Question

State Ohm's law in words. For what type of circuit elements does it correctly describe the...

State Ohm's law in words. For what type of circuit elements does it correctly describe the behavior?

Does a light bulb have a constant resistance? Explain. Is a light bulb ohmic?

Does a resistor have a constant resistance? Explain. Is a resistor ohmic?

Homework Answers

Answer #1

. Ohm's Law deals with the relationship between voltage and current in an ideal conductor. This relationship states that:

The potential difference (voltage) across an ideal conductor is proportional to the current through it.

The constant of proportionality is called the "resistance", R.

Ohm's Law is given by:

  • V = I R

where V is the potential difference between two points which include a resistance R. I is the current flowing through the resistance. For biological work, it is often preferable to use the conductance, g = 1/R; In this form Ohm's Law is:

  • I = g V

2. Material that obeys Ohm's Law is called "ohmic" or "linear" because the potential difference across it varies linearly with the current.

3. Ohm's Law can be used to solve simple circuits. A complete circuit is one which is a closed loop. It contains at least one source of voltage (thus providing an increase of potential energy), and at least one potential drop i.e., a place where potential energy decreases. The sum of the voltages around a complete circuit is zero.

4. An increase of potential energy in a circuit causes a charge to move from a lower to a higher potential (ie. voltage). Note the difference between potential energy and potential.

Because of the electrostatic force, which tries to move a positive charge from a higher to a lower potential, there must be another 'force' to move charge from a lower potential to a higher inside the battery. This so-called force is called the electromotive force, or emf. The SI unit for the emf is a volt (and thus this is not really a force, despite its name). We will use a script E, the symbol , to represent the emf.

a)

A light bulb is not an ohmic resistor since the current through it varies NON-LINEARLY with the applied voltage.

Ohmic resistors have a linear relationship between applied voltage and the resulting current.

b)

The term, ohmic resistor, is actually a redundancy. Resistors by definition have a (more or less) constant resistance, expressed in ohms. Other devices whose resistance is dependent on some variable have other, distinct, designations. Thermistors show a great variation (sometimes negative) with temperatures, and varistors show a large variation of resistance with applied voltage.

It can be argued that all resistors are thermistors, since all known materials have resistivities which vary with temperature, so ohmic resistor might be used to refer to an ideal resistor, whose resistance would be invariate under all conditions.

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