Magnetic field lines around a bar magnet form closed curves. Outside the magnet they emerge from the north pole and enter the south pole; inside the magnet they go from south pole to north pole. The lines are crowded near the poles where the magnetic field is strongest.
Magnetic field lines are closed continuous curves. Outside a magnet they go from north pole to south pole, and inside the magnet from south pole to north pole. The tangent at any point gives the direction of the magnetic field. The closer the lines, the stronger the field. No two magnetic field lines intersect.
If two magnetic field lines intersected, the magnetic field at the point of intersection would have two directions, which is impossible. Therefore magnetic field lines never intersect.
Using the right-hand rule, if the current appears clockwise when viewed from above the table, the magnetic field inside the loop is directed downward, into the table. Outside the loop, the field lines return upward, out of the table.
A uniform magnetic field is represented by straight, parallel and equally spaced magnetic field lines, all pointing in the same direction.
- 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.
The magnetic field inside a long solenoid is nearly uniform.
(d) is the same at all points.
- a. mass
- b. speed
- c. velocity
- d. momentum
A magnetic field changes the direction of motion of a charged particle, so velocity and momentum change. It does not change mass or speed.
(c) velocity and (d) momentum
The displacement of rod AB increases in all three cases. The magnetic force on a current-carrying conductor increases when current increases, when the magnetic field is stronger, and when the length of the conductor in the field is greater.
- a. towards south
- b. towards east
- c. downward
- d. upward
For a positive charge, force is in the direction of v × B. With velocity towards west and force towards north, Fleming's left-hand rule gives magnetic field upward.
(d) upward
Two common safety measures are electric fuses and earthing of metallic appliances.
Current required by the oven is I = P/V = 2000/220 = 9.1 A approximately. This is greater than the circuit rating of 5 A, so the circuit is overloaded.
The circuit will be overloaded and the fuse may blow or the circuit breaker may trip.
Do not connect too many appliances, especially high-power appliances, to the same socket or circuit at the same time. Use proper fuses/circuit breakers and separate circuits for high-power appliances.
- 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.
Magnetic field lines around a current-carrying straight conductor are concentric circles centred on the wire.
(d) The field consists of concentric circles centred on the wire.
- a. reduces substantially.
- b. does not change.
- c. increases heavily.
- d. vary continuously.
During a short circuit, resistance becomes very low, so current increases sharply.
(c) increases heavily.
(a) True.
(b) False. The green insulated wire is usually the earth wire, not the live wire.
Magnetic fields can be produced by permanent magnets and by electric current flowing through conductors such as wires, loops or solenoids.
The force is largest when the direction of current is perpendicular to the direction of the magnetic field.
The magnetic field is vertically downward. Since electrons are negatively charged, the force on them is opposite to that predicted for conventional current; applying Fleming's left-hand rule gives a downward magnetic field.
(i) Right-hand thumb rule: hold the conductor in the right hand with thumb in the direction of current; curled fingers show the direction of magnetic field lines.
(ii) Fleming's left-hand rule: stretch thumb, forefinger and middle finger of the left hand mutually perpendicular; forefinger gives field, middle finger gives current and thumb gives force/motion.
(iii) Fleming's right-hand rule: stretch thumb, forefinger and middle finger of the right hand mutually perpendicular; forefinger gives field, thumb gives motion and middle finger gives induced current.
A short circuit occurs when the live wire and neutral wire come into direct contact, or when resistance in a circuit becomes very low. This causes a sudden large current to flow.
The earth wire provides a low-resistance path for leakage current to flow safely into the ground. Metallic appliances are earthed so that if the live wire touches the metal body, current flows to earth and the fuse blows, preventing electric shock.