10th Class Physics Notes 2026 — Chapter 19 – Federal Board
Class 10 Physics – Chapter 19: Electromagnetism
Board: Federal Board (FBISE)
Multiple Choice Questions
1. The presence of a magnetic field can be detected by a:
A. Small mass
B. Stationary positive charge
C. Stationary negative charge
D. Magnetic compass
Correct Answer: (D) Magnetic compass
2. A DC motor converts:
A. Mechanical energy into electrical energy
B. Mechanical energy into chemical energy
C. Electrical energy into mechanical energy
D. Electrical energy into chemical energy
Correct Answer: (C) Electrical energy into mechanical energy
3. Which of the following devices works on the principle of electromagnetic induction?
A. Magnetic compass
B. Motor
C. Transformer
D. Oscilloscope
Correct Answer: (C) Transformer
4. Voltage in the secondary coil of a transformer does not depend upon:
A. Frequency of source
B. Primary voltage
C. Power losses
D. Turns ratio
Correct Answer: (A) Frequency of source
5. Which of the following quantities remains constant in an ideal step-up transformer?
A. Current
B. Voltage
C. Power
D. Heat
Correct Answer: (C) Power
6. A step-up transformer has a transformation ratio of 3:2. What is the voltage in the secondary coil if the voltage in the primary coil is 30 V?
A. 15 V
B. 45 V
C. 90 V
D. 300 V
Correct Answer: (B) 45 V
Calculation:
7. Toward which magnetic pole of Earth is the north pole of a compass needle attracted?
A. The north pole of a compass needle is attracted to the north magnetic pole of Earth, which is located near the geographic North Pole of Earth.
B. The north pole of a compass needle is attracted to the south magnetic pole of Earth, which is located near the geographic North Pole of Earth.
C. The north pole of a compass needle is attracted to the north magnetic pole of Earth, which is located near the geographic South Pole of Earth.
D. The north pole of a compass needle is attracted to the south magnetic pole of Earth, which is located near the geographic South Pole of Earth.
Correct Answer: (B) The south magnetic pole of Earth, located near the geographic North Pole.
8. A step-up transformer:
A. Increases the input current
B. Increases the output voltage
C. Has more turns in the primary
D. Has fewer turns in the secondary coil
Correct Answer: (B) Increases the output voltage
9. A step-up transformer is used before electricity is transmitted by overhead cables. Select the statement that explains why this is done.
A. It increases the voltage to increase the speed at which electricity travels.
B. It increases the voltage to reduce energy loss in the cables.
C. It increases the current to increase the speed at which electricity travels.
D. It increases the current to reduce energy loss in the cables.
Correct Answer: (B) It increases the voltage to reduce energy loss in the cables.
10. Which of the following does not change in an ordinary transformer?
A. Power
B. Frequency
C. Current
D. Voltage
Correct Answer: (B) Frequency
Constructed Response Questions
1. In which direction must the current flow through the conductor to create the magnetic field shown in the diagram: up or down?
Answer: Up.
According to the Right-Hand Grip Rule, when the fingers of the right hand curl in the direction of the magnetic field lines, the thumb points in the direction of the current. In the given diagram, the thumb points upwards.
Note: The diagram was not included in the provided material.
2. In the following diagram, will there be a North pole or a South pole at the left side of the solenoid? Also draw its magnetic field, showing arrows to indicate its direction.
Answer: North pole.
According to the Right-Hand Grip Rule for solenoids, when the fingers curl in the direction of conventional current flowing through the coils, the thumb points towards the North pole.
Therefore, the left end of the solenoid acts as the North pole. Magnetic field lines emerge from the North pole and enter the South pole.
Note: The diagram was not included in the provided material.
3. A current-carrying conductor is placed in a magnetic field, as shown.
a) In which direction will the conductor move: up, down, left or right?
Answer: Up.
According to Fleming’s Left-Hand Rule, the magnetic field is directed from North to South (right to left), and the current flows into the page. The resulting magnetic force acts upwards.
b) If the magnetic field is reversed, in which direction will the conductor move?
Answer: Down.
Reversing the magnetic field reverses the direction of the magnetic force. Therefore, the conductor moves downwards.
c) If both the magnetic field and the current are reversed, in which direction will the conductor move?
Answer: Up.
Reversing both the magnetic field and the current maintains the original direction of the magnetic force. Therefore, the conductor moves upwards.
Note: The original diagrams for these questions were not included in the provided material.
Short Response Questions
1. How does a generator differ from a motor, and what is their relationship to each other?
A generator converts mechanical energy into electrical energy using electromagnetic induction, whereas an electric motor converts electrical energy into mechanical energy using the magnetic force on a current-carrying conductor.
Both devices have a fundamentally similar internal construction, consisting of coils rotating within a magnetic field.
Because of this structural reversibility, a motor can act as a generator when its armature is mechanically turned, demonstrating the reverse conversion of energy.
2. Why is a transformer ineffective with DC voltage?
A transformer relies on electromagnetic induction, which requires a continuously changing magnetic flux to induce an electromotive force (EMF) in the secondary coil.
Direct current (DC) produces a constant magnetic field that does not change with time. Therefore, the rate of change of magnetic flux is zero:
Consequently, no secondary voltage is induced. Continuous DC can also overheat or damage the primary winding.
3. Explain the relationship between the direction of a current-carrying conductor and the magnetic field it produces.
The direction of the circular magnetic field lines around a straight current-carrying conductor depends on the direction of the electric current.
According to the Right-Hand Grip Rule, if you point your right thumb in the direction of the current, your curled fingers show the direction of the magnetic field.
Reversing the direction of the electric current also reverses the direction of the magnetic field lines.
4. Describe the interaction between a current-carrying conductor and a magnetic field, and explain how to determine the direction of the force exerted.
When a current-carrying conductor is placed in an external magnetic field, the magnetic field produced by the current interacts with the external field. This interaction creates a force on the conductor.
The magnitude of this force depends on the current, magnetic field strength, length of the wire, and the angle between the conductor and the magnetic field.
The direction of the force is determined using Fleming’s Left-Hand Rule:
Thumb represents Force.
Forefinger represents Magnetic Field.
Middle finger represents Current.
5. Why is high voltage used for long-distance transmission of electrical power?
Power loss in transmission cables occurs mainly as heat due to Joule heating:
By increasing the transmission voltage to a very high level, the current flowing through the transmission lines is reduced for the same amount of transferred power.
Since power loss is proportional to the square of the current, reducing the current minimizes energy losses over long distances.
6. Design an experiment to generate an electric current using a coil of wire and a bar magnet.
Connect a sensitive galvanometer or ammeter across the two ends of a closed coil of insulated copper wire.
Push the North pole of a permanent bar magnet into the centre of the coil.
Observe that the galvanometer needle deflects to one side.
Pull the magnet out of the coil in the opposite direction.
Observe that the galvanometer needle deflects in the opposite direction.
This experiment shows that relative motion between a magnet and a coil induces an electric current.
7. What factors influence the strength of an induced electromotive force (EMF)?
According to Faraday’s Law, the induced EMF is directly proportional to the rate of change of magnetic flux linking the coil.
The induced EMF can be increased by:
Increasing the number of turns in the coil.
Increasing the strength of the magnetic field.
Increasing the speed of relative motion between the magnet and the coil.
8. Compare and contrast the two primary types of transformers.
A step-up transformer increases voltage and has more turns in the secondary coil than in the primary coil:
A step-down transformer decreases voltage and has fewer turns in the secondary coil than in the primary coil:
Both transformers operate on the principle of mutual electromagnetic induction and use a common laminated iron core.
In an ideal transformer, input power equals output power:
Therefore, an increase in voltage is accompanied by a decrease in current, and vice versa.
9. Identify three devices that operate on the principle of electromagnetic induction.
Transformers: Change alternating voltages using mutual induction between primary and secondary coils.
AC Generators (Alternators): Rotate coils within magnetic fields to induce alternating electric current.
Induction Cooktops: Use changing magnetic fields to induce eddy currents that heat suitable cookware.
Long Response Questions
1. Design an experiment to qualitatively demonstrate the existence of a magnetic field around a current-carrying conductor.
Objective:
To demonstrate that an electric current produces a circular magnetic field around a conductor, as shown by Oersted’s discovery.
Apparatus Required:
A straight rigid copper wire
A DC power supply or battery
A switch
Connecting wires
Small magnetic compasses
A horizontal cardboard sheet
Experimental Procedure:
Pass the straight copper wire vertically through a small hole in the centre of a horizontal cardboard sheet.
Connect the ends of the wire in series with a battery and a switch to establish an electrical circuit.
Place several small magnetic compasses in a circular pattern around the wire on the cardboard.
Close the switch to allow current to flow through the conductor.
Observe the immediate deflection of the compass needles.
Open the switch and observe that the compass needles return to their original North-pointing alignment.
Reverse the battery connections to reverse the current direction and observe that the compass needle orientations also reverse.
Scientific Conclusion:
The deflection of the compass needles proves that an electric current generates a magnetic field around a conductor. The direction of this magnetic field depends on the direction of current flow.
2. Describe an experimental setup to induce an EMF in a circuit using a changing magnetic field. How can you measure the induced EMF?
Apparatus:
A solenoid coil
A sensitive centre-zero galvanometer
A bar magnet
Connecting wires
Experimental Setup:
Connect the terminals of the solenoid to form a closed electrical circuit with the galvanometer. Initially, keep the bar magnet stationary near the coil.
Inducing EMF:
Move the bar magnet towards or away from the coil. This motion changes the magnetic flux through the loops of the wire over time.
According to Faraday’s Law:
The changing magnetic flux induces an electromotive force, which drives a current through the circuit and causes the galvanometer needle to deflect.
Measuring Induced EMF:
The magnitude of the induced EMF can be measured directly using a calibrated voltmeter connected across the coil terminals.
It can also be determined indirectly by measuring the induced current with the galvanometer and applying Ohm’s Law:
This method requires the resistance of the coil to be known.
3. Explain Lenz’s Law in your own words, and provide a real-world application to illustrate its application.
Definition:
Lenz’s Law states that the direction of an induced current is always such that the magnetic field produced by it opposes the change in magnetic flux that caused the current.
It is a consequence of the law of conservation of energy.
Real-World Application: Electromagnetic Braking (Eddy Current Brakes)
Heavy trains and roller coasters use eddy current braking systems.
An electromagnet produces a changing magnetic field near a metal track or wheel.
This changing magnetic field induces swirling eddy currents inside the metal.
According to Lenz’s Law, these eddy currents produce a magnetic field that opposes the relative motion causing them.
This creates a braking force that slows down the moving object without direct physical contact.
This system provides smooth braking and reduces mechanical wear and tear.
4. Define an alternating current (AC) generator. Draw a labelled diagram and explain its components and operation. How does it convert mechanical energy into electrical energy?
Definition:
An AC generator is an electromechanical device that converts mechanical energy into electrical energy in the form of alternating current using electromagnetic induction.
Components:
Magnetic Field Poles (N and S): Permanent magnets or electromagnets that provide a stable external magnetic field.
Armature Coil: A rectangular coil of insulated copper wire wound on an iron core that rotates within the magnetic field.
Slip Rings: Two continuous metal rings connected to the ends of the rotating armature coil.
Carbon Brushes: Stationary graphite blocks that maintain sliding electrical contact with the rotating slip rings and transfer current to the external circuit.
Operation and Energy Conversion:
When mechanical torque rotates the armature coil, its wires cut through the external magnetic field lines. This produces a continuous change in magnetic flux, inducing an alternating EMF and current in the coil.
The direction of the induced current changes after every half-rotation. The slip rings and carbon brushes transfer the alternating current to the external circuit.
Thus, the generator converts mechanical energy into electrical energy.
5. What is an AC motor? Draw a labelled diagram and explain its components and operation. How does it convert electrical energy into mechanical energy?
Definition:
An AC motor is an electrical machine that converts alternating electrical energy into mechanical rotational energy using magnetic forces.
Components:
Stator: The stationary outer part containing electromagnets or windings supplied with alternating current to produce a rotating magnetic field.
Rotor (Armature): The rotating inner assembly consisting of conducting bars or coils mounted on a shaft.
Slip Rings / Commutator: Contacts used to deliver or distribute electrical current to the rotor windings, depending on the motor design.
Operation and Energy Conversion:
When alternating current is supplied to the motor, magnetic fields are produced in its stator and rotor. The interaction between these magnetic fields generates a force on the current-carrying conductors.
This magnetic force produces a turning torque that causes the rotor to spin continuously.
Thus, electrical energy supplied to the motor is converted into useful mechanical work.
6. Define a transformer. Explain its principle of operation, components, and applications. How does it change the AC voltage?
Definition:
A transformer is a static electromagnetic device used to increase or decrease alternating voltage and current without changing the frequency.
Principle of Operation:
A transformer operates on the principle of mutual induction.
An alternating current in the primary coil produces a continuously changing magnetic flux. This magnetic flux is guided through an iron core and induces an alternating EMF in the secondary coil.
Components:
Primary Coil: The input winding connected to the AC source.
Secondary Coil: The output winding connected to the load.
Laminated Iron Core: A magnetic core made of stacked insulated sheets. It provides a path for magnetic flux and reduces eddy current energy losses.
How It Changes AC Voltage:
The voltage transformation is governed by the turns ratio equation:
Where:
\(V_s\) = Secondary voltage
\(V_p\) = Primary voltage
\(N_s\) = Number of turns in the secondary coil
\(N_p\) = Number of turns in the primary coil
For a step-up transformer:
The secondary voltage is greater than the primary voltage.
For a step-down transformer:
The secondary voltage is lower than the primary voltage.
Applications:
Power transmission grids use step-up transformers to increase voltage for long-distance transmission.
Household appliance adapters use step-down transformers to reduce mains voltage for electronic devices.
Transformers are also used in electrical isolation circuits.
Numerical Response Questions
1. A step-up transformer has a turns ratio of 1:100. An alternating supply of 20 V is connected across the primary coil. What is the secondary voltage?
Given Data:
Turns ratio:
Therefore:
Primary voltage:
To Find:
Secondary voltage \(V_s=?\)
Formula:
Calculation:
Answer: 2000 volts.
2. A step-down transformer has a turns ratio of 100:1. An AC voltage of amplitude 170 V is applied to the primary. What is the voltage across the secondary?
Given Data:
Turns ratio:
Therefore:
Primary voltage:
To Find:
Secondary voltage \(V_s=?\)
Formula:
Calculation:
Answer: 1.7 volts.
3. Calculate the input voltage if it draws 0.5 A. Also calculate the turns ratio of the primary to secondary coil if a transformer is rated for an output of 100 W and 8.3 A. Also identify the type of transformer.
Given Data:
Input current:
Output power:
Output current:
Assuming an ideal transformer:
To Find:
Input voltage \(V_p=?\)
Turns ratio \(N_p:N_s=?\)
Type of transformer
Calculations:
1. Input Voltage (\(V_p\)):
Using:
Substituting the values:
2. Secondary Voltage (\(V_s\)):
Using:
3. Turns Ratio (\(N_p:N_s\)):
Using:
Therefore:
4. Type of Transformer:
Since the primary voltage (200 V) is greater than the secondary voltage (12.05 V), it is a step-down transformer.
Answer: Input voltage = 200 V; turns ratio (primary to secondary) = 16.6:1; step-down transformer.
4. Calculate the output power and output current of the following step-down transformer. The value of input current is 0.050 A. Assume that the transformer is 100% efficient.
Given Data:
Primary voltage:
Primary current:
Secondary voltage:
Efficiency:
Therefore:
To Find:
Output power \(P_{\text{out}}=?\)
Output current \(I_s=?\)
Calculations:
1. Input Power (\(P_{\text{in}}\)):
Using:
2. Output Power (\(P_{\text{out}}\)):
Since the transformer is 100% efficient:
3. Output Current (\(I_s\)):
Using:
Answer: Output power = 11.5 W; output current = 1.9 A.
5. A transformer in an adapter steps up a voltage of 115 V to the 230 V needed for a hair dryer. A current of 5 A flows through the hair dryer. Assuming that the transformer is 100% efficient, calculate the current drawn from the mains supply.
Given Data:
Primary voltage:
Secondary voltage:
Secondary current:
Efficiency:
Therefore:
To Find:
Input current \(I_p=?\)
Formula:
For a 100% efficient transformer:
Calculation:
Answer: 10 A.
Notes by Sir Dilawar
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