A step-up transformer turns 120 V into 240 V. Twice the voltage, out of the same box. Did you just get something for nothing?
In earlier grades you saw that energy cannot be created or destroyed. And back in the Circuits topic you saw electrical power: P = IV, power is current times voltage. Those two facts answer the question.
An electric scooter's motor runs at 30 V and carries 5 A. What is its power, in W?
A transformer creates no energy. It only passes power along, from the primary circuit to the secondary circuit.
So the secondary's output power can never exceed the primary's input power.
Power is current times voltage. If the voltage goes up and the power cannot, the current must come down.
The doubling transformer takes in 120 V at 4 A: input power 120 × 4 = 480 W.
Its secondary gives 240 V. The most it can supply is 2 A, because 240 × 2 = 480 W: the same power, never more.
A transformer that doubles the voltage can supply at most half the current.
A transformer steps the voltage up four times. The current into its primary is 10 A. What is the biggest current the secondary can supply, in A?
Carys says a step-up transformer gives you extra power for free, because the voltage grows. Is Carys correct?
A transformer triples the voltage. The current into its primary is 9 A. What is the biggest current the secondary can supply?
The output power can never exceed the input power.
The voltage goes up three times, so the biggest current comes down three times.
9 A ÷ 3 = 3 A
The secondary can supply at most 3 A.
A student designs a transformer meant to put out more power than goes in. Why must the design fail?
Energy cannot be created or destroyed.
A transformer only passes power along.
So the output power can never exceed the input power.
No transformer can put out more power than it takes in.
Tomas says no transformer can supply more power than it takes in. Is Tomas correct?
A transformer doubles the voltage. What happens to the biggest current its secondary can supply?
A substation transformer steps 120 V up to 480 V. The current into its primary is 6 A. What is the biggest current its secondary can supply, in A?
Why can a step-up transformer not raise the power?