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Answers to Readers' Questions on Physical Science:

Ohm's Law

The following 9 comments and questions have been sent in. They are listed according to date.

List of first 10 items

• Is Ohm's Law really a law?
• What happens to the resistance of a conductor?
• How does a capacitor maintain polarity?
• Is electricity generated or produced?
• Why is I the symbol for current?
• Why doesn't Ohm's law apply to electric motors?
• Do light bulbs obey Ohm's law?
• Can we use ohms law in AC circuits?
• Use of Ohm's Law

Next 10

Is Ohm's Law really a law?

Question

November 9, 2009

dear sir!
i wants to know that is ohms law is really a law
if not, discuss it deeply
thanks for participation

ASAD - Pakistan

18636

Answer

In the sciences, they often refer to certain equations as "laws". A better expression might be to call it a "rule". Ohm's Law or Ohm's Rule concerns the relationship between voltage, resistance and current in a simple electrical circuit.

Although the rule or relationship holds most of the time, there are extreme voltages, currents or thin wires where it breaks down. Thus, it is a rule or law for only common situations.

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What happens to the resistance of a conductor?

Question

August 19, 2009

what happens to the resistance of the conductor if potensial difference across it is doubled ?

Ranjith - India

18163

Answer

The resistance is primarily fixed. If you double the potential difference (voltage) the current (amperes) will double according to V = IR.

But note that if the conductor overheats, the resistance can increase.

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How does a capacitor maintain polarity?

Question

January 2, 2009

HOW A CAPACITOR VOLTAGE MAINTAIN THE SAME POLARITY DURING CHARGING AND DISCHARGING OF CAPACITOR.

inayatu - Nigeria

17038

Answer

A capacitor can be used to store charges, either from static electricity or DC electricity. Since the current cannot pass across the dielectric spacer, the discharge is through the wires, away from the capacitor.

If AC electricity charges the capacitor, it changes polarity with each cycle, but with a delay or phase shift.

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Is electricity generated or produced?

Question

February 21, 2008

Is electricity generated or produced?

Justin - USA

15412

Answer

Both are correct. You would probably call electricity from a generator as being generated, but saying it is produced is also OK. Electricity from a battery is created or produced but not generated.

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Why is I the symbol for current?

Question

January 15, 2008

Can I know the reason why the symbol for current is I?

Eugene - Philippines

15144

Answer

Since "c" is used for the speed of light and "e" might be confused with energy, someone just decided to use I for electric current. They could have used "C" or even "A" for amperes.

It is a puzzle. I don't have any good reason. But also, why did they use "c" for the speed of light, instead of another letter?

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Why doesn't Ohm's law apply to electric motors?

Question

December 17, 2007

Tell me why ohm's law does not apply to electric motors?

Miguel - USA

14987

Answer

Ohm's Law states the relationship between voltage, current and resistance in an electrical wire. It does apply to the wire in a motor. But if you try to measure the relationship with the motor in the circuit, you are adding a factor that goes beyond Ohm's Law. Often resistance is replaced by impedance, which takes into account added factors.

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Do light bulbs obey Ohm's law?

Question

October 5, 2006

do light bulbs obey ohms law

adam - Grenada

12080

Answer

If you know the voltage and current into a light bulb, you can use Ohm's Law to calculate its resistance.

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Can we use ohms law in AC circuits?

Question

September 25, 2006

Can we use ohms law in AC circuits?so in an AC circuit voltage decreases what happened to the current?If it decrease or increase?According to ohms law the current is directly propotional to voltage .But in trans formers , we know that P=V*I. That means when voltage increases current decrease ?How can we explain this using ohms law?

karippars - India

12006

Answer

Ohm's Law works in AC circuits as long as there are no capacitors or inductors in the circuit. They will change the resistance to become impedance.

If there are only resistors in the AC circuit, Ohm's Law works fine. Otherwise, it becomes more complex.

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Use of Ohm's Law

Question

February 16, 2006

what would you use ohms law for in the current day practise

james - United Kingdom

10108

Answer

Ohm's law gives the relationship between voltage, current and resistance. If you know two of them, you could find the third value on an electrical appliance.

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