowing, its needle deflects.
This happens because of the magnetic effect of current. An electric current flowing through a wire creates its own magnetic field around the wire. This newly created magnetic field interacts with the magnetic field of the compass needle, exerting a force on it and causing it to deflect from its usual North-South alignment.
(c) The number of primary windings and secondary windings of a transformer are not the same.
The primary function of a transformer is to change voltage levels. This change is achieved based on the ratio of the number of turns in the secondary coil to the primary coil (\(V_s/V_p = N_s/N_p\)). If the number of windings were the same (\(N_p = N_s\)), the voltage ratio would be 1, meaning the output voltage would be the same as the input voltage (\(V_s = V_p\)), and the device would not function as a transformer.
(d) An electromagnet is used in the electric bell.
An electromagnet is a temporary magnet that can be switched on and off with an electric current. This property is essential for an electric bell’s operation. When the circuit is complete, the electromagnet activates and pulls a metal hammer to strike the bell. This movement simultaneously breaks the circuit, deactivating the electromagnet. A spring then pulls the hammer back, which completes the circuit again, and the process repeats rapidly, causing the continuous ringing sound.
(e) The core of a transformer is laminated.
A changing magnetic field in the solid iron core of a transformer induces unwanted circular currents called eddy currents. These currents generate a significant amount of heat (due to the \(I^2R\) effect), which wastes energy and reduces the transformer’s efficiency. The core is laminated—made of thin, insulated iron sheets—to break the path of these eddy currents, thereby minimizing their formation and reducing energy loss.
(f) Transformers are used in mobile chargers.
Mains electricity is supplied at a high a.c. voltage (e.g., 220V), while mobile phone batteries require a very low d.c. voltage (e.g., 5V) to charge. A mobile charger contains a step-down transformer to first reduce the high a.c. voltage to a low a.c. voltage. This low voltage is then converted from a.c. to d.c. by a circuit called a rectifier before being supplied to the battery.
4. Answer the following questions:
(a) The frequency of a.c. in our country is 50 Hz, what does it mean?
A frequency of 50 Hz (Hertz) means that the alternating current completes 50 full cycles of changing its direction and magnitude every second. In each cycle, the current flows in one direction and then in the opposite direction. Therefore, the direction of the current reverses 100 times per second (50 times in the positive direction and 50 times in the negative direction).
(b) Draw the time graph of direct current and alternating current.
Direct Current (d.c.): A graph of voltage vs. time shows a straight horizontal line, indicating constant voltage and direction.
Alternating Current (a.c.): A graph of voltage vs. time shows a sine wave, indicating that the voltage and direction change cyclically.
(c) Draw the magnetic field lines around the current-carrying straight wire and solenoid.
• Straight Wire: The magnetic field lines are concentric circles around the wire, with the direction determined by the right-hand thumb rule.
• Solenoid: The magnetic field lines are parallel and uniform inside the solenoid, making it a strong and uniform field. Outside, the lines loop from the North pole to the South pole, resembling the field of a bar magnet.
(d) Explain the following rules.
(i) Maxwell’s right-hand thumb rule to show the direction of the magnetic field produced when an electric current flows through a straight wire.
Imagine you are holding a current-carrying straight wire in your right hand so that your thumb points in the direction of the electric current. The direction in which your fingers curl around the wire indicates the direction of the magnetic field lines.
(ii) Maxwell’s right-hand grip rule to find the direction of magnetic field lines of force around a solenoid.
Imagine you are holding a current-carrying solenoid in your right hand so that your fingers are curled in the direction of the current flowing through the coils. Your outstretched thumb will then point towards the North pole of the magnetic field produced by the solenoid.
(e) What is the magnetic effect of current?
The magnetic effect of current is the phenomenon where an electric current flowing through a conductor produces a magnetic field in the space around it.
(f) Define magnetic flux.
Magnetic flux is the measure of the total number of magnetic field lines passing perpendicularly through a given surface area. It quantifies the amount of magnetic field penetrating a surface. Its SI unit is the Weber (Wb).
(g) How can the magnetic field produced around a straight current-carrying wire be demonstrated by using iron dust, cardboard, and a conducting straight wire? Explain it.
1. Take a flat piece of cardboard and make a small hole in its center.
2. Pass a straight conducting wire through the hole, ensuring the cardboard is perpendicular to the wire.
3. Connect the ends of the wire to a power source (like a battery) through a switch.
4. Sprinkle iron filings evenly on the surface of the cardboard.
5. Close the switch to allow current to flow through the wire and gently tap the cardboard.
6. The iron filings will align themselves in concentric circles around the wire. This circular pattern visibly demonstrates the shape and presence of the magnetic field produced by the current.
(h) Draw the magnetic field developed around a straight current-carrying wire.
(This is the same as the first part of question 4c)
(i) What is a Solenoid? Draw a picture showing the magnetic field developed around a solenoid.
A solenoid is a long coil of insulated wire wound in the form of a helix or cylinder. When an electric current is passed through it, it produces a nearly uniform magnetic field in its interior and acts like a bar magnet. (The drawing is the same as the second part of question 4c)
(j) Write two uses of the solenoid.
1. Electromagnets: Solenoids are widely used to make electromagnets, which are key components in devices like electric bells, relays, circuit breakers, and door locks.
2. Actuators: In mechanical applications, they are used as actuators to convert an electrical signal into linear motion, such as in car door locks and automated valves.
(k) Which effects are demonstrated in the given figures? (Assuming the figures depict a motor and a generator setup)
• One figure likely demonstrates the Motor Effect, where an electric current in a magnetic field produces motion (converting electrical to mechanical energy).
• The other figure likely demonstrates Electromagnetic Induction, where motion within a magnetic field produces an electric current (converting mechanical to electrical energy).
(l) A simple electric motor constructed by using a coil, paper clips, a dry cell, and a permanent magnet, is shown in the figure. Explain its working process based on the motor effect.
The working process is based on the motor effect.
1. Current Flow: When the circuit is completed, current from the dry cell flows through the coil.
2. Force on Coil: The coil is placed in the magnetic field of the permanent magnet. According to Fleming’s Left-Hand Rule, the side of the coil where the current flows in one direction experiences an upward force, while the other side, with current in the opposite direction, experiences a downward force.
3. Rotation: These two opposing forces create a turning effect (torque) on the coil, causing it to rotate.
4. Continuous Rotation: A simple commutator (like the scraped ends of the coil wire touching the paper clips) reverses the direction of current in the coil every half rotation, ensuring the torque continues to act in the same direction and the coil keeps spinning.
(m) What is electromagnetic induction?
Electromagnetic induction is the phenomenon of producing an electromotive force (EMF) or voltage across an electrical conductor in a changing magnetic field. If the conductor is part of a closed circuit, an induced current will flow.
(n) Study the given picture and write what happens in the following situations. (This describes Faraday’s experiment with a magnet and a solenoid connected to a galvanometer.)
(i) As the bar magnet is slowly introduced into the solenoid.
A small current is induced in the solenoid, causing a small deflection in the galvanometer.
(ii) While introducing the bar magnet rapidly into the solenoid.
A larger current is induced because the rate of change of magnetic flux is greater. This causes a larger deflection in the galvanometer in the same direction as before.
(iii) Holding the bar magnet stationary inside the solenoid.
There is no change in the magnetic flux linked with the coil. Therefore, no current is induced, and the galvanometer shows zero deflection.
(iv) On pulling the bar magnet quickly out of the solenoid.
A large current is induced, but in the opposite direction to when it was introduced. The galvanometer shows a large deflection in the opposite direction.
(o) State Faraday’s law of electromagnetic induction.
Faraday’s laws of electromagnetic induction are:
1. First Law: Whenever the amount of magnetic flux linked with a closed circuit changes, an EMF (and hence a current) is induced in the circuit. This induced EMF lasts only as long as the change in flux is taking place.
2. Second Law: The magnitude of the induced EMF is directly proportional to the rate of change of the magnetic flux linked with the circuit.
(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 the dynamo.
The dynamo works on the principle of electromagnetic induction. The magnitude of the induced current depends on the rate of change of magnetic flux, which is directly related to the speed of rotation of the dynamo’s coil (driven by the bicycle’s tire).
• Bright Light: When the cycle moves fast, the coil rotates rapidly, causing a high rate of change of magnetic flux and inducing a large current, making the bulb glow brightly.
• Dimmed Light: When the cycle slows down, the coil’s rotation slows, reducing the rate of change of flux. This induces a smaller current, and the bulb dims.
• Turned Off: When the cycle comes to rest, the coil stops rotating. There is no change in magnetic flux, so no current is induced, and the bulb turns off.
(q) What can be done to increase the magnitude of current produced by a dynamo? Write any two ways.
1. Increase the speed of rotation: Rotating the coil faster increases the rate at which magnetic flux lines are cut, inducing a larger current.
2. Increase the strength of the magnetic field: Using a stronger magnet creates a denser magnetic field, so more flux lines are cut for the same speed of rotation, inducing a larger current. (Another way is to increase the number of turns in the coil.)
(r) Prepare a research report on any two sources of electricity in Nepal (Hydropower station, solar power plant) including their capacity, type of electricity produced, and transmission.
Research Report: Major Electricity Sources in Nepal
1. Hydropower Station: Kali Gandaki ‘A’
• Capacity: It is one of Nepal’s largest hydropower plants with an installed capacity of 144 Megawatts (MW).
• Type of Electricity Produced: It produces Alternating Current (a.c.) using large alternators (a.c. generators) driven by turbines, which are spun by the force of water from the Kali Gandaki River. The standard frequency is 50 Hz.
• Transmission: The electricity generated at a high voltage is stepped up by transformers to an even higher voltage (e.g., 132 kV). It is then transmitted across the country through the national grid’s high-tension power lines to various substations, where the voltage is stepped down for distribution to homes and industries.
2. Solar Power Plant: Nuwakot Solar Power Station
• Capacity: This is the largest solar power plant in Nepal, with an installed capacity of 25 Megawatts (MW).
• Type of Electricity Produced: Solar panels generate Direct Current (d.c.). This d.c. power is then converted into Alternating Current (a.c.) using large inverters because the national grid and our appliances operate on a.c.
• Transmission: After conversion to a.c., the voltage is stepped up using transformers and synchronized with the national grid. It is then transmitted through the same high-voltage transmission lines to supply power to the integrated Nepal Power System, helping to meet energy demands, especially during peak daytime hours.
(s) What is a transformer?
A transformer is a static electrical device that works on the principle of mutual induction to change the voltage level of an alternating current. It can “step up” (increase) or “step down” (decrease) voltage while transferring electrical energy from one circuit to another without changing the frequency.
(t) Write the type of transformers J and K shown in the figure. (Assuming the figures show standard transformer diagrams)
• Transformer J: If it shows more turns in the secondary coil than the primary coil (\(N_s > N_p\)), it is a Step-up transformer.
• Transformer K: If it shows fewer turns in the secondary coil than the primary coil (\(N_s < N_p\)), it 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.
Step-up Transformer
• Diagram: A core with a primary coil having few turns and a secondary coil having many turns.
• Uses:
1. At power stations to increase voltage for efficient long-distance power transmission.
2. In microwave ovens to generate the very high voltage needed to operate the magnetron.
Step-down Transformer
• Diagram: A core with a primary coil having many turns and a secondary coil having few turns.
• Uses:
1. In mobile phone and laptop chargers to reduce the mains voltage (220V) to a safe, low level (e.g., 5V or 20V).
2. At distribution substations in towns and cities to lower the high voltage from transmission lines before supplying it to households.
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.
Given: Primary Voltage (\(V_p\)) = 220 V
Secondary Voltage (\(V_s\)) = 20 V
Number of primary turns (\(N_p\)) = 550
Number of secondary turns (\(N_s\)) = ?
Formula: \(V_s / V_p = N_s / N_p\)
Solution:
\(20 / 220 = N_s / 550\)
\(N_s = (20 / 220) \times 550\)
\(N_s = (1 / 11) \times 550\)
\(N_s = 50\)
Answer: The number of secondary windings is 50.
(b) The number of secondary windings of the coil of a transformer 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?
Given: Primary Voltage (\(V_p\)) = 220 V
Relation of turns: \(N_s = 10 \times N_p\)
Secondary Voltage (\(V_s\)) = ?
Formula: \(V_s / V_p = N_s / N_p\)
Solution:
\(V_s / 220 = (10 \times N_p) / N_p\)
\(V_s / 220 = 10\)
\(V_s = 10 \times 220 = 2200 V\)
Answer: The secondary voltage obtained is 2200 V.
(c) The ratio of the number of the primary winding to the number of secondary windings of a transformer is 22:1. If an adapter with that transformer is connected to an electric circuit having a potential difference of 220V, calculate the output voltage so obtained.
Given: Primary Voltage (\(V_p\)) = 220 V
Ratio of turns: \(N_p / N_s = 22 / 1\)
Secondary Voltage (\(V_s\)) = ?
Formula: \(V_s / V_p = N_s / N_p\)
Solution:
We are given \(N_p / N_s = 22 / 1\), so the inverse is \(N_s / N_p = 1 / 22\).
\(V_s / 220 = 1 / 22\)
\(V_s = 220 / 22\)
\(V_s = 10 V\)
Answer: The output voltage obtained is 10 V.