Electricity and Magnetism— Class 10 Science Guide [New Syllabus]
UNIT-11
1. Chose the correct option for the following questions:
(a) Which of the following is the source of a.c?
(i) drycell
(ii) solar panel
(iii) dynamo
(iv) voltaic cell
(b) which scientist discovered the magnetic effect of electric current?
(i) Michael Faraday
(ii) Hans Christian Oersted
(iii) John Ambrose Fleming
(iv) James Clerk Maxwell
(c) Which is the direction of the magnetic field when current is flowing upwards through a conducting straight wire?
(i) Anticlockwise direction
(ii) Clockwise direction
(iii) Perpendicular to the direction of electric current
(iv) Opposite to the direction of electric current
(d) Which of the following statements is true for the current source shown in the figure?
 |
| B |
 |
| A |
(i) The value of electric current produced by A is constant.
(ii) The frequency of electric current produced by B is constant.
(iii) The brightness of the lamp fluctuates if the current produced by A is used.
(iv) The direction of electric current produced by B changes constantly.
(e) On which of the following principles the working of a transformer based?
(i) Electromagnetic induction
(ii) Mutual induction
(iii) Motor effect
(iv) Lighting effect of current.
(f) Which is the transformer’s formula?
(i) Vp/Vs = Ns/Np
(ii) Vs/Np = Ns/Vp
(iii) Vs/Vp = Np/Ns
(iv) Vs/Ns = Vp/Np
2. Differentiate between:
(i) A.C. and D.C.
| A.C. (Alternating Current) | D.C. (Direct Current) |
|---|---|
| 1. In A.C., the direction of current alternates periodically. | 1. In D.C., the current flows in a single, constant direction. |
| 2. A.C. is used for long-distance power transmission. | 2. D.C. is mainly used in batteries and electronic devices. |
| 3. The voltage in A.C. varies sinusoidally over time. | 3. The voltage in D.C. remains constant over time. |
| 4. A.C. can be easily transformed to different voltage levels using transformers. | 4. D.C. cannot be easily transformed to different voltage levels without complex circuits. |
| 5. Example: Household electricity, power grids, etc. | 5. Example: Batteries, solar panels, electronic circuits, etc. |
(ii) Dynamo and Generator
| Dynamo | Generator |
|---|---|
| 1. A dynamo is a small device used to generate electricity, primarily used in low-power applications. | 1. A generator is a larger machine used to generate electricity on a larger scale, typically for industrial use or power plants. |
| 2. A dynamo produces direct current (DC) electricity. | 2. A generator produces alternating current (AC) electricity, although some can produce DC as well. |
| 3. Dynamo uses a simple, permanent magnet to induce current. | 3. A generator typically uses electromagnets or a rotating magnetic field to induce current. |
| 4. Dynamos are more commonly used in small applications like bicycle lights, or small machines. | 4. Generators are used for larger power generation, such as in electricity grids, backup power systems, and large machines. |
| 5. Example: Dynamo used in older bicycle lights or hand-crank generators. | 5. Example: Large power generators used in power stations, or portable generators used in homes or construction sites. |
(iii) Motor and Generator
| Electric Motor | Electric Generator |
|---|---|
| 1. An electric motor converts electrical energy into mechanical energy. | 1. An electric generator converts mechanical energy into electrical energy. |
| 2. Electric motors are used to drive machines, fans, and other electrical appliances. | 2. Electric generators are used to produce electricity in power plants, backup power systems, and portable generators. |
| 3. Motors work on the principle of electromagnetic induction, where a current-carrying conductor experiences a force in a magnetic field. | 3. Generators work on the principle of electromagnetic induction, where mechanical motion (e.g., turning a coil in a magnetic field) induces an electric current. |
| 4. In an electric motor, electrical energy is supplied to generate movement. | 4. In an electric generator, mechanical energy (usually from a turbine or engine) is used to generate electricity. |
| 5. Example: Fans, washing machines, electric cars, etc. | 5. Example: Power plants, wind turbines, hydroelectric dams, portable generators, etc. |
(iv) Step-up Transformer and Step-down Transformer
| Step-Up Transformer | Step-Down Transformer |
|---|---|
| 1. A step-up transformer increases the voltage from primary to secondary coil. | 1. A step-down transformer decreases the voltage from primary to secondary coil. |
| 2. It has more turns in the secondary coil compared to the primary coil. | 2. It has fewer turns in the secondary coil compared to the primary coil. |
| 3. Step-up transformers are commonly used in power transmission to step up voltage for long-distance transmission. | 3. Step-down transformers are used to reduce voltage for safe usage in homes and businesses. |
| 4. The output voltage is greater than the input voltage. | 4. The output voltage is lower than the input voltage. |
| 5. Example: Power plants, high-voltage transmission lines. | 5. Example: Household electrical appliances, low-voltage applications. |
3. Give reasons:
(a) When a ceiling fan is connected to the circuit of the solar panel, the fan does not rotate.
→ Many ceiling fans are designed for AC power, while solar panels often output DC power due to which the ceiling fan doesn’t rotate.
(b) When a magnetic compass is placed near a circuit in which an electric current is flowing, its needle deflects.
→ The needle of magnetic compass deflects due to the magnetic effect produced by the electric current.
(c) Electromagnet is used in the electric bell.
→ An electromagnet is used in an electric bell to attract the hammer and strike the bell when current flows, producing sound, and to release the hammer when the current stops, enabling repeated ringing.
(d) The number of primary windings and secondary windings of a transformer are not the same.
→ Transformer is a device which is used to either increase or decrease a.c. output voltage. If the primary windings and secondary windings are made equal, the transformer neither increases nor decreases the a.c. output voltage. This means transformer has no work. Hence, the number of primary and secondary windings of transformer are not made equal.
(e) The core of a transformer is laminated.
→ There is loss of current while inducing current in secondary coil of transformer due to which current produces more heat on transformer. Hence, the core of a transformer is laminated to reduce the heating effect of transformer
(f) Transformers are used in mobile chargers.
→ Transformers are used in mobile chargers to convert high voltage from power outlet to a lower and safer voltage suitable for charging the mobile battery.
4. Answer the following questions:
(a) The frequency of a.c. in our country is 50 Hz. What does it mean?
→ The frequency of a.c. in our country is 50 Hz means that the direction of the a.c. changes 50 times per second.
OR, It means that the polarity of the current is alternated by 50 times in 1 second.
(b) Draw the time graph of direct current and alternating current.
→
 |
| For Alternating current |
 |
| For Direct Current |
(c) Draw the magnetic field lines around the current-carrying straight wire and solenoid.
→
 |
| Magnetic field around a current carrying solenoid |
 |
| Magnetic field around current carrying st. wire |
(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.
→ Maxwell’s right hand thumb rule states that if you hold a straight current-carrying wire in your right hand with your thumb pointing in the direction of electric current, the direction in which your fingers curl around the wire will indicate the direction of the magnetic field lines produced around it.
(ii) Maxwell’s right hand grip rule to find the direction of magnetic lines of force around a solenoid.
→ Maxwell’s right hand grip rule states that if you hold a solenoid in your right hand with your fingers curling around it in the direction of the electric current, your thumb will point in the direction of the magnetic field lines inside the solenoid. This means that the curled fingers show the direction of circular magnetic lines around the solenoid’s coil, while the extended thumb indicates the north pole of the solenoid, showing the overall magnetic field direction. This rule helps to determine the direction of magnetic lines of force created by the solenoid.
(e) What is magnetic effect of current?
→ The formation of a magnetic field around a conducting wire when there is electric current in it is called magnetic effect of current.
(f) Define magnetic flux.
→ The total number of magnetic lines of force passing through the surface area within the magnetic field represents the magnetic flux.
(g) How can the magnetic field produced around straight current carrying wire be demonstrated by using iron dust, cardboard, and conducting straight wire? Explain it.
→ To demonstrate the magnetic field produced around a straight current-carrying wire using iron dust, cardboard, and a conducting straight wire, these steps should be followed:
· Sprinkle a small amount of iron dust evenly over the cardboard sheet surrounding the wire. Make sure the filings cover a large area.
· Connect the two ends of the wire to a battery using conducting wires.
Observation: When the current passes through the wire, the iron dusts will align themselves along circular paths around the wire. These circular patterns represent the magnetic field lines produced by the current in the wire.
Explanation:
- When current flows through the conducting wire, it generates a magnetic field around it. The magnetic field is circular in shape and its direction can be determined using the right-hand rule (thumb pointing in the direction of current, fingers curl in the direction of magnetic field).
- The iron dusts act as small magnetic needles, aligning themselves along the magnetic field lines. The pattern of these dusts shows the shape and direction of the magnetic field around the wire.
Conclusion:
This experiment visually demonstrates that a current-carrying wire creates a magnetic field in the surrounding space, and the pattern of the field is circular, which can be observed by the alignment of iron dusts around the wire.
(h) Draw the magnetic field developed around a straight current-carrying wire.
→ See Question (c)
(i) What is solenoid? Draw a picture showing the magnetic field developed around a solenoid.
→ The solenoid is a long cylindrical coil of wire consisting of a large no. of turns bound together very tightly. See Question (c) for figure portion
(j) Write two uses of solenoid.
→ Solenoids are used in MRI machines which help in creating MRI images.
→ They are used in automatic locks (door locks for example).
→ They are used in circuit breakers to protect equipment.
(k) Which effects are demonstrated in the given figure?
(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 motor effect.
Current flow in the Coil: When the circuit is completed, current flows from the dry cell through the coil via paper clips, which acts as supports and conductors.
Motor Effect: The current in the coil creates magnetic field around it. When this coil is placed within magnetic field of permanent magnet, it experiences a force due to interaction between magnetic fields.
Rotation: According to Fleming’s left hand rule, the direction of the force causes the coil to rotate. This rotation motion continues as long as there is current flowing, creating continuous spin in one direction functioning as a basic motor.
(m) What is electromagnetic induction?
→ The phenomenon in which an electromotive force will be set up on a coil due to relative motion between coil and magnet is known as electromagnetic induction.
(n) Study the given picture and write what happens in the following situations.
→ A small temporary current is induced in the coil, detected by slight deflection in galvanometer.
(ii) While introducing the bar magnet rapidly into the solenoid
→ Moving magnet rapidly increases rate of change in magnetic flux causing larger induced current. The galvanometer needle shows more deflection indicating a stronger current.
(iii) Holding the bar magnet stationary inside the solenoid
→ By holding bar magnet stationary inside the solenoid, there is no change in magnetic flux due to which no current is induced. The galvanometer shows null deflection.
(iv) On pulling the bar magnet quickly out of the solenoid
→ Pulling out magnet rapidly causes change in magnetic flux in opposite direction due to which needle of galvanometer deflects in opposite direction, showing a reverse current.
(o) State faraday’s law of electromagnetic production.
→ It states that, When there is relative motion between a conductor and a magnet, an emf is induced in the conductor and the magnitude of such emf is directly proportional to the rate of change of the magnetic flux linked with the conductor.
(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.
→ Dynamo is a device which converts mechanical energy into electrical energy and works on the principle of electromagnetic induction. The dynamo produces electricity when the bicycle tire spins. Faster spinning makes more electricity and slower spinning makes less. When we pedal quickly, dynamo generates more electricity, so the bulb glows brightly. When we slow down, it produces less, and the bulb dims. When the bicycle stops, the dynamo stops, due to which no electricity flows and bulb turns off.
(Note: Verify this answer to your teacher)
(q) What can be done to increase the magnitude of current produced by a dynamo? Write any two ways.
→ The magnitude of current produced by dynamo can be increased in following ways:
· By increasing the number of turns in the coil.
· By increasing the rate of rotation of magnet.
· By increasing strength of magnetic field.
(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 should be made by students themselves after visiting such sources on their educational tour.)
(s) What is a transformer?
→ Transformer is a device which is used to increase or decrease the voltage of an alternating current.
(t) Write type of transformers J and K shown in figure.
(u) Draw a block diagrams of the step-up transformer and step-down transformer and write two uses of each.
→ See Question no 27 below for uses of these
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.
→ Here,
No of primary turns (Np) = 550
No of secondary turns (Ns) = ?
Primary Voltage (Vp) = 220V
Secondary Voltage (Vs) = 20V
We Know, Vp/Vs = Np/Ns
Or, 220/20 = 550/Ns
Or, Ns = 550 x 20 / 220 = 50 turns.
(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?
→ Here,
No of primary windings (Np) = a (let)
No of secondary windings (Ns) = 10a
Primary Voltage (Vp) = 220V
Secondary Voltage (Vs) = ?
We Know, Vp/Vs = Np/Ns
Or, 220/Vs = a/10a
Or, Vs = 220 x 10 = 2200V
(c) The ratio of the number of the primary winding to the secondary windings of a transformer is 2:1. If an adapter with that transformer is connected to an electric circuit having a potential difference of 220V, calculate the output voltage so obtained.
→ Here,
No of primary windings (Np) = 2a (let)
No of secondary windings (Ns) = a
Primary Voltage (Vp) = 220V
Secondary Voltage (Vs) = ?
We Know, Vp/Vs = Np/Ns
Or, 220/Vs = 2a/a
Or, Vs = 220/2 = 110V
POINTS TO REMEMBER
Ø The conducting path through which electricity flows is called electric circuit.
Ø A.C means alternating current and D.C means direct current.
Ø Effects of electricity are heating effect, lighting effect, chemical effect and magnetic effect.
Ø The magnet which is formed by passing electricity in a solenoid is called electromagnet.
Ø Solenoid is made by winding a length of insulated wire.
Ø In electric bell, electromagnet is used.
Ø Galvanometer is a device which is used to detect electric current in a circuit.
Ø Bicycle dynamo is based on the principle of electromagnetic induction.
Ø When current carrying conductor is placed in a magnetic field, it starts to move. This is called motor effect.
Ø Transformers convert high ac voltage to low ac voltage and vice versa.
Ø Transformer is based on principle of mutual induction.
Ø Core of transformer is laminated by joining U shaped iron plates.
Ø Core is insulated by varnish or shellac.
SOME IMPORTANT BONUS QUESTIONS
1. What is electricity? How are electric charges produced?
→ The energy of charged particles flowing through an electric conductor is called electricity.
→ Electric charges are produced by gain or loss of electrons from valence shell of an atom.
2. Define current. Write its SI unit.
→ The rate of flow of charge or electrons through an electric circuit is called current. Its SI unit is Ampere.
3. How is strength of electromagnet increased?
→ The strength of electromagnet is increased by:
- Increasing the strength of current through the coil
- Increasing the number of coils in solenoid
- Using soft magnetic material in solenoid
4. Write three reasons for why electromagnet is widely used?
→ Following are the reasons for wide use of electromagnet:
- It is temporary magnet so according to need we can switch on and off the power supply
- The strength of electromagnet can be increased or decreased according to need.
- It can be designed in any shape and size.
5. The use of soft iron in an electromagnet is better, why?
→ The use of soft iron increases the strength of electromagnet. So, to make powerful electromagnet, soft iron is used.
6. The naked wire is not used while making an electromagnet, why?
→ If naked wire is used then the iron rod also acts as a conductor and short circuit takes place instead of showing magnetic effect of electricity to the molecules of the rod. Hence, naked wire is not used to make electromagnets.
7. How does electric bell work?
→ As the switch is on, the U shaped soft iron acts as electromagnet. So that springy plate gets attracted towards electromagnet. At the same times, hammer hits the gong and sound is produced. Now, circuit opens and U shaped electromagnet becomes iron by losing magnetic property. So the plate again connects the circuits. This process repeats continuously as the switch is on.
8. What is AC?
→Alternating current (ac) is one which changes its magnitude continuously and reserves its direction periodically.
9. The frequency of ac is 50Hz, what does it mean?
→The frequency of ac is 50Hz means that the polarity of the current is alternated by 50 times in one second.
10. Write the source of alternating current.
→Alternating current is produced by generator or a dynamo.
11. What is dc?
→The current whose polarity does not change with time is called direct current (dc).
12. How does dynamo work?
→Dynamo consists of rotating head with the magnet inside it. An insulated copper wire wounded around a soft iron is kept below the magnet. This all system is sealed within an aluminum pot. When head rotates with the help of wheel, magnet also moves. This causes the magnetic flux changes and emf is induced. Thus induced current is flown through the wire to the bulb.
13. What change will occur in the brightness of bulb when the bicycle is turned first slowly and then speeded up in dynamo and why?
→ Lowering the speed of rotations of wheel of bicycle lowers the rate of change in magnetic flux. So that current will be less and brightness decreases. Consequently, increasing the speed of wheel, magnetic flux changes rapidly and current will be more. Hence, brightness increases.
14. What happens when ac is passed into the coil of dynamo?
→ When ac is passed into the coil of dynamo, magnet comes in motion due to motor effect.
15. Soft iron core is used in dynamo or generator. Why?
→ Soft iron core is used in dynamo or generator because it helps to induce more current in the circuit.
16. How does generator produce current?
→ The electromagnet is connected with a rotor and rotor is connected to turbine. When turbine is moved then rotor rotates along with electromagnet. Thus, the coil wound around the soft iron crosses the magnetic lines of force and emf is induced so that current is produced.
17. What happens when the speed of rotor in a generator is doubled?
→ As we know that, induced emf is directly proportional to the speed of rotor so induced current is also doubled when the speed of rotor in a generator is doubled.
18. List out the similarities between dynamo and generator.
→The similarities between dynamo and generator are as follows:
- Both depend on the principle of electromagnetic induction.
- Dynamo and generator both are ac source.
19. How can you increase the strength of dynamo or generator?
→ The strength of dynamo or generator can be increased by the following ways:
- By increasing the number of turns of wire in a coil.
- By using soft iron core or by increasing the strength of magnet.
- By increasing the rotation speed of dynamo.
- By decreasing the distance between coil and magnet.
20. What is meant by motor effect?
→ When current flows through a wire is placed in a magnetic field, a movement is developed in the wire. This phenomenon is called motor effect.
21. Why does a current carrying conductor kept in magnetic field shift from its position?
→The current currying conductor kept in external magnetic field shifts from its initial position due to attraction and repulsion between external magnetic field and that of the current around the wire.
22. What is electric motor? On which principle is it based?
→A device which converts electrical energy into mechanical energy is called electric motor. It is used in industries and in domestic purpose. Electric motor is based in motor effect.
23. What is transformer? Mention its types.
→Transformer is a device which is used to increase or decrease the voltage of an alternating current. It is of two types: step-down transformer and step-up transformer.
24. What do you mean by mutual induction?
→ The process of inducing emf in a coil due to change in ac current flowing in a neighboring coil is called mutual induction.
25. Transformer changes only ac current not de current. Why?
→ Transformer works on the principle of mutual induction and for mutual induction polarity of current should change continuously. This is possible only for ac current not for dc current. So, transformer works only in ac not in dc current.
26. What is meant by step down and step up transformer?
→ Step-down transformer: the transformer whose input ac voltage is more than that of the output ac voltage is called step-down transformer. Hence, number of turns in secondary coil is less than that of primary coil.
→ Step-up transformer: The transformer whose output ac voltage is more than that of input ac voltage is called step-up transformer. Here, number of turns in secondary coil is more than that of primary coil.
27. Write the uses of step down and step up transformer.
→ Step down transformer is used by electric authority while distributing electricity.
→ Step up transformer is used:
- In household electrical appliances. For example, TV, fridge etc.
- In office to maintain potential for computers.
- In factories, industries etc.
28. What is used to laminate or insulate the iron plates in transformer?
→Varnish or shellac is used to laminate or insulate the iron plates in transformer.
29. Core of transformer is laminated, why?
→ Core of transformer is laminated because while inducing current in the secondary coil there is loss of current. This current produces more heat on transformer. So, to reduce heating effect of transformer it must be laminated.
30. What is the condition of primary turns and secondary turns in a transformer?
→ In step up transformer, primary turns (Np) is less than secondary turns (Ns). Similarly, in step down transformer, primary turns (Np) is more than that in secondary turns (Ns).
31. Which transformer is used to transmit the electricity for long distance?
→ Low ac voltage should be changed into high ac voltage by using step up transformer to transmit electricity to a long distance.
32. The number of turn in primary coil are never made equal to number of turns in secondary coil in a transformer, why?
→ The transformer is a device either to increase or to decrease the ac output voltage. If the number of turns in primary coil and the number of turns in secondary coil are made equal then the transformer neither increases nor decreases the ac output voltage. This means transformer has no work. Hence, number of turns in primary secondary coils are made different.
33. Which transformer is used to play a radio of rating 12V in ac mains supply? why?
→ Step down transformer is used to play the radio of 12V. It is because, input supply at house hold is 220V so that the step down transformer in the adaptor changes it into 12V.
34. 'The use of ac is limited without transformer' Justify it.
→To transmit the ac voltage from power station to long distance and to run different electrical devices of different voltage rating, the voltage should either to increase or to decrease. For this transformer is needed. Without transformer, there is no change in ac voltage. So, the use of ac current is limited without transformer.
35. Why is soft iron core used in transformer?
→ Soft iron core is used in transformer because the soft iron core has good magnetic permeability as a result concentrates magnetic lines of force and reduces energy loss.
36. What are ways of energy loss in transformer?
→ In a transformer, for ideal case, input energy = output energy I₁ × V₁ = I2 × V2 This condition is violated due to energy loss in transformer by following ways:
- Wastage of energy due to resistance of windings.
- Wastage of energy due to heating effect.
- Wastage of energy due to leakage of field lines.
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