High Voltage Low current from a step up transformer

[john liang]

Hi,

From what I have read, if a bulb X is connected to the input voltage of a step-up transformer, and another similar bulb Y is connected to the output voltage (secondary coil), then Y will be brighter than X.

From Joule's law of P = V I, it seems that the output voltage will be higher and the current will be lower.

My question is, if the current in the secondary coil is lower, how come the bulb Y is brighter?

Thanks.

 

[Bill_K]

The current in the secondary is not a given quantity. It's determined by the voltage E and the resistance R of the bulb. Therefore the correct form of Joule's Law to use is P = E²/R, showing that the power increases as the voltage is increased.

 

[john liang]

The current in the secondary is not a given quantity. It's determined by the voltage E and the resistance R of the bulb. Therefore the correct form of Joule's Law to use is P = E2/R, showing that the power increases as the voltage is increased. - Bill_K

Isn't it a fact that if the output voltage of the secondary coil is say 5x higher than the input voltage of the primary coil, the output current is 1/5 of the input current?
Thanks

 

[Redbelly98]

Welcome to Physics Forums.

The current in the primary coil is different than the current in the lightbulb connected across it. They are in parallel with each other, so they have the same voltage but not (necessarily) the same current.

Therefore, your correct statement about the relative currents in the primary and secondary does not apply to the currents in the lightbulbs.

 

[arydberg]

The current in the secondary is variable depending on the load connected to it. The maxim current the secondary can supply is 1/5 of the input current. The input current is also variable and depends on how the transformer is loaded. You need to go back to E = IR and understand how current changes with load.