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Chapter 13: Magnetic Effects Of Electric Current
In the previous chapter on electricity, we learned about the heating effect of electric current. Besides producing heat, electric current can also produce magnetic effects. This link between electricity and magnetism is fundamental to many technologies.
It was discovered by Hans Christian Oersted in 1820 that an electric current flowing through a metallic wire causes a compass needle placed nearby to deflect. This showed that an electric current produces a magnetic field around the conductor.
This chapter explores the magnetic effects of electric current, magnetic fields, how they are produced by current-carrying conductors, and the fascinating phenomenon of electromagnetic induction, which is the basis for generating electricity.
Magnetic Field And Field Lines
A magnet exerts a force on magnetic materials or other magnets in the region surrounding it. This region is called a magnetic field.
A compass needle is a small magnet that aligns itself with the Earth's magnetic field, pointing approximately North and South. The ends of the compass needle are called the north pole (points towards North) and the south pole (points towards South). Like magnetic poles repel each other, while unlike poles attract.
We can visualise the magnetic field around a magnet using iron filings or a compass. Iron filings sprinkled around a bar magnet arrange themselves in a pattern of curves, which represent the magnetic field lines. These lines indicate the direction and strength of the magnetic field.
Properties of Magnetic Field Lines:
- Magnetic field lines emerge from the north pole of a magnet and merge at the south pole.
- Inside the magnet, the direction of field lines is from the south pole to the north pole, forming closed curves.
- The direction of the magnetic field at any point is the direction the north pole of a compass needle would point if placed at that point.
- The relative strength of the magnetic field is indicated by the degree of closeness of the field lines. Where field lines are closer (crowded), the magnetic field is stronger (e.g., near the poles). Where field lines are farther apart, the field is weaker.
- No two magnetic field lines ever intersect each other. If they did, it would mean that at the intersection point, a compass needle would point in two directions simultaneously, which is impossible.
Magnetic Field Due To A Current-carrying Conductor
As discovered by Oersted, an electric current in a conductor creates a magnetic field around it. The pattern and strength of this magnetic field depend on the current, the distance from the conductor, and the shape of the conductor.
Magnetic Field Due To A Current Through A Straight Conductor
A straight wire carrying electric current produces a magnetic field around it. Using iron filings or a compass, we can observe the pattern of this field.
The magnetic field lines around a straight current-carrying wire are concentric circles centered on the wire. The direction of the magnetic field reverses if the direction of the current in the wire is reversed.
The strength of the magnetic field produced by a straight current-carrying wire:
- Increases as the current through the wire increases.
- Decreases as the distance from the wire increases (the concentric circles become larger and farther apart).
Right-hand Thumb Rule
The direction of the magnetic field around a straight current-carrying conductor can be easily determined using the Right-Hand Thumb Rule (also known as Maxwell's Corkscrew Rule).
Rule: Imagine holding the straight current-carrying conductor in your right hand with your thumb pointing 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.
Magnetic Field Due To A Current Through A Circular Loop
When a straight wire is bent into a circular loop and current is passed through it, each segment of the loop contributes to the magnetic field. The field lines around a current-carrying circular loop have a specific pattern.
Near the wire at any point on the loop, the field lines are concentric circles. As you move towards the centre of the loop, these circles become larger, and at the centre, the field lines are nearly straight and parallel. The direction of the field lines within the loop is the same, adding up to create a stronger magnetic field at the center compared to points farther away.
If the circular loop has $n$ turns (forming a coil), the magnetic field produced by the coil is $n$ times stronger than that produced by a single loop, because the current in each turn flows in the same direction, and their magnetic fields add up.
Magnetic Field Due To A Current In A Solenoid
A solenoid is a coil of many closely wrapped circular turns of insulated copper wire in the shape of a cylinder.
When electric current passes through a solenoid, it produces a magnetic field. The pattern of the magnetic field lines around a current-carrying solenoid is similar to that of a bar magnet.
- One end of the solenoid acts as a magnetic north pole, and the other end acts as a magnetic south pole.
- The magnetic field lines inside the solenoid are in the form of parallel straight lines. This indicates that the magnetic field is uniform (same strength and direction) at all points inside the solenoid.
A strong magnetic field produced inside a solenoid can be used to magnetise a piece of magnetic material, like soft iron, placed inside it. This forms an electromagnet. Electromagnets are temporary magnets that are magnetic only when electric current flows through the coil.
Force On A Current-carrying Conductor In A Magnetic Field
Since a current-carrying conductor produces a magnetic field and exerts a force on a magnet, it follows (by Newton's Third Law) that a magnet also exerts an equal and opposite force on a current-carrying conductor placed within its magnetic field. This effect was demonstrated by Andre Marie Ampere.
When a current-carrying conductor is placed in a magnetic field, it experiences a force. This force can cause the conductor to move.
Experimental Observation: A suspended aluminium rod carrying current is placed in a magnetic field (e.g., between the poles of a horseshoe magnet). The rod is displaced, indicating a force acting on it. Reversing the direction of the current or reversing the direction of the magnetic field reverses the direction of the force and displacement.
The magnitude of the force acting on the conductor is largest when the direction of the current is perpendicular to the direction of the magnetic field. In this situation, the force is perpendicular to both the current and the magnetic field.
The direction of the force can be determined using Fleming's Left-Hand Rule.
Fleming's Left-Hand 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.
Devices that use this principle of force on a current-carrying conductor in a magnetic field to convert electrical energy into mechanical energy include electric motors, loudspeakers, and measuring instruments.
Electric Motor
An electric motor is a rotating device that converts electrical energy into mechanical energy. It is a fundamental component in many electrical appliances.
Principle of Electric Motor: An electric motor works on the principle that a current-carrying conductor experiences a force when placed in a magnetic field. This force causes the conductor (part of a coil) to move, producing rotation.
Working of a Simple Electric Motor:
- A rectangular coil (armature) is placed in a magnetic field (usually from a permanent magnet or electromagnet).
- Electric current is supplied to the coil through conducting brushes (X, Y) and a split ring (commutator).
- The split ring (commutator) consists of two halves (P, Q) and acts as a device that reverses the direction of current flow through the coil periodically (every half rotation).
- Current flows through the coil (e.g., ABCD). Applying Fleming's left-hand rule, the force on arm AB is downwards, and the force on arm CD is upwards. These forces form a couple, causing the coil to rotate.
- As the coil rotates and reaches the half-way point, the split ring reverses the direction of current in the coil (now DCBA). This reversal also reverses the direction of the forces on AB (upwards) and CD (downwards).
- The reversed forces continue to rotate the coil in the same direction.
- This process repeats with each half rotation, resulting in the continuous rotation of the coil and its attached axle.
Commercial motors often use electromagnets, coils with many turns, and a soft iron core (armature) to enhance power. Electric motors are used in fans, refrigerators, mixers, washing machines, cars, computers, etc.
Electromagnetic Induction
Following the discovery of the magnetic effect of electric current, scientists like Michael Faraday investigated the reverse possibility: whether moving magnets or changing magnetic fields could produce electric current. This led to the discovery of electromagnetic induction.
Electromagnetic Induction: The phenomenon of inducing an electric current (called induced current) in a conductor when it is placed in a changing magnetic field.
Experimental Observations:
- Moving a magnet towards or away from a coil connected to a galvanometer causes a momentary deflection in the galvanometer needle, indicating an induced current in the coil. The direction of deflection (current) reverses when the magnet's direction of motion is reversed. The deflection is zero when the magnet is stationary relative to the coil.
- Moving a coil towards or away from a stationary magnet also induces current.
- Changing the current in one coil (primary coil) placed near another coil (secondary coil) also induces a momentary current in the secondary coil. When the current in the primary coil is steady, no current is induced in the secondary coil.
These experiments demonstrate that a potential difference (induced potential difference) and hence an induced current are produced in a conductor whenever the magnetic field associated with it is changing. The change in magnetic field lines linking the conductor is the cause of the induced current.
The magnitude of the induced current is largest when the direction of motion of the conductor (relative to the field) is at right angles to the direction of the magnetic field.
The direction of the induced current can be determined using Fleming's Right-Hand Rule.
Fleming's Right-Hand Rule: Stretch the thumb, forefinger, and middle finger of your right hand so that they are mutually perpendicular to each other.
- If the forefinger points in the direction of the magnetic field.
- And the thumb points in the direction of the motion of the conductor.
- Then the middle finger will show the direction of the induced current.
Electromagnetic induction is the underlying principle behind electric generators, which produce electricity by rotating conductors in magnetic fields.
Electric Generator
An electric generator is a device that converts mechanical energy into electrical energy. It works based on the principle of electromagnetic induction.
Working of an Electric Generator:
- A rectangular coil (armature) is rotated mechanically (using a turbine, engine, etc.) in a uniform magnetic field (produced by a permanent magnet or electromagnet).
- As the coil rotates, the conducting arms cut the magnetic field lines, inducing a potential difference and current in the coil according to Fleming's Right-Hand Rule.
- The ends of the coil are connected to rings (slip rings in AC generator, split rings in DC generator) which are in contact with stationary brushes. These brushes transfer the induced current to the external circuit.
Types of Generators:
- AC Generator: Uses slip rings (R₁, R₂). As the coil rotates, the direction of current in the coil reverses every half rotation. Slip rings ensure that the current flowing into the external circuit also reverses its direction periodically. This produces an alternating current (AC), which changes direction at regular intervals. Most power stations generate AC.
- DC Generator: Uses a split ring commutator (similar to the one in a DC motor). The split ring reverses the connections to the coil in the external circuit every half rotation, ensuring that the current flows in the same direction in the external circuit. This produces a direct current (DC), which flows in one direction only.
In India, AC supply has a frequency of 50 Hz, meaning the current reverses direction 100 times per second. AC can be transmitted over long distances more efficiently than DC, with less energy loss.
Domestic Electric Circuits
Our homes receive electric power through a main supply, which is distributed through various circuits within the house to power appliances.
Components of Domestic Circuits:
- Live wire: Usually has red insulation. Carries the potential of the supply (typically 220 V in India).
- Neutral wire: Usually has black insulation. Is normally at zero potential (ground potential).
- Potential difference: Between live and neutral wires is 220 V (in India).
- Main Fuse: A safety device connected in series with the main supply at the meter board. Protects the entire house circuit from excessive current.
- Electricity Meter: Measures the electrical energy consumed.
- Main Switch: Allows the entire circuit to be turned ON or OFF.
- Distribution Circuits: Electricity is distributed to separate circuits in the house, often with different current ratings (e.g., 5 A for lights, fans; 15 A for heavy appliances like geysers, ACs).
- Earth wire: Usually has green insulation. Connected to a metal plate buried deep in the earth near the house. Provides a low-resistance path to the earth for safety, especially for appliances with metal bodies. If the live wire accidentally touches the metal body, the current flows to the earth instead of shocking a person touching the appliance.
Connection of Appliances: Appliances in domestic circuits are connected in parallel across the live and neutral wires. This ensures that:
- Each appliance receives the same voltage (220 V).
- Each appliance can be switched ON/OFF independently.
- If one appliance fails, it does not affect the others.
Safety Devices:
- Electric Fuse: Protects appliances and circuits from short-circuiting and overloading. Melts and breaks the circuit when current exceeds its rating.
- Earthing: Connecting metal bodies of appliances to the earth wire provides safety against electric shock.
Overloading and Short-circuiting:
- Overloading: Occurs when the total current drawn by all appliances connected to a circuit exceeds the maximum allowable current for that circuit. Can be caused by connecting too many devices or accidental voltage hikes. Results in excessive heating of wires, potentially causing fire.
- Short-circuiting: Occurs when the live wire and neutral wire come into direct contact (e.g., due to damaged insulation). This creates a path of very low resistance, causing a very large current to flow abruptly. This also leads to excessive heating and potential damage.
Fuses protect against both overloading and short-circuiting by melting and breaking the circuit when the current becomes dangerously high.
Intext Questions
Page No. 224
Question 1. Why does a compass needle get deflected when brought near a bar magnet?
Answer:
Page No. 228
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. 229 - 230
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. 231 - 232
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 13.7, 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. 233
Question 1. State Fleming’s left-hand rule.
Answer:
Question 2. What is the principle of an electric motor?
Answer:
Question 3. What is the role of the split ring in an electric motor?
Answer:
Page No. 236
Question 1. Explain different ways to induce current in a coil.
Answer:
Page No. 237
Question 1. State the principle of an electric generator.
Answer:
Question 2. Name some sources of direct current.
Answer:
Question 3. Which sources produce alternating current?
Answer:
Question 4. Choose the correct option.
A rectangular coil of copper wires is rotated in a magnetic field. The direction of the induced current changes once in each
(a) two revolutions
(b) one revolution
(c) half revolution
(d) one-fourth revolution
Answer:
Page No. 238
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. The phenomenon of electromagnetic induction is
(a) the process of charging a body.
(b) the process of generating magnetic field due to a current passing through a coil.
(c) producing induced current in a coil due to relative motion between a magnet and the coil.
(d) the process of rotating a coil of an electric motor.
Answer:
Question 3. The device used for producing electric current is called a
(a) generator.
(b) galvanometer.
(c) ammeter.
(d) motor.
Answer:
Question 4. The essential difference between an AC generator and a DC generator is that
(a) AC generator has an electromagnet while a DC generator has permanent magnet.
(b) DC generator will generate a higher voltage.
(c) AC generator will generate a higher voltage.
(d) AC generator has slip rings while the DC generator has a commutator.
Answer:
Question 5. 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 6. State whether the following statements are true or false.
(a) An electric motor converts mechanical energy into electrical energy.
(b) An electric generator works on the principle of electromagnetic induction.
(c) The field at the centre of a long circular coil carrying current will be parallel straight lines.
(d) A wire with a green insulation is usually the live wire of an electric supply.
Answer:
Question 7. List two methods of producing magnetic fields.
Answer:
Question 8. 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.
Answer:
Question 9. When is the force experienced by a current–carrying conductor placed in a magnetic field largest?
Answer:
Question 10. 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 11. Draw a labelled diagram of an electric motor. Explain its principle and working. What is the function of a split ring in an electric motor?
Answer:
Question 12. Name some devices in which electric motors are used.
Answer:
Question 13. A coil of insulated copper wire is connected to a galvanometer. What will happen if a bar magnet is
(i) pushed into the coil,
(ii) withdrawn from inside the coil,
(iii) held stationary inside the coil?
Answer:
Question 14. 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:
Question 15. 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 16. Explain the underlying principle and working of an electric generator by drawing a labelled diagram. What is the function of brushes?
Answer:
Question 17. When does an electric short circuit occur?
Answer:
Question 18. What is the function of an earth wire? Why is it necessary to earth metallic appliances?
Answer: