Transcript Discussion 11

Last week …
A simple electric circuit consists of
-a current source, such as a car battery
-a resistor, such as a light bulb, or heater
Voltage drop = Resistance * Current
Power = (Voltage drop) * Current
A simple circuit: A battery
creates an voltage V which
pushes the current i through the
resistor R. In the wires, voltage
and current are constant.
Units:
Current in Amperes A
Voltage in Volts V
Resistance in Ohm Ω
Honors credit paper:
Due last day of finals
Magnets
Context: Magnetic fields make an electro-motor turn.
A compass needle points to the North pole of the earth.
Definition:
A magnet is a material or object that produces a
magnetic field. A method to detect a magnetic field is Iron filings that have oriented
to scatter iron filings and observe their pattern.
in the magnetic field produced
by a bar magnet
Discussion:
-Each magnet has a North and a South pole. Opposite
poles attract each other, equal poles repel each other.
-A "hard" or "permanent" magnet is one that stays
Lecture demos:
magnetized, such as the rock “loadstone” or iron
-Load stone
(ferromagnet).
-Bar magnet
-A "soft" or "impermanent" magnet is one that loses its
-Broken magnet
memory of previous magnetizations.
-A material without a permanent magnetic moment
can, in the presence of magnetic fields, be attracted
(paramagnetic: aluminum), or repelled (diamagnetic:
graphite, super conductors).
Electromagnets
Context: Electric current creates a magnetic field.
Definition:
An electromagnet is a wire coil in which the magnetic
field is produced by the flow of an electric current.
Discussion:
- When the current is off, there is no magnetic field.
When the current changes, the magnetic field changes.
- An electromagnet creates a force and a torque on
other magnets, including other electromagnets. This
can be used to build electro motors.
- A changing magnetic field induces a current, and thus
a magnetic field in a second coil (magnetic brakes).
-The direction of magnetic field in the second coil is
opposite to the magnetic field in the first coil
(repulsive force  E&M cannon).
Iron filings that have oriented
in the magnetic field produced
by a coil carrying a current
Lecture demos:
- Helmholtz coil
-E&M cannon
-Faraday effect
- Motor 1, Motor 2
- magnetic brake
Transformers
Context: High voltage power lines save energy.
Definition:
A transformer are two coils that transfer electrical
energy from one circuit to another through magnetic
coupling. A changing current in the first coil (the
primary ) creates a changing magnetic field; in turn,
this magnetic field induces a changing voltage in the
second coil (the secondary).
Discussion:
- The secondary induced voltage V2 is scaled from the
primary V1 by a factor ideally equal to the ratio of the
number of turns of wire in their respective windings
(N2, N1): V2/V1 = N2/N1
-Power line: energy loss = resistance * current2
Energy transferred = voltage * current
 High voltage power lines are energy efficient
A transformer (2 coils) reduces
The voltage from 10000V to
110V.
Lecture demos:
-Transformer
- E&M cannon
- high voltage power line
Magnets & transformers
A magnet is a material or object that produces a
magnetic field. A method to detect a magnetic field is
to scatter iron filings and observe their pattern.
An electromagnet is a wire coil in which the magnetic Iron filings that have oriented
field is produced by the flow of an electric current.
in the magnetic field produced
by a bar magnet & a coil
A transformer are two coils that transfer electrical
energy from one circuit to another through magnetic
coupling. A changing current in the first coil (the
primary ) creates a changing magnetic field; in turn,
this magnetic field induces a changing voltage in the
second coil (the secondary).