MAGNETIC FIELDS AND
ELECTRIC CURRENT
Electromagnetism: when an electric
current flows through a wire a
magnetic field is created.
1
Conductors



A magnetic field is created
whenever an electric current flows
through a conductor
The conductor can be a “straightline” conductor (i.e. an uncoiled
wire)
Or it can be a coiled wire called a
solenoid or a coil
2
Solenoid




Wire wrapped around a
core (tube)
Have a current flowing
through them
Have a magnetic field
Their magnetic fields
look like the magnetic
field around a bar
magnet.
Electromagnet
3
Cores





A Core is the object that is inserted
into the solenoid, creating an
electromagnet.
Different materials influence the
strength of the electromagnet.
Different metals can be used for the
core: iron, steel, nickel or cobalt.
Iron is most commonly used because
when you turn off the electricity it
demagnetizes.
Metals like steel remain magnetized
thus creating a permanent magnet.
4
The “Hand” Rules


There are two sets of “hand” rules that can be used to
determine the direction of the magnetic field around an
electrical conductor
Your choice of which set of rules to use depends on
which system you use to describe electric current:

or




Electron Flow
Conventional Current
Both systems are correct, and both work, but you must
be careful not to mix them up
The “Left Hand Rules” are covered in slides 6 to 11
The “Right Hand Rules” are covered in slides 12 to 17
5
The Left Hand Rules
6
Electron Flow


If you use “electron flow” to
describe the direction of the current,
it flows from the negative to the
positive terminal
You must use the “Left Hand Rules”
7



There are two left “hand rules” that
will help you to determine the
direction of the magnetic fields
created by electric currents.
The 1st Left Hand Rule is used with a
straight line or uncoiled wire.
The 2nd Left Hand Rule is used with a
solenoid (or coil)
8
Straight Line Conductors (Uncoiled
Conductors)
1.
2.
3.
4.
5.
Find the positive and
negative ends of the
wire
The electrons flow from
negative to positive
Point the thumb of your
left hand to + with hand
flat on page
Your knuckles will be on
the NORTH
Your fingertips will be
on the SOUTH
+
Direction of the
electron flow
9
Solenoids (or coils)
1.
2.
3.
4.
5.
Find the positive and
negative ends of the wire
The electrons flows from
negative to positive
Draw (or imagine) arrows
on the front of the coil
pointing in the direction of
the electron flow.
Place your hand on the page
with your fingers pointing in
the direction of the electron
flow (in the direction of the
arrows).
Your thumb will point
towards the north end of
the solenoid.
Up behind the core and
Down in front of the core.
N
+
S
-
10

The Left Hand Rules will help you
with all of the electromagnetic
problems …got it?
11
The Right Hand Rules
12
Conventional Current


If you use “conventional current”
to describe the direction of the
current, it is said to flow from the
positive to the negative terminal
You must use the “Right Hand
Rules”
13



There are two “hand rules” that will help
you to determine the direction of the
magnetic fields created by electric
currents.
The 1st Right Hand Rule is used with a
straight line or uncoiled wire.
The 2nd Right Hand Rule is used with a
solenoid (or coil)
14
Straight Line Conductors (Uncoiled
Conductors)
1.
2.
3.
4.
5.
Find the positive and
negative ends of the
wire
The conventional
current flows from
positive to negative
Point thumb of your
right hand in the
direction of the
conventional current
Wrap your fingers
around the wire
Your fingers will point in
the direction of the
magnetic field
Direction of the
magnetic field
Direction of the
Conventional Current
+
15
Solenoids (or coils)
1.
2.
3.
4.
5.
Find the positive and negative
ends of the wire
The conventional current flows
from positive to negative
Draw (or imagine) arrows on
the front of the coil pointing in
the direction of the
conventional current.
Wrap your right hand around
the solenoid with your fingers
pointing in the direction of the
electron flow (in the direction
of the arrows).
Your thumb will point towards
the north end of the solenoid.
Up behind the core and
Down in front of the core.
S
N
-
+
16

The Right Hand Rules will help you
with all of the electromagnetic
problems …got it?
17
Solenoid Example - # 1
1.
2.
3.
-
+
4.
5.
Find the positive and
negative ends of the
wire
Determine the
direction of the
electric current
Wrap your fingers
around the coil
pointing in the same
direction as the
current
Thumb points “N”.
Other end is “S”.
Lines of Magnetic
force run N to S
(Nerd to Sexy).
18
Solenoid Example # 2
1.
2.
3.
+
-
4.
5.
Find the positive
and negative
ends of the wire
Determine the
direction of the
electric current
Wrap your fingers
around the coil
pointing in the
same direction as
the current
Thumb points “N”.
Other end is “S”
Lines of Magnetic
force run N to S
(Nerd to Sexy).
19
Determining the Direction of the Magnetic
Field around a Straight Line Conductor
+
+
+
+
-
-
20
Effects of a Straight Line Conductor on
a Compass
Determine the
direction of the
Magnetic Field
-
+
Compass
Compass arrow
Points in the same
direction as the
Magnetic Filed
21
The Continuous Magnetic Field Around a
Straight-line Conductor
-
The
magnetic
+ field
If
we
reverse
the
flows in a continuous
terminals,
the
circle around
the will
wire
magnetic
field
perpendicular to it.
flow
in the opposite
 Determine the
direction
direction of the
 And
the magnets
magnetic
field
point the
other
 will
Compasses
placed
way…
around the wire will


point in the direction of
the magnetic field
+
22
Determine the direction that the
compass would point if it were
placed on top of the wires as
illustrated.
Straight Line Conductor
Examples
+
B
A
+
-
-
+
+
D
C
-
23





Hand Rule for Straight Line Conductors –
Your thumb points in the direction of the
electric current
Your fingers will point in the direction of
the magnetic field
North and South are never on the ends
of the wire
There is no North or South Pole. The
magnetic field flows in a continuous circle
around the wire.
The magnetic field flows in the same
direction as the compass points.
24