Learning Objectives: What is a Transformer? How is a current induced in the secondary coil? Is there a relationship between the number of coils and the output voltage?

How can we induce a current in a circuit? I wonder how the charger charges the toothbrush up! They are both made of plastic so the charger should not be able to transfer any electric charges to the toothbrush, right? I think it might work with electromagnetic induction, but I cannot imagine how. One would need to open one up and see!

Wires, coils and solenoids

What name can we give to a transformer that has more windings in the primary coil than in the secondary coil? Step Down Transformer Step Up Transformer Optimus Prime Megatron Well done! A Step Down transformer has less windings in the secondary coil than in the primary coil.

Why does a transformer only work if an A.C. Voltage is input across the Primary Coil? A changing magnetic field is needed to induce a current in the Secondary Coil D.C. Current is not high enough to induce a magnetic field in the Primary Coil D.C. currents are too dangerous for the user A magnetic field cutting through the secondary coil is needed to induce a current Well done! An A.C. current will generate a changing magnetic field in the Primary Coil and this will induce a changing (alternating) current in the secondary coil because of Lenz’s Law.

Why is a laminated core useful in a transformer? It channels the magnetic field lines to the Secondary Coil It’s there to hold the primary and secondary coils together It carries the current from the primary to the secondary coil It ensures no magnetic field lines reach the secondary coil Well done! A laminated core is not essential, but it highly improves its performance, because it channels all the magnetic field lines generated around the Primary Coil to the Secondary Coil.

What is the relationship linking V 1, N 1, V 2 and N 2 ? Well done! The Primary Voltage divided by the Secondary Voltage equals the no of Primary Windings divided by the no of Secondary Windings.

When is the relationship from the previous question false? When the transformer is not 100% efficient When there are too many windings in the Primary Coil When the Secondary Voltage is too high When the transformer is 100% efficient Well done! If the transformer is not 100% efficient the power output in the Secondary Coil would not be the same as the input power in the Primary Coil and the equation would not work.

What is the Secondary Voltage in a transformer when V 1 = 20 V, N 1 = 100 and N 2 = 2000? V 2 = 400 V V 2 = 20 V V 2 = 200 V V 2 = 4000 V Well done! V 1 / N 1 x N 2 = 20/100 x 2000 = 400 V

What is the Secondary Voltage in a transformer whose input power P = 1500 W and Secondary Current = 30 A? V 2 = 50 V V 2 = 10 V V 2 = 45 V V 2 = V Well done! The output power = input power and P = VI, so: V = P/I = 1500/30 = 50 V

A current flows in the primary coil Blue TeamRed Team A changing magnetic field is induced in the primary coil The iron core channels the field lines to the secondary coil The iron core stays magnetised, so a V s is induced The motor effect causes the secondary coil to move The field lines cut through the secondary coil & a V s is induced A fixed magnetic field runs through the iron core Most field lines are lost but some reach the secondary coil An AC current flows in the primary coil 1 A current flows in the primary coil A changing magnetic field is induced in the primary coil The iron core channels the field lines to the secondary coil The iron core stays magnetised, so a V s is induced The motor effect causes the secondary coil to move The field lines cut through the secondary coil & a V s is induced A fixed magnetic field runs through the iron core Most field lines are lost but some reach the secondary coil An AC current flows in the primary coil 2 A current flows in the primary coil A changing magnetic field is induced in the primary coil The iron core channels the field lines to the secondary coil The iron core stays magnetised, so a V s is induced The motor effect causes the secondary coil to move The field lines cut through the secondary coil & a V s is induced A fixed magnetic field runs through the iron core Most field lines are lost but some reach the secondary coil An AC current flows in the primary coil 4 A current flows in the primary coil A changing magnetic field is induced in the primary coil The iron core channels the field lines to the secondary coil The iron core stays magnetised, so a V s is induced The motor effect causes the secondary coil to move The field lines cut through the secondary coil & a V s is induced A fixed magnetic field runs through the iron core Most field lines are lost but some reach the secondary coil An AC current flows in the primary coil 3 A current flows in the primary coil A changing magnetic field is induced in the primary coil The iron core channels the field lines to the secondary coil The iron core stays magnetised, so a V s is induced The motor effect causes the secondary coil to move The field lines cut through the secondary coil & a V s is induced A fixed magnetic field runs through the iron core Most field lines are lost but some reach the secondary coil An AC current flows in the primary coil 1 A current flows in the primary coil A changing magnetic field is induced in the primary coil The iron core channels the field lines to the secondary coil The iron core stays magnetised, so a V s is induced The motor effect causes the secondary coil to move The field lines cut through the secondary coil & a V s is induced A fixed magnetic field runs through the iron core Most field lines are lost but some reach the secondary coil An AC current flows in the primary coil 2 A current flows in the primary coil A changing magnetic field is induced in the primary coil The iron core channels the field lines to the secondary coil The iron core stays magnetised, so a V s is induced The motor effect causes the secondary coil to move The field lines cut through the secondary coil & a V s is induced A fixed magnetic field runs through the iron core Most field lines are lost but some reach the secondary coil An AC current flows in the primary coil 4 A current flows in the primary coil A changing magnetic field is induced in the primary coil The iron core channels the field lines to the secondary coil The iron core stays magnetised, so a V s is induced The motor effect causes the secondary coil to move The field lines cut through the secondary coil & a V s is induced A fixed magnetic field runs through the iron core Most field lines are lost but some reach the secondary coil An AC current flows in the primary coil 3

Learning outcomes What is a transformer? A Transformer is made of two coils and a laminated soft iron core. How is a current induced in the secondary coil? The primary coil carries an A.C. current which induces a changing magnetic field cutting through the secondary coil, where an A.C. current is induced. Is there a relationship between the number of coils and the output voltage?

Answer the following questions. 1.How many windings would a laptop transformer have in the primary coil, if V2 = 15 V and N2 = 250? (Hint: What is the mains voltage?) 2.The power output on the transformer box is rated at 90 W. What is the working current of the laptop? 3.What is the total resistance of the laptop? Homework