Transformers and Electrical Safety

Transformers and Electrical Safety

By Rachel Wampler

Electrical Safety

• The voltage in a standard U.S. wall outlet is approximately 110V

• Many electrical appliances can not handle that many volts.

  • ​Computer speakers 9 V

  • Projections TV 15000 V

  • Phone charger 5 V

• A transformer changes the voltage for the appliance.​

Tansformers

• A primary coil creates a magnetic field in the iron core.

• Since the current in the coil is AC, the B-field is always changing.

• The iron makes the B-field go through the secondary coil.

• The changing B-filed means the flux in the secondary coil is also changing and thus induces an emf.​

1. Induced emf

2. Primary emf

3. Dividing

4. Transformer equation

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But P = IV...

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Transformers

Transformers

• A transformer that steps up the voltage, steps down current and vise versa.

• To keep electrical lines from getting too hot, the electrical companies use transformers to step up the voltage up to 11000V. The box on the electrical pole is a transformer that steps the voltage down to 220V. ​

Multiple Choice

A TV requires 15000V and 0.01 A to accelerate the electron beam.  The outlet in the house supplies 120V.  The primary coil of the transformer in the TV has 100 turns. 

A. How many turns should the secondary coil have?

B. How much current does the TV draw from the outlet?

• A. 12500 turns

  B. 1.25 A

• A. 15000 turns

  B. 0.01 A

• A. 15000 turns

  B. 1.25 A

• A. 12500 turns

  B. 0.01 A

Electrical Safety

• A standard electrical plug in the U.S. has:

  • Two grounds

    • ​White wire

      • Wide prong

      • Return through ground

    • ​Green wire

      • 3rd prong

      • Grounds the case​

  • "Hot" wire

    • Black/red

      • Carries the higher voltage​

Electrical Safety

• Circuit Breaker

  • If the current load gets too large, an electromagnet pulls a switch to stop the current

  • Stops wires from getting hot in short circuits that lead to fires.​

• Ground Fault Interrupter (GFCI)

• Both ​sides (hot and neutral) are wrapped around a metal toroid like transformer, but the number of loops are equal.

• Normally the induced current is 0 since the two sides cancel.

• If an imbalance occurs (like current going through a person and into the ground), an electromagnet pulls a switch​

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Electrical Safety