| Latest Science NCERT Notes and Solutions (Class 6th to 10th) | ||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 6th | 7th | 8th | 9th | 10th | ||||||||||
| Latest Science NCERT Notes and Solutions (Class 11th) | ||||||||||||||
| Physics | Chemistry | Biology | ||||||||||||
| Latest Science NCERT Notes and Solutions (Class 12th) | ||||||||||||||
| Physics | Chemistry | Biology | ||||||||||||
Chapter 12 Magnetic Effects Of Electric Current
In the previous chapter, we explored the heating effects of electric current. Beyond producing heat, electric current has other significant effects. A fundamental observation is that an electric current-carrying wire behaves like a magnet, producing a magnetic field around it. This phenomenon, first discovered by Hans Christian Oersted, revealed a crucial link between electricity and magnetism.
This chapter delves into magnetic fields produced by electric currents, electromagnetic effects, and related concepts like electromagnets and the forces experienced by current-carrying conductors in magnetic fields. We will also touch upon domestic electric circuits and associated safety measures.
Magnetic Field And Field Lines
We are familiar with the properties of magnets, such as attraction and repulsion between poles. A compass needle is essentially a small bar magnet whose ends point towards geographic north and south (north pole and south pole). When a compass needle is brought near a magnet, it gets deflected because it experiences a magnetic force.
Question 1. Why does a compass needle get deflected when brought near a bar magnet?
Answer:
A compass needle itself is a small magnet. When it is brought near a bar magnet, it experiences a magnetic force due to the magnetic field produced by the bar magnet. This force causes the compass needle to align itself with the direction of the magnetic field lines at that point, resulting in a deflection.
A magnet influences the region surrounding it. This region where the force of the magnet can be detected is called a **magnetic field**. We can visualize the pattern of the magnetic field using iron filings or a compass.
When iron filings are sprinkled around a bar magnet and the surface is gently tapped, the filings arrange themselves in a pattern of curves. These curves represent **magnetic field lines**.
Alternatively, a compass can be used to trace magnetic field lines. By placing a compass at various points around a magnet and marking the direction indicated by its north pole, a series of points can be joined to draw a field line.
Properties of magnetic field lines:
- Magnetic field lines have both direction and magnitude.
- The direction of a magnetic field line at any point is the direction in which a hypothetical isolated north pole would move if placed at that point.
- By convention, magnetic field lines **emerge from the north pole** of a magnet and **merge at the south pole** outside the magnet.
- Inside the magnet, the direction of field lines is from the south pole to the north pole. Thus, magnetic field lines are **closed curves**.
- The relative strength of the magnetic field is indicated by the **closeness of the field lines**. Where the field lines are crowded (closer together), the magnetic field is stronger, and the force exerted on another magnet is greater.
- **No two magnetic field lines ever intersect** each other. If they did, it would mean that at the point of intersection, the compass needle (or a north pole) would point in two different directions simultaneously, which is impossible.
Magnetic Field Due To A Current-Carrying Conductor
Hans Christian Oersted's experiment in 1820 demonstrated that an electric current produces a magnetic field. This fundamental discovery established the connection between electricity and magnetism.
The pattern and strength of the magnetic field produced by a current-carrying conductor depend on the shape of the conductor and the magnitude of the current.
Magnetic Field Due To A Current Through A Straight Conductor
When electric current flows through a straight wire, it produces a magnetic field around it. Activity 12.5 (using iron filings sprinkled on cardboard pierced by a wire) shows that the magnetic field lines around a straight current-carrying wire are **concentric circles centered on the wire**.
The strength of the magnetic field produced by a straight current-carrying wire depends on:
- **Current (I):** The magnitude of the magnetic field increases as the current through the wire increases.
- **Distance from the conductor (r):** The magnitude of the magnetic field decreases as the distance from the wire increases ($B \propto 1/r$). The concentric circles representing the field lines become larger and spread further apart as you move away from the wire.
Right-Hand Thumb Rule
The **Right-Hand Thumb Rule** provides a simple way to determine the direction of the magnetic field lines around a straight current-carrying conductor.
Rule: Imagine holding the straight current-carrying conductor in your **right hand** such that your **thumb points in the direction of the electric current**. Then, the direction in which your **fingers wrap around** the conductor gives the direction of the magnetic field lines.
If the direction of the current is reversed, the direction of the magnetic field is also reversed. This rule is also known as Maxwell's corkscrew rule.
Magnetic Field Due To A Current Through A Circular Loop
When a straight current-carrying wire is bent into a **circular loop**, the magnetic field lines around it form a different pattern. At every point on the circular loop, the magnetic field lines are concentric circles around that point of the wire.
As you move towards the center of the loop, the arcs of these circles become larger and appear straighter. At the very **center of the circular loop**, the magnetic field lines are approximately **straight and perpendicular** to the plane of the loop.
Using the Right-Hand Thumb Rule at different points along the loop, you can see that the magnetic field lines inside the loop are all in the **same direction**. Outside the loop, they are in the opposite direction.
The strength of the magnetic field at the center of a current-carrying circular loop depends on:
- **Current (I):** Directly proportional to the current.
- **Radius of the loop (r):** Inversely proportional to the radius.
- **Number of turns (n):** If the circular loop has multiple turns (a coil), the magnetic field is $n$ times stronger than that produced by a single turn, as the field from each turn adds up.
Question 1. Draw magnetic field lines around a bar magnet.
Answer:
Question 2. List the properties of magnetic field lines.
Answer:
Properties of magnetic field lines:
- They are closed curves.
- They emerge from the north pole of a magnet and merge at the south pole (outside the magnet).
- Inside the magnet, they run from the south pole to the north pole.
- The direction of the magnetic field at any point is given by the tangent to the field line at that point.
- The strength of the magnetic field is proportional to the relative closeness of the field lines (more crowded lines mean a stronger field).
- No two magnetic field lines intersect each other.
Question 3. Why don’t two magnetic field lines intersect each other?
Answer:
If two magnetic field lines were to intersect each other at a point, it would mean that at that specific point, the magnetic field has two different directions simultaneously. This is impossible because a compass needle (which aligns with the magnetic field) can only point in one unique direction at any given location. Therefore, magnetic field lines do not intersect.
Question 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:
Imagine holding the wire at any point along the circular loop in your right hand, with your thumb pointing in the direction of the clockwise current at that specific point.
- **Inside the loop:** Your fingers will curl into the table (assuming the loop is on the table). This means the magnetic field inside the loop is directed **inwards**, perpendicular to the plane of the loop.
- **Outside the loop:** Your fingers will curl outwards (above the plane of the table). This means the magnetic field outside the loop is directed **outwards**, perpendicular to the plane of the loop.
Alternatively, for a circular loop, you can curl your fingers in the direction of the current around the loop. Then your extended thumb will point in the direction of the magnetic field *inside* the loop. For a clockwise current, your thumb points inwards (into the table).
Question 2. The magnetic field in a given region is uniform. Draw a diagram to represent it.
Answer:
A uniform magnetic field is one that has the same strength and direction at all points in a region. This is represented by drawing parallel and equidistant straight lines in the direction of the field, with arrows indicating the direction.
Magnetic Field Due To A Current In A Solenoid
A **solenoid** is a coil of many circular turns of insulated copper wire wrapped closely in the shape of a cylinder. When electric current flows through a solenoid, it produces a magnetic field.
The pattern of the magnetic field lines around a current-carrying solenoid is very similar to the magnetic field around a **bar magnet**. The field lines are closed curves. Outside the solenoid, the field lines are spread out, emerging from one end and merging into the other, resembling the field around a bar magnet's poles.
Inside the solenoid, the magnetic field lines are in the form of **parallel straight lines**. This indicates that the magnetic field is **uniform** (same strength and direction) at all points inside a long straight solenoid.
One end of the current-carrying solenoid behaves as a magnetic north pole, and the other end behaves as a magnetic south pole. The polarity can be determined using the Right-Hand Thumb Rule: if you curl the fingers of your right hand in the direction of the current through the turns of the coil, your extended thumb points towards the magnetic north pole of the solenoid.
A strong magnetic field produced inside a solenoid can be used to magnetise a piece of magnetic material, such as soft iron, placed inside the coil. This magnet so formed is called an **electromagnet**. Electromagnets are temporary magnets; the magnetism exists only as long as current flows through the coil.
Question 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:
Based on the pattern of field lines inside a long straight solenoid (parallel straight lines), the magnetic field inside it is uniform. Therefore, the correct option is **(d) is the same at all points**.
Force On A Current-Carrying Conductor In A Magnetic Field
We have seen that a current-carrying conductor creates a magnetic field. This field can exert a force on a magnet nearby. The reverse is also true: a magnetic field can exert a force on a current-carrying conductor placed within it. This force is called the **magnetic force** or **Lorentz force** (in general).
Activity 12.7 demonstrates this: an aluminium rod carrying current, suspended in a magnetic field (e.g., between the poles of a horseshoe magnet), experiences a force that causes it to be displaced. The direction of the force changes if the direction of the current is reversed or if the direction of the magnetic field is reversed.
Experiments show that the magnitude of the force on the conductor is largest when the direction of the current is **perpendicular** to the direction of the magnetic field. In this condition, the force is perpendicular to both the current and the magnetic field.
The direction of the force on a current-carrying conductor placed perpendicular to a magnetic field can be determined using **Fleming's Left-Hand Rule**.
Rule: Stretch the thumb, forefinger, and middle finger of your **left hand** so that they are mutually perpendicular to each other.
- If the **forefinger** points in the direction of the **magnetic field**.
- And the **middle finger** points in the direction of the **electric current**.
- Then the **thumb** will point in the direction of the **force (motion)** acting on the conductor.
This principle (force on a current-carrying conductor in a magnetic field) is the basis for the operation of many devices that involve the movement of a coil within a magnetic field, such as **electric motors**, loudspeakers, and measuring instruments like galvanometers.
Example 12.2. An electron enters a magnetic field at right angles to it, as shown in Fig. 12.14. The direction of force acting on the electron will be
(a) to the right.
(b) to the left.
(c) out of the page.
(d) into the page.
Answer:
The electron is moving horizontally to the right. The magnetic field is directed vertically upwards.
The direction of electric current is conventionally taken as opposite to the direction of flow of electrons. So, the direction of current is horizontally to the left.
Applying Fleming's Left-Hand Rule:
- Forefinger (Field) points upwards.
- Middle finger (Current) points horizontally to the left.
- The thumb will point in the direction of the force.
If you align your left hand as described, your thumb will point **into the page**.
The correct option is **(d) into the page**.
Question 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:
When a charged particle (like a proton) moves freely in a magnetic field, the magnetic force acting on it is always perpendicular to its velocity. A force perpendicular to velocity causes a change in the direction of the velocity but does not change the magnitude of the velocity (speed).
- (a) Mass: Mass is an intrinsic property and does not change. (False)
- (b) Speed: Speed is the magnitude of velocity. Since the force is perpendicular to velocity, it does no work on the particle, and its kinetic energy and speed remain constant. (False)
- (c) Velocity: Velocity is a vector quantity (speed + direction). Since the direction of motion can change (e.g., the particle moves in a circular or helical path in a uniform magnetic field), the velocity vector changes. (True)
- (d) Momentum: Momentum is mass $\times$ velocity. Since mass is constant and velocity changes, momentum also changes. (True)
The properties of a proton that can change are **(c) velocity** and **(d) momentum**.
Question 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 force experienced by a current-carrying conductor in a magnetic field is directly proportional to the current flowing through it, the strength of the magnetic field, and the length of the conductor (when they are perpendicular). $F \propto I$, $F \propto B$, $F \propto l$.
- (i) If the current in rod AB is increased, the force on the rod will **increase**. This will result in a larger displacement of the rod.
- (ii) If a stronger horse-shoe magnet is used, the magnetic field strength (B) is increased. The force on the rod will **increase**. This will result in a larger displacement of the rod.
- (iii) If the length of the rod AB is increased, the force on the rod will **increase**. This will result in a larger displacement of the rod.
Question 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:
A positively charged particle moving constitutes an electric current. The direction of current is the same as the direction of motion of the positive charge. So, the current is towards the west.
The particle is deflected towards north, so the force is towards north.
We use Fleming's Left-Hand Rule: Stretch your left hand's thumb, forefinger, and middle finger mutually perpendicular.
- Middle finger (Current) points towards West.
- Thumb (Force) points towards North.
Now, check the direction of the forefinger (Magnetic Field). If you point your middle finger west and your thumb north, your forefinger will point **downward**.
The correct option is **(c) downward**.
Domestic Electric Circuits
Electric power in homes is supplied through a main supply (mains). This supply typically consists of two main wires: a **live wire** (usually with red insulation, carrying the current from the power station) and a **neutral wire** (usually with black insulation, completing the circuit back to the power station). In many countries, the potential difference between the live and neutral wires is 220 V.
In a house, these wires first go through an electricity meter and a main fuse, then through a main switch, which connects them to the internal wiring of the house. Houses are wired with separate circuits for different purposes. Common ratings are 15 A circuits for high-power appliances (geysers, air coolers) and 5 A circuits for low-power appliances (bulbs, fans, TVs).
Appliances in domestic circuits are always connected in **parallel** to each other across the live and neutral wires. This ensures that each appliance receives the full supply voltage (e.g., 220 V) and can operate independently. Each appliance also has its own switch.
An important safety measure in domestic circuits is the **earth wire**. It has green insulation and is connected to a metal plate buried deep in the earth near the house. Metallic bodies of appliances (like irons, toasters, refrigerators) are connected to the earth wire. If there is a fault causing the live wire to touch the metallic body, the current flows through the earth wire to the ground (which offers a low-resistance path). This prevents the user from getting a severe electric shock if they touch the faulty appliance and also causes a large current to flow, which melts the fuse or trips the circuit breaker, disconnecting the power supply.
**Electric fuse** is another crucial safety device (discussed in Chapter 11). It protects circuits and appliances from damage due to **overloading** or **short-circuiting**. A fuse is a wire with a specific low melting point, connected in series. Overloading occurs when too many appliances are connected to a single circuit or there is a voltage hike, causing excessive current. Short-circuiting happens when the live and neutral wires directly touch (due to damaged insulation), leading to a very large current. In both cases, the high current causes the fuse wire to heat up and melt, breaking the circuit and stopping the dangerous current flow.
Question 1. Name two safety measures commonly used in electric circuits and appliances.
Answer:
Two safety measures commonly used in electric circuits and appliances are:
- **Electric Fuse:** Protects against damage due to overloading and short-circuiting by melting and breaking the circuit when current exceeds a safe limit.
- **Earthing:** Connecting the metallic body of an appliance to the earth wire. This provides a safety path for current leakage to flow to the ground, preventing electric shock to the user.
Other safety measures include Miniature Circuit Breakers (MCBs), proper insulation of wires, and proper wiring practices.
Question 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:
Given: Power of the oven, P = 2 kW = 2000 W.
Supply voltage, V = 220 V.
Circuit current rating (maximum safe current), $I_{rating}$ = 5 A.
First, calculate the current drawn by the oven when operated at 220 V:
Using the formula $P = VI$, the current $I_{oven} = P/V = 2000\text{W} / 220\text{V} = 200/22\text{ A} = 100/11\text{ A} \approx 9.09\text{ A}$.
The oven draws approximately 9.09 A of current when operating.
The circuit has a current rating of only 5 A. This means the maximum current it can safely handle is 5 A.
Since the current drawn by the oven (9.09 A) is significantly greater than the circuit's current rating (5 A), this will cause **overloading** of the circuit. If the circuit has a fuse or MCB rated at or below 5 A, the **fuse will melt or the MCB will trip**, interrupting the circuit and stopping the flow of current to prevent damage. If there is no adequate protection, the wires could overheat, leading to insulation damage or fire.
Therefore, I expect the fuse to blow or the MCB to trip, and the oven will not operate. This also highlights that a 2 kW oven is not suitable for a circuit rated only for 5 A.
Question 3. What precaution should be taken to avoid the overloading of domestic electric circuits?
Answer:
Precautions to avoid overloading of domestic electric circuits include:
- **Avoiding connecting too many appliances** simultaneously to a single socket or circuit, especially high-power appliances.
- Using circuits with **appropriate current ratings** for the appliances being used. High-power appliances should be connected to circuits designed for them (e.g., 15 A circuit).
- Ensuring that wiring in the house is of **adequate thickness and quality** to handle the expected current load.
- Using **properly rated fuses** or **MCBs** in each circuit as safety devices.
- Repairing or replacing faulty appliances or wiring promptly to prevent short-circuiting, which leads to overloading.
Intext Questions
Page No. 196
Question 1. Why does a compass needle get deflected when brought near a bar magnet?
Answer:
Page No. 200
Question 1. Draw magnetic field lines around a bar magnet.
Answer:
Question 2. List the properties of magnetic field lines.
Answer:
Question 3. Why don’t two magnetic field lines intersect each other?
Answer:
Page No. 201 - 202
Question 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:
Question 2. The magnetic field in a given region is uniform. Draw a diagram to represent it.
Answer:
Question 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:
Page No. 203 - 204
Question 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:
Question 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:
Question 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:
Page No. 205
Question 1. Name two safety measures commonly used in electric circuits and appliances.
Answer:
Question 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:
Question 3. What precaution should be taken to avoid the overloading of domestic electric circuits?
Answer:
Exercises
Question 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:
Question 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:
Question 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.
Answer:
Question 4. List two methods of producing magnetic fields.
Answer:
Question 5. When is the force experienced by a current–carrying conductor placed in a magnetic field largest?
Answer:
Question 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:
Question 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:
Question 8. When does an electric short circuit occur?
Answer:
Question 9. What is the function of an earth wire? Why is it necessary to earth metallic appliances?
Answer: