Question

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?

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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