Here’s a summary of the key points from the “Magnetic Effects of Electric Current” chapter in 10th-standard science:
- Electric Current and Magnetic Field: When an electric current flows through a conductor, it creates a magnetic field around the conductor.
- Magnetic Field Lines: These invisible lines represent the direction and strength of the magnetic field. The closer the lines are packed together, the stronger the magnetic field.
- Right-Hand Rule: A rule to determine the direction of the magnetic field around a current-carrying straight wire. Curl your thumb in the direction of the current flow, and your fingers will wrap around the wire indicating the direction of the magnetic field.
- Magnetic Field of a Solenoid: When a current flows through a coil of wire (solenoid), it creates a stronger magnetic field inside the coil. The strength increases with the number of turns in the coil and the current flowing through it.
- Electromagnets: Electromagnets are temporary magnets made by wrapping a coil of wire around a soft iron core. When current flows through the coil, the iron core becomes magnetized. The magnetic strength can be controlled by varying the current or the number of turns in the coil.
- Applications: Magnetic effects of electric current have numerous applications such as electromagnets in loudspeakers, electric motors, MRI machines, and doorbells.
This summary covers the main concepts of the chapter. You might also encounter:
- Force on a current-carrying conductor in a magnetic field (concept used in electric motors)
- Fleming’s left-hand rule (another rule to determine the direction of force on a current-carrying conductor in a magnetic field)
Questions (Page 197)
1. Why does a compass needle get deflected when brought near a bar magnet ?
Ans : A compass needle gets deflected when brought near a bar magnet because:
- Compass needle is a magnet: The needle in a compass is a small, permanent magnet. It has a north and south pole, just like a bar magnet.
- Magnets attract and repel: Permanent magnets exert a force on each other. Opposite poles (north and south) attract, while similar poles (north-north or south-south) repel.
- Magnetic field interaction: A bar magnet creates an invisible magnetic field around it. This field consists of lines of force that point from the north pole to the south pole of the magnet.
Questions (Page 200)
1. Draw magnetic field lines around a bar magnet.
Ans :
2. List the properties of magnetic lines of force.
Ans :
- Continuous and Closed Loops: Magnetic lines of force are continuous loops. They originate from the north pole of a magnet and terminate at the south pole, never having an endpoint or breaking off.
- Direction: The direction of the tangent to a magnetic field line at any point indicates the direction of the magnetic field at that point.
- Never Intersect: Magnetic lines of force never intersect with each other. If they did, it would indicate points of uncertainty in the magnetic field, which doesn’t occur.
- Strength and Density: The density of magnetic field lines is an indicator of the strength of the magnetic field. Areas with more concentrated lines represent a stronger magnetic field, while sparser lines indicate a weaker field.
- Not Physical Lines: Magnetic lines of force are a conceptual tool used to visualize the magnetic field. They don’t physically exist but represent the influence of the magnetic field on tiny magnetic dipoles.
- Affect Moving Charges: Magnetic fields exert a force on moving charged particles. The direction and magnitude of this force are determined by the interaction of the particle’s charge, velocity, and the strength and direction of the magnetic field.
3. Why don’t two magnetic lines of force intersect each other?
Ans : Magnetic field lines don’t intersect each other because of a fundamental principle of magnetic fields: a magnetic field has a well-defined direction at every point.
Questions(Page 201-202)
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 magnetic field inside and outside the loop.
Ans : Right-hand rule: curl fingers clockwise (current), thumb shows magnetic field.
Inside loop: magnetic field points straight upwards.
Outside loop:
- Above loop: magnetic field points outwards (away from table).
- Below loop: magnetic field points downwards (towards table).
2. The magnetic field in a given region is uniform. Draw a diagram to represent it.
Ans :
3. 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
Ans : (d) is the same at all points
Questions (Page 203-204)
1. Which of the following property of a proton can change while it moves freely in a magnetic field?
a) Mass
b) Speed
c) Velocity
d) Momentum
Ans :
c) Velocity
d) Momentum
2. A positively-charged particle projected towards west is deflected towards north by a magnetic field. The direction of the magnetic field is
a) Towards south
b) Towards east
c) Downward
d) Upward
Ans :
(d) Upward
Questions (Page 205)
1. Name two safety measures commonly used in electric circuits and appliances.
Ans :
- Fuse: A fuse is a thin wire or strip of metal designed to melt and interrupt the circuit if the current exceeds a safe limit. This prevents overheating and potential fire hazards in case of overload or short circuit.
- Earthing: Earthing (or grounding) is a process of connecting the metallic body of an appliance to the ground. This provides a path for any leakage current to flow safely to the ground instead of passing through a person who might come in contact with the appliance if there’s a fault.
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.
Ans :
- Power and Current Relationship: We can use the formula for power (P) to find the current (I) that the oven would draw:
P = V * I
where:
- P is the power (2 kW = 2000 W)
- V is the voltage (220 V)
- I is the current (unknown)
- Calculating Current for the Oven:
I = P / V = 2000 W / 220 V ≈ 9.09 A (rounded to two decimal places)
- Current Rating vs. Oven Draw:
The oven is designed to draw a current of approximately 9.09 A, which is significantly higher than the circuit’s current rating of 5 A.
- Fuse Protection:
The fuse in the circuit is designed to melt and break the circuit if the current exceeds its safe limit.
Conclusion:
Since the oven draws more current (9.09 A) than the circuit is rated for (5 A), the fuse will likely blow to protect the wiring from overheating and potential fire hazards. The oven won’t function, and you’ll need to replace the fuse with one rated for a current that can safely handle the oven’s power consumption.
3. What precaution should be taken to avoid the overloading of domestic electric circuits?
Ans :
- Know circuit rating (amps).
- Check appliance power (watts).
- Don’t exceed circuit capacity (watts / volts x amps).
- Avoid overloading outlets with multi-plugs.
- Unplug unused appliances.
- Use appliances wisely (don’t run too many high-power devices together).
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.
Ans : (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.
Ans : (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
(b) A wire with a green insulation is usually the live wire of an electric supply
Ans :
(a) False
(b) False
4. List two methods of producing magnetic fields
Ans :
- Permanent magnets: These are materials like iron, nickel, or cobalt that have a magnetization that persists even in the absence of an electric current. They have a north and south pole and attract or repel other magnets depending on their poles.
- Electromagnets: These are temporary magnets created by coiling a wire and passing an electric current through it. The magnetic field exists only as long as the current flows. The strength of the magnetic field can be controlled by varying the current or the number of turns in the coil.
5. When is the force experienced by a current-carrying conductor placed in a magnetic field the largest?
Ans :
he force experienced by a current-carrying conductor placed in a magnetic field is the largest when the following condition is met:
The angle between the direction of the current flow in the conductor and the direction of the magnetic field is 90 degrees.
Here’s the explanation:
- The force exerted on a current-carrying conductor in a magnetic field is governed by a principle called the Lorentz force.
- This force depends on several factors, including the current (I), the length of the conductor (L), and the strength of the magnetic field (B).
- There’s also an angle (θ) between the current direction and the magnetic field direction.
The force can be expressed by the formula:
F = I * L * B * sin(θ)
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?
Ans : Since the electron beam is deflected to your right side and you’re facing the front wall, the magnetic field must be pointing out of your back (towards the left).
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.
Ans :
(i) Magnetic field produced around a straight conductor-carrying current:
- Right-Hand Rule: Curl your fingers of your right hand so that they wrap around the conductor in the direction of the current flow. Your thumb will then point in the direction of the magnetic field lines around the conductor.
(ii) Force experienced by a current-carrying straight conductor placed in a magnetic field which is perpendicular to it:
- Right-Hand Rule (Again!): Extend your right hand with your fingers pointing in the direction of the current flow (in the conductor) and your thumb extended perpendicularly. The magnetic field should be oriented such that your thumb points in the direction of the magnetic field lines. Your palm will then point in the direction of the force exerted on the conductor by the magnetic field.
(iii) Current induced in a coil due to its rotation in a magnetic field:
- Fleming’s Right-Hand Rule: Extend your right hand with the thumb, index finger, and middle finger pointing at right angles to each other. Point your thumb in the direction of the magnetic field, your index finger in the direction of the motion of the conductor (rotation of the coil), and your middle finger will then point in the direction of the current induced in the coil (due to the motion and magnetic field).
8. When does an electric short circuit occur?
Ans : Short circuit happens when live and neutral wires touch accidentally. This causes a surge in current, overheating wires, and potential fire. It can be caused by damaged wires, water damage, overloading circuits, or faulty appliances.
9. What is the function of an earth wire? Why is it necessary to earth metallic appliances?
Ans :
- Leakage Current Path: In some situations, there might be a leakage of current from the live wire to the metal body of an appliance due to internal faults or damaged insulation.
- Shock Prevention: The earth wire provides a low-resistance path for this leakage current to flow directly to the ground (earth) instead of passing through a person who might touch the appliance if it’s faulty.
- Safety Measure: Earthing prevents a potential shock hazard by diverting the current away from the user’s body.
Why Earth Metallic Appliances?
Earthing is particularly important for metallic appliances because metal is a good conductor of electricity. If a fault occurs, the metal body could become energized, posing a shock risk. Earthing ensures that any leakage current is safely channeled away from the appliance and the user.
