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National Grid demo (asset code : E-Transf-2)

Applicable Levels : GCSE

Applicable Examination Boards : AQA, OCR21C , WJEC

AIM

An introduction to the use of the transformer and to demonstrate the benefits of transmitting electrical power at high voltage.

APPARATUS

a.c. power supply 2V – 12V
Two pea bulbs (2.5V 0.75W) or bulbs (12V 24W)
a.c.voltmeters range 20V or 600V
Transformer coils two each of 120 + 120 turns and 1200 + 1200 turns
Two iron C-cores with metal clip
One metre of 28 swg Constantan wire as the power lines mounted on two dowel rods
Connecting leads
Arrangement X uses 2.5V bulbs; Arrangement Y uses 12V bulbs.

SAFETY

If using arrangement Y you must take extra precautions as the high voltage transmission is at approximately mains 230V. You must check that the laboratory is protected by an Residual Current Circuit Breaker (RCCB). You could instead use the specialised national grid simulation apparatus as described in E-Transf-2.

METHOD

A. Connect up circuit 1 as shown in the diagram by clicking here.
B. Adjust the a.c. voltage to match the bulb that you are using and switch on.
C. Compare the brightness of the two bulbs.
D. Measure the a.c. voltage before the power line and after the power line.

E. Do not mention anything about the transformer coils just connect up circuit 2 as shown in the diagram.
F. Use the same a.c. voltage as with circuit 1 and switch on.

G. Compare the brightness of the two bulbs.
H. Measure the a.c. voltage at the input to the first transformer (Col 1) and the output voltage of the second transformer (Col 8), i.e. the voltage across the bulbs.
J. Measure the a.c. voltage before the power line (Col 4) and after the power line (Col6).

RESULTS

Circuit 1.

Input Voltage/V Power of bulb/W Low voltage transmission/V Output Voltage /V
2.5 0.75 approx 2.5 0.9
12 24 approx 12 1.4
 

Circuit 2.

1 2 3 4 5 6 7 8
Input Voltage/V Power of bulb/W Turns ratio Step-up output voltage/V High voltage transmission/V Step-down input voltage/V Turns ratio Final Output Voltage /V
2.5 0.75 1:10 22 approx 20 18 10:1 1.7
12.1 24 1:20 220 approx 200 180 20:1 9.1

EXPERIMENT NOTES

After finishing the class experiment E-Transf-1 and learning the transformer equation, (Click here to see it) show them the coils used and ask the pupils to describe what happens in the first transformer and in the second transformer.

Refer to the voltages measured, or better still show them the model grid again and after measuring the voltages again, justify the pupil comments.

APPLICATIONS

The National Grid Takes alternating current from power stations (coal, gas, oil, nuclear) and delivers it to factories and homes. The output voltage from a power station is usually about 25 000 V. The voltage in a house is 230 V. Transformers are used to change the voltages.

The output voltage of the power station is stepped up by transformers before being supplied to the National Grid. At the consumer end the voltage must be stepped down before being supplied to homes and factories.

A small current at high voltage delivers the same power as a large current at a low voltage. For example 1000 W of electrical power can be delivered by a ;
Current of 10 A at a voltage of 100 V [Remember Power (W) = Current (A) x Voltage (V)] or
Current of 1 A at a voltage of 1000 V.

The heating effect of a large current due to cable resistance is much greater than with a small current. The equation for the heating effect in a resistor is
Power (W) = (Current in A)2 x Resistance (Ω)

The advantage of a 10 fold increase in voltage is a 10 fold decrease in current for the same power delivered, and a 100 times decrease in the heat produced.

EXAM SPECIFICATION REFERENCES

AQA P3 13.9
EDEXCEL
OCR21C P5.4
OCR21C P3 2b
WJEC P3 2

QUESTIONS

  • 1.   Ask pupils to explain the dim bulb connected after the power line (Step C). Expect the answer of energy loss.
  • Remind them that when a current meets resistance energy is transferred or shifted, so energy is transferred in the power line and lost as heat.
  • 2.  Measuring the input and output voltages to the power line (Step D) confirms this energy loss, remembering that the voltage is equal to the energy carried per coulomb.
  • Note the voltage drop over the power line.
  • 3.  Ask pupils to explain the brightness of the second bulb connected after the power line (Step G).
  • Expect the answer of reduced energy loss in the power line in circuit 2.
  • 4.  The voltage across the second bulb (Step H) explains why the second bulb is brighter than in circuit 1.
  • The voltage drop is less than in circuit 1 so there is less energy loss.
  • 5.  After performing step J just say that the step-up transformer increases the voltage and the second transformer decreases the voltage.
  • The pupils now need to do the experiment E-Transf-1 to find out the arrangements of coils needed to step-up the voltage and to step-down the voltage. Any further comments and explanations about the National Grid should be reserved until after the pupils have obtained their results.