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Magnetic fields and electric currents A magnetic field circulates around a current-carrying wire.

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Presentation on theme: "Magnetic fields and electric currents A magnetic field circulates around a current-carrying wire."— Presentation transcript:

1 Magnetic fields and electric currents A magnetic field circulates around a current-carrying wire.

2 Moving charges create a magnetic field Right hand rule to get direction of field around an electric current

3 Magnetic fields and electric currents Tiny iron filings align with the magnetic field to show the field lines around these current-carrying wires.

4 Moving charges create a magnetic field Solenoid – use right hand rule to get direction of field around electric current in wire loops

5 Magnetic fields and electric currents A coil of current carrying wire is called a solenoid. The magnetic field lines circle the wire and cancel in the spaces between the wires. Out side and inside the coil, a strong magnetic field is produced where the field lines get closer together.

6 Electromagnets By placing ferromagnetic material inside a coil of wire, we can make the magnetic field inside the solenoid even stronger. When the current is turned on, this configuration becomes an electromagnet.

7 Force on a moving charge (Lorentz force) The force that a magnetic field exerts on a moving charge is perpendicular to the direction of the charge, and also perpendicular to the direction of the field. So if a proton were moving in the direction shown, which way would the magnetic field deflect it? Direction of magnetic field (B) The Lorentz force is zero if the magnetic field is parallel or anti-parallel to the velocity

8 Force on a current carrying wire Electrons moving through a wire that is in a magnetic field also feel a force (recall that the direction of the current is the direction that positive charges move, or opposite to the direction that electrons move). In that case, the current-carrying wire feels a force exerted on it. N N

9 Electric motors A magnetic field exerts a force on a current loop, at right angels to the direction of the field and to the direction of the current.

10 Electric motors As the current loop turns, the magnetic field continues to exert a force on it.

11 Electric motors When the loop turns far enough, the force would work to stop turning it, so the polarity of the current is switched to run the other way. This makes the force on the loop such that it continues to turn. This is the basic mechanism of an electric motor.

12 Electric motors A simple electric motor operates using switching magnetic fields to push and pull a rotator with magnets affixed to it.

13 Motor demos Homopolar motor - demo Battery and generator as a motor

14 Magnetic field of the Sun The magnetic field is evident in this photo from the sun's surface. Remember that magnetic fields exert a force on moving charged particles that deflects them perpendicular to the field. This causes them to spiral along the strong magnetic field lines of the sun, mapping out the field as they release light.

15 Magnetic field of the Sun The solar wind contains charged particles which distort Earth's magnetic field. The magnetic field of Earth deflects charged particles in the solar wind. Huge magnetic storms on the sun can hurl gigantic clouds of plasma into space. If they happen to come our way, they can cause problems like surges in power grids and breakdown of satellite communication.

16 Aurorae Charged particles in the solar wind can collide with particles in Earth's atmosphere, especially near the north and south magnetic poles. When they do, they excite atoms which then return to ground state, emitting light. We see the eerie streaming flows of color that result. They are called the aurora borealis and aurora australis for the northern and southern phenomena respectively.

17 Aurorae The colors of the aurorae change with height in the atmosphere, depending on the type of gas found at that height.

18 Magnetism history People used to think electricity and magnetism were the same, and sometimes students conflate or confuse positive/negative & north/south. Some distinctions: A non-neutral net charge produces an electric field. Moving charges produce a magnetic field. An electric field always exerts a force on a charge in a direction parallel to the field. A magnetic field only exerts a force on a moving charge, and the force is perpendicular to the direction of the field. Single electric (positive or negative) charges exist. Single magnetic poles do not seem to exist.

19 The distinction between electricity and magnetism depends on your frame of reference Lab frame of reference: Consider a neutral, current-carrying wire containing moving electrons (producing a magnetic field and no electric field), with a single electron moving parallel to the wire at the same speed as the drift velocity (v) of the electrons in the wire. There is no electrical force on the electron (because the wire is neutral), but there is a magnetic force pushing the the electron towards the wire. Electron (moving) frame of reference: In the frame of reference of the electron outside the wire, it feels no magnetic field. But according to special relativity, in the frame of reference of the electrons drifting, the spacing between the protons in the wire is less (length contraction), and so the positive charge density is greater than the negative charge density. There is an overall positive charge in the wire, so an electric field attracts the electron to the wire. v current e v = drift velocity of electrons in wire above wire, B is into page


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