High Voltage Low current from a step up transformer

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Discussion Overview

The discussion centers around the behavior of a step-up transformer, specifically addressing the relationship between voltage, current, and brightness of bulbs connected to the primary and secondary coils. Participants explore concepts related to Joule's law and the effects of load on current and voltage in the context of electrical circuits.

Discussion Character

  • Technical explanation, Conceptual clarification, Debate/contested

Main Points Raised

  • One participant notes that bulb Y connected to the secondary coil of a step-up transformer appears brighter than bulb X connected to the primary, despite the lower current in the secondary coil.
  • Another participant argues that the current in the secondary is determined by the voltage and resistance of the bulb, suggesting that higher voltage leads to increased power output.
  • A participant questions the relationship between output voltage and current, proposing that if the output voltage is significantly higher, the output current should be proportionally lower.
  • One response clarifies that the current in the primary coil differs from the current in the lightbulb connected to it, indicating that they are in parallel and thus have the same voltage but potentially different currents.
  • Another participant emphasizes that the current in the secondary is variable and depends on the load, stating that the maximum current the secondary can supply is inversely related to the input current.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between voltage, current, and brightness of the bulbs, indicating that the discussion remains unresolved with multiple competing perspectives on the topic.

Contextual Notes

Participants reference Joule's law and the behavior of electrical components under varying loads, but there are unresolved assumptions regarding the definitions of current and voltage in the context of the transformer and connected loads.

john liang
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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.
 
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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.
 
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
 
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.
 
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.
 

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