No current, compass points to north Current, compass deflected Electromagnetism No current, compass points to north Current, compass deflected In 1820 Hans Ørsted noticed that a wire carrying an electric current caused a compass needle to deflect. 2

Mag Field ~ long straight Wire B Right Hand Rule #2 I Thumb points with I Fingers curl with B The magnitude of B depends on the distance r from the current: r 0 = 4 x 10-7 Tm/A permeability of free space

Magnetic Field from a Wire What is the direction of the magnetic field created by the current flowing down through this wire? A. B. C. D. E. F.

dot is coming out of the page Since it’s difficult to draw in 3-D, adopt the following symbols: dot is coming out of the page cross is going into the page

Magnetic Field from a Wire What is the direction of the magnetic field created by the current flowing down through this wire? A. B. C. D.

Complete the diagrams below: Add field arrows Add current direction Add current direction Add field arrows Electric current into the page Electric current out of the page 8

+ – A B What is the direction of the magnetic field at points A and B due to the current in the wire? A B Into screen Out of screen Out of screen Into screen Into screen Zero field Out of screen Zero field

A jumper cable used to start a stalled vehicle carries a 65-A current A jumper cable used to start a stalled vehicle carries a 65-A current. How strong is the magnetic field 6.0 cm away from it? I p r A power line carries a current of 95 A along the tops of 8.5 m-high poles. What is the magnitude of the magnetic field produced by this wire at the ground? r I

(roughly the value of earth’s magnetic field) [3]If a wire carries a current of 480 A, how far from the wire will the magnetic field have a value of 5.0 x 10-5 T ? (roughly the value of earth’s magnetic field)

Consider a circular current… and use RHR #2 to determine the direction of the magnetic field at the center of the loop: I B I B B B B x or B B At the center of the loop: Radius of loop

If there are many circular loops: N = number of loops

Solenoid A solenoid is a coil of wire carrying an electric current. The magnetic field is similar in shape to that around a bar magnet. N S 15

The right-hand grip rule (for solenoid) Grip the coil with the RIGHT hand. The fingers are placed in the direction that the current flows around the coil. The thumb points towards the north pole end of the coil. N S 16

L I N B

Complete the diagrams below: 1. Locate north 2. Locate south 3. Add current direction S N N 4. Add coils N 18

Solenoids and Electromagnets inside: x x x x x x x x x x x x B I For a long, ideal solenoid: B = 0n I n = turns/length

Solenoid of Wire with Current If an iron bar was placed inside of this solenoid, which side would be the North pole Right Left Can’t be determined Both Neither

Magnetic Field in a Solenoid What is the direction of the magnetic field created by current flowing this solenoid? A. B. C. D.

Electromagnet Which way would the current need to be flowing in order to make the top end of the iron bar the North pole of the electromagnet? top wire  bottom wire  top wire  bottom wire C. Can’t be determined D. Don’t know

What should be the orientation of the terminals on the battery? Positive on left, negative on right Negative on left, positive on right Field will not look like this at any orientation, it will be out of or into the page.

This solenoid has a soft iron core which is more permeable and as a result, creates a much stronger magnetic field.  As a result, it is a much stronger electromagnet.  the number of turns of wire around a solenoid is directly related to the magnetic field strength of the solenoid. the magnetic field strength of a solenoid is directly related to the current through the coil.

Application: ELECTROMAGNETS A wire is wrapped an ordinary piece of unmagnetized iron so that a solenoid forms with the iron inside of it. The ends of the wire are connected to a DC voltage source. Direct current flows through the wires in the solenoid. A uniform magnetic field forms inside the solenoid. The magnetic domains in the unmagnetized iron align with the magnetic field. The iron behaves like a magnet with its own north and south poles. Application: ELECTROMAGNETS S N 5. The magnetic field is lost when the wires are disconnected from the voltage source (because the current stops).

Electric bell When the push switch is closed current flows around the circuit turning on the electromagnet. The soft iron armature is pulled towards the electromagnet and the hammer hits the gong. This causes the contact switch to open cutting off the electric current. The spring now pulls the armature back again closing the contact switch. Current now flows again and the hammer hits the gong again. push switch spring electromagnet contact switch soft iron armature hammer gong 26

Domestic circuit breakers Current normally flows between terminals A and B through the contact and the electromagnet. When the current in a circuit increases, the strength of the electromagnet will also increase. This will pull the soft iron armature towards the electromagnet. As a result, spring 1 pulls apart the contact and disconnecting the circuit immediately, and stopping current flow. A B 2 1 Domestic circuit breakers The reset button can be pushed to bring the contact back to its original position to reconnect the circuit