Physics
Electricity and Heat
Sensors:
Loggers:
Voltage, Current
Any EASYSENSE
Logging time: 30 seconds
34a Ohm’s law (current vs. voltage)
Read
The current through a resistor depends upon the applied voltage. When the voltage is changed the current
also changes.
In this investigation, you are going to apply different voltages across a resistor and then measure the current
flowing through the resistor.
In this version of the experiment the current is plotted against voltage directly.
Preliminary question
Draw a sketch graph to show how you expect the current to change as the voltage increases. Make the Yaxis the current.
Potential
Divider
Current Sensor
Input 1
Input 2
Current
Voltage
Voltage Sensor
What you need
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
An EASYSENSE logger
A Smart Q Voltage sensor ±12 or 20 V.
A Smart Q Current sensor ±1 A.
Resistor 22 Ω.
2.5 V 0.3 A lamp.
3 V battery pack.
22 Ω potentiometer.
Wires / patch leads
LDR for the extension activity.
Thermistor for the extension activity e.g. an NTC disk thermistor 300 Ω at 250ºC and 24 Ω at 1,000ºC.
What you need to do
1. Assemble the apparatus as shown. Leave the switch open to protect the cells. Connect the Current
sensor to input 1 and the Voltage sensor to input 2 of the logger.
2. From the EasySense software’s Home screen select Open Setup (or File, Open Setup). Open the
file Data Harvest Investigations (Edition 2) \ Setup files \ Physics_Electricity and Heat_L3 V2 \ 34a
Ohms Law (Current vs. Voltage).
3. From the Tools menu select Test Mode, the readings of the sensors will be displayed in the data
boxes.
4. Switch the current on.
Electricity and Heat
34a - 1 (V2)
5. Make sure that the voltage reading is zero.
6. Click on Start, and slowly increase the current and voltage values using the potentiometer. Aim to
get to the maximum value after about 15 seconds and then slowly reduce so that it is at zero before
the end of the 30 seconds.
7. If the curves are reasonably smooth, name the file and save it. If not repeat.
Results and analysis
You will have obtained graphs of Current vs. Voltage. Use Sensor Settings from Options to scale the
graphs.
Questions
The questions refer to the graph of Current vs. Voltage.
1.
2.
3.
4.
Describe in detail how the current changes as the voltage increases.
State the relationship between voltage and current for the resistor.
Refer to Ohm’s law in the Learning Summary. Does your resistor obey Ohm’s law?
Resistance is the ability of a material to oppose the flow of an electric current. It can be calculated
using the equation:
V
R=
Where:
I
R = resistance in ohms (Ω)
V = voltage in volts (V)
I = current in amps (A)
Calculate the resistance of your resistor for a medium value of the current.
5. Does the resistance vary as the current increases? To answer this question you can use the
Gradient function in the toolbar and then generate a resistance data set as follows:
To use the gradient function
Select Options, then the X-Axis tab. Select the button next to Time.
Select the Gradient icon and then run the curser along the graph. You will see the value of the
gradient in the box at the top. The value of the gradient is equal to the reciprocal’ (1/R) of the
resistance, because
R=
V
, so
I
I=
V
, which is the same as,
R
I=
1
×V
R
(y = m x + c, where c = 0)
6. Describe, in detail, how the resistance changes as the current increases
To generate a resistance data set from the voltage and current data
•
•
•
•
•
•
From the Tools menu, select Post-log Function.
From the Preset functions select Electricity, then Resistance.
Select the correct channels for the current and voltage channels (data).
Name = Resistance
Units = ohms
Multiplier = follow the information in the wizard to correct for the range of the sensors.
Select Options, then the X-Axis tab. Select the button next to Channel. Click on the axis labels to
make the X-axis Current and the Y-axis Resistance.
7. Which method i.e. using the gradient function or generating resistance data, do you think gives you
the most useful way of seeing how the resistance changes as the current changes? Explain why you
think so.
Electricity and Heat
34a - 2 (V2)
Extension activity
a. Repeat the procedure outlined in this experiment with a small lamp. It is advisable to allow time for
the temperature of the filament to stabilise at each value of the voltage. Use SnapShot and increase
the voltage in 0.2 V intervals from 0 V. Analyse the data as above.
•
•
•
Is the relationship between I and V, for the lamp filament, the same as for the resistor above?
What happens to the resistance as the current increases?
What happens to the temperature of the filament as the current increases?
2. Therefore, what happens to the resistance of the metal filament as the temperature increases?
3. Repeat the experiment for other types of resistor e.g. an LDR, a thermistor.
Learning summary
Ohm’s law
From your results, you have learned the following.
For the 22 Ω resistor;
•
•
•
The graph of I vs. V was a straight line through the origin.
Therefore, the voltage was directly proportional to the current.
The resistance was independent of the voltage and current.
This resistor therefore obeyed Ohm’s law which states: The current flowing in a resistor is directly
proportional to the voltage across it, provided the temperature remains constant.
Resistance
As we have stated earlier resistance is the ability of a material to resist the flow of current. Electrons moving
in the material interact with atoms and give them energy. Heat is produced and the temperature of the
material will rise if the heat cannot be released quickly enough.
We calculate resistance using the equation: -
R=
V
I
The unit of resistance is the ohm, which is named after the physicist Georg Simon Ohm who discovered the
relationship between current and voltage. This relationship became known as Ohm’s law.
Resistance of metals
If you measured the resistance of the bulb filament, you would have found out that its resistance increased
markedly as it got hotter. The resistance of metals increases as the temperature increases.
Different types of resistor
There are a variety of resistors because not all materials obey Ohm’s law.
•
Ohmic resistors
These are resistors that obey Ohm’s law. They are, typically, made of metals or carbon.
•
Light Dependant Resistors - LDR
Resistors that change their resistance when the light falling on them varies. The resistance increases as
light intensity drops. These have widespread use in light control circuits e.g. switching on street lamps in
the evening, and light meters.
•
Thermistors
Resistors whose resistance decreases as the temperature increases. Used in temperature measuring
devices and temperature control systems, e.g. to measure the temperature of the water in a car cooling
system.
Electricity and Heat
34a - 3 (V2)