# Sensing the speed and acceleration of a train: results

## Teachers' notes

This activity occurs part of the way through the A2 unit Transport on Track and provides an opportunity to look at electromagnetic induction and use the data collected, with additional information, to calculate speeds and acceleration.

1. A voltage is induced across a coil when the magnetic flux/field changes in it.
2. As the flux/field increases in the coil it produces a voltage across it in one direction. As the flux/field decreases it produces a voltage in the opposite direction.
3. The trace for A is smaller in voltage (negative and positive) than B and is produced over a longer time. OR The trace for B is higher in voltage (negative and positive) than A and is produced over a shorter time. The higher voltage (positive and negative) is a result of a faster rate of change of flux/field due to the quicker movement of the magnet over the coil. Similarly the shorter trace for B indicates quicker movement of the magnet over the coil.
4. The answers to Q4, Q5 and Q6 will vary with the trace obtained but, for the trace provided in Figure A17.2, the following would be correct.
5. Time to cross coil A is 0.28 s. The distance moved by the magnet will be 2.50 cm so the average velocity is 2.50 cm/0.28 s = 8.93 cm s-1
6. Time to cross coil B is 0.16 s so the average velocity is 2.50 cm/0.16 s = 15.6 cm s-1
7. Time taken to move from the centre of coil A to the centre of coil B is given by:
8. (0.35 - 0.08)s = 0.27 s
9. Acceleration = (15.6 - 8.93) cm s-1/0.27 s = 24.7 cm s-2

## Technicians' notes

Apparatus

• Track
• Clockwork locomotive
• Amplifier
• BNC plug to 4 mm plugs (Pico M1029)
• PC running PicoScope

For making the track

Figure T17.1

• Wood approximately 120 cm x 5 cm x 2 cm
• PVC channelling high impact 25 mm x 8 mm x 2 m B&Q ref: DGCH100/2
• 4 mm socket green JPR 705-232
• 4 mm socket blue JPR 705-233
• Search coils Maplin JC25C (2)
• Connecting wire
• Rubber feet JPR 885-060
• Wire netting staples (2)
• Glue

Use a staple gun to fix the channelling to the wood. Hammer a wire netting staple into the middle of the ‘track’ at each end to act as a stop. Drill two holes through the channelling and wood 10 cm apart centrally in the track and 5 cm each side of its centre point. Glue the search coils into these holes with their windings in the same direction and their tops flush with the top surface of the channelling. Drill another two holes in the side of the wood each 1.5 cm from the centre. Insert the two 4 mm sockets into these holes. Drill a further two holes from the underside of the wood to make contact with the solder tags of the 4 mm sockets. Solder wires to the 4 mm sockets’ solder tags. Connect (by soldering) the wires from the top of each search coil to the wire from the blue socket, and the wires from the bottom of each search coil to the wire from the green socket. Staple these wires to the base of the wood.

For modifications to the locomotive

• Hornby Percy Clockwork 0-4-0 Loco R9035 (Thomas the Tank Engine series) toy shop
• button magnet 12 mm diameter x 4 mm thickness Scientific and Technical Supplies Ltd MA370.20 or similar from some toy shops
• 5 cm 18 swg copper wire
• double-sided sticky tape

Remove the linkages between the wheels. Remove the front coupling. Replace the plastic cross-link just in front of the front wheels by a piece of 18 swg copper wire threaded through the holes left by the removal of the plastic cross-link “ this will hold the clockwork mechanism in place more firmly. Stick the button magnet in place just beneath the front buffer using double-sided tape.

For making the amplifier

Figure T17.2

• 4 mm socket green JPR 705-232 (2)
• 4 mm socket blue JPR 705-233
• 4 mm socket red JPR 705-230 (2)
• 4 mm socket black JPR 705-231 (2)
• box MB2 black 100 mm x 76 mm x 41 mm JPR 880-025B
• cermet trimmer 10 k JPR 306-10k
• 8 pin IC socket JPR 715-005
• TL071 op-amp JPR TL071
• pcb NE531 RS 434-065
• 1 k 0.4 W resistor Maplin TX20W (2)
• 100 k 0.4 W resistor Maplin TX44X
• nuts and bolts
• connecting wire
• dual supply -15 V to 0 to + 15 V Unilab 022.114 or use two 9 V PP3 batteries connected via battery snaps

Label the Input and Output sockets on the box lid and also the Gain of the amplifier - in this case (Gain * -100).

Initially set the input to the amplifier to 0 V and adjust the trimmer so that the output voltage is as near to 0 V as can be achieved. If you need to know the exact gain of the amplifier place a known voltage, say 1 mV, across the input and see how large the output voltage is.

The gain required depends on the strength of the magnet used. The stronger it is the less gain is needed and vice versa.

NOTE: If, on testing, the train does not trigger the taking of a trace, turn the button magnet over and re-stick.

Suppliers

JPR Electronics Ltd.
Dunstable LU5 5DD
UK

Tel: 01582 470000
Fax: 01582 470001
E-mail: sales@jprlec.co.uk

Maplin Electronics
Freepost smu 94
PO Box 777
Rayleigh
Essex SS6 8LU
UK

Tel: 0870 264 6000
Fax: 0870 264 6001

RS
PO Box 99
Corby
Northants NN17 9RS
UK

Tel: 01536 201201
Fax: 01536 201501

Scientific and Chemical Supplies Limited
Carlton House
Bilston
West Midlands WV14 0QZ
UK

Tel: 01902 402402
Fax: 01902 402343
E-mail: scs@schichem.co.uk

UNILAB
Lynn Lane
Shenstone
Lichfield
Staffordshire WS14 0EE
UK

Tel: 01543 483064
Fax: 01543 483056
E-mail: sales@unilab.philipharris.co.uk