Get summaries, questions, answers, solutions, notes, extras, PDF and guides for Chapter 12 Magnetic Effects of Electric Current: Class 10 Science textbook, which is part of the syllabus for students studying under SEBA (Assam Board), NBSE (Nagaland Board), TBSE (Tripura Board), CBSE (Central Board), MBOSE (Meghalaya Board), BSEM (Manipur Board), WBBSE (West Bengal Board), and all other boards following the NCERT books. These solutions, however, should only be treated as references and can be modified/changed.
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Summary
Electric current can do more than just create heat. A wire that carries electricity also behaves like a magnet. A scientist named Hans Christian Oersted discovered this when he noticed a compass needle moving near an electric wire. This showed that electricity and magnetism are linked. This important link is used in many devices we use, such as radios and televisions.
A magnet has an area around it where its magnetic force can be felt. This area is called a magnetic field. We can imagine this field by looking at how iron filings arrange themselves around a magnet, forming patterns of lines. These are known as magnetic field lines. These lines show the direction and strength of the magnetic field. By agreement, field lines are shown coming out of a magnet’s north pole and going into its south pole. Inside the magnet, they travel from the south pole back to the north pole, making them complete, closed loops. An important property of these lines is that they never cross each other. If they did, it would mean a compass at that point would try to point in two directions at once, which is impossible.
When electricity flows through a straight wire, it creates a magnetic field in the form of circles around the wire. A simple way to find the direction of this field is the “Right-Hand Thumb Rule.” If you imagine holding the wire with your right hand, with your thumb pointing in the direction of the electric current, the way your fingers curl around the wire shows the direction of the magnetic field. The magnetic field becomes stronger if the current increases or if you are closer to the wire. If this wire is bent into a circular loop, the magnetic field lines inside the loop all point in the same direction. A coil made of many such loops, called a solenoid, acts very much like a bar magnet when electricity passes through it. The magnetic field inside a solenoid is strong and uniform. An electromagnet is made by placing a piece of material like soft iron inside a current-carrying solenoid.
If a wire carrying an electric current is placed within the magnetic field of another magnet, the wire will experience a force. This force can cause the wire to move. The direction of this movement depends on the direction of the current in the wire and the direction of the magnetic field. “Fleming’s Left-Hand Rule” is a way to determine the direction of this force. If you stretch the first finger, second finger, and thumb of your left hand so they are at right angles to each other, and your first finger points in the direction of the magnetic field, and your second finger points in the direction of the current, then your thumb will point in the direction of the force or motion. This principle is used in electric motors.
In our homes, the electricity we use is supplied through wires. Typically, there is a ‘live’ wire, often with red insulation, and a ‘neutral’ wire, often with black insulation. For safety, there is also an ‘earth’ wire, usually with green insulation. This earth wire is connected to the metal casings of appliances. If a fault causes the live wire to touch the metal casing, the earth wire provides a safe path for the current to flow to the ground, preventing electric shocks. Fuses are another vital safety feature in circuits. If too much current flows, for example, due to a ‘short circuit’ where the live and neutral wires accidentally touch, the thin wire inside the fuse melts and breaks the circuit. This stops the flow of excessive current, protecting appliances from damage and reducing the risk of fire.
Textbook solutions
Intext Questions and Answers I
1. Why does a compass needle get deflected when brought near a bar magnet?
Answer: A compass needle gets deflected when brought near a bar magnet because the compass needle is, in fact, a small bar magnet. The bar magnet exerts a magnetic force on the compass needle. This interaction occurs because like magnetic poles repel each other, while unlike poles of magnets attract each other, causing the compass needle to deflect.
Intext Questions and Answers II
1. Draw magnetic field lines around a bar magnet.
Answer:
2. List the properties of magnetic field lines.
Answer: The properties of magnetic field lines are:
(i) The direction of the magnetic field is taken to be the direction in which a north pole of the compass needle moves inside it.
(ii) By convention, field lines emerge from the north pole and merge at the south pole.
(iii) Inside the magnet, the direction of field lines is from its south pole to its north pole.
(iv) Magnetic field lines are closed curves.
(v) The relative strength of the magnetic field is shown by the degree of closeness of the field lines. The field is stronger where the field lines are crowded.
(vi) No two field-lines are found to cross each other.
3. Why don’t two magnetic field lines intersect each other?
Answer: Two magnetic field lines don’t intersect each other because if they did, it would mean that at the point of intersection, the compass needle would point towards two directions, which is not possible.
Intext Questions and Answers III
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.
Answer: To find out the direction of the magnetic field inside and outside the circular loop of wire, the right-hand rule is applied. Imagine that you are holding a current-carrying straight conductor, which can be considered as a segment of the loop, 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. For the circular loop of wire lying in the plane of the table with the current passing through it clockwise, by applying the right-hand rule, it is easy to check that every section of the wire contributes to the magnetic field lines in the same direction within the loop. The direction of the magnetic field inside the loop is directed into the plane of the table, and outside the loop, the magnetic field is directed out of the plane of the table.
2. The magnetic field in a given region is uniform. Draw a diagram to represent it.
Answer:
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.
Answer: The correct option is (d).
Explanation: The magnetic field inside a long straight solenoid-carrying current is the same at all points. The field lines inside the solenoid are in the form of parallel straight lines. This indicates that the magnetic field is the same at all points inside the solenoid. That is, the field is uniform inside the solenoid.
Intext Questions and Answers IV
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
Answer: (c) velocity and (d) momentum.
Explanation:
As explained by Fleming’s left-hand rule (page 9), the magnetic force on a moving charged particle is always perpendicular to its direction of motion.
- A force perpendicular to the motion changes the particle’s direction but does not change its speed.
- Velocity is a vector that includes both speed and direction. Since the direction changes, the velocity changes.
- Momentum is also a vector (mass × velocity). Since the velocity changes, the momentum also changes.
- The proton’s mass and speed remain constant.
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?
Answer: The displacement of the rod AB will increase in all three cases.
Explanation: Activity 12.7 demonstrates that a current-carrying conductor in a magnetic field experiences a force, which causes displacement. The magnitude of this force is directly proportional to:
- (i) The current (I) in the conductor.
- (ii) The strength of the magnetic field (B).
- (iii) The length (L) of the conductor within the magnetic field.
Therefore, increasing the current, using a stronger magnet, or increasing the length of the rod will all result in a larger force, and consequently, a greater 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
Answer: (d) upward
Explanation: This can be determined using Fleming’s Left-Hand Rule (page 9):
- Current (Middle Finger): The particle is positively charged and moving west, so the conventional current is towards the west.
- Force (Thumb): The particle is deflected towards the north.
- Magnetic Field (Forefinger): When you align your left hand with your thumb pointing north and your middle finger pointing west, your forefinger points upward.
Intext Questions and Answers V
1. Name two safety measures commonly used in electric circuits and appliances.
Answer: Two safety measures commonly used in electric circuits and appliances are the earth wire and the electric fuse. The earth wire, which has insulation of green colour, is usually connected to a metal plate deep in the earth near the house; this is used as a safety measure, especially for those appliances that have a metallic body, to ensure that any leakage of current to the metallic body of the appliance keeps its potential to that of the earth, and the user may not get a severe electric shock. An electric fuse is an important component of all domestic circuits that prevents damage to the appliances and the circuit due to overloading by stopping the flow of unduly high electric current; the Joule heating that takes place in the fuse melts it to break the electric circuit.
2. An electric oven of 2 kW power rating is operated in a domestic electric circuit (220 V) that has a current rating of 5 A. What result do you expect? Explain.
Answer: For an electric oven of 2 kW power rating operated in a domestic electric circuit of 220 V, the current it will draw is calculated as Power (P) / Voltage (V), which is 2000 W / 220 V, approximately 9.09 A. Since this current of 9.09 A is higher than the circuit’s current rating of 5 A, the circuit will experience overloading. In such a situation, where the current in the circuit abruptly increases to an unduly high value, the electric fuse is designed to protect the circuit and appliance. The Joule heating that takes place in the fuse, due to this unduly high electric current, will melt the fuse, thereby breaking the electric circuit. This prevents possible damage to the electric oven and the household wiring.
3. What precaution should be taken to avoid the overloading of domestic electric circuits?
Answer: A precaution that should be taken to avoid the overloading of domestic electric circuits is to avoid connecting too many appliances to a single socket, as sometimes overloading is caused by this.
Exercise Questions and Answers
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.
Answer: (d) The field consists of 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.
Answer: (c) increases heavily.
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.
Answer: True.
(b) A wire with a green insulation is usually the live wire of an electric supply.
Answer: False.
4. List two methods of producing magnetic fields.
Answer: Two methods of producing magnetic fields are:
(i) Using a magnet, as a magnetic field exists in the region surrounding a magnet.
(ii) Passing an electric current through a metallic wire, as a metallic wire carrying an electric current has associated with it a magnetic field.
5. When is the force experienced by a current-carrying conductor placed in a magnetic field largest?
Answer: The force experienced by a current-carrying conductor placed in a magnetic field is largest (or the magnitude of the force is the highest) when the direction of current is 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?
Answer: The direction of current is taken opposite to the direction of motion of electrons. Thus, the current is from the front wall towards the back wall. The deflection (force) is to the right side. Applying Fleming’s left-hand rule, where the first finger points in the direction of the magnetic field, the second finger in the direction of current, and the thumb points in the direction of motion or force, the direction of the magnetic field is vertically downwards.
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.
Answer: (i) The rule is the Right-Hand Thumb Rule.
Reason: Imagine you are holding a current-carrying straight conductor in your right hand such that your thumb points in the direction of the current. The direction in which your fingers wrap around the conductor gives the direction of the magnetic field lines.
(ii) The rule is Fleming’s Left-Hand Rule.
Reason: Stretch the thumb, forefinger, and middle finger of your left hand so that they are mutually perpendicular. If the forefinger points in the direction of the magnetic field and the middle finger points in the direction of the current, then the thumb will point in the direction of the force (or motion) experienced by the conductor.
(iii) The rule is Fleming’s Right-Hand Rule.
Reason: Stretch the thumb, forefinger, and middle finger of your right hand so they are mutually perpendicular. If the thumb points in the direction of the motion of the conductor, and the forefinger points in the direction of the magnetic field, then the middle finger will point in the direction of the induced current.
8. When does an electric short circuit occur?
Answer: An electric short circuit occurs when the live wire and the neutral wire come into direct contact. This can happen when the insulation of wires is damaged or there is a fault in the appliance. In such a situation, the current in the circuit abruptly increases.
9. What is the function of an earth wire? Why is it necessary to earth metallic appliances?
Answer: The function of an earth wire is to serve as a safety measure, especially for those appliances that have a metallic body. The earth wire, which has insulation of green colour, is usually connected to a metal plate deep in the earth near the house. The metallic body of the appliance is connected to the earth wire, which provides a low-resistance conducting path for the current.
It is necessary to earth metallic appliances because it ensures that any leakage of current to the metallic body of the appliance keeps its potential to that of the earth. This prevents the user from getting a severe electric shock.
Extras
Additional MCQs (Knowledge Based)
1. The direction of magnetic field lines inside a bar magnet is from its _____________ pole to its _____________ pole.
A. North, South
B. South, North
C. East, West
D. West, East
Answer: B. South, North
25. The primary purpose of an electric fuse in a circuit is to protect appliances from damage due to _____________ .
A. Low voltage
B. High resistance
C. Overloading
D. Power factor changes
Answer: C. Overloading
Additional MCQs (Competency Based)
1. Assertion (A): When an electric current passes through a metallic wire placed nearby a compass, the compass needle gets deflected.
Reason (R): An electric current-carrying wire behaves like a magnet.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R does not explain 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.
30. To demonstrate the force on a current-carrying conductor in a magnetic field, an aluminium rod AB is suspended horizontally. Arrange the following steps to observe the force.
(i) Pass a current through the aluminium rod.
(ii) Place a strong horseshoe magnet such that the rod is between its poles.
(iii) Connect the aluminium rod in series with a battery and a key.
(iv) Observe the displacement of the rod.
(a) (iii) → (ii) → (i) → (iv)
(b) (ii) → (iii) → (i) → (iv)
(c) (iii) → (i) → (ii) → (iv)
(d) (ii) → (i) → (iii) → (iv)
Answer: (b) (ii) → (iii) → (i) → (iv)
Additional Questions and Answers
1. What is a compass needle made of?
Answer: A compass needle is made of a small magnet.
23. How does increasing the number of turns in a circular coil affect the magnetic field produced at its centre?
Answer: If there is a circular coil having n turns, the field produced is n times as large as that produced by a single turn. This is because the current in each circular turn has the same direction, and the field due to each turn then just adds up.
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