Y10 24-25 Chapter 21 Electromagnetic Induction PDF

CHAPTER 21 ELECTROMAGNETIC INDUCTION SUTARTO 2024/2025 LEARNING OUTCOMES q Describing how an e.m.f. is induced in a circuit q Identifying factors affecting the magnitude and direction of an induced e.m.f. q Describing the design of an a.c. generator q Describing...

CHAPTER 21 ELECTROMAGNETIC INDUCTION SUTARTO 2024/2025 LEARNING OUTCOMES q Describing how an e.m.f. is induced in a circuit q Identifying factors affecting the magnitude and direction of an induced e.m.f. q Describing the design of an a.c. generator q Describing the construction of a transformer q Using the transformer equation q Explaining how transformers work q Using the power equation for a transformer 21.1 GENERATING ELECTRICITY A motor is a device for transforming electrical energy into mechanical (kinetic) energy. To generate electricity, we need a device that will do the opposite: it must transform mechanical energy into electrical energy. That device is called a generator. All generators have three things in common: 1. a magnetic field (provided by magnets or electromagnets) 2. a coil of wire (fixed or moving) 3. movement (the coil and magnetic field move relative to one another).  21.1 GENERATING THE PRINCIPLES OF ELECTROMAGNETIC INDUCTION “ The process of generating electricity from motion is called ELECTRICITY electromagnetic induction " 1. Move one pole of the magnet downwards past the wire, and a current flows. 2. Move the magnet back upwards, and a current flows in the opposite direction. Alternatively, the magnet can be stationary, and the wire can be moved up and down next to it. 1. Reverse the magnet to use the opposite pole, and the current flows in the opposite direction. 2. Hold the magnet stationary next to the wire or coil, and no current flows. They must move relative to each other, or nothing will happen. 21.1 GENERATING ELECTRICITY AN A.C. GENERATOR A simple a.c. generator, which produces alternating current. In principle, this is like a d.c. motor, working in reverse. The axle is made to turn so that the coil spins around in the magnetic field, and a current is induced. The other difference is in the way the coil is connected to the circuit beyond. A d.c. motor uses a split ring commutator, whereas an a.c. generator uses slip rings. 21.1 GENERATING ELECTRICITY AN A.C. GENERATOR This generator produces alternating current (a.c.). This means that the current is not direct current (d.c.), which always flows in the same direction. Instead, an alternating current flows back and forth. Half of the time, the current flows in the positive direction. Then it flows in the opposite direction. The frequency of an a.c. supply is the number of cycles it produces each second. 21.1 GENERATING ELECTRICITY INDUCTION AND FIELD LINES The idea of field lines helps us to understand the 3. If the magnet is moved quickly, the lines are factors that affect the magnitude and direction of the cut more quickly and a bigger e.m.f. is induced. induced e.m.f. 4. A coil gives a bigger effect than a single wire, 1. If the magnet is stationary, there is no cutting of because each turn of wire cuts the magnetic field lines and so no e.m.f. is induced. field lines and each therefore contributes to the induced e.m.f. 2. If the magnet is further from the wire, the field lines are further apart and so fewer are cut, giving a smaller e.m.f. 21.1 GENERATING ELECTRICITY WAYS TO INCREASE THE VOLTAGE GENERATED BY AN A.C. GENERATOR 1. turn the coil more rapidly Direction of the induced e.m.f. 2. use a coil with more turns of wire An induced current always flows in such a 3. use a coil with a bigger area way that its magnetic field opposes the change that causes it. 4. use stronger magnets. Each of these has the effect of increasing the rate at which magnetic field lines are cut, and so the induced e.m.f. is greater.  21.2 POWER LINES AND TRANSFORMERS Electricity is usually generated at a distance from where it is used. If you look on a map, you may be able to trace the power lines that bring electrical power to your neighborhood. q High-voltage electricity leaves the power station. When the power lines approach the area where the power is to be used, they enter a local distribution centre. q Here the voltage is reduced to a less hazardous level, and the power is sent through more cables (overhead or underground) to local substations. q In the substation, transformers reduce the voltage to the local supply voltage, typically 230 V. 21.2 POWER LINES AND TRANSFORMERS WHY USE HIGH VOLTAGES? The high voltages used to transmit When a current flows in a wire or cable, some of the electrical power around a country are energy it is carrying is lost because of the cable’s dangerous. That is why the cables that resistance - the cables get warm. A small current wastes carry the power are supported high less energy than a high current. above people, traffic and buildings on tall pylons. If we can reduce the current to half its value (by doubling the voltage), the losses will be one-quarter of There is a good reason for using high their previous value. This is because power losses in cables voltages. It means that the current are proportional to the square of the current flowing in flowing in the cables is relatively low, the cables: and this wastes less energy. q double the current gives four times the losses q three times the current gives nine times the losses. 21.2 POWER LINES AND TRANSFORMERS TRANSFORMERS A transformer is a device used to increase or decrease the voltage of an electricity supply. Every transformer has three parts: q a primary coil - the incoming voltage VP is connected across this coil q a secondary coil this provides the voltage VS to the external circuit q an iron core this links the two coils. 21.2 POWER LINES AND TRANSFORMERS TRANSFORMERS q A step-up transformer increases the voltage, so there are more turns on the secondary than on the primary. q A step-down transformer reduces the voltage, so there are fewer turns on the secondary than on the primary. 21.3 HOW TRANSFORMERS WORK Transformers ONLY work with alternating current (a.c.). To understand why this is, we need to look at how a transformer works (Figure 21.12). It makes use of electromagnetic induction. 1. The primary coil has alternating current flowing through it. It is thus an electromagnet and produces an alternating magnetic field. 2. The core transports this alternating field around to the secondary coil. 3. Now the secondary coil is a conductor in a changing magnetic field. A current is induced in the coil. 21.3 HOW TRANSFORMERS WORK THINKING ABOUT POWER If a transformer is 100% efficient, no power is lost in its coils or core. This is a reasonable approximation, because well- designed transformers waste only about 0.1% of the power transferred through them. This allows us to write an equation relating the primary and secondary voltages, VP and VS., to the primary and secondary currents, IP and IS, flowing in the primary and secondary coils, using P = IV.