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Magnetism Can you read through last lessons slide show? Magnetism ALL magnets have two poles NORTH seeking pole SOUTH seeking pole Breaking a magnet produces two magnets! N N S N S S N S Opposites attract! Opposite poles attract and like poles repel Magnetic materials Magnetic materials Iron (steel), Cobalt and Nickel Magnetic induction Magnetic induction When a magnetic material is close to a magnet, it becomes a magnet itself magnet S N S We say it has induced magnetism N Hard and Soft Magnetism Soft Magnetism Purebefore iron is a soft magnetic material after S N S N S NN Not a magnet Iron nail It is easy to magnetise but loses its magnetism easily Hard Magnetism Steelbefore is a hard magnetic material after S N S S NN S N Steel paper clip It is harder to magnetise, but keeps its magnetism (it is used to make magnets!) N It’s a magnet! Magnetic field Magnets and electric currents produce magnetic fields around them. In a magnetic field, another magnet, a magnetic material or a moving charge will experience a magnetic force. www.physchem.co.za Magnetic field lines We can represent the magnetic field around a wire or magnet using field lines. Magnetic field lines The arrows show the direction a compass needle would point at that point in the field. Magnetic field lines The arrows show the direction a compass needle would point at that point in the field. The closer the field lines are, the stronger the magnetic force felt Note that magnetic field is a vector quantity Earth’s Magnetic Field Remember the North of a compass needle points to the geographic north pole (i.e. the geographic North pole is a magnetic south pole!) Moving charges (currents) Moving charges (electric currents) also produce a magnetic field Conventional current – electrons flow in the opposite direction http://www.sciencebuddies.org Magnetic field around a straight wire Stronger field closer to wire Magnetic field around a flat circular coil http://physicsed.buffalostate.edu Magnetic field around a solenoid The Motor Effect When a current is placed in a magnetic field it will experience a force. This is called the motor effect. The Motor Effect The direction of the force on a current in a magnetic field is given by Flemming’s left hand rule. Thumb = Motion First finger = Field direction Centre finger = Conventional Current Sample question In this example, which way will the wire be pushed? (red is north on the magnets) Another sample question! An electron approaches a bar magnet as shown. What is the direction of the force on the electron? S N D.C.Motor Commutator ensures that every half rotaion the current direction reverses in the coil D.C.Motor Defining Magnetic Field B The size of the force on a wire in a field depends on the size of the field (B), the length of wire in the field (L) and the current in the wire (I) Defining Magnetic Field B In other words , F α BIL, or F = kBIL Defining Magnetic Field B F = kBIL We can make k = 1 by defining the Tesla as the magnetic field when the force on 1 m of wire carrying a current of 1 A is 1 N. Force on a current in a field Thus the force on a length L of wire carrying a current I in a magnetic field B is given by F = BILsinθ where θ is the angle between the current and the magnetic field. The force on a moving charge in a magnetic field Since a current experiences a force in a magnetic field, and a current is just made of moving charges, moving charges themselves must experience a force in a magnetic field. www.nearingzero.net The force on a moving charge in a magnetic field Consider a positive charge q moving with speed v. v q Magnetic field B out of the slide The force on a moving charge in a magnetic field In time Δt the charge will have moved a distance L = vΔt v q The force on a moving charge in a magnetic field The current is given by I = q/Δt v q The force on a moving charge in a magnetic field Given that v q F = BILsinθ F = B(q/Δt)vΔt = Bvqsinθ The force on a moving charge in a magnetic field The fact that this force is always at right angles to the velocity means that the charge will move in a circle (if the speed is constant) q v Note; If the force is perpendicular to the motion, no work is done. Homework – Due Monday Let’s do some questions! Page 344 onwards Questions 2, 6, 7, 9, 13, 15, 20.