Course description

8.7 Ohm's law:

q  Ohm's Law states that:

At a constant temperature the potential difference (voltage) across an ideal conductor is proportional to the current through it.

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

Ø  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.

Ø  For biological work, it is often preferable to use the conductance, g = 1/R; In this form Ohm's Law is: I = g V

Ø 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.

Ø  This so-called force is called the electromotive force, or emf.

Ø  The SI unit for the emf is a volt.

                  Ԑ = I (R + r)

An emf source of 6.0 V is connected to a purely resistive lamp and a current of 2.0 amperes flows. All the wires are resistance-free. What is the resistance of the lamp?

Soln.: The gain of potential energy occurs as a charge passes through the battery, that is, it gains a potential of   Ԑ = 6.0 V. No energy is lost to the wires, since they are assumed to be resistance-free. By conservation of energy, the potential that was gained (i.e.   Ԑ = V = 6.0V) must be lost in the resistor. So, by Ohm's Law:

V = I R, R=V/I, R = 3.0 Ω

Problem: For the circuit shown below find the Voltage (V), the Current (I), and the Resistance (R).

Soln.:

Voltage   [ V = I x R ] = 2 x 12Ω = 24V

Current   [ I = V ÷ R ] = 24 ÷ 12Ω = 2A

Resistance   [ R = V ÷ I ] = 24 ÷ 2 = 12 Ω 

What will i learn?