Transcript File - Science with Ms. Tantri

Electricity & Magnetism
How are electricity & magnetism related?
Warm-up: True or False?
1. Magnets attract all metals.
2. If you cut a magnet in half, one end will be a north
pole, the other end will be a south pole.
3. North poles are made of positive electric charges,
and south poles are made of negative electric
charges.
Conventional Current vs. Electron Flow
 Electron Flow Model:
 Electrons flow from the
negative terminal to the
positive terminal of the
source.
 Conventional Current:
 From Benjamin Franklin’s
Kite Experiment
 Positive charges were
thought to carry current
Relating Electricity & Magnetism
 When a current flows through the
wire, you should have noticed that
this causes the needle of the
compass to move perpendicular to
the wire.
Watch This
 What does this imply?
Oersted’s Discovery
 Hans Oersted first related
electricity and magnetism in 1819
when he noticed that the needle
of a compass deflects when placed
closed to a current carrying
conductor
Oersted’s Principle:
 Electric charges moving through a straight
conductor generate a circular magnetic field
around the conductor
Oersted’s Discovery…
 The field around a straight conductor:
 Forms concentric circles around the conductor
 Has a direction that depends on the direction of the current
 Has a strength that decreases the further you are from the
conductor
 Gets stronger when the current is stronger
Oersted’s Discovery…
• Cross section of the field around a straight conductor
(note how the field weakens further away from the wire):
How can you predict the direction of the magnetic
field around a line conductor?
Use right hand rule for a straight conductor
• Rule is based on the flow of conventional current
•
•
To apply the rule:
1. Point your thumb in the direction
of conventional current flow
2. Your fingers curl in direction of
the magnetic field
Relating Electricity & Magnetism
 Watch this video until 0:17.
 What will happen when the switch is pulled?
a) Nothing
b) The wires will always be attracted
c) The wires will always be repelled
d) Sometimes they will attract, and sometimes they
will repel
Relating Electricity & Magnetism
 In the video, what happened
when the switch is pulled?


When in series, the wires repel
When in parallel, the wires attract
Coiled Conductors?
 Use the right hand rule to predict what the magnetic field
will look like in each case. The + and – signs show the
direction of conventional current.
Coiled Conductors?
Look Here
Coiled Conductors
• A solenoid is a coiled conductor.
• The magnetic field created by a current flowing through
a solenoid is similar to the field of a bar magnet.
Electromagnets are electrically powered bar magnets,
that can be turned on and off by turning the current on
and off.
• The field of an electromagnet can be increased by:
• Increasing the number of loops in the coil
• Increasing the current
• Introducing a core made of a material that can be
magnetized
How can you predict the direction of the magnetic
field produced by a solenoid?
•
•
•
Use right hand rule for a solenoid
Based on the flow of conventional current
To apply the rule:
• Wrap your fingers in the direction of the conventional
current flow
• Your thumb will point north
Recall: Magnetic Domains
 Domains in a
non-magnetized material
 Domains in a
magnetized material
Strength of an Electromagnet
Magnetic Field of a Solenoid
 Rank the magnetic field just outside the RIGHT end of the
electromagnet from largest to smallest. (Some may be
equal.)
Explain your ranking!
Magnetic Field of a Loop
Magnetic Field of a Series of Loops
 Hypothesize:
Use the right hand rule to predict what the magnetic field
will look like in each case.
current
flow
current
flow
Homework
 Pg. 556 #1, 2, 3, 5, 6, 7
 Pg. 562 #1, 2, 3, 4, 7
 Read and take notes on Pg. 561, “Applications of
Solenoids”
 Really Old School But Cool Video