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Magnetism and Electromotive Force. Magnets. General Properties of Magnets. 1. have polarity, north and south poles 2. poles attract is they are different and repel if they are alike – like electric charges. Magnetic Fields. 1. Field lines are drawn to represent the magnetic field.
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General Properties of Magnets 1. have polarity, north and south poles 2. poles attract is they are different and repel if they are alike – like electric charges
Magnetic Fields 1. Field lines are drawn to represent the magnetic field. a. Field lines come out of the North pole of a magnet (field lines are away from positive charges) b. Field lines go into the South pole of a magnet (field lines are toward negative charges) c. Field lines are greatest in number at the poles
N S Field of a Permanent Magnet
The Earth’s Magnetic Field • The northern hemisphere of the Earth contains the south pole of the “Earth magnet”. • The Earth's magnetic field is similar to that of a bar magnet tilted 11 degrees from the spin axis of the earth. • The Earth's core is not magnetic. So how did the Earth get its magnetic field? • Magnetic fields surround electric currents, so we surmise that circulating electric currents in the Earth's molten metallic core are the origin of the magnetic field. • A current loop gives a field similar to that of the earth.
More magnetic properties 3. Permanent magnets a. do not lose their polarity b. AlNiCo – aluminum, nickel, cobalt 4. Non-permanent magnets a. can be magnetized by induction b. iron, cobalt, nickel alone
Domains At the atomic level, we see that magnetism is produced due to the alignment of the domains of groups of atoms. Before magnetism After magnetism
. Superconductors repel magnets by expelling the field lines from their surfaces. There is no magnetic flux inside a superconductor.
Two Electromagnetism Laws Ampere’s Law • Electrical currents create magnetic fields • Right Hand Rule Faraday’s Law • Changing magnetic fields in a conductor generates electricity • Currents are induced in conductors when the conductors either… (a) move in a magnetic field or (b) are in the presence of a changing magnetic field.
Electromagnetism Oersted (Denmark, 1800’s)showed magnetic effects could be created by moving charges. (Current carrying wires are surrounded by magnetic fields.) Using the first right hand rule, we can see why current carrying wires will attract if the current is traveling in the same direction in both wires and repel if the current is traveling in opposite directions.
The First Right Hand Rule The direction of the magnetic field can be established by using the first right hand rule. a. Place your right hand on the wire with your thumb extended in the direction of conventional current flow (positive charges move) b. Curl you hand around the wire. Your fingers now point in the direction of the magnetic field. c. The magnetic field goes around the wire.
The right-hand rule gives the direction of the magnetic field (N to S).
The Second Right Hand Rule The North pole around a coil can be determined by using the second right hand rule. a. Curl the fingers around the coil pointing in direction of current flow. b. The direction your thumb points is the direction of the North pole
Magnetic Field Near a Coil Electromagnets can be strengthened by adding a soft iron core which becomes magnetized when current is turned on. A coil of current carrying wire has a magnetic field making it an electromagnet.
Forces Caused by Magnetic Fields Faraday discovered that the force on a current carrying wire in a magnetic field is at right angles to both the current and the magnetic field. The direction of the force on the wire can be determined using the third right-hand rule.
The Third Right Hand Rule Using an open right hand, point the thumb in the direction of the current flow and the fingers in the direction of the magnetic field. The force is visualized as coming out of the palm of the hand
The magnitude of the force can be calculated using the equation: F = BIL The strength of the magnetic field (magnetic induction) is calculated using the equation: B = F/IL
Sample Problem If a wire bearing a current of 10 A lies perpendicular to a uniform magnetic field and a force of 0.2 N is found to exist on a section of that wire 80 cm long, what is the magnitude of the magnetic field? • F = BIL • B = F/IL • B = 0.2 N/ (10 A x 0.8 m) • B = 0.025 T
Magnetic Field Strength Magnetic field strength (actually the density of the flux lines ) is measured in teslas (T), equivalent to 1 N/A·m (1 T is very large.)
Strengths of various magnetic fields. • 1. The strength of the earth’s magnetic field is about 5 x 10-4 T • 2. A lab magnet is about 0.01 T • 3. Only the most powerful of electromagnets have a field strength of 1 T • 4. Surface of a neutron star 108 T • 5. In interstellar space 10-10 T
The Earth’s Magnetic Field Earth’s magnetic field (1) 5 x 10-4 T (2) points into the earth in the northern hemisphere
Devices which incorporate the effect of magnetic fields on current carrying wires Loudspeakers – a coil of current carrying wire is forced into and out of a magnetic field which causes a paper cone to vibrate Galvanometers a. measure very small amounts of current b. may measure as little as 50 A c. can be converted to ammeters or voltmeters Electric Motors – use a loop which can rotate 360 as a result of changing the direction of the current just as the loop reaches vertical
Force on a Single Charged Particle The force of the magnetic field on current carrying wires is a result of the force exerted on individual charges. Cathode ray tubes use this concept to direct electrons to the inside surface of the screen where they strike the phosphor producing a picture.
This force can be calculated using the formula F = Bqv The direction of the force on the particle can be predicted using the third right hand rule by extending the thumb in the direction of the velocity of the positive charge.
1. The force is perpendicular to the velocity of the particle and the magnetic field. 2. Charges moving parallel to the magnetic field experience no force. 3. Maximum force is experienced when the particle is moving perpendicular to the field 4. The magnitude of the force is directly proportional to q and to v 5. The direction of the force depends upon the sign of the charge.
Sample Problem A He2+ ion travels at right angles to a magnetic field of 0.80 T with a velocity of 105 m/s. Find the magnitude of the magnetic force on the ion. F = Bqv F = (0.80 T)2(1.6 x 10-19 )(105 m/s) F = 2.56 x 10-14 N
Sample Problem A proton moves perpendicularly to a magnetic field that has a magnitude of 4.2 x 10-2 T. What is the speed of the particle if the magnitude of the magnetic force on it is 2.4 x 10-14 N? F = Bqv so v = F/Bq V = (2.4 x 10-14 N)/[(4.2 x 10-2 T)(1.6 x 10-19 C)] V = 3.57 x 106 m/s
INDUCTION: Creating Electric Current from Changing Magnetic Fields 1. Michael Faraday and Joseph Henry showed that current could be produced by moving magnets. 2. Electromagnetic induction is the production of potential difference by moving a wire through a magnetic field.
Faraday’s Law of Induction • A loop of wire is connected to a current meter… • If a magnet is moved toward or away from the loop, a current is measured. • If the magnet is stationary, no current exists. • If the magnet is stationary and the wire loop is moved, a current is measured. \A current is set up in the loop as long as there is relative motion between the magnet and the loop.
Electromotive Force 1. EMF is not a force but rather an increase in potential and is measured in volts. 2. EMF can be calculated using the equation EMF = BLv
Devices which incorporate the concept of magnetic fields producing current 1. Microphones – movement of a diaphragm causes a wire to move into and out of a magnetic field inducing current. 2. Electric generators a. convert kinetic energy into electrical energy. b. mechanical movement of an armature inside a magnetic field causes current to flow within the wire.
