Y12 Chapter 24 - Magnetic Fields and Electromagnetism PDF

CHAPTER 24 Magnetic Fields and Electromagnetism SUTARTO 2024/2025 Learning Outcomes You should be able to: q describe a magnetic field as an example of a field of force caused by moving charges or permanent magnets q use field lines to represent a field and sketch...

CHAPTER 24 Magnetic Fields and Electromagnetism SUTARTO 2024/2025 Learning Outcomes You should be able to: q describe a magnetic field as an example of a field of force caused by moving charges or permanent magnets q use field lines to represent a field and sketch various patterns q determine the size and direction of the force on a current- carrying conductor in a magnetic field q define magnetic flux density and know how it can be measured q explain the origin of the forces between current-carrying conductors and find the direction of these forces. 24.1 Magnetic force A current-carrying wire is surrounded by a magnetic field. This magnetic field will interact with an external magnetic field, giving rise to a force on the conductor, just like the fields of two interacting magnets.  24.1 Magnetic Force  24.2 Magnetic flux density The strength of a magnetic field is known as its DEFINITION (for a long straight wire) magnetic flux density, with symbol B. It is The magnetic flux density at a point in space is measured in Tesla (T). the force experienced per unit length by a long straight conductor carrying unit current and 1 Tesla definition placed at right angles to the field at that point. The magnetic flux density is 1 T when a wire carrying a current of 1 A placed at right angles For a uniform magnetic field, the flux density B to the magnetic field experiences a force of 1 N is defined by the equation: per meter of its length. B = F/IL General Definition for Magnetic Flux Density, B Magnetic flux density is defined in terms of the magnetic force experienced by a current-carrying conductor placed at right angles to a magnetic field. Question 24.1 1. A current of 0.20 A flows in a wire of length 2.50 m placed at right angles to a magnetic field of flux density 0.06 T. Calculate the force on the wire. 2. A 20 cm length of wire is placed at right angles to a magnetic field. When a current of 1.5 A flows in the wire, a force of 0.015 N acts on it. Determine the flux density of the field. 3. A wire of length 50 cm carrying a current lies at right angles to a magnetic field of flux density 5 mT. (a) If 1018 electrons pass a point in the wire each second, what current is flowing? (b) What force acts on the wire? 24.4 Currents crossing fields At right angle Whenever an electric current cuts across magnetic field lines, a force is exerted on the current-carrying conductor. This helps us to remember that a conductor experiences no force when the current is parallel to the field. 24.4 Currents crossing fields At an angle other than 900 To calculate the force, we need to find the component of the magnetic flux density B at right angles to the current. This component is B sin θ, where θ is the angle between the magnetic field and the current or the conductor. F = BIL sin 𝜽 Question 24.4 1. A wire of length 50 cm carrying a current of 2.4 A lies at right angles to a magnetic field of flux density 5.0 mT. Calculate the force on the wire. 2. The coil of an electric motor is made up of 200 turns of wire carrying a current of 1.0 A. The coil is square, with sides of length 20 cm, and it is placed in a magnetic field of flux density 0.05 T. (a) Determine the maximum force exerted on the side of the coil. (b) In what position must the coil be for this force to have its greatest turning effect? (c) List four ways in which the motor could be made more ‘powerful’ – that is, have greater torque. Question 24.4 3. What force will be exerted on each of the currents shown in the following figure, and in what direction will each force act? 24.5 Force between currents The origin of the magnetic force (model 1) 24.5 Force between currents Hence there will be a force on I2 (the BIL force), and we can find its direction using Fleming’s left-hand rule. The arrow shows the direction of the force, which is towards I1. Similarly, there will be a BIL force on I1, directed towards I2. These two forces are equal and opposite to one another. They are an example of an action and reaction pair, as described by Newton’s third law of motion Question 24.5 Two flat circular coils of wire are set upside by side, as shown in Figure 26.25. They are connected in series so that the same current flows around each, and in the same direction. Will the coils attract or repel one another? Explain your answer, first by describing the coils as electromagnets, and secondly by considering the forces between parallel currents. What will happen if the current is reversed in both coils?