In-Text Questions & Answers
Section 12.1: Magnetic Field and Field Lines
A compass needle is a small bar magnet. When brought near a bar magnet, it experiences a magnetic force due to the magnetic field exerted by the bar magnet in the surrounding region. This magnetic force causes the needle to deflect.
The magnetic field lines emerge from the north pole and merge at the south pole outside the magnet, while inside the magnet, they move from the south pole to the north pole to form closed curves.
The properties of magnetic field lines are:
- They emerge from the north pole and merge at the south pole outside the magnet. Inside the magnet, their direction is from its south pole to its north pole.
- Magnetic field lines are continuous closed curves.
- The relative strength of the magnetic field is shown by the degree of closeness of the field lines; the field is stronger where the lines are crowded.
- No two field lines intersect each other.
Two magnetic field lines do not intersect each other because if they did, it would mean that at the point of intersection, a compass needle would point towards two different directions at the same time, which is impossible.
Section 12.2: Magnetic Field due to a Current-Carrying Conductor
Applying the right-hand thumb rule:
- Inside the loop: The magnetic field lines point vertically downwards into the plane of the table.
- Outside the loop: The magnetic field lines point vertically upwards out of the plane of the table.
A uniform magnetic field is represented by parallel, equidistant straight lines with arrowheads indicating the direction of the field.
The magnetic field inside a long straight solenoid-carrying current
Answer: (d) is the same at all points.
Explanation: The field lines inside a current-carrying solenoid are in the form of parallel straight lines, which indicates that the magnetic field is uniform and the same at all points inside it.
Section 12.3: Force on a Current-Carrying Conductor in a Magnetic Field
Answer: (c) velocity and (d) momentum
Explanation: When a proton moves in a magnetic field, it experiences a magnetic force that acts perpendicular to its direction of motion. This force changes the direction of motion of the proton without changing its speed or mass. Since velocity depends on direction, velocity changes. Consequently, momentum (mass × velocity) also changes.
The displacement of rod AB reflects the magnitude of force acting on it:
- (i) If current in rod AB is increased: The displacement of the rod increases because the force acting on a current-carrying conductor increases with an increase in current.
- (ii) If a stronger horse-shoe magnet is used: The displacement of the rod increases because the force increases with an increase in the strength of the magnetic field.
- (iii) If length of the rod AB is increased: The displacement of the rod increases because a longer conductor in a magnetic field experiences a greater magnetic force.
Answer: (d) upward
Explanation: The direction of current is the same as the direction of motion of positively charged particles (towards West). The force acts towards North. According to Fleming's left-hand rule, stretching the forefinger, middle finger (pointing West), and thumb (pointing North) places the forefinger pointing vertically upwards. Therefore, the magnetic field direction is upward.
Section 12.4: Domestic Electric Circuits
Two safety measures commonly used in electric circuits and appliances are:
- Electric Fuse: Prevents damage to appliances and circuits caused by overloading or short-circuiting by melting and breaking the circuit when excessive current flows.
- Earthing (Earth Wire): Protects users from severe electric shocks by providing a low-resistance path for any leaking current from appliances with metallic bodies to the ground.
Calculations:
Power of the electric oven, $P = 2\text{ kW} = 2000\text{ W}$
Supply voltage, $V = 220\text{ V}$
Current drawn by the oven, $I = \frac{P}{V} = \frac{2000}{220} \approx 9.09\text{ A}$
Result & Explanation:
The current required by the electric oven ($9.09\text{ A}$) is significantly higher than the circuit's current rating of $5\text{ A}$. This causes overloading of the circuit. Due to excessive current, the Joule heating effect will cause the electric fuse to melt and break the circuit, or it may heat up the wiring and cause damage.
The precautions to avoid overloading include:
- Do not connect too many high-power electrical appliances to a single socket.
- Avoid using high-power appliances simultaneously on the same circuit.
- Ensure that electrical wires with appropriate current ratings and high-quality insulation are used, and damaged insulation is replaced promptly.
- Use an electric fuse or circuit breaker of appropriate capacity in the circuit.
End of Chapter Exercises
Answer: (d) The field consists of concentric circles centred on the wire.
Answer: (c) increases heavily.
Explanation: A short circuit occurs when a live wire comes into direct contact with a neutral wire, leading to an abrupt and heavy increase in current.
- The field at the centre of a long circular coil carrying current will be parallel straight lines.
- A wire with a green insulation is usually the live wire of an electric supply.
(a) True — At the centre of a circular coil, the arcs of field lines appear as straight parallel lines representing a uniform magnetic field.
(b) False — A wire with green insulation is the earth wire; the live wire usually has red insulation cover.
Two methods of producing magnetic fields are:
- Using permanent magnets (e.g., a bar magnet).
- Passing an electric current through a conductor, such as a straight wire, a circular loop, or a solenoid (electromagnet).
The force experienced by a current-carrying conductor placed in a magnetic field is largest when the direction of the current is at right angles ($90^\circ$) to the direction of the magnetic field.
Answer: The direction of the magnetic field is vertically downward.
Explanation:
- The electron beam moves from the back wall to the front wall. Since current flows opposite to the direction of motion of electrons, the conventional current is directed from the front wall to the back wall.
- The force (deflection) acts towards the right side.
- Applying Fleming's left-hand rule: align the middle finger pointing towards the back wall (current) and the thumb pointing to the right (force). The forefinger then points vertically downward. Thus, the magnetic field is directed downwards.
- magnetic field produced around a straight conductor-carrying current
- force experienced by a current-carrying straight conductor placed in a magnetic field which is perpendicular to it
- current induced in a coil due to its rotation in a magnetic field
(i) Right-Hand Thumb Rule: Imagine holding a current-carrying straight conductor 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 magnetic field lines.
(ii) Fleming's Left-Hand Rule: Stretch the thumb, forefinger, and middle finger of your left hand mutually perpendicular to each other. If the forefinger points in the direction of the magnetic field and the middle finger in the direction of current, the thumb will point in the direction of motion or force acting on the conductor.
(iii) Fleming's Right-Hand Rule: Stretch the thumb, forefinger, and middle finger of your right hand mutually perpendicular to each other. If the forefinger points in the direction of the magnetic field and the thumb points in the direction of motion of the conductor, then the middle finger points in the direction of the induced current.
An electric short circuit occurs when the live wire and the neutral wire come into direct contact with each other. This can happen when the insulation cover of wires is damaged or when there is a technical fault in an electrical appliance, causing the current in the circuit to increase abruptly.
Function of an earth wire: The earth wire acts as a safety measure by providing a low-resistance conducting path for electric current into the earth.
Necessity of earthing metallic appliances: Metallic body appliances (e.g., electric press, toaster, refrigerator) are earthed so that if any current leaks to the metal body due to faulty insulation, the potential of the body stays equal to that of the earth. The current flows harmlessly to the ground, protecting the user from receiving a severe electric shock.
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