DHS EM

Dunman High School (Senior High Physics Department) 9646 Physics (2014) Topic 15: Electromagnetism 15-1 H2 Topic 15 Electromagnetism Magnetic field lines showed by iron fillings An exploded view of an electric motor Large Hadron Collider (LHC) Tunnel: A 17 mile long tunnel underlying the border between Switzerland and France

Dunman High School (Senior High Physics Department) 9646 Physics (2014) Topic 15: Electromagnetism 15-2 Learning Objectives Content  Force on a current-carrying conductor  Force on a moving charge  Magnetic fields due to currents  Force between current-carrying conductors Learning Outcomes Candidates should be able to a. Show an appreciation that a force might act on a current -carrying conductor placed in a magnetic field. b. Recall and solve problems by using the equation F = BIL sin θ, with directions as interpreted by Fleming's left-hand rule. c. Define magnetic flux density and the tesla. d. Show an understanding of how the force on a current -carrying conductor can be used to measure the flux density of a magnetic field, using a current balance. e. Predict the direction of the force on a charge moving in a magnetic field. f. Recall and solve problems by using F = B q v sin θ. g. Describe and analyse deflections of beams of charged particles by uniform electric and uniform magnetic fields. h. Explain how electric and magnetic fields can be used in velocity selection for charged particles. i. Sketch flux patterns due to a long straight wire, a flat circular coil and a long solenoid. j. Show an understanding that the field due to a solenoid may be influenced by the presence of a ferrous core. k. Explain the forces between current -carrying conductors and predict the directions of the forces.

Dunman High School (Senior High Physics Department) 9646 Physics (2014) Topic 15: Electromagnetism 15-3 15.1 Magnetic Fields Besides the gravitational field and the electric field, another field of force in nature is the magnetic field. A magnetic field is a region of space where a magnetic pole, a current-carrying conductor or a moving charge particle will experience a force. A magnetic field can be produced by: (i) Permanent magnets (ii) Current-carrying conductors The magnetic field around a permanent magnet can be seen by sprinkling some iron filings around it. In 1820, the Danish physicist, Hans Christian Oersted first discovered that magnetic fields are produced by electric currents. It is the motion of the electric charge in the conductor that produces the magnetic field. 15.1.1 Magnetic Field Lines A magnetic field may be re presented by a series of lines called magnetic field lines, each having a specific direction. The direction of a magnetic field line at any point in the field shows the direction of the force that a ‘free’ magnetic north pole would experience at that point. The following are characteristics of magnetic field lines: i. They are imaginary. ii. By convention, magnetic field lines leave the north pole and enter the south pole of a magnet. Note that the field lines do not start or end at the north or south poles of a magnet, they continue to go from south pole to the north pole in the magnet. iii. Do not touch or intersect one another. iv. Can be straight lines or curves. The tangent to a curved field line at a point indicates the direction of the magnetic field at that point. This tangent also indicates the direction of the magnetic flux density B at that point. direction of magnetic field at point P. P

Dunman High School (Senior High Physics Department) 9646 Physics (2014) Topic 15: Electromagnetism 15-4 v. They are represented by crosses or dotted circles in a 2-D plan view. vi. If the lines are parallel and evenly spaced, the magnetic field is uniform. Otherwise, the field will vary in strength from one point to another. vii. The closeness of the lines indi cates the strength of the field; the field is said to be strong if the lines are crowded very closely together and weak when they are widely separated from one another. Uniform Field Non-Uniform Field Strong Field Weak Field A non-uniform field X Y The field is stronger at X than at Y Magnet field around a bar magnet x x x x x x x x x x x x x x x x x Magnetic field pointing into the paper. Magnetic field pointing out of the paper.

Dunman High School (Senior High Physics Department) 9646 Physics (2014) Topic 15: Electromagnetism 15-5 15.2 Magnetic Flux Pattern A conductor carrying an electric current produces a magnetic field. The pattern formed by the field lines is known as a magnetic flux pattern. Field patterns and the expressions for the associated magnetic flux densities for different current - carrying conductors are given below: 15.2.1 Magnetic Field Generated by a Long Straight Wire When a long straight wire carries a current, a magnetic field is generated around the wire in concentric circles as shown. Isometric View Plan view I B I B X

Dunman High School (Senior High Physics Department) 9646 Physics (2014) Topic 15: Electromagnetism 15-6 The direction of the magnetic field lines can be determined by the right-hand grip rule as illustrated in the diagram; when the thumb of the right hand points in the direction of the current, the fingers indicate the direction of the magnetic field lines. The d istance between successive circles increases as one move outwards from the wire; this indicates that the field is weaker as one moves away from the wire. The magnetic flux density B at any point which is at a perpendicular distance d from the wire carrying current I is given by: d IB o   2 where o = permeability of free space (vacuum) = 7104  H m-1 Example 1 The figures below shows 3 points close to a wire carrying current of 12 A. For point a, calculate the magnetic flux density and state whether the magnetic field points into or out of the plane of the page. Without calculation, compare the magnitude of the magnetic flux density at the three points. d I = 12 A b a c 5 m 5 m 12 m Using B = d I   2 0 , Ba = 74 10 12 2 (5)    = 4.8 x 10-7 T Using Right Hand Grip Rule, Ba points into the page. Ba = Bc > Bb

Dunman High School (Senior High Physics Department) 9646 Physics (2014) Topic 15: Electromagnetism 15-7 15.2.2 Magnetic Field Generated by a Flat Circular Coil When a current passes through a flat circular coil, the magnetic flux pattern of the generated field is as shown below. Magnetic field lines around a circular coil Iron filings sprinkled around a circular coil. Field lines at the centre of the coil are straight and perpendicular to the plane of the coil. The direction of the field lines is gi ven by the right -hand grip rule, which in this case the direction of the current is represented by the curled fingers and the thumb indicates the direction of the magnetic field acting through the centre of the coil. I B

Dunman High School (Senior High Physics Department) 9646 Physics (2014) Topic 15: Electromagnetism 15-8 The magnetic flux density at the centre, P, for a current-carrying flat circular coil of N turns and radius r is: r NIB = 2 0 15.2.3 Magnetic Field Generated by a Solenoid If a long straight wire is bent into a coil of several closely spaced loops, the resulting device is called a solenoid. A solenoid is also known as an electromagnet, because it acts like a magnet only when a current passes through its wire. Field lines w ithin a solenoid are straight and parallel to the axis of the coil, indicating that the field