Science Class- 10
Class 10 - Chapter 12: Magnetic Effects of Electric Current
Q1. Which of the following correctly describes the magnetic field near a long straight wire?
Answer: (d) The field consists of concentric circles centred on the wire.
A current-carrying straight conductor produces magnetic field lines in the form of concentric circles around it. The direction of these field lines can be determined using the right-hand thumb rule.
Q2. At the time of short circuit, the current in the circuit
Answer: (c) Increases heavily.
During a short circuit, the live and neutral wires come into direct contact, causing the resistance of the circuit to become very small. As a result, a very large current flows through the circuit.
Q3. 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.
Near the centre of a circular coil, the magnetic field lines are nearly parallel and straight, indicating a strong and uniform magnetic field.
(b) A wire with a green insulation is usually the live wire of an electric supply.
Answer: False.
The green wire is the earth wire. The live wire is generally red or brown in colour.
Q4. List two methods of producing magnetic fields.
Magnetic fields can be produced by:
- Using a permanent magnet.
- Passing electric current through a conductor or coil.
Q5. When is the force experienced by a current-carrying conductor placed in a magnetic field largest?
The force on a current-carrying conductor is maximum when the conductor is placed perpendicular to the magnetic field.
Mathematically,
F = BIL sin θ
The force is maximum when θ = 90° because sin 90° = 1.
Q6. 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-hand side. What is the direction of magnetic field?
The electron beam is moving from the back wall towards the front wall and is deflected towards the right.
Since electrons are negatively charged, the direction of force is opposite to that predicted by Fleming's Left-Hand Rule.
Applying the rule, the magnetic field must be directed vertically downward.
Q7. State the rule to determine the direction of
(i) Magnetic field produced around a straight conductor carrying current:
The Right-Hand Thumb Rule is used.
If the right-hand thumb points in the direction of current, the curled fingers give the direction of magnetic field lines.
(ii) Force experienced by a current-carrying conductor placed in a magnetic field:
Fleming's Left-Hand Rule is used.
If the thumb, forefinger and middle finger of the left hand are stretched mutually perpendicular to each other:
- Forefinger indicates the magnetic field.
- Middle finger indicates the current.
- Thumb indicates the force or motion.
(iii) Current induced in a coil due to its rotation in a magnetic field:
Fleming's Right-Hand Rule is used.
If the thumb, forefinger and middle finger of the right hand are held mutually perpendicular:
- Forefinger indicates magnetic field.
- Thumb indicates motion of conductor.
- Middle finger indicates induced current.
Q8. When does an electric short circuit occur?
An electric short circuit occurs when the live wire and neutral wire come into direct contact due to damaged insulation or faulty wiring.
This results in a sudden flow of a very large current, which may cause overheating and fire.
Q9. What is the function of an earth wire? Why is it necessary to earth metallic appliances?
The earth wire provides a low-resistance path for leakage current to flow safely into the ground.
Metallic appliances are earthed to protect users from electric shock.
If the live wire accidentally touches the metal body of an appliance, the leakage current flows through the earth wire instead of passing through a person touching the appliance.
This makes the use of electrical appliances safe.