Comprehension Check 1. An electron is moving at right angles to uniform magnetic field; if the electron is moving at.010c, determine the magnitude of the force if: 1.B = nT (~Earth’s magnetic field) 2.B = 5.0mT (~fridge magnet) 3.B = 1.0T (~rare earth magnet) 4.B = 3.0T (~superconducting magnet)

Comprehension Check 1. F = 2.5x N 2. F = 2.4x N 3. F = 4.8x N 4. F = 1.4x N

Charged Particle Applications and Torque Physics 12

Force on a Current Carrying Wire in a Magnetic Field Take the equation for the force experienced by a charged particle moving in a magnetic field and the definition of current to develop an equation for a current carrying wire in a magnetic field

Torque Torque is the cross product of radius and force The units are Nm but are not joules! The direction is positive for ccw and negative for cw

Torque on a Current Carrying Loop

Galvanometer The force experience by a current carrying wire in a magnetic field can be used to build a galvanometer Based on the length of wire, the strength of the field, the tension in the spring and the current, different readings are obtained

Charged Particle placed in a B- Field When a charged particle is placed in a magnetic field, it experiences a force based on the cross product of its velocity and the magnetic field intensity Therefore, a charged particle experiences no force if it is not moving

Circular Motion When a charged particle is moving in a magnetic field, it always experiences a force that is at right angles to the velocity This results in a change in the direction of the velocity but not its magnitude As a result, this force will provide a centripetal acceleration towards the centre of the circular path

How can we calculate centripetal acceleration?

Centripetal Force Like the centripetal acceleration, the centripetal force is always directed towards the centre of the circle The centripetal force can be calculated using Newton’s Second Law of Motion

Charged Particle moving in an B- Field A charged particle, moving with an initial velocity enters a magnetic field as shown in the diagram at the right and will follow a circular path as a result of the b-field We can solve this problem through the use the centripetal motion

Lorentz Force and Velocity Selector The Lorentz Force gives the method of calculating the total force acting on the charged particle When the velocity is equal to the ratio of E/B, the particle passes straight through the fields, otherwise it is deflected

Mass Spectrometer A mass spectrometer can be built using a velocity selector followed by an area of uniform magnetic field. Particles that have the correct velocity will pass through the velocity selector and into the magnetic field where they will be curved. Develop an equation that will allow you solve for the ratio m/q based on the magnetic field, speed of the particle and radius of curvature

e/m Thompson used a similar technique to measure the ratio of charge to mass ratio of electrons Using variable electric and magnetic fields, it is possible to investigate the charge of the electron in comparison to its mass

Cyclotron In a cyclotron, a charged particle is accelerated in an alternating electric field and passes into an area of uniform magnetic field which turns it in a circle and it passes back into the magnetic field As velocity increases, the radius gets larger and larger