Magnetic Forces & Fields Topic 12.1

Outcomes You will describe magnetic interactions in terms of forces and fields You will compare gravitational, electric and magnetic fields (caused by permanent magnets and moving charges) in terms of their sources and directions You will describe how the discoveries of Oersted and Faraday form the foundation of the theory relating electricity to magnetism You will describe, qualitatively, a moving charge as the source of a magnetic field and predict the orientation of the magnetic field from the direction of motion

Magnetic Poles Two types: North pole and South Pole Law of Magnetic Poles: Like poles repel, opposite poles attract

Compasses A compass will point OUT OF THE magnetic NORTH and INTO THE magnetic SOUTH (or in the direction of the magnetic field) The Earth’s North pole is actually the magnetic South pole and the Earth’s South pole is the magnetic North pole

Magnetic Fields A magnetic field is the area surrounding a magnet that exerts a force on other magnets or substances that can be magnetised Magnetic fields, like gravitational and electric fields, are vector fields The direction of the Magnetic Field at any point in space is the direction a compass needle points when placed in the field at that point (towards the “north” or MAGNETIC south) “Out of North and into South”

Magnetic Fields

Comparing Magnetic & Electric Fields

Oersted & Electromagnetism Hans Christian Oersted discovered that a current carrying wire influenced the orientation of a compass needle Discovered the relationship between electricity and magnetism An electric current (moving charges - electrons) will produce a magnetic field *Current in a straight line caused = circular magnetic field *Current in a circular fashion = straight magnetic field

Thumb = current Fingers = magnetic field HAND RULE #1 This rule is for straight current-carrying conductors: Fingers wrap around the conductor with the thumb pointing in the direction of the current. For electron flow, use your left hand. For conventional current (positive particle) use your right hand. Thumb = current Fingers = magnetic field “out of N. into S.”

HAND RULE #1 Draw the magnetic field around the current-carrying wires below.

HAND RULE #2 Solenoid: Determines the direction of the magnetic field inside the solenoid. To use this rule, your fingers wrap around the solenoid in the direction of the current and your thumb points in the direction of the magnetic field inside the solenoid (direction of north pole) For electron flow, use your left hand. For conventional current (positive particle) use your right hand.

HAND RULE #2

Interaction of Current Carrying Conductors Current flowing in opposite directions through parallel conductors will create a force that repels the conductors. This is because the magnetic fields between the conductors point in the same direction The opposite is true for current flowing in the same direction

WORKBOOK USES CONVENTIONAL CURRENT!!! Homework p.592 # 2, 5-10 p.55 #1-10 (workbook) WORKBOOK USES CONVENTIONAL CURRENT!!!