Download SPH 3U Lesson 6

SPH 3U Lesson 6: Oersted’s Principle (the Right-Hand Rule) and Solenoids
Recall that magnetic substances are magnetic because they have an electric current do to the alignment of
their atom’s electrons.
So what about when we pass an electric current through a conductor?
a) Hook up a power supply with two alligator clips attached to each other to make a complete circuit. Don’t
turn it on yet.
b) Get two compasses, and line then up so that they point north.
c) Position one of the wires over top of the compasses, in line with their needles pointing north.
d) What will happen when you turn on the power supply?
e) Turn on the power supply and slowly increase the voltage. Draw a diagram below, then use it to try and draw
the magnetic field lines.
f) Try reversing the current (Switch the alligator clips connected to the power supply). What happens?
Oersted’s Principle: When a charge is moved through a straight conductor, a circular magnetic field is created
around the conductor.
Right-Hand Rule (RHR) for a straight conductor: If you hold the conductor in your right hand, with your right
thumb pointing in the direction of the conventional current, your curled fingers will point in the direction of
the magnetic field lines.
Conventional current: Current defined as flowing out of the positive terminal of a source and back into the
negative terminal.
Conventional current was conceived during the discovery of electricity. We now know that it is not true;
because electrons can move, we know that they flow out of the negatively charged terminal and back into the
positive terminal. However, since many electricity rules were correctly discovered in the “old” days, we still use
conventional current in many cases (such as when applying the RHR).
So, conventional current is from positive to negative, whereas electron flow is from negative to positive.
Thankfully, the direction of current never matters as far as calculating V, I, or R, in any problems no matter
what the circuit design.
Use the Right-Hand Rule to predict which way the compass needles point in our demonstration.
Method of Drawing Current Direction and Magnetic Fields: We need a way to represent these 3-D situations
conveniently and accurately:
Imagine that the conventional current is represented by an arrow. If the arrow is coming at you (if the current
is travelling out of the page towards you), you would only see the tip of the arrow. If the arrow was going away
from you (current travelling into the page away from you) you would see a cross made by the feathers on the
tail of the arrow.
Current out of the page:
Circle is the wire, dot shows
that conventional current goes out of page
(towards you)
Current into the page:
Circle is the wire, cross shows that conventional current
goes into the page (away from you)
Use the Right-Hand Rule to add magnetic field lines to both diagrams above.
Oersted’s discovery allowed us to use electricity to control magnetism (turn it on and off, switch its direction)
allowing us to create all manner of devices.
Hmwk: pg. 556 #s 1, 2, 5
Solenoids
Ampere’s Experiment: Ampere (the scientist) ran currents through two separate straight conductors. What do
you think happened?
Video: https://www.youtube.com/watch?v=he9G1wtSV6c
Explanation:
What effect would a current have through a loop of wire? Grab one of the loop apparatuses and some
compasses and check it out.
Diagram/Explanation:
A solenoid is a conductor that is coiled into loops. The magnetic field created when current is passed through it
is similar to that of a bar magnet:
Also see diagram pg. 560
We can use a solenoid to make an electrically powered bar magnet- an electromagnet, which is a device that
produces a magnetic field when current is passed through it.
The strength of an electromagnet can be increased by increasing the number of loops in the coil, increasing the
current, or putting a core of material that is easily magnetized (such as iron) that the loops are wrapped
around.
Right-Hand Rule for a Solenoid: The fingers of your right hand wrap around the coils in the direction of
conventional current, and your right thumb points in the direction of the north magnetic pole of the field.
Read the applications pg. 561, then answer pg. 562 #s 1 – 4.
Solenoid Investigations
1st: Use one of the wire-nail solenoids to pick up some iron nails. Pick them up, then turn off your power
supply. What happens? Why?
2nd: Safety: See how hot the smaller solenoid with the black wire gets
3rd: Demonstrate the small speaker
4th: Obtain one of the large solenoids. Hook it up to a power supply and a voltmeter. Get a bar magnet.
Turn on the power supply and dial it up until you get a voltage reading. Move the N end of the bar magnet into
and out of the end of the solenoid. What happens?
Now do the same with the S end. Is there any difference?
Turn off the power supply. Put the N end of the magnet inside the solenoid, then turn on the power supply.
What happens?
Try the same thing with the S end (turn the power off in between)