Day 4: Magnetism & Electricity
Danish Physicist Hans Oersted made an accidental discovery in 1819. He had a compass
(with its needle pointing, as usual, to magnetic South) next to a wire. He connected a
wire to the battery and found that the wire “induced” the compass needle to swing away
from magnetic north. Thus, he found that:
When current flows through a wire, a magnetic field is created around the wire.
In-class Experiment
A
B
A
+
wire loop
-
1. Lie a compass on a table. Find the direction of
“magnetic north”. Explain how you know what
direction this is.
_______________________________________
2. Which direction (CW or CCW) would the
current move in the above figure? ______
Indicate this direction with an arrow marked “I“
3. Which direction (CW or CCW) would electrons
move? _____ Indicate this direction with an
arrow marked “e- “
4. Place the loop of wire UNDER the compass as
shown. Then, hook up the battery (while
watching the needle). What happened to the
compass?? Indicate this on the drawing above.
_________________________________________________
_________________________________________________
5. Grab section AB of the wire with you’re your
RIGHT hand, thumb pointing in the direction of
the CURRENT. Which way are your fingers
curling ABOVE the wire? Are they in the same
direction as the needle or the opposite direction?
B
+
-
A) Which direction (CW or CCW) would the
current move in the above figure? ______
Indicate this direction with an arrow marked “I“
B) Which direction (CW or CCW) would
electrons move? _____ Indicate this direction
with an arrow marked “e-I“
C)
Place the loop of wire ON TOP OF the
compass as shown. Then, hook up the battery
(while watching the needle). What happened to
the compass?? Indicate this on the drawing
above.
__________________________________
__________________________________
E) Grab section AB of the wire with you’re your
RIGHT hand, thumb pointing in the direction of
the CURRENT. Which way are your fingers
curling UNDER the wire? Are they in the same
direction as the needle or the opposite direction?
__________________________________
__________________________________
_________________________________________________
_________________________________________________
6. What should happen if you reverse the
POLARITY of the battery? Think about it!!!!
Now, do it. Did what you expect to happen
actually happen?
F) Again, what should happen if you reverse the
POLARITY of the battery? Think about it!!!!
Now, do it. Did what you expect to happen
actually happen? Explain below.
__________________________________
__________________________________
How do we find the direction of the magnetic field created by a current carrying wire?
We use the RIGHT HAND RULE. Since we will learn three right-hand rules in this
chapter, we will call this Right-Hand Rule #2 (RHR-2).
What is the magnitude of the magnetic field? It changes, of course, since the strength
decreases as you move away from the wire. We can calculate the magnitude of the
B-field using
B
o I
2d
I is the current (measured in Amperes), d is the perpendicular distance measured from the
center of the wire (in meters), and 0 is a constant known as the magnetic
permeability of free-space, which is equal to
4  10 7 T 
m
A
What is “T”. It is the unit of Magnetic Field Strength, which is known as the Tesla. It
should be noted that 1T 1N /(C ms ) .
If we were to graph the strength of the magnetic field around a current carrying wire as a
function of distance, we would get

It should be noted that the equation above is only valid for straight, very long wires. The
equation can be derived, but only using calculus techniques that will be learned in AP 1-2
Right-Hand-Rule #2 (RHR-2) Practice Worksheet
For a current carrying wire, hold the wire in the palm of your hand with your
_______________ in the direction of the _______________. Your _______________
will curl around the wire indicating the ________________________________. The
sign convention for magnetic fields uses the symbol ______ to denote “into the paper”
and the symbol _______ to denote “out of the paper”.
1. Given the battery combination below, indicate the direction of the electrons, the
current, and the magnetic field around the wire between pts A and B.
a)
b)
A
+
-
A
-
B
+
B
2. Given the current carrying wires shown below, indicate the direction of the
magnetic field at points A, B, & C.
a)
B
D
B
D
A
b)
A
C
C
3. Given the two current carrying wires at the
right (in which both wires have currents
flowing out of the paper), describe the
contribution due to each wire and the net
effect at each of the points.
1
A
2
B
Point A: ____________________________________________________
Point B: ____________________________________________________
Point C: ____________________________________________________
C
4. For the current-carrying conductors shown (with their magnetic fields), indicate
the direction of electron flow in each wire.
5. Four current-carrying wires are shown below. The direction of the conventionalcurrent is shown. Draw the magnetic field lines around each one.
6. REVIEW: Using the magnetic-domain
model, draw what the magnetic domains
would look like in each of the substances
shown below.
Unmagnetized substance
Magnetized Substance
7. Draw the magnetic field generated around the single loop and around the
SOLENOID (coiled wire) below.
I
Day 4: Charged Particles Moving Through Magnetic Fields
Accelerated: Read pp. 770-772 of Holt Physics and do problems 1-10 (skip 3) below.
Honors: Read pp. 770-772 of Holt Physics and complete ALL problems below.
1) Fully explain why we use RHR-2 and how to use RHR-2.
2. Find the strength of the magnetic field at a distance of 4 cm from a wire carrying a
current of 15A.
3. Two wires are shown below (perpendicular to the plane of the page), each carrying 5A
of current that is flowing INTO the page, as indicated by the “X”.
1
A
2
B
C
5 cm
a) Find the magnetic field at pt A (both magnitude and direction) due to wire #1 only.
b) Find magnetic field at pt A (both magnitude and direction) due to wire #2 only.
c) Find the net magnetic field at pt A (both magnitude and direction) due to both wires.
d) Find the net magnetic field at pt B (both magnitude and direction) due to both wires.
e) Find the net magnetic field at pt C (both magnitude and direction) due to both wires.
4. Draw the complete magnetic field around each of the objects shown below.
N
S
e- flow
shown
After reading pp. 770-772 in the HOLT Physics book, answer the following questions.
5. What does the magnetic field of a current-carrying loop resemble?
6. What is a Solenoid?
7. What does a solenoid act like when current flows through it?
8. How could you increase the magnetic field inside a solenoid?
9. Compare and Contrast the magnetic field inside a solenoid to the field outside the
solenoid.
10. Explain why, using the concept of DOMAINS, the magnetic field inside a solenoid is
increased by inserting an iron core at its center.