Case Based Questions: Magnetic Effects of Electric Current

Hans Christian Oersted, in 1820, observed that when an electric current flows through a conductor, a nearby compass needle deflects. This experiment showed a link between electricity and magnetism.

(a) What conclusion did Oersted draw from his experiment? (1 Mark)
(b) How can the direction of the magnetic field around a straight conductor be determined? (2 Marks)
(c) What happens to the magnetic field strength if the current in the conductor is increased? (1 Mark) 
OR
(c) What happens to the direction of the magnetic field when the current direction is reversed? (1 Mark)

Ans: 
(a) Oersted concluded that a current-carrying conductor produces a magnetic field around it and that electric current and magnetism are linked.
(b) The direction of the magnetic field around a straight conductor is found using the Right-Hand Thumb Rule:

• Hold the conductor in your right hand with the thumb pointing in the direction of the conventional current (from positive to negative).
• The direction in which the fingers curl around the conductor shows the direction of the magnetic field lines (they form concentric circles around the wire).

(c) If the current in the conductor is increased, the magnetic field strength around the conductor becomes stronger.
OR
(c) If the direction of the current is reversed, the direction of the magnetic field around the conductor also reverses.

Q2: Read the source below and answer the questions that follow:

A student set up an experiment using a straight wire, a battery, and a compass. When current was passed, the compass needle deflected.

(a) What shape do magnetic field lines form around a straight conductor? (1 Mark)
(b) How does the strength of the magnetic field change with distance from the wire? (2 Marks)
(c) What happens to the field lines when the current is doubled? (1 Mark) 
OR
(c) How can the direction of the magnetic field be reversed? (1 Mark)

Ans: 
(a) The magnetic field lines form concentric circles centred on the straight conductor.
(b)

• The magnetic field is much stronger close to the wire and becomes weaker as the distance from the wire increases.
• As you move away, the circles get larger and the field at those larger circles is weaker.

(c) If the current is doubled, the magnetic field strength around the wire increases (approximately in proportion to the current), so the field becomes stronger.
OR
(c) The direction of the magnetic field can be reversed by reversing the direction of the current in the conductor.

Q3: Read the source below and answer the questions that follow:

In an experiment, a circular wire loop was connected to a battery, and iron filings were sprinkled around it. The filings aligned in circular patterns.

(a) How does the magnetic field behave at the center of a current-carrying circular loop? (1 Mark)
(b) What factors affect the strength of the magnetic field in a circular loop? (2 Marks)
(c) What happens to the field strength if the number of turns in the loop is increased? (1 Mark) 
OR
(c) How does increasing the radius of the loop affect the magnetic field? (1 Mark)

Ans:
(a) At the centre of a current-carrying circular loop the magnetic field is strong and the field lines there are nearly straight and approximately perpendicular to the plane of the loop.
(b) The magnetic field at the centre of a circular loop depends mainly on:

• The current through the loop - a larger current produces a stronger magnetic field.
• The number of turns in the coil - more turns produce a stronger resultant field at the centre.

(c) Increasing the number of turns increases the field strength at the centre because each turn contributes to the total magnetic effect.
OR
(c) If the radius of the loop is increased, the magnetic field at the centre becomes weaker because the same current and turns are spread over a larger area.

Q4: Read the source below and answer the questions that follow:

A scientist demonstrated that a current-carrying solenoid behaves like a bar magnet. He placed a compass near the solenoid's ends and observed north and south poles.

(a) What is a solenoid? (1 Mark)
(b) How does a solenoid produce a uniform magnetic field? (2 Marks)
(c) How can a solenoid be used to create a strong electromagnet? (1 Mark) 
OR
(c) How does a solenoid's field compare to that of a bar magnet? (1 Mark)

Ans:
(a) A solenoid is a long coil of insulated wire wound in the form of a helix or cylindrical shape.
(b) A solenoid produces a nearly uniform magnetic field inside because the field lines produced by each turn add together and run parallel within the coil, giving a strong and nearly uniform field inside while the field outside is similar to that of a bar magnet.
(c) A strong electromagnet can be made by:

• Increasing the current through the coil.
• Increasing the number of turns of the coil.
• Placing a soft iron core inside the solenoid to concentrate and strengthen the field.

OR
(c) The magnetic field of a solenoid is similar to that of a bar magnet: it has distinct north and south poles at the ends and field lines emerging from one end and entering the other.

Q5: Read the source below and answer the questions that follow:

In households, fuses and circuit breakers prevent damage caused by short circuits or overloading.

(a) What is the purpose of a fuse in an electric circuit? (1 Mark)
(b) How does a fuse protect electrical appliances? (2 Marks)
(c) Why should high-power appliances be connected separately in a house? (1 Mark) 
OR
(c) What is the function of an earth wire in a household circuit? (1 Mark)

Ans:
(a) The purpose of a fuse is to protect the circuit by breaking the connection when the current exceeds a safe value.
(b) Purpose of fuse :

• A fuse contains a thin wire that melts when the current goes above the fuse rating, thereby opening the circuit.
• By breaking the circuit, a fuse prevents overheating, damage to appliances and reduces the risk of fire.

(c) High-power appliances draw large currents; connecting them on separate circuits prevents one circuit from being overloaded and reduces the chance of frequent tripping or damage.
OR
(c) An earth wire provides a low-resistance path for fault or leakage current to flow into the ground, which helps prevent electric shocks to users.

Q6: Read the source below and answer the questions that follow:

During a science fair, a student demonstrated that when a current flows through a conductor placed in a magnetic field, the conductor experiences a force and moves. This is the working principle of many electrical devices, including electric motors.

(a) What happens when a current-carrying conductor is placed in a magnetic field? (1 Mark)
(b) Explain how the direction of force on the conductor can be determined. (2 Marks)
(c) What happens if the direction of the current is reversed? (1 Mark) 
OR
(c) Why does a charged particle moving in a magnetic field experience a force? (1 Mark)

Ans:
(a) A current-carrying conductor placed in a magnetic field experiences a force; the force acts perpendicular to both the direction of the current and the magnetic field and may cause the conductor to move.
(b) The direction of the force is found using Fleming's Left-Hand Rule:

• Forefinger points in the direction of the magnetic field (from north to south).
• Middle finger points in the direction of the conventional current (from positive to negative).
• Thumb then points in the direction of the force (motion) on the conductor.

(c) If the direction of the current is reversed, the direction of the force also reverses, so the conductor moves in the opposite direction.
OR
(c) A charged particle moving in a magnetic field experiences a force because the motion of the charge interacts with the magnetic field; this force is always perpendicular to both the velocity of the particle and the magnetic field.

Q7: Read the source below and answer the questions that follow:

During a physics experiment, Priya moved a magnet in and out of a coil connected to a galvanometer. She observed a deflection in the galvanometer, which meant a current was induced in the coil.

(a) What is electromagnetic induction? (1 Mark)
(b) Explain how an induced current is produced in a coil. (2 Marks)
(c) What happens if the magnet is moved faster inside the coil? (1 Mark) 
OR
(c) How can the direction of the induced current be reversed? (1 Mark)

Ans:
(a) Electromagnetic induction is the process by which an electric current is produced in a conductor when the magnetic field linked with it changes.
(b) An induced current is produced as follows:

• When the magnet is moved into or out of the coil, the magnetic field through the coil changes with time.
• This change in magnetic field (change in magnetic flux) induces an electromotive force in the coil and so a current flows, which is shown by the galvanometer.

(c) If the magnet is moved faster, the magnetic field through the coil changes more rapidly and the induced current becomes larger.
OR
(c) The direction of the induced current reverses when the direction of motion of the magnet is reversed (for example, when the magnet is pulled out instead of pushed in).

Q8: Read the source below and answer the questions that follow:

A student set up an experiment where a coil was connected to a galvanometer. When a second coil carrying current was brought near it, the galvanometer needle deflected, showing the presence of an induced current.

(a) What causes the needle of the galvanometer to deflect in this experiment? (1 Mark)
(b) Explain how a changing magnetic field induces a current in the second coil. (2 Marks)
(c) What happens if the second coil is moved away instead of closer? (1 Mark) 
OR
(c) How can the induced current be increased in this setup? (1 Mark)

Ans:
(a) The galvanometer needle deflects because a changing magnetic field from the first (current-carrying) coil induces a current in the nearby second coil.
(b) The effect occurs as follows :

• When the current in the first coil changes or the coil is moved, the magnetic field around it changes in time.
• The changing magnetic field through the second coil changes the magnetic flux linked with it, which induces an emf and hence a current in the second coil that the galvanometer detects.

(c) If the second coil is moved away, the magnetic flux through it changes in the opposite sense (or reduces), so the induced current will change accordingly and the galvanometer needle will deflect in the opposite direction or show a smaller deflection depending on the motion.
OR
(c) The induced current can be increased by:

• Using a stronger magnet or increasing the current in the first coil.
• Increasing the number of turns in the second coil.
• Moving the coil or changing the current more rapidly so the flux changes faster.

Q9: Read the source below and answer the questions that follow:

During a summer afternoon, Ravi's house experienced a power outage when too many high-power appliances were switched on simultaneously. The electrician explained that this was due to overloading.

(a) What is short-circuiting in a domestic circuit? (1 Mark)
(b) How does overloading cause power failures? (2 Marks)
(c) How can we prevent short-circuiting in electrical circuits? (1 Mark) 
OR
(c) Why is it necessary to use fuses or MCBs in household circuits? (1 Mark)

Ans:
(a) Short-circuiting is a fault that occurs when the live and neutral (or live and earth) wires come into direct contact, allowing a very large current to flow through the circuit.
(b) Overloading happens when too many appliances draw current from the same circuit. This raises the current above the safe limit and can lead to:

• Excessive heating of wires.
• Damage to appliances and wiring.
• Tripping of protective devices or blown fuses, causing power outages and, in severe cases, fire hazards.

To avoid such failures, the total load on a circuit should not exceed its rated capacity and heavy appliances should be distributed across separate circuits.

(c) Short-circuiting can be minimised by:

• Using good-quality insulation on wires so that conductors do not touch accidentally.
• Avoiding loose or exposed connections and ensuring proper installation by a qualified electrician.

OR
(c) Fuses and MCBs are necessary because they automatically break the circuit when the current exceeds safe limits, protecting wiring and appliances and preventing fires.

Q10: Read the source below and answer the questions that follow:

While setting up a new washing machine, Sita's electrician explained the importance of earthing to prevent electric shocks.

(a) What is the purpose of earthing in domestic electrical circuits? (1 Mark)
(b) How does earthing protect electrical appliances and users? (2 Marks)
(c) Why is the earth wire connected to the metal body of appliances? (1 Mark) 
OR
(c) What happens if an appliance is not properly earthed? (1 Mark)

Ans:
(a) Earthing provides a safe low-resistance path for fault or leakage current to flow into the ground, thereby reducing the risk of electric shock.
(b) Earthing protects appliances and users as follows:

• If a live wire inside an appliance touches its metal body, the leakage current flows through the earth wire to the ground instead of passing through a person who touches the appliance.
• This large fault current will usually cause the fuse to blow or the MCB to trip, isolating the appliance and preventing injury and further damage.

(c) The earth wire is connected to the metal body so that any leakage current is safely diverted to the ground and does not pass through the user.
OR
(c) If an appliance is not properly earthed, a person touching its metal body may receive an electric shock because the leakage current could pass through the person to reach the ground.