P2bL4 Ohm's Law

Key Words

Current - movement of charge

Diode - component that allows only one way flow of a current.

Lamp - bulb.

Potential Difference - energy per unit charge

Variable resistor - a resistor whose resistance can be changed.

Voltage - potential difference, "electrical pressure".

Grade E

Ohm's Law states:

The potential difference (voltage) across the ends of a conductor is directly proportional to the current, provided the temperature stays the same.

We can take readings from an experiment like this:

If you are not sure what the symbols mean, go back and review them.

We change the variable resistor and take data off the voltmeter and ammeter.  The switch makes sure that the battery doesn't go flat, or the resistor heat up.  So it is turned off between readings.

If we plot them on a graph, we get:

Note:

• The graph is a straight line that goes through the origin.  No voltage, no current.

• It's a line of best fit; the points don't fit exactly onto a straight line.  No wonky donkeys, please.

• If the voltage doubles, so does the current.  The relationship is directly proportional.

• We have assumed that the temperature has stayed the same.

We can give the equation:

resistance = volts

                   amps

Grade C

If we reverse the voltage, the current reverses too, so we can extend the characteristic graph as below:

For a lamp, the graph is like this:

As the voltage goes up, the current goes up, but not as much.  The resistance increases as the bulb gets hot.

The diode acts as a one-way electrical valve, allowing the current to go one way only.  The diode conducts when it is forward-biased, but does not conduct when it's reverse-biased.

Below about 0.5 V, very little current flows.  Between 0.5 and 0.6 V, there is a sudden increase in the current.  This is the activation voltage.

If the reverse voltage is too big (about 30 V), the diode suddenly conducts.  It's a bit like the non-return valve bursting.  This is called the breakdown voltage, and can destroy the diode.

Grade A

For a resistor, the key thing is to keep the temperature the same.  A conductor that obeys Ohm's Law is called an Ohmic conductor.  If we plot the voltage against the current, the gradient gives us the resistance.

For a light bulb the resistance gets bigger as the filament gets hotter.  The electrons collide with vibrating metal atoms, transferring energy to make them vibrate more.  The more the vibration, the more the chances are of a collision.  The wire gets hotter and hotter until it glows white hot.  If the voltage is too big, the filament will melt and break.  A light bulb acts as a non-ohmic conductor

Often the tungsten starts to evaporate from the filament and is deposited on the glass of the bulb.  The filament gets thinner and even hotter at the weak point.  Eventually it breaks and the bulb is a dud.  You can see the black deposits on an old bulb.

Diodes when forward biased have quite a low resistance, so can pass a big current.  With semi-conductors, the resistance goes down as they get hotter.  They can go into thermal runaway and will be destroyed.  Light emitting diodes are very sensitive, so need a current limiting resistor in series.