MOTORS AND GENERATORS - Contextual Outline Modern society is geared to using electricity. Electricity has characteristics that have made it uniquely appropriate for powering a highly technological society. There are many energy sources that can be readily converted into electricity. In Australia, most power plants burn a fuel, such as coal, or use the energy of falling water to generate electricity on a large scale. While it is relatively economical to generate electric power at a steady rate, there are both financial and environmental issues that should be considered when assessing the long- term impact of supplying commercial and household power. Electricity is also relatively easy to distribute. Electricity authorities use high-voltage transmission lines and transformers to distribute electricity to homes and industries around each state. Voltages can be as high as 5 x 10 5 volts from power stations but by the time this reaches homes, the electricity has been transformed to 240 volts.

The design of a motor for an electrical appliance requires consideration of whether it will run at a set speed, how much power it must supply, whether it will be powered by AC or DC and what reliability is required. The essentials of an electric motor are the supply of electrical energy to a coil in a magnetic field causing it to rotate. The generation of electrical power requires relative motion between a magnetic field and a conductor. In a generator, mechanical energy is converted into electrical energy while the opposite occurs in the electric motor. The electricity produced by most generators is in the form of alternating current. In general AC generators, motors and other electrical equipment are simpler, cheaper and more reliable than their DC counterparts. AC electricity can be easily transformed into higher or lower voltages making it more versatile than DC electricity. This module increases students’ understanding of the applications and uses of physics and the implications for society and the environment.

1. Motors use the effect of forces on current-carrying conductors in magnetic fields Jacaranda Before starting, make sure you are familiar with ‘REVIEW OF MAGNETIC FIELDS’ P

Moving charged particles in a magnetic field experience a force This happens because the magnetic field created by the moving charged particle interacts with the existing field N.B. If the velocity has a component parallel to the magnetic field, that component is not subject to the force and is retained while the other component is accelerated. It depends on the field strength, the charge size and the velocity. e.g. Conventional current, alpha particles e.g. Electron flow, beta particles Identify that the motor effect is due to the force acting on a current-carrying conductor in a magnetic field

Discuss the effect on the magnitude of the force on a current-carrying conductor of variations in: – the strength of the magnetic field in which it is located – the magnitude of the current in the conductor – the length of the conductor in the external magnetic field – the angle between the direction of the external magnetic field and the direction of the length of the conductor where  is the angle between the magnetic field and the conductor So F is directly proportional to B, I, l and sin  I  B Perform a first-hand investigation to demonstrate the motor effect Jacaranda Experiment 6.2 N.B. magnetic field may be created by permanent magnet or electromagnet Solve problems and analyse information about the force on current-carrying conductors in magnetic fields using F = BI l sin 

Describe qualitatively and quantitatively the force between long parallel current- carrying conductors: The right-hand grip rule gives the direction of the magnetic field surrounding conductor 1. d l B This shows the field that conductor 2 experiences due to conductor 1 The right-hand push rule gives the direction of the force experienced by conductor 2 F Conductor 2 has the same effect on conductor 1, so they ATTRACT. Use the same approach to show that opposite currents repel F F F

Describe qualitatively and quantitatively the force between long parallel current- carrying conductors: Conductor 2 experiences a magnetic field due to conductor 1 d l B Conductor 2 experiences a force due to this magnetic field F Substituting for B: rearranging gives Force per unit length:

Two straight current-carrying conductors are placed parallel to each other, 20 cm apart. = 2 A = 5 A (a)Determine the force per unit length of the 2 A wire on the 5 A wire. (b)Describe what will happen to the magnitude of the force as the 2 A wire is rotated 90 o until it is perpendicular with the 5 A wire. 20 cm M.Edwards 25/4/02 Solve problems using:

(a)F/l = kI 1 I 2 /d F/l = (2.0 x x 2 x 5) / 0.2 F/l = 1 x N/m (attraction). 1 mark (b)The force will decrease as the 2 A wire is rotated and will be zero when the two wires are perpendicular to each other. 1 mark 20 cm M.Edwards 25/4/02 Solve problems using:

(OPTIONAL EXERCISE) Undertake a first hand investigation, devised by the teacher, to observe Oersted’s experiment. Students record the method, their observations, and the significance of the experiment, after teacher led discussion. Record information, after teacher led discussion, to explain the force between the wires as an example of the motor effect Jacaranda Experiment 6.3