CBSE Class 10 Science Magnetic Effects of Electric Current VBQs Set 03

Magnetic Field and Field Lines

Question. Why do two magnetic field lines not intersect each other?
Answer: Two magnetic field lines do not intersect each other because if they did, it would mean that at the point of intersection, the compass needle would point in two different directions (two tangents at the same point), which is not possible.

Question. Design an activity to demonstrate that a bar magnet has a magnetic field around it.
Answer: Fix a sheet of white paper on a drawing board. Place a bar magnet in the centre. Sprinkle some iron filings uniformly around the bar magnet. Tap the board gently. You will observe that the iron filings arrange themselves in a specific pattern of curved lines. This pattern represents the magnetic field around the magnet.

Question. What are magnetic field lines? Justify the following statements:
(a) Two magnetic field lines never intersect each other.
(b) Magnetic field are closed curves.
Answer: Magnetic field lines are imaginary lines used to represent the direction and strength of a magnetic field at various points.
(a) If they intersected, a compass needle would point in two directions at the point of intersection, which is impossible.
(b) Field lines emerge from the North pole and enter the South pole outside the magnet, and move from South to North inside the magnet, forming continuous loops.

Magnetic Field Due to a Current-Carrying Conductor

Magnetic Field due to Current through a Straight Conductor

Question. Assertion (A) : The magnetic eld lines around a current carrying straight wire do not intersect each other.
Reason (R) : The magnitude of the magnetic field produced at a given point increases as the current through the wire increases.
(a) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A)
(b) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A)
(c) Assertion (A) is true, but Reason (R) is False.
(d) Assertion (A) is false, but Reason (R) is true.
Answer: (b) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A)

Question. Name the instrument used to detect the presence of a current in a circuit.
Answer: Galvanometer (or Ammeter).

Question. List two factors on which the strength of magnetic field at a point due to a current carrying straight conductor depends. State the rule that determines the direction of magnetic field produced in this case.
Answer: Factors: (i) Amount of current flowing through the conductor (\( B \propto I \)). (ii) Distance of the point from the conductor (\( B \propto 1/r \)). The rule used is the Right-Hand Thumb Rule.

Question. A compass needle is placed near a current carrying straight conductor. State your observation for the following cases and give reasons for the same in each case.
(a) Magnitude of electric current is increased.
(b) The compass needle is displaced away from the conductor.
Answer: (a) Observation: The deflection of the compass needle increases. Reason: Magnetic field strength is directly proportional to the current.
(b) Observation: The deflection of the compass needle decreases. Reason: Magnetic field strength decreases as the distance from the current-carrying conductor increases.

Question. State how the magnetic field produced by a straight current carrying conductor at a point depends on (a) current through the conductor (b) distance of point from conductor.
Answer: (a) It is directly proportional to the current (\( I \)). (b) It is inversely proportional to the distance (\( r \)) from the conductor.

Right Hand Thumb Rule

Question. (i) A magnetic compass shows a deflection when placed near a current carrying wire. How will the deflection of the compass get affected if the current in the wire is increased ? What does it indicate ?
(ii) State Right Hand Thumb rule.
Answer: (i) The deflection increases. It indicates that the strength of the magnetic field produced by the current increases with the increase in the magnitude of current.
(ii) Right-Hand Thumb Rule: Imagine that you are holding a current-carrying straight conductor in your right hand such that the thumb points towards the direction of current. Then your fingers will wrap around the conductor in the direction of the field lines of the magnetic field.

Question. (a) State Right Hand Thumb rule to find the direction of the magnetic field around a current carrying straight conductor.
(b) How will the magnetic field be affected on:
(i) increasing the current through the conductor
(ii) reversing the direction of flow of current in the conductor?
Answer: (a) [Rule stated as above].
(b) (i) The strength of the magnetic field increases. (ii) The direction of the magnetic field lines will be reversed.

Magnetic Field due to Current through a Circular Loop and Solenoid

Question. For a current in a long straight solenoid, N and S poles are created at the two ends. Among the following statements, the incorrect statement is :
(a) the magnetic field lines inside the solenoid are in the form of straight lines, which indicates that the magnetic field is uniform at all points inside the solenoid.
(b) the strong magnetic field produced inside the solenoid can magnetize the soft iron placed inside it.
(c) the pattern of the magnetic field associated with a current carrying solenoid is different from the pattern of the magnetic field around a bar magnet.
(d) The N and S poles exchange positions when the direction of current through the solenoid is reversed.
Answer: (c) the pattern of the magnetic field associated with a current carrying solenoid is different from the pattern of the magnetic field around a bar magnet.

Question. What is an electromagnet ?
Answer: An electromagnet is a temporary magnet consisting of a coil of insulated copper wire wrapped around a soft iron core, which acts as a magnet only when an electric current flows through the coil.

Question. Give reason for the following
(i) There is either a convergence or a divergence of magnetic field lines near the ends of a current carrying straight solenoid.
(ii) The current carrying solenoid when suspended freely rests along a particular direction.
Answer: (i) This happens because the ends of the solenoid behave like the North and South poles of a magnet. Field lines emerge (diverge) from the North pole and enter (converge) at the South pole.
(ii) A current-carrying solenoid behaves like a bar magnet. When suspended freely, it aligns itself in the North-South direction, just like a magnetic needle.

Question. Find the direction of magnetic field due to a current carrying circular coil held:
(i) vertically in North - South plane and an observer looking it from east sees the current to flow in anticlockwise direction,
(ii) vertically in East - West plane and an observer looking it from south sees the current to flow in anticlockwise direction,
(iii) horizontally and an observer looking at it from below sees current to flow in clockwise direction.
Answer: (i) Towards East. (ii) Towards South. (iii) Downwards.

Question. (a) What is an electromagnet? List any two uses.
(b) State the purpose of soft iron core used in making an electromagnet.
(c) List two ways of increasing the strength of an electromagnet if the material of the electromagnet is fixed.
Answer: (a) [Definition as above]. Uses: Electric bells, lifting heavy iron scrap in cranes.
(b) The soft iron core increases the strength of the magnetic field many times because it gets magnetized easily.
(c) (1) Increasing the number of turns in the coil. (2) Increasing the magnitude of the current flowing through it.

Question. What are magnetic field lines? List three characteristics of these lines. Describe in brief an activity to study the magnetic field lines due to a current carrying circular coil.
Answer: Characteristics: (1) They form closed continuous loops. (2) They never intersect. (3) Their degree of closeness represents the field strength. Activity: Pass a circular coil through a cardboard. Sprinkle iron filings. Pass current. filings will form concentric circles around the wires and become straight at the center.

Force on a Current-Carrying Conductor in a Magnetic Field

Question. An alpha particle enters a uniform magnetic eld as shown. The direction of force experienced by the alpha particle is
(a) towards right
(b) towards left
(c) into the page
(d) out of the page
Answer: (c) into the page

Question. A constant current ows in a horiontal wire in the plane of the paper from east to west as shown in the gure. The direction of the magnetic eld will be north to south at a point
(a) directly above the wire
(b) directly below the wire
(c) located in the plane of the paper on the north side of the wire
(d) located in the plane of the paper on the south side of the wire
Answer: (a) directly above the wire

Question. Assertion : A current carrying straight conductor experiences a force when placed perpendicular to the direction of magnetic eld.
Reason : The net charge on a current carrying conductor is always zero.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true and R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Answer: (b) Both A and R are true and R is not the correct explanation of A.

Ampere (1775 - 1836) grasped the significance of Oersted’s discovery. He carried out a large series of experiments to explore the relationship between current electricity and magnetism. On the basis of experiments, he hypothesised that all magnetic phenomena are due to circulated electric currents. Later on many devices such as electromagnets, electric motors, microphones, electric generators, etc. were developed on the basis of magnetic phenomena.

Question. A magnetic needle is a/an :
(a) isolated north pole pivoted at its centre of mass.
(b) isolated south pole pivoted at its centre of mass.
(c) ordinary needle made of soft iron and pivoted at its centre of mass.
(d) small bar magnet pivoted at its centre of mass.
Answer: (d) small bar magnet pivoted at its centre of mass.

Question. A freely suspended magnet always rests in geographically north and south direction because :
(a) the Earth has two poles.
(b) the Earth behaves as a huge magnet.
(c) the magnetic north pole of the Earth’s magnet is located very close to its south pole.
(d) the magnetic south pole of the Earth’s magnet is located very close to its south pole.
Answer: (b) the Earth behaves as a huge magnet.

Question. When a current flows through a straight conductor, a magnetic field is produced around it. Consider the following statements about this field :
I. The direction of the magnetic field of a current carrying straight conductor is determined by right-hand thumb rule.
II. A charged body placed in this field experiences a force whose direction is given by Fleming’s left-hand rule.
III. The magnetic field lines around a current carrying straight conductor are in the form of concentric circles with the conductor as the centre.
The correct statement(s) is/are :
(a) I only
(b) III only
(c) I and II
(d) I and III
Answer: (d) I and III

Question. The strength of magnetic field of a current carrying solenoid is
(a) minimum at its ends
(b) uniform inside it at all points
(c) maximum at its centre
(d) zero at its centre
Answer: (b) uniform inside it at all points

Question. Which one of the following particles would not experience a force while moving perpendicular to a uniform magnetic field ?
(a) A neutron
(b) An alpha particle
(c) An electron
(d) A proton
Answer: (a) A neutron

Question. State the effect of a magnetic field on the path of a moving charged particle.
Answer: A magnetic field exerts a force on a moving charged particle in a direction perpendicular to its velocity and the magnetic field. This force causes the path of the particle to become curved (circular or helical).

Question. When is the force experienced by a current - carrying straight conductor placed in a uniform magnetic field
(i) Maximum ;
(ii) Minimum ?
Answer: (i) The force is maximum when the conductor is placed perpendicular to the direction of the magnetic field (angle = 90°).
(ii) The force is minimum (zero) when the conductor is placed parallel or anti-parallel to the direction of the magnetic field (angle = 0° or 180°).

Question. (i) Name and state the rule to determine the direction of force experienced by a current carrying straight conductor placed in a uniform magnetic field which is perpendicular to it.
(ii) An alpha particle while passing through a magnetic field gets projected towards north. In which direction will an electron project when it passes through the same magnetic field?
Answer: (i) Fleming's Left-Hand Rule: Stretch the thumb, forefinger and middle finger of your left hand such that they are mutually perpendicular. If the first finger points in the direction of magnetic field and the second finger in the direction of current, then the thumb will point in the direction of motion or the force acting on the conductor.
(ii) Since the alpha particle (positively charged) is projected towards North, the direction of force is towards North. An electron is negatively charged, so the direction of current is opposite to its motion. For the same magnetic field, the force on an electron will be in the opposite direction to that on an alpha particle. Therefore, the electron will be projected towards South.

Question. (i) A straight cylindrical conductor is suspended with its axis perpendicular to the magnetic field of a horse-shoe magnet. The conductor gets displaced towards left when a current is passed through it. What will happen to the displacement of the conductor if the
(1) current through it is increased ?
(2) horse-shoe magnet is replaced by another stronger horse-shoe magnet ?
(3) direction of current through it is reversed?
(ii) Name and state the rule for determining the direction of force on a current carrying conductor in a magnetic field.
Answer: (i) (1) The magnitude of displacement will increase because force is directly proportional to current.
(2) The magnitude of displacement will increase because force is directly proportional to magnetic field strength.
(3) The direction of displacement will be reversed (it will move towards the right).
(ii) Fleming's Left-Hand Rule. [Rule stated as above].

Question. A current carrying conductor is placed in a magnetic field. Now answer the following.
(i) List the factors on which the magnitude of force experienced by conductor depends.
(ii) When is the magnitude of this force maximum?
(iii) State the rule which helps in finding the direction of motion of conductor.
(iv) If initially this force was acting from right to left, how will the direction of force change if:
(a) direction of magnetic field is reversed?
(b) direction of current is reversed?
Answer: (i) The magnitude of force depends on (a) Current flowing through the conductor (\( I \)), (b) Strength of the magnetic field (\( B \)), and (c) Length of the conductor (\( l \)). [\( F = BIl \sin \theta \)]
(ii) The force is maximum when the conductor is placed perpendicular to the direction of the magnetic field.
(iii) Fleming’s Left-Hand Rule.
(iv) (a) The force will act from left to right (reversed). (b) The force will act from left to right (reversed).

Question. State whether an alpha particle will experience any force in a magnetic field if (alpha particles are positively charged particles)
(i) it is placed in the field at rest.
(ii) it moves in the magnetic field parallel to field lines.
(iii) it moves in the magnetic field perpendicular to field lines.
Justify your answer in each case.
Answer: (i) No force. A magnetic field only exerts force on moving charges.
(ii) No force. When the direction of motion is parallel to the field lines, the angle \( \theta = 0^\circ \), and since \( F = qvB \sin \theta \), the force is zero.
(iii) Maximum force. When moving perpendicular to field lines (\( \theta = 90^\circ \)), \( \sin 90^\circ = 1 \), resulting in the maximum possible force.

Electric Generator (A.C and D.C)

Question. Write the frequency of alternating current (AC) in India. How many times per second it changes its direction?
Answer: The frequency of AC in India is 50 Hz. It changes its direction 100 times per second.

Question. How is the type of current that we receive in domestic circuit different from the one that runs a clock?
Answer: Domestic circuits receive Alternating Current (AC), which changes its direction periodically. A clock runs on Direct Current (DC) obtained from a cell or battery, which flows in only one direction.

Question. (i) Why is an alternating current (A.C.) considered to be advantageous over direct current (D.C.) for the long distance transmission of electric power?
(ii) How is the type of current used in household supply different from the one given by a battery of dry cells?
(iii) How does an electric fuse prevent the electric circuit and the appliances from the possible damage due to short circuiting or overloading.
Answer: (i) AC can be transmitted over long distances without much loss of electric energy using transformers to step up or step down the voltage.
(ii) Household supply is AC (alternating current), whereas current from a battery of dry cells is DC (direct current).
(iii) An electric fuse is made of a material with a low melting point. In case of short-circuiting or overloading, the excessive current produces heat (\( H = I^2Rt \)), which melts the fuse wire. This breaks the circuit and protects the appliances.

Question. Define alternating current and direct current. Explain why alternating current is preferred over direct current for transmission over long distances.
Answer: Alternating current (AC) is a current whose direction and magnitude change periodically with time. Direct current (DC) flows only in one direction. AC is preferred for long-distance transmission because it can be stepped up to very high voltages to reduce energy loss during transmission and then stepped down for domestic use.

Domestic Electric Circuits

Question. At the time of short circuit, the electric current in the circuit
(a) vary continuously
(b) does not change
(c) reduces substantially
(d) increases heavily.
Answer: (d) increases heavily.

Question. Give reason for the following : The burnt out fuse should be replaced by another fuse of identical rating.
Answer: Replacing a burnt-out fuse with one of an identical rating ensures that the safety limit of the circuit is maintained. If a fuse of higher rating is used, it will not melt during a current surge, potentially causing fire or damage to appliances.

Question. Justify the following statements :
(a) Tungsten is used exclusively for filaments of electric lamps.
(b) Series arrangement is not used for domestic circuits.
(c) Copper and aluminium wires are usually employed for electricity transmission.
Answer: (a) Tungsten has a very high melting point (\( 3380^\circ\text{C} \)) and high resistivity, allowing it to become white-hot and emit light without melting.
(b) In series, if one appliance fails, the whole circuit breaks and others stop working. Also, each appliance needs a different current and voltage which is not possible in series.
(c) Copper and aluminium are used because they are excellent conductors with very low electrical resistivity, minimizing energy loss as heat.

Question. Give reasons for the following:
(a) It is dangerous to touch the live wire of the main supply rather than neutral wire.
(b) In household circuit, parallel combination of resistances is used.
(c) Using fuse in a household electric circuit is important.
Answer: (a) The live wire is at a high potential (220 V), while the neutral wire is at zero potential. Touching the live wire allows current to flow through the body to the earth, causing a fatal shock.
(b) Parallel combination ensures that each appliance gets the same voltage and can be operated independently with its own switch.
(c) A fuse is essential as it acts as a safety device, breaking the circuit in case of overloading or short-circuiting to prevent fire.

Question. (a) Fuse acts like a watchman in an electric circuit. Justify this statement.
(b) Mention the usual current rating of the fuse wire in the line to (i) lights and fans (ii) appliance of 2 kW or more power.
Answer: (a) A fuse monitors the current in the circuit. If the current exceeds a safe limit, the fuse wire melts and breaks the circuit, preventing damage to appliances and electrical fires, much like a watchman protects a house.
(b) (i) 5 A for lights and fans. (ii) 15 A for high-power appliances like geysers and ACs.

Question. (a) State Fleming’s Left-hand rule.
(b) List three characteristic features of the electric current used in our homes.
(c) What is a fuse? Why is it called a safety device?
(d) Why is it necessary to earth metallic electric appliances?
Answer: (a) [Rule stated as above].
(b) (1) It is Alternating Current (AC). (2) It has a frequency of 50 Hz. (3) The potential difference is 220 V.
(c) A fuse is a short length of wire made of a low-melting-point alloy. It is a safety device because it melts and breaks the circuit if the current becomes dangerously high.
(d) Earthing provides a low-resistance path for leakage current. If the live wire touches the metal casing, the current flows to the earth rather than through a person touching the appliance, preventing electric shocks.

Question. (a) Name two safety measures commonly used in an electric circuit and appliances.
(b) What precaution should be taken to avoid the overloading of domestic electric circuits?
Answer: (a) (1) Electric fuse. (2) Earthing.
(b) Precaution: Avoid connecting too many high-power electrical appliances to a single socket simultaneously.

Magnetic Field and Field Lines

Question. A student fixes a white sheet of paper on a drawing board. He places a bar magnet in the centre and sprinkles some iron filings uniformly around the bar magnet. Then he taps gently and observes that iron filings arrange themselves in a certain pattern.
(a) Why do iron filings arrange themselves in a particular pattern?
(b) Which physical quantity is indicated by the pattern of field lines around the bar magnet?
(c) State any two properties of magnetic field lines.

Answer: (a) The magnet exerts its influence in the region surrounding it. Therefore, the iron filings experience a force. The force thus exerted makes iron filings to arrange in a pattern.
(b) The pattern indicates the magnetic field of the bar magnet.
(c) Properties of magnetic field lines: (i) Magnetic field lines emerge from the North pole and merge at the South pole outside the magnet. (ii) Two magnetic field lines never intersect each other.

Magnetic Field Due to a Current-Carrying Conductor

Question. Which of the following pattern correctly describes the magnetic field around a long straight wire carrying current?
(a) straight lines perpendicular to the wire.
(b) straight lines parallel to the wire.
(c) radial lines originating from the wire.
(d) concentric circles centred around the wire.
Answer: (d) concentric circles centred around the wire.

Question. Assertion : On freely suspending a current – carrying solenoid, it comes to rest in Geographical N-S direction.
Reason : One end of current carrying straight solenoid behaves as a North pole and the other end as a South pole, just like a bar magnet.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true and R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Answer: (a) Both A and R are true and R is the correct explanation of A.

Question. A compass needle is placed near a current carrying wire. State your observations for the following cases and give reasons for the same in each case-
(a) Magnitude of electric current in wire is increased.
(b) The compass needle is displaced away from the wire.

Answer: (a) Observation: The deflection of the compass needle increases. Reason: The strength of the magnetic field produced by the current-carrying wire is directly proportional to the magnitude of the current (\( B \propto I \)).
(b) Observation: The deflection of the compass needle decreases. Reason: The strength of the magnetic field produced by the current-carrying wire decreases as the distance from the wire increases (\( B \propto 1/r \)).