Transformers

Aim: To investigate whether the output voltage depends on the number of turns on each coil.

Part A — Does the output depend on the turns at all?

1. Set the supply to a.c., the input voltage to 6.0 V, and both coils to 100 turns. Read both voltmeters and note what you see.
2. Change only N_s — try 200 turns, then 400. Does the secondary voltmeter reading change?
3. Put N_s back to 100 and change only N_p instead. Does the output voltage respond to this coil too?

Part B — Which way does each coil push the output?

1. Keep N_p at 100 and the input at 6.0 V. Record a table row for N_s = 50, 100, 200 and 400.
2. Now keep N_s at 100 and record a row for N_p = 50, 100, 200 and 400.
3. Look at your table. Which changes make the output voltage bigger than the input, and which make it smaller?

Part C — How exactly are they connected?

1. For every row in your table, use a calculator to fill in the V_s ÷ V_p and N_s ÷ N_p columns.
2. Compare the two columns, row by row. What do you notice?
3. Test your idea: predict the secondary reading for N_p = 50, N_s = 200 with a 12.0 V input, then set it up and check the voltmeter.
4. Does your rule survive a change of input voltage? Repeat one of your rows with a different V_p and add it to the table.

Part D — The d.c. mystery

1. Set up any combination that gives a healthy output on a.c., then flick the supply switch to d.c.
2. What happens to each voltmeter reading? Record the row in your table.
3. A transformer is just two separate coils of wire on an iron core — there is no electrical connection between them. Suggest how energy gets from one coil to the other, and why the type of supply might matter.
4. What does your answer suggest about whether a transformer could ever be run from a battery?