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In this segment of "Physics in Motion",
we're gonna talk about Ohm's law.
Ohm is the last name
of a German physicist and mathematician,
who, back in the 19th century,
solved a mystery that led to major advances in our ability
to use electricity safely and reliably.
Ohm figured out how three basic aspects of electricity
relate to each other.
Knowing how these three properties affect each other
is what lets engineers design circuits.
The paths that we use to send electricity
where we want it to go and do.
The three properties that Ohm studied are voltage,
current, and resistance.
Let's break it down.
Voltage is the difference in electrical potential
between two points.
It's like the pressure of a water pump
pushing current through a loop,
so the charge can be used to do work.
Current is the rate at which charge is flowing
in a circuit.
Similar to water flowing through a
Resistance is a material's tendency to resist
the flow of charge or current.
Like rocks or sticks blocking a channel of water.
And if you know two of these three,
you can figure out the last one
using the law Ohm discovered.
So, what Ohm did, back in the 1820's,
was to record a lot of data
measuring different voltages and wire links,
which led to his discovery about their relationship.
In his day, the materials used in these experiments
were handmade and not very exact.
So, results varied.
Scientists like Ohm were working with early forms
of batteries and wires,
but Ohm worked hard to make his work consistent
and easy to replicate
so that he knew his data were correct.
After years of work,
he felt he had proof of something
that people had been unsure about.
The longer the wire he used in his experiments,
the less current flowed through it.
That was because resistance increases
as the wire becomes longer.
That's important because it helps us to understand
how much electrical power is being used in the system.
The higher the resistance, the more heat is generated.
In the case of an old-school incandescent bulb,
the heat makes the filament shine brighter,
for instance.
It may seem simple now, but it was a breakthrough.
Today, we can measure voltage, current,
and resistance with this.
A multimeter.
Let's do a simple circuit test.
A circuit,
which you'll hear more about later on in this unit,
is a closed loop through which electrical charge
can continuously flow.
We'll make our circuit with a battery, some wire,
a light bulb, and a resistor.
When we connect the circuit,
that means I connect this wire here...
and the loop is closed, the bulb lights up.
Now, let's find out what the current is in the resistor,
and also measure its resistance.
If we connect a multimeter into the circuit, like this...
we can make it measure current,
which is expressed in Amps.
And it's reading out 176 milliamps,
or 0.176 Amps.
Now, if we change the multimeter settings...
we can measure the resistance in a unit called Ohms.
Named after our guy, Georg.
And it looks like the resistor's resistance is...
1.5 Ohms.
By Ohm's law, when we multiply these two together,
we get the calculated voltage of 0.26 volts.
In Ohm's day, measuring what we just did
would have been much more difficult.
But with modern equipment, it's relatively easy.
We can do a little something similar
to what Ohm did when he spent all that time recording data.
Let's measure the currents of some wires
of different materials and lengths.
Here's copper, aluminum, and steel.
Using our trusty multimeter,
let's hook up the leads and see what we get.
And then, we'll fill in the points on our graph
of voltage versus current, like this.
Adding the measurements for each material.
When we look at the ratio of the current to the voltage,
we see that when current changes,
the voltage changes.
And within each type of metal,
the straight line tells us the ratio is about the same.
That's the basic principle that Ohm discovered.
His work led to the realization
that current through a conductor,
like the metals seen in this graph,
is directly proportional to the voltage
across the same span of wire.
The ratio of voltage to current,
the slope of these lines,
is equal to the resistance through the wire.
The more recognizable version of his law
looks like this.
Voltage equals current times resistance.
There are a few exceptions to Ohm's law.
Not all materials are ohmic,
meaning they don't follow Ohm's law.
Their voltage and current aren't related
in the same way as other materials.
Doubling one doesn't necessarily double the other.
An example of a non-ohmic material is an LED light.
It can use much smaller current
to provide the same light as an old-fashioned filament bulb,
saving energy.
Even though not all materials follow Ohm's law,
it's still used by electrical engineers
all over the world.
One more thing about Ohm.
When he first published his findings
about electrical resistance in 1827,
the reaction was so bad
among the thought-leaders of the day,
he quit his job.
It was almost 15 years before he was recognized
for his work that changed the world.
It's a good lesson in sticking with things,
as well as a lesson on how to analyze electrical circuit.
That's it for this segment of "Physics in Motion".
We'll see you next time.
For more practice problems, lab activities,
and note-taking guides,
check out the "Physics in Motion" toolkit.