Magnetic Flux of a Current Carrying Wire E-practical
A realistic 3D virtual physics experiment
A virtual experiment that investigates the field strength of a magnetic field by observing the force it exerts on a current carrying wire; zero the scales, switch on the power, adjust the voltage, take readings from the scales - just as with an actual experiment.
Realistic experimental technique
E-practicals are 3d, highly realistic emulations based on a blend of theory and the actual practicals, giving an authentic experience of doing the actual practical in a laboratory. Students operate the apparatus, make measurements and obtain their own results. It is designed to reproduce the experience of performing the practical rather than simply demonstrating the underlying physics. Like a real experiment, it can be done badly. Careless setup or measurements can produce poor results, so students have to apply experimental technique and take care with their measurements.
Useful when laboratory access is limited
Ideal for preparation before a practical, revision afterwards, missed practicals, home study, or demonstrating an experiment when setting up the real apparatus isn't practical.
Works directly in a browser
No installation is required. Windows, Mac and suitable mobile devices can run the WebGL e-practical.
Watch it in Action
Watch a video of the experiment here.
This shows how to use it and how to collect the data.
School/Department licence £9.99
Valid for one year.
Introduction
This experiment determines the field strength of a magnetic field by observing the force it exerts on a current-carrying wire. This surprisingly simple experiment works by creating a magnetic field by using permanent magnets. A wire is connected to a power supply and the force on the wire is measured using simple kitchen style electronic scales. Weight is actually a force, so it is easily measured using kitchen scales. But what about the force due to the weight rather than the magnetic field, how do we ignore that? Easy, we zero the kitchen scales when it is only holding the magnet.
The objective
To find how the strength of a magnetic field due to a wire varies with the current through the wire.
The apparatus
You will need:
• A power supply
• Copper wire
• Digital kitchen-type scales with zero function
• Multimeter
• Wires and crocodile clips
• Two magnets in a U-shaped yoke
• Wooden mounting blocks
The power source is connected in series with an ammeter and a copper wire.
The copper wire is supported between an arrangement of magnets in a U-shaped yoke, which provides a close approximation to a uniform field. The yoke containing the magnets sits on the scales, which measure the force on the yoke from the magnetic field.
The variables
The independent variable is the current. The dependent variable is the strength of the magnetic field as measured by the scales.
The Physics
The force on a single charge q moving at speed v due to a magnetic field B at right angles to the movement of the charge is given by:
F = qvB
If there are n charges per unit volume in our wire of length L and cross-sectional area A, then the total number of charges N is given by:
N = nLA
Therefore, the total force will be given by:
F = NqvB = nLAqvB
The current in a wire i is given by:
i = nqvA
This is substituted into the previous equation to give:
F =BiL
Where B is the magnetic field, L is the length of the wire in the field (which is the length of the magnets), and i is the current. Therefore, the magnetic field is given by:
B = F/Li
When you plot the mass registered on the scales against the current, the gradient of your graph gives you a value for m over i. Consider forces on the wire. The force from the magnetic field upwards as measured by the scales is BiL.
The force downwards on the wire due to gravity is:
F = mg
The forces must balance, as the wire does not move; therefore:
BiL = mg
Therefore, the result for B is given by:
B = mg/iL
The gradient of the graph, mass against current, is m/I; substituting this into the previous equation gives you a value for the magnetic field.
The method
The electronic scales indicate the weight of the yoke and magnet assembly. We want to ignore this weight and concentrate on the force due to the current in the wire and the magnetic field. Zero the scales. The scales will now only show any additional weight due to the force from the interaction of the magnetic field and the current in the wire. Normally we would expect the wire to be pushed upwards, but the wire is being held rigidly in the two brackets on either side of the scales, so the force will press on the scales and cause a positive reading.
Switch on the power supply. With the power on, set the maximum current to a little over 5 amps. You should now adjust the voltage until the meter reads as close to 1 amp as you can get, as shown here:
Note the reading of the amps and the weight indicated on the scales. Now repeat this for currents of 2, 3, 4, and 5 amps.
What the Students do
You can zero the scales by clicking on the red zero knob.
You can switch on the power supply by clicking the red on/off button on the supply.
The length of the wire is 8.5 centimeters.
The Results
Complete this table of results:
Plot a graph of m against i and calculate the gradient. Use the formula given in the Physics section to calculate the strength of the magnetic field.
You should obtain a result of around 0.05 Tesla.
Further Discussion
Would the experiment work as well with a spring balance?
Justify your answer.
This section is adapted from material developed by Dr Robert Lucas and is related to the book High School and Undergraduate Physics Practicals, published by CRC Press.