CBSE Class 10 Science Magnetic Effects of Electric Current Important Questions Set 02

Answer: (c) the same at all points. Inside a long solenoid the field lines are parallel, so the field is uniform.

Teacher's Note:
1. Parallel and equally spaced field lines mean a uniform field.
2. The field is not uniform near the ends or outside the coil.

Answer: (b) use of fuse. A fuse melts and breaks the circuit when the current becomes too large.

Teacher's Note:
1. Earthing protects against shock, not against excess current.
2. Stabilizers and meters do not cut off the supply on overload.

Answer: (d) In India, the AC changes direction after every 1/50 second. Indian AC has a frequency of 50 Hz, so it reverses direction every 1/100 second, not 1/50 second.

Teacher's Note:
1. In each cycle the current reverses twice.
2. So 50 cycles per second means 100 reversals per second.

Answer: (b) concentric circles with the wire at the centre. The field forms closed circles around the wire, with the direction given by the right-hand thumb rule.

Teacher's Note:
1. The field is perpendicular to the current.
2. Oersted's compass experiment shows this circular pattern.

Answer: (c) 50 Hz. The supply is at 220 V and 50 Hz in India.

Teacher's Note:
1. Frequency is the number of complete cycles per second.
2. 60 Hz is used in some other countries.

Answer: (c). A is true because the field at any point has only one direction. R is false because field lines do have a definite direction, from north to south outside the magnet.

Teacher's Note:
1. If two lines crossed, there would be two field directions at that point.
2. The direction of a field line is shown by the path of a free north pole.

Answer: (a). Both are true and the parallel, equally spaced lines are exactly what makes the field uniform. So R explains A.

Teacher's Note:
1. Uniform field means the same magnitude and direction at every point.
2. The lines are close together and parallel in the central part of a long solenoid.

Answer: A coil of many circular turns of insulated copper wire wrapped closely in the shape of a cylinder is called a solenoid.

Teacher's Note:
1. Its magnetic field is similar to the field of a bar magnet.
2. Current through it makes one end a north pole and the other a south pole.

Answer: (a) Inside the magnet the lines run from the south pole to the north pole.
(b) Outside the magnet they run from the north pole to the south pole.

Teacher's Note:
1. This convention makes the field lines closed curves.
2. The direction is the path a free north pole would take.

Answer: An electromagnet is a strong temporary magnet made by winding insulated copper wire on a soft iron core and passing current through the coil. Soft iron is used because it is magnetised easily when current flows and loses its magnetism quickly when the current is switched off.

Teacher's Note:
1. The soft iron core greatly increases the field of the coil.
2. Steel would not be suitable because it keeps its magnetism.

Answer: The right-hand thumb rule: hold the conductor in the right hand with the thumb along the current. The curled fingers show the direction of the field lines.

Teacher's Note:
1. The field lines are concentric circles around the conductor.
2. Reversing the current reverses the direction of the field.

Answer: DC flows in one direction only, while AC reverses its direction periodically. The AC supply in India has a frequency of 50 Hz (it changes direction every 1/100 second).

Teacher's Note:
1. A cell or battery is a source of DC and an AC generator is a source of AC.
2. AC can be transmitted over long distances with less loss of energy.

Answer: A fuse is a safety device containing a wire of low melting point. When an unusually high current flows because of a short circuit or overloading, the wire melts and breaks the circuit, which prevents damage to the appliance and wiring.

Teacher's Note:
1. It is connected in series with the live wire.
2. It must be replaced with one of the correct rating after it blows.

Answer: A current-carrying loop produces a magnetic field around it, so it behaves like a magnet. The needle is a small magnet and it experiences a force from this field, which deflects it.

Teacher's Note:
1. Oersted first observed this effect with a straight wire.
2. The deflection reverses if the direction of current is reversed.

Answer: 1. Current: the field increases as the current increases.
2. Number of turns: the field increases with more turns, because the field of each turn adds up.
3. Radius: the field at the centre decreases when the radius of the coil is larger.

Teacher's Note:
1. The field at the centre is directly proportional to current and number of turns.
2. It is inversely proportional to the radius of the coil.

Answer: Stretch the thumb, forefinger and middle finger of the left hand mutually perpendicular. If the forefinger points along the field and the middle finger along the current, the thumb shows the direction of the force on the conductor. The force is maximum when the current is perpendicular to the field.

Teacher's Note:
1. The force is zero if the current is parallel to the field.
2. Reversing either the current or the field reverses the force.

Answer: Electromagnetic induction is the production of an induced potential difference (and current) in a coil when the magnetic field linked with it changes. Fleming's right-hand rule gives the direction: with forefinger along the field and thumb along the motion of the conductor, the middle finger shows the induced current.

Teacher's Note:
1. The change may be caused by relative motion between a magnet and a coil.
2. A steady field with no relative motion induces no current.

Answer: The three wires are the live wire (red), the neutral wire (black) and the earth wire (green). The earth wire connects the metal body of an appliance to the ground, so any leakage current flows to the earth and the user is saved from a severe shock.

Teacher's Note:
1. Newer wiring uses brown for live and blue for neutral.
2. The potential difference between live and neutral wires is 220 V.

Answer: Short circuiting happens when the live and neutral wires touch each other, and overloading happens when too many appliances are connected to one socket. In both cases the current in the circuit becomes very large and may cause a fire. Using a fuse of the right rating and avoiding too many appliances on one socket prevents this.

Teacher's Note:
1. Both cause excessive heating of the wires.
2. An MCB can be used in place of a fuse.

Answer: 1. They emerge from the north pole and merge at the south pole outside the magnet.
2. Inside the magnet they run from south to north, so they are closed curves.
3. No two field lines cross each other.
4. Closer lines mean a stronger field and widely spaced lines mean a weaker field.
5. The tangent at any point on a line gives the direction of the field at that point.

Teacher's Note:
1. The field is strongest near the poles where the lines are crowded.
2. The lines are imaginary but they represent the real field.

Answer: When current flows in a solenoid, its field pattern is like that of a bar magnet, with one end behaving as a north pole and the other as a south pole. Inside, the field lines are parallel straight lines, so the field is uniform. The strength increases if the current is raised, the number of turns per unit length is increased, or a soft iron core is placed inside.

Teacher's Note:
1. The polarity can be found with the right-hand thumb rule applied to the coil.
2. A solenoid with a soft iron core is an electromagnet.