NCERT Class 10 Science – Wide View Edition

NCERT CLASS 10 SCIENCE • CHAPTER 12 • MAGNETIC EFFECTS OF ELECTRIC CURRENT • WIDE VIEW EDITION

Magnetic Effects

Detailed Solutions Q1 – Q13

Which of the following correctly describes the magnetic field near a long straight wire carrying current?

(a) The field consists of straight lines perpendicular to the wire.
(b) The field consists of straight lines parallel to the wire.
(c) The field consists of radial lines originating from the wire.
(d) The field consists of concentric circles centred on the wire.

Answer: (d) The field consists of concentric circles centred on the wire.

Explanation: According to the Right-Hand Thumb Rule, if you hold a current-carrying wire in your right hand with the thumb pointing in the direction of the current, your fingers wrap around the wire in the direction of the magnetic field lines, forming concentric circles.

At the time of short circuit, the current in the circuit:

(a) reduces substantially.
(b) does not change.
(c) increases heavily.
(d) vary continuously.

Answer: (c) increases heavily.

Explanation: A short circuit occurs when the live wire comes in direct contact with the neutral wire. This makes the resistance of the circuit almost zero. According to Ohm’s Law ($I = V/R$), if $R$ tends to zero, the current $I$ becomes extremely large.

State whether the following statements are True or False.

(a) The field at the centre of a long circular coil carrying current will be parallel straight lines.

TRUE

Reason: As we move towards the centre of the loop, the concentric circles become larger and larger. At the very centre, the arcs appear as straight lines.

(b) A wire with a green insulation is usually the live wire of an electric supply.

FALSE

Reason: Green insulation indicates the Earth wire. The Live wire usually has Red or Brown insulation.

List two methods of producing magnetic fields.

1. Using Permanent Magnets: Naturally occurring magnets (like lodestone) or artificial magnets (Bar magnets, Horseshoe magnets) produce a magnetic field.

2. Using Electromagnets (Current carrying conductors): Passing electricity through a straight wire, a circular loop, or a solenoid produces a magnetic field around it.

How does a solenoid behave like a magnet? Can you determine the north and south poles of a current–carrying solenoid with the help of a bar magnet? Explain.

Behavior: A solenoid is a coil of many circular turns of insulated copper wire. When current flows through it, the magnetic field lines inside are parallel straight lines (uniform field), and outside they look exactly like a bar magnet. One end acts as the North Pole and the other as the South Pole.

Determining Poles: Yes, using a bar magnet:

1. Bring the North Pole of a bar magnet near one end of the solenoid.

2. If it is repelled, that end of the solenoid is the North Pole.

3. If it is attracted, that end is the South Pole.

[Image of solenoid magnetic field lines]

When is the force experienced by a current–carrying conductor placed in a magnetic field largest?

The force is largest when the direction of the electric current is perpendicular ($90^\circ$) to the direction of the magnetic field.

If the current flows parallel to the magnetic field, the force is zero.

Imagine that you are sitting in a chamber with your back to one wall. An electron beam, moving horizontally from back wall towards the front wall, is deflected by a strong magnetic field to your right side. What is the direction of the magnetic field?

We use Fleming’s Left-Hand Rule.

1. Current ($I$): Electrons move Back $\rightarrow$ Front. Conventional current moves opposite, so Front $\rightarrow$ Back (Towards the wall behind you).

2. Force ($F$): The beam is deflected to the Right.

3. Field ($B$): Align your left hand:

– Middle finger (Current) points Back.

– Thumb (Force) points Right.

– Your Index finger (Field) will point Downwards.

Answer: The direction of the magnetic field is vertically downwards.

[Image of Fleming’s Left Hand Rule diagram]

State the rule to determine the direction of a (i) magnetic field produced around a straight conductor-carrying current, (ii) force experienced by a current-carrying straight conductor placed in a magnetic field.

(i) Right-Hand Thumb Rule: Hold the wire in your right hand with the thumb pointing in the direction of current. The direction in which your fingers curl gives the direction of magnetic field lines.

(ii) Fleming’s Left-Hand Rule: Stretch the thumb, forefinger and middle finger of your left hand mutually perpendicular. If the forefinger points in the direction of the magnetic field and the middle finger in the direction of current, then the thumb will point in the direction of motion (force).

What is the function of an earth wire? Why is it necessary to earth metallic appliances?

Function: The earth wire (green insulation) provides a low-resistance path for electric current to flow into the ground.

Necessity: It is a safety measure for appliances with metallic bodies (like toaster, fridge). If there is any leakage of current to the metallic body, the earth wire transfers this current safely to the ground. This prevents the user from getting a severe electric shock.

Q10

Two circular coils A and B are placed closed to each other. If the current in the coil A is changed, will some current be induced in the coil B? Give reason.

Answer: Yes.

Reason: When current in coil A changes, the magnetic field associated with it also changes. Since coil B is close by, the magnetic field lines passing through coil B also change. This changing magnetic field induces a potential difference (and thus a current) in coil B. This phenomenon is called Electromagnetic Induction.

Q11

Explain what is short-circuiting and overloading in an electric supply.

Short-Circuiting: Occurs when the Live wire and Neutral wire come into direct contact (due to damaged insulation). Resistance drops to zero, and a huge current flows, often causing sparks or fire.

Overloading: Occurs when too many appliances are connected to a single socket, or when high-power appliances are used simultaneously. The total current drawn exceeds the capacity of the wire/fuse, leading to overheating.

Q12

What is the role of the split ring in an electric motor?

The split ring acts as a Commutator. It reverses the direction of the current in the coil after every half rotation. This reversal of current ensures that the force on the coil arms keeps pushing it in the same direction, allowing the motor to rotate continuously.

[Image of electric motor split ring commutator]