Electromagnetic Forces and Fields Lesson 2 : Wires carrying current close to each other.

Magnetic Flux Density A – higher density of field lines, greater B, greater Fm B – lower density of field lines, lesser B, lesser Fm Greatest density of field lines is near the poles of a magnet A B

Magnetic lines of force are continuous The direction of a magnetic field line outside a magnet is N to S and inside is from S to N

Earth’s Magnetic Field The Earth’s magnetic field is similar to a bar magnet The field is the result of the fluid motion of iron within the earth’s core (read text for new theories)

Magnetic Fields in Solenoids When current flows through each loop of wire in a coil a magnetic field is set up in such a way that it makes the coil act like a single bar magnet. - the magnetic field around a straight wire is very weak - to make the field stronger, we can bend the wire into many loops

Solenoid the current goes in the same direction for each wire the magnetic field of each wire adds up to create a much stronger field

Second Left Hand Rule Fingers curl around the coil in the direction of the electron flow. Outstretched thumb points to the end of the coil that acts as the north end of a fixed magnet.

Example: Is the direction of the current shown, e¯ flow or conventional current?

Solution Need to use right hand to B field direction as shown: ∴ is conventional current.

Example Determine position of each compass

Solution

Electromagnets A solenoid can be made to be a much stronger magnet by placing a ferromagnetic material (usually steel) inside the coil. Electromagnets have many uses. Magnetizing ferromagnetic material Bells Relay switches

Electric Bell

Electromagnetic Relay Changes current to another circuit

Magnetic Effects of Current Ampere investigated the interaction between two current-carrying wires parallel to each other. What is the effect on the wires?

Answer The direction of the fields is different therefore the fields attract each other. If the field directions are the same they would repel.

Current Current is the rate at which e¯ flow through a wire. Units: C/s or ampere (A) Symbol: I

Current and B field The B field around a current carrying wire depends in part on the current in the wire. F = magnetic force (N) I = current (A) or (C/s) l = length (perpendicular to B) B = magnetic field strength in tesla (T)

Definition of an Ampere If two wires, each one meter long and one meter apart, carry current so that the force between the two wires is 2.00 x 10-7 N, the current in each wire is one ampere.

Example A 30 cm long wire is placed in a 0.50 T B field that is directed out of the page. The e¯ flow is to the right, with a current of 1.2 A. Determine the force acting on the wire.

Solution What is the direction of the force?

3rd Left Hand Rule

Example Diagram