Physics II

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### Transcript

Ohm's law relates voltage, current, and resistance for an electrical component, or a circuit.

Voltage, V, current, I, and resistance, R, are fundamental to the operation of all electronic devices like radios, music players and computers as well as simpler electrical applications like house wiring, fuse boxes and home lighting. The circuits in all of these cases behave predictably and can be designed rationally because of Ohm's law.

This video will introduce circuit terminology, symbols and diagrams, and then demonstrate how to wire a simple circuit. In addition, the current through a component and the voltage across a component will be measured.

The output of a power supply or battery has both positive and negative terminals, which respectively define high and low values of a quantity called electric potential. The difference in this electric potential is voltage, measured in volts. To learn more about these terminologies, please see the video on electric potential in this collection.

A circuit is a network of components connected together to perform a particular function. Current is the movement of a quantity of charge per second and is measured in amperes, or amps, for short. Interestingly, only electrons, which have negative charge, move through wires in a circuit. Because of their negative charge, electrons flow in the direction opposite to that of current. Electrical current can only flow through wires and components connected in a complete loop, much like water current from a reservoir through a pump, into a water wheel and back into the reservoir.

To some degree all electrical elements impede the flow of current, like the bottleneck in a pipe that reduces the flow of water. Resistance describes this phenomenon, and is measured in ohms. Ohm's law defines resistance as voltage across a component divided by current through the component.

For components specifically called resistors, the resistance is approximately constant. The resistor on a common circuit board is typically a small cylindrical object with bands representing a color code for the resistance. By Ohm's law, the current through a constant resistance is directly proportional to the applied voltage and inversely proportional to resistance. In reality, the resistance of most materials generally increases as temperature increases.

The resistance of some devices, like diodes, also varies with the operating condition-that is, voltage and current-as well as other factors. A diode is a device that, to a very good approximation, allows current to flow in one direction only. As a result, it behaves as a one-way valve, passing current through a very low resistance in the "forward" direction and prohibiting current with an extremely high resistance in "reverse."

A light emitting diode, also known as an "LED", is a diode that is illuminated with the flow of forward current. Like a simple diode, an LED does not pass current in the reverse direction, in which case it is not illuminated.

With a simple relationship among voltage, current and resistance, Ohm's law-often expressed as V equals I times R-makes it possible to calculate any one of these quantities if the other two are known.

This video will show that the voltage across a component and the current through it may be easily measured. These experiments will also demonstrate Ohm's law in various circuits and illustrate the relationship between current and voltage for a resistor, a light bulb, and a light emitting diode.

The measurement apparatus consists of a voltage sensor, a current sensor, a power supply, a computer controlled measurement system, and the components to be tested.

To load the measurement software, double-click on the "Ohm's Law" icon on the computer desktop. After the program loads, the screen should display a graph, a table and in the lower left corner, boxes with the voltage and current measurements.

Click the "Zero" button and select "Zero All Sensors" to remove offsets in the data acquisition system. Voltage and current readings should be zero when the leads to the apparatus are not connected to anything.

Select the data that will be plotted by clicking on the axis label and checking the desired option. For the y-axis only, uncheck items that will not be plotted. Set the graph to show current on the y-axis and voltage on the x-axis.

Next, set the y-axis scale to negative 0.3 to positive 0.3 amps, and set the x-axis scale to negative 5 to positive 5 volts.

A resistance box, which can be set for different values of resistance, is used in the first experiment in order to observe how current varies with voltage for a resistor.

Set the resistance box to 100 ohms. Then wire together the power supply, voltage and current sensors, and resistance box as shown in the circuit diagram. Finally, set both the current and voltage outputs of the power supply to the maximum values.

Click the green arrow icon to start data collection. Slowly reduce the power supply voltage to its minimum value. Then, reverse the leads at the power supply and slowly increase the voltage back to its maximum value. This should result in a graph of current versus voltage, spanning the range from -5 to +5 volts. Repeat this process until the graph is free of noise, then store the data.

For the 100 ohm resistance, the plot of current versus voltage is a straight line. Perform a linear fit to the data and record the slope of the line. The slope should be very close to 0.0100 amp/volt, the inverse of the resistance.

Now set the resistance box to 200 ohms and repeat the experiment to obtain another plot of current versus voltage over the range of -5 to +5 volts. This time the slope should be very close to 0.00500 amp/volt, again the inverse of the resistance.

For a constant resistance, Ohm's law states that current through the resistor is proportional to the applied voltage and inversely proportional to resistance. This is apparent in the data for both the 100 ohm and 200 ohm resistors.

For the next experiment, replace the resistor box with a small incandescent light bulb, as shown in the schematic diagram. Set the voltage control on the power supply to maximum and begin data collection. Slowly reduce voltage to the minimum value then slowly increase voltage back to the maximum. The computer will display a plot of current versus voltage over the range of about +0.7 to +5 volts.

Reverse the power supply leads, set the voltage control to maximum, and repeat the process of reducing the voltage to the minimum value and increasing it to the maximum value again. The computer will display a plot of current versus voltage over the range of about -0.7 to -5 volts.

The plot of current versus voltage for the light bulb is not nearly as linear as for the resistors. The graph also shows that in general the current is higher at a given voltage when voltage is increasing, compared to what it is at the same voltage when voltage is decreasing.

When the voltage is increasing, the filament is warming up. With a filament that starts cooler, the resistance is lower and the current is higher. When the voltage is decreasing, the filament is cooling down from a higher temperature, so it has a higher resistance and a lower current at the same operating point.

Now plot current versus time instead of current versus voltage. To do this, change the horizontal axis to measure time.

Adjust the voltage to its maximum, so the bulb glows brightly. Then turn off the power supply. Click on the green arrow to start data collection and then turn the power supply back on.

Current through the light bulb is high immediately after the power supply is turned on, then drops to a lower, constant value. While the bulb is off, the filament is at room temperature and has a relatively low resistance.

When the bulb is first turned on, the current jumps to a high level because of that low resistance. However, the resistance of the filament increases significantly with temperature-as the filament heats, resistance grows and current drops. Eventually its temperature stabilizes and current is constant.

Finally, set the axis to display current versus voltage again and use a light-emitting diode-an "LED"-in place of the light bulb. The maximum current for common LED's is around 30 mA, so the current must be carefully monitored to prevent burning out the LED.

Use the procedure from the prior experiments to obtain a graph of current as a function of voltage for the LED. First apply positive voltage across the LED, and adjust the power supply voltage from maximum to minimum. Then switch the power supply leads and adjust the voltage from minimum back to maximum to observe the directionality of the LED.

The resulting plot shows that an LED allows current to flow only when the voltage is positive and greater than a certain threshold. Once the diode "turns on," current increases quickly as voltage increases. However, no current flows for negative voltage. This behavior demonstrates how an LED acts like a one-way valve for current.

Electronic gadgets are omnipresent in today's world, and Ohm's law has a role to play in every single one of these gadgets.

For instance, the bulb in a flashlight is designed to work with two 1.5-volt batteries in series. Therefore, a light bulb with a suitable resistance must be chosen, so that the batteries provide an appropriate amount of current to make the bulb shine brightly, without burning out. Ohm's law helps guide that choice of bulb.

Another application of Ohm's law is to limit the current supplied to a particular device, perhaps to reduce the risk of electric shock, or to protect the device itself. Ohm's law tells us that, for a given voltage, the higher the resistance, the lower the current. Therefore, by placing a resistance in series with the device, we can limit the current flowing through the device and thus prevent any potential damage.

You've just watched JoVE's introduction to Ohm's law. You should now understand the relationship between the voltage across an electrical component, its resistance, and the resulting current through it, as well as the differences in the electrical behavior of resistors, light bulbs, and light emitting diodes. Thanks for watching!