Magnetic Effects of Electric Current — Class 10 Science
Science · Chapter 12
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Chapter summary
Key ideas & terms
- Magnetic field
- The region around a magnet or current where a magnetic force acts.
- Fleming’s left-hand rule
- Gives the force direction on a conductor in a magnetic field.
- Electromagnetic induction
- Production of current by a changing magnetic field.
- Electric generator
- A device converting mechanical energy into electrical energy.
Extra questions & answers
What is the function of an earth wire in domestic circuits?
It provides a low-resistance path to the ground, protecting users from electric shock if a fault makes the appliance’s body live.
State the principle of an electric motor.
A current-carrying conductor in a magnetic field experiences a force, which rotates the coil — converting electrical energy into mechanical energy.
What is a magnetic field, and how is its direction shown?
The region around a magnet or current-carrying conductor where a magnetic force acts; its direction is shown by magnetic field lines, which run from north to south outside a magnet (refer to the field-line diagram in your NCERT textbook).
State the right-hand thumb rule.
If a current-carrying conductor is held in the right hand with the thumb pointing in the direction of the current, the curled fingers show the direction of the magnetic field around it.
What is the magnetic field due to a current-carrying solenoid like?
The field inside a solenoid is uniform and resembles that of a bar magnet, with one end acting as a north pole and the other as a south pole.
State Fleming’s left-hand rule and where it is applied.
If the thumb, forefinger and middle finger of the left hand are held mutually perpendicular, the forefinger shows the field, the middle finger the current, and the thumb the force; it gives the force direction in an electric motor.
What is the principle of an electric motor?
A current-carrying coil placed in a magnetic field experiences a force that rotates it, converting electrical energy into mechanical energy.
What is electromagnetic induction?
The production of an induced current in a coil due to a change in the magnetic field around it; its direction is given by Fleming’s right-hand rule.
What is the function of the three wires — live, neutral and earth — in a domestic circuit?
The live wire carries current to the appliance, the neutral completes the circuit, and the earth wire provides a safe path for current to the ground, protecting users from shock.
What are fuses and short circuits?
A fuse is a safety device that melts and breaks the circuit when too much current flows; a short circuit is the accidental contact of live and neutral wires, causing a sudden large current.
QUESTION
1. Why does a compass needle get deflected when brought near a bar magnet?
A compass needle is itself a small magnet. When it is brought near a bar magnet, the bar magnet’s magnetic field exerts a force on the poles of the needle, causing the needle to turn and align with the field.
QUESTIONS
1. Draw magnetic field lines around a bar magnet.
The magnetic field lines should be drawn as closed curves. Outside the bar magnet, they emerge from its north pole and enter its south pole. Inside the magnet, they travel from the south pole to the north pole. The lines should be closer together near the poles, where the magnetic field is stronger.
2. List the properties of magnetic field lines.
Magnetic field lines form closed continuous curves. Outside a magnet they travel from the north pole to the south pole, while inside the magnet they travel from the south pole to the north pole. The tangent to a field line at any point gives the direction of the magnetic field there. Closely spaced lines indicate a stronger magnetic field, and no two magnetic field lines intersect each other.
3. Why don’t two magnetic field lines intersect each other?
If two magnetic field lines intersected, the magnetic field at the point of intersection would have two different directions. A compass needle placed there would then have to point in two directions simultaneously, which is impossible.
QUESTIONS
1. Consider a circular loop of wire lying in the plane of the table. Let the current pass through the loop clockwise. Apply the right-hand rule to find out the direction of the magnetic field inside and outside the loop.
Viewed from above the table, curl the fingers of the right hand in the clockwise direction of current. The thumb then points vertically downward. Therefore, the magnetic field inside the loop is directed into the table. Outside the loop, the field lines return in the opposite direction and are directed upward, out of the table.
2. The magnetic field in a given region is uniform. Draw a diagram to represent it.
A uniform magnetic field is represented by several straight, parallel and equally spaced lines, with arrowheads on all the lines pointing in the same direction.
3. Choose the correct option.
The magnetic field inside a long straight solenoid-carrying current
(a) is zero.
(b) decreases as we move towards its end.
(c) increases as we move towards its end.
(d) is the same at all points.
(d) is the same at all points. Inside a long current-carrying solenoid, the magnetic field lines are parallel and equally spaced, showing that the field is nearly uniform.
QUESTIONS
1. Which of the following property of a proton can change while it moves freely in a magnetic field? (There may be more than one correct answer.)
(a) mass (b) speed
(c) velocity (d) momentum
The correct options are (c) velocity and (d) momentum. The magnetic force changes the direction of the proton’s motion but not its speed or mass. Since velocity and momentum are vector quantities, they change when their direction changes.
2. In Activity 12.7, how do we think the displacement of rod AB will be affected if (i) current in rod AB is increased; (ii) a stronger horse-shoe magnet is used; and (iii) length of the rod AB is increased?
The displacement of rod AB will increase in all three cases. Increasing the current increases the magnetic force on the rod. A stronger horse-shoe magnet provides a stronger magnetic field and therefore a greater force. Increasing the length of the rod lying in the magnetic field also increases the force and hence the displacement.
3. A positively-charged particle (alpha-particle) projected towards west is deflected towards north by a magnetic field. The direction of magnetic field is
(a) towards south (b) towards east
(c) downward (d) upward
The correct option is (d) upward. For a positively charged particle moving towards the west and experiencing a force towards the north, the magnetic field must be vertically upward according to Fleming’s left-hand rule.
QUESTIONS
1. Name two safety measures commonly used in electric circuits and appliances.
Two commonly used safety measures are an electric fuse and earthing. A fuse breaks the circuit when an excessive current flows, while earthing provides a low-resistance path through which leakage current can flow safely into the ground.
2. An electric oven of
The current drawn by the oven is
This is greater than the circuit’s rated current of
3. What precaution should be taken to avoid the overloading of domestic electric circuits?
Too many appliances should not be connected to or operated from a single socket or circuit at the same time. High-power appliances should be connected to properly rated separate circuits, and suitable fuses or circuit breakers should be used.
EXERCISES
1. Which of the following correctly describes the magnetic field near a long straight wire?
(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.
The correct option is (d). The magnetic field lines near a long straight current-carrying wire form concentric circles centred on the wire.
2. At the time of short circuit, the current in the circuit
(a) reduces substantially.
(b) does not change.
(c) increases heavily.
(d) vary continuously.
The correct option is (c) increases heavily. During a short circuit, the live and neutral wires come into direct contact, producing a path of very low resistance and causing a sudden large increase in current.
3. 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.
(b) A wire with a green insulation is usually the live wire of an electric supply.
(a) True. Near the centre of a current-carrying circular coil, the magnetic field lines appear as nearly parallel straight lines. (b) False. A green-insulated wire is usually the earth wire, not the live wire.
4. List two methods of producing magnetic fields.
A magnetic field can be produced by using a permanent magnet and by passing an electric current through a conductor, coil or solenoid.
5. When is the force experienced by a current–carrying conductor placed in a magnetic field largest?
The force is largest when the direction of current in the conductor is perpendicular, or at right angles, to the direction of the magnetic field.
6. 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 magnetic field?
The magnetic field is directed vertically upward. The conventional-current direction is opposite to the motion of the electrons. Applying Fleming’s left-hand rule with the force towards the right gives the magnetic-field direction as upward.
7. 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 which is perpendicular to it, and (iii) current induced in a coil due to its rotation in a magnetic field.
(i) The right-hand thumb rule is used. Hold the conductor in the right hand with the thumb pointing in the direction of current; the curled fingers show the direction of the magnetic field. (ii) Fleming’s left-hand rule is used. Stretch the thumb, forefinger and middle finger of the left hand mutually perpendicular to one another. If the forefinger shows the magnetic-field direction and the middle finger shows the current direction, the thumb shows the direction of force. (iii) Fleming’s right-hand rule is used. Stretch the thumb, forefinger and middle finger of the right hand mutually perpendicular to one another. If the forefinger shows the magnetic-field direction and the thumb shows the motion of the conductor, the middle finger shows the direction of induced current.
8. When does an electric short circuit occur?
An electric short circuit occurs when the live wire and neutral wire come into direct contact, usually because of damaged insulation or a fault in an appliance. This creates a very low-resistance path and causes a sudden large current to flow.
9. What is the function of an earth wire? Why is it necessary to earth metallic appliances?
The earth wire connects the metallic body of an appliance to the ground through a low-resistance path. If the live wire touches the metallic body because of a fault, the leakage current flows safely to earth instead of through the user. This keeps the appliance’s body at approximately earth potential and protects the user from severe electric shock.