Magnetic Fields Starter questions Draw the magnetic field around a bar magnet Draw the magnetic field between and around two north poles brought close together. Draw the magnetic field between and around a north and a south pole brought close together. Draw the magnetic field through and around a solenoid (coil of wire) Use the right hand grip rule to draw the magnetic filed around a straight current carrying wire if the current is flowing for top to bottom. From these diagrams, what tells us if a magnetic field is strong or weak? What happens to a current carrying wire if it is placed in a magnetic field? What do you know about Fleming's left hand rule? What is magnetic flux and magnetic flux density?

Magnetic Fields Magnetic field lines can be represented by field lines. The field lines go from north to south and the closer together they are the stronger the field.

Current Carrying Wire When current flows through a wire, a magnetic field is created around the wire. This magnetic field forms concentric circles around the wire. The closer the circles are together, the stronger the field. To identify the direction of the magnetic field around a wire we use the RIGHT HAND RULE

Solenoid Magnetic Field If a current carrying wire is wrapped around to make a solenoid then the magnetic field is the same as that for a bar magnet.

Looking Through a Solenoid

Magnetic Field directions

Flemings Left Hand Rule If a wire carrying a current is placed in a magnetic field, the wire experiences a force on it. If the wire is at right angles to the magnetic field then it receives a maximum force. If the wire is parallel with the magnetic field then it receives no force. The direction of the force is identified by using Flemings left hand rule.

Let's practice!

Calculating the Force F = BIL sin θ The size of the force on the wire can be calculated by: F = BIL sin θ Where F = force on wire (N), B = magnetic field strength (T), I = current (A), L = length of wire(m) and θ = angle between magnetic field lines and wire. If wire and magnetic field are at right angles sin θ = 1 TASK : Using F = BIL derive a word definition for the Tesla

Definitions One Tesla, T is the magnetic flux density when a wire of length one metre and carrying a current of one ampere at a right angle to the field experiences a force of one newton. Magnetic Flux Density,B is defined by the equation F = BIL sin θ: Where F = force on wire of length L carrying a current I at an angle θ to the field. Magnetic field strength is also called Magnetic Flux Density It is a vector as it has magnitude and DIRECTION http://www.magnet.fsu.edu/

Practice Questions 1) A 4cm length of wire carrying a current of 3A runs perpendicular to a magnetic field of strength 2 x 10-5 T. a) calculate the force on the wire b) If the wire is rotated so that it is at 30 degrees to the direction of the field - what would the size of the force be? 2) Two wires are placed in a uniform magnetic field of 0.25T. Both wires are 0.5 m long and carry a current of 4A. One wire is at right angles to the direction to the magnetic field and the other is at an angle of 40 degrees. Calculate the size of the force on each wire.

Charged Particles in a Magnetic Field Current is the flow of negatively charged particles, therefore if a current carrying wire experiences a force in a magnetic field so to will charged particles. This force can be calculated by: F =BQv Where F = force (N), B = magnetic field strength (T), Q = charge (C), v = velocity (m/s) The force on a moving charge in a magnetic field is a circular path. BQv = mv2/r (centripetal force)

Practice Exam Questions Qu 4 on page 109 of course text book