Topic 6.3 Magnetic Force and Field. 2 hours

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Linda Letitia Cummings
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1 Topic 6.3 Magnetic Force and Field 2 hours 1

2 Magnetic Fields A magnetic field is said to exist at a point if a compass needle placed there experiences a force. The appearance of a magnetic field can be obtained with the use of iron filings or plotting compasses. The direction of the field is given by the direction that the compass needles point. The figure below demonstrates the use of iron filings and plotting compasses to detect the magnetic fields of a bar magnet and bar magnets used in combination. The compass needles shown for the single bar magnet point along the lines of magnetic flux. The magnetic flux through a given surface is proportional to the number of magnetic B field lines that pass through the surface. The magnetic field produced for the two like poles have no magnetic field at some point P. If there are no lines of magnetic flux, there is no magnetic field. 2

3 Magnetic Field LInes Magnetic field lines leave at the north pole of a bar magnet and enter at its south pole. 3

Electricity and Magnetism The Danish physicist, Hans Christian Oersted (1777 1851), in 1819, showed conclusively that there existed a relationship between electricity and magnetism.

4 Electricity and Magnetism The Danish physicist, Hans Christian Oersted ( ), in 1819, showed conclusively that there existed a relationship between electricity and magnetism. He placed a magnetic needle on a freely rotating pivot point beneath and parallel to a conducting wire. He aligned the compass needle and wire so that it lay along the earth s magnetic north south orientation. When no current was flowing in the wire, there was no deflection in the needle. However, when the current was switched on, the needle swung to an east west direction almost perpendicular to the wire. When he reversed the direction of the current, the needle swung in the opposite direction. 4

Up to this stage, all forces were believed to act along a line joining the sources such as the force between two masses, the force between two charges or the force between two magnetic poles.

5 Up to this stage, all forces were believed to act along a line joining the sources such as the force between two masses, the force between two charges or the force between two magnetic poles. With Oersted s findings, the force did not act along the line joining the forces but rather it acted perpendicular to the line of action. On closer examination and analysis, it was determined that the conducting wire produced its own magnetic field. The magnetic needle, upon interaction with the conducting wire s magnetic field, turns so that it is tangentially (not radially) perpendicular to the wire. Therefore, the magnetic field produced by the conducting wire produces a circular magnetic field. 5

6 The Right Hand Rule for a Current Carrying Wire 6

7 The Magnetic Field Around a Current Carrying Loop 7

8 The Magnetic Field Around a Current Carrying Loop 8

9 The Magnetic Field Around a Solenoid 9

Solenoids The strength of the magnetic field inside a solenoid can be increased by: 1. Increasing the current flowing. 2. Increasing the number of coils. 3. Inserting a soft iron core in the coil.

10 Solenoids The strength of the magnetic field inside a solenoid can be increased by: 1. Increasing the current flowing. 2. Increasing the number of coils. 3. Inserting a soft iron core in the coil. When a soft iron core is inserted into a solenoid and the current is switched on, an electromagnet is produced. If the current is switched off, the solenoid loses its magnetic properties. We say it is a temporary magnet in this case. 10

11 Right Hand Rule for a Current Loop or Solenoid 11

12 Force on a Current Carrying Wire Suppose a long straight current carrying wire is hung perpendicular to the direction of the magnetic field between the poles of a U shaped magnet, as shown below. If a conventional current is then allowed to flow in the wire in a downwards direction, the wire experiences a force and it tends to want to be catapulted out of the magnet. This is known as the motor effect and this effect is put to practical use in electric motors. 12

13 The Motor Effect The reason for the movement is due to the interaction of the two magnetic fields that of the magnet and the magnetic field produced by the current carrying wire. If the current was reversed, then the wire would be catapulted inwards. 13

14 The Right Hand Rule for Force on a Current Carrying Wire 14

15 And now some math When an electric current flows in a conductor, and the conductor is placed in a magnetic field, the force on the conductor is due to the individual forces on each of the individual charges in the conductor. The magnitude of the magnetic force F is found to be directly proportional to: 1. the strength of the magnetic field B measured in teslas (T) 2. the current flowing in the wire I measured in amperes (A) 3. the length of the conductor in the magnetic field L measured in metres (m). So that F = BIL 15

16 Field, Force, and Current This force is greatest when the magnetic field is perpendicular to the conductor. Sometimes the wire in the magnetic field is at an angle θ to the magnetic field. In this case F = BIL sinθ Therefore, as θ decreases, so too does the force. When θ = 0 the current in the conductor is moving parallel to the magnetic field and no force on the conductor occurs. 16

17 Field, Force, and Charged Particles Given that Recall that current is charge per unit time and speed is distance per unit time, now we get Which is usually written as 17

18 Example An electron is moving with a speed of m s 1 in a direction that is at right angles to a uniform magnetic field of T. Calculate a. the force exerted on the electron. b. the radius of the path of the electron. 18

Section 3: Mapping Magnetic Fields. In this lesson you will

Section 3: Mapping Magnetic Fields In this lesson you will state the Law(s) of magnetic forces use iron filings to map the field around various configurations of bar magnets and around a horse shoe magnet