Magnetism

Synopsis on Magnetism and Electromagnetism Magnetic field – the area over which a magnetic force exists, designated by magnetic field lines) Field lines – a diagram of continuous lines drawn to illustrate the direction of a force on a test mass, or charge (also called lines of force).Like magnetic poles repel; unlike poles attract. A charged particle moving in a magnetic field is subject to a force given by . Ampere: When the attractive force per unit length between two long parallel wires placed one meter apart and carrying the same electric current is N/m, that current is defined to be exactly one ampere. Right-Hand Rules: The direction of the magnetic field about a wire is given by a right-hand rule: If the thumb of the right hand is taken as the direction of current, the curled fingers point in the direction of the magnetic field about that current. Fleming’s left Hand Rule for motor: First finger - direction of magnetic field (N-S) SeCond finger - direction of current (positive to negative) ThuMb movements of the wire or Force Fleming’s right hand rule for generator- The Thumb represents the direction of Motion of the conductor. The First finger represents the direction of the Field. The Second finger represents the direction of the induced or generated Current (in the classical direction, from positive to negative). Properties of magnets- 1.Magnet attracts objects of iron, cobalt and nickel. , 2.The force of attraction of a magnet is greater at its poles than in the middle. ,3. Like poles of two magnets repel each other. ,4. Opposite poles of two magnets attracts each other. ,5. If a bar magnet is suspended by a thread and if it is free to rotate, its South Pole will move towards the North Pole of the earth and vice versa. Characteristics of magnetic lines of force- Magnetic lines of force start from the North Pole and end at the South Pole. , 1.. They are continues through the body of magnet, 2.. Magnetic lines of force can pass through iron more easily than air. 3. Two magnetic lines of force can not intersect each other. 4. They tend to contract longitudinally. 5. They tend to expand laterally. Important parts of an electric bell are: 1. Electromagnet 2. Armature 3. Spring 4. Armature rod 5. Hammer

6. Gong

Galvanometer-It is an electromechanical instrument which is used for the detection of electric currents through a circuit. Being a sensitive instrument, Galvanometer can not be used for the measurement of heavy currents. There are five essential parts of a Galvanometer. 1. A U-shaped permanent magnet with concave poles. 2. Flat rectangular coil of wire. 3. A soft iron cylinder. 4. A pointer or needle. 5. A scale. Ammeter is an electrical measuring device, which is used to measure electric current through the circuit. An ammeter is always connected in series to a circuit Voltmeter is an electrical measuring device, which is used to measure potential difference between two points in a circuit Voltmeter is always connected in parallel to a circuit Generators – machines that convert mechanical energy into electrical energy Electric motor – an apparatus that converts electrical energy into mechanical energy

M.C.Qs on Magnetism and Electromagnetism 1. The _____________ rule gives us the direction of the magnetic moment produced by a current loop. a. clockwise , b, thumbs, c. left-hand, d. right-hand 2. An __________ is a device for measuring currents. a. ammeter , b. Coulomb meter ,c. voltmeter ,d. altimeter 3. An ________________ is a current measuring device that uses the magnetic field produced by the current being measured to deflect a magnetized compass needle. a. calorimeter ,b. galvanometer , c. potentiometer, d. Coulomb gauge 4. Heating a magnet will a. weaken it b. strengthen itc. have no effect d. produces new poles. 5. The region around a magnet in which a magnetic effect can be experienced is called a. electric field b. gravitational field c. magnetic field d. electromagnetic field. 6. When current passes through a straight wire the magnetic fields is a. straight b. circular around the wire c. opposite to the direction of current d. perpendicular to the direction of current 7. When an electron traveling in a uniform magnetic field it experience a. no force b. voltage c. a force d. current 8. Mass of an electron in kg is a. 91x10-27 b. 1x10-27 c. 9.1x10-31 d. 0.911x10-27 9. With in the magnet the magnetic lines of forces are from a. south to north b. north to south c. north to north d. south to south 10. Magnetic lines of forces are stronger in a. water b. iron c. air d. wood 11. Parallel magnetic lines of forces are ___________ curved magnetic lines of forces a. stronger than b. weaker than c. same as d. cancelled 12. In electric bell we use a. permanent magnet b. electromagnet c. no magnet d. all are correct 13. Electric bell is an example of a. electrochemical device b. electromagnetic device c. electro couple device d. mechanical device 14. Electrical bell is operated with a. Alternating current b. direct current c. pulsating current d. all are correct 15. When the charge passes through the uniform magnetic field the magnitude of the force experience by the charge is a. F=qvBsinθ b. F=qvBcosθ c. F=vBsinθ d. F=qvsinθ

16. Galvanometer is a device by which we can measure a. high current b. high voltagec. low current d. low voltage

17. To modify the galvanometer into __________resistance in parallel a. small value b. high value c. shunt

an

ammeter

we

have

to

connect

d. series resistor

18 To modify the galvanometer into a voltmeter we have to connect a resistance in a. parallel b. high value in series c. shunt d. . low value in series 19. Ammeter is used to measure . a. high current b. high voltage c. low current d. low voltage 20. Voltmeter is used to measure a. high current b. high voltage c. low current d. low voltage 21. To measure the voltage across the resistor we have to connect the voltmeter a. in series b. in series c. first parallel then series d. first series then parallel 22. To measure the current through the resistor we have to connect the ammeter a. in series b. in series c. first parallel then series d. first series then parallel 23. The direction of the current in two straight wires are opposite to each other the wires will a. repel each other b. repel each other c. first attract then repel d. first repel then attract 24. When the magnet placed in the core of the solenoid it is probable that a. magnet may demagnetized b. magnet may become strong c. no change d. (a) & (b) are both correct 25. When the current passes through the solenoid the magnetic field will be a. strong inside b. weak inside1 c. strong outside d. zero 26. When the current is straight the magnetic field will be a. straight parallel to the current b. circular around the wire c. zero d. all are correct 27. When the current is moving in circular path the magnetic field will be a. straight b. parallel to the current c. circular d. zero 28. Motor works on the principle in which we convert a. electrical to mechanical b. mechanical to electrical c. electrical to electrical d. mechanical to mechanical 29. Generator works on the principle in which we convert a. electrical to mechanical b. mechanical to electrical c. electrical to electrical d. mechanical to mechanical 30. The field in which we study the relation between electricity and magnetism is called a. magnetism b. electromagnetism c. electricity d. electrodynamics