D.C. ELECTRIC MOTORS 1. Motors use the effect of forces on current-carrying conductors in magnetic fields

F d Define torque as the turning moment of a force using: Solve problems and analyse information about simple motors using: Increasing F or d increases the turning effect (torque) Substituting: Torque from each side So total Torque, Torque for each loop and for n loops If the coil is at an angle  to a uniform magnetic field Describe the forces experienced by a current-carrying loop in a magnetic field and describe the net result of the forces  F  F d M.Edwards 25/4/02

Below is a diagram of a coil of wire attached to a split-ring commutator and a power source. A student placed some permanent magnets at A and B and the motor started spinning. Attaching a torque meter to the axle, they were able to determine the torque at various angles (the angle as shown in the diagram is 0 o ). The results are shown at right. (a) Graph the results in such a way that there is a linear relationship between the two variables. (b) Given that there were 200 turns of wire, a current of 3 A, the coils were 2cm by 2cm, and using the graph, determine the magnetic field strength. (c) If the student did not have permanent magnets, how could they have created the magnetic field?

(a)2 marks (b)2 marks gradient = ( )/(1-0) = gradient = nBIA B = 0.022/nIA B = 0.022/200 x 3 x 0.02 x 0.02 B = 0.09 T (c)2 marks They could have created an electromagnet by wrapping a wire around an iron core many times and attaching the ends so that it was in parallel with the motor.

Identify that the required magnetic fields in DC motors can be produced either by current-carrying coils or permanent magnets Where are the permanent magnets placed? Why? Can we place an electromagnet in the same circuit as the motor? How? What are the advantages and disadvantages of permanent and electromagnets? What is the shape and direction of a magnetic field produced by a current-carrying coil?

Identify data sources, gather and process information to qualitatively describe the application of the motor effect in: – the galvanometer – the loudspeaker Jacaranda PHYSICS IN FOCUS P.112 Student exploration of large galvanometers Student exploration of car stereo speakers E.g. ammeter: The galvanometer can be converted into an ammeter by using a very small resistor in parallel with the galvanometer and changing the scale appropriately. Most of the current will flow through the small resistor, with the remainder through the galvanometer. This allows larger currents to be measured without the needle moving off the scale. voltmeter: Make sure you find the answer to this too!

(b)2 marks The galvanometer can be converted into an ammeter by placing a very small resistor in parallel with the galvanometer and changing the scale appropriately. Most of the current will flow through the very small resistor and only a small amount of the current will flow through the galvanometer. This allows larger currents to be measured.

Describe the main features of a DC electric motor and the role of each feature armature N F d M.Edwards 25/4/02 magnets (stator) S coil axle The magnets are stationary and are known as the stator. The coil, armature and axle rotate and are known as the rotor. brush Split-ring commutator Source of EMF HSC Practical Research Task Internet DC motor animation by Walter Fendt

NS F M.Edwards 25/4/02 The right hand push rule tells us this rotor will move anticlockwise. side A side B Conventional current in side B is currently out of the page I I When  reaches 90 degrees, the split ring will contact the other brush, reverse polarity and current in side B will be into the page  F  A B So the rotor continues to rotate in an anticlockwise direction A F B HSC Practical Research Task M.Edwards 25/4/02 Describe the main features of a DC electric motor and the role of each feature

Question 1 The diagram below shows a DC motor. (a)Explain why the motor spins. (b)Explain why the split-ring commutator is employed. (c)If the coil was 2 cm long and 2 cm wide, consisted of 50 turns of wire carrying a current of 5 mA in a magnetic field of magnitude 0.6 T, what would be the magnitude of the maximum torque generated and the direction the coil turned? (d)How could this device be modified in order to create a galvanometer?

1(a)1 mark Due to flow of electrons in the magnetic field, side AB will experience a downwards force. Similarly, CD will experience an upwards force. This will cause the coil to spin in an anticlockwise direction. (b)3 marks If there was no split ring commutator, side AB would always experience a downwards force and side CD would always experience an upwards force. After a quarter of a turn, the force would no longer cause the coil to spin anticlockwise but clockwise resulting in an oscillating coil. To make the device useful, the coil must spin in one direction continuously. This can only be obtained by changing the direction of the force every half cycle. The split ring commutator allows the current to be changed every half cycle, hence the torque is always in the same direction. (c)1 mark  = nBIAcos  = 50 x 0.6 x x 0.02 x 0.02cos90 =6 x Nm spinning in an anticlockwise direction. (d)2 marks A spring could be connected to the axle of the motor so that the torque generated by the motor effect is countered by the opposing torque of the spring. The more the coil is turned, the greater the torque of the spring. The coil would come to rest when these torques were in equilibrium. The degree to which the coil had moved would be proportional to the magnitude of the current. A pointer attached to the axle could indicate on a scale the magnitude of the current.