IB Physics 12

Mr. Jean

December 15th, 2014

AC vs. DC Motors:

• Electric motors are mostly AC because our electric energy for industry and home is transmitted as AC.

• DC motor – starter motor on a car.

AC Generators:

• Generators are essentially the same design as motors. – The mechanical energy input to a generator turns the

coil in the magnetic field. This produces an emf (voltage). A sinusoidal voltage output.

• The mechanical energy may come from:– i. Steam– ii. Wind– iii. Waterfall– iv. Electric motor

DC Generator: • The commutator must change the AC

flowing into its armature into DC.

• Commutators keep the current flowing in one direction instead of back and forth.

Power Production

• Generators were built by Tesla to generate electricity reliably and in large quantities.

• Most of today’s energy sold is in the form of AC because it can easily be transformed from one voltage to another.

High Voltage with Low Current:

• Power is transmitted at high voltages and low current without much energy loss (heating of wire) because it can be stepped down from the plant to many cities, to a city, to the household.

• Household typical outlet is 120 V AC.

Transformers: • Transformers are used to transfer energy

from one circuit to another by means of mutual inductance between two coils.

Types of Transformers:

• Transformers consist of a primary coil (input) and a secondary coil (output).

• Step-up Transformer – Secondary has more turns – greater electric potential (V) induced, caused

by the lowering for current (I)

Types of Transformers:

• Transformers consist of a primary coil (input) and a secondary coil (output).

• Step-down Transformer – primary has more turns– Thus less electric potential (V) induced

causing a greater current (I).

Transformers:

• Transferring energy from one coil to the other OR the rate of transferring energy is the power.

• The power used in the secondary is supplied by the primary.

• LAW OF CONSERVATION OF ENERGY

Transformers:

• Brightstorm: Transformers– http://www.youtube.com/watch?v=4aYhAbzS

wvs

Sample Questions:

• At the Equator near Earth’s surface, the magnetic field is approximately 50.0 μT northward and the electric field is about 100 N/C downward in fair weather. – Find the gravitational, electric, and magnetic

forces on an electron with an instantaneous velocity of 6.00x106 m/s directed to the east in this environment.

Find the Gravitational Force

Find the electric force

Find the magnetic forces

Traveling Through a field:

• A proton travels with a speed of 3.0x106 m/s at an angle of 37° with the direction of a magnetic field of 0.30 T in the +y direction. – What are (a) the magnitude of the magnetic

force on the proton?– (b) the proton’s acceleration?

Example: • Sodium ions (Na+) move at 0.851 m/s

through a bloodstream in the arm of a person standing near a large magnet. The magnetic field has a strength of 0.254 T and makes an angle of 51.0° with the motion of the sodium ions. The arm contains 100 cm3 of blood with 3.00x1020 Na+ ions per cubic centimeter. If no other ions were present in the arm, what would be the magnetic force on the arm?

Example: • A current I = 15 A is directed along the

positive x axis and perpendicularly to a magnetic field. The conductor experiences a magnetic force per unit length of 0.12 N/m in the negative y direction. Calculate the magnitude and direction of the magnetic field in the region through which the current passes.

Example:

• A single-turn square loop of wire, 2.00 cm on a side, carries a counterclockwise current of 0.200 A. The loop is inside a solenoid, with the plane of the loop perpendicular to the magnetic field of the solenoid. The solenoid has 30 turns per centimeter and carries a counterclockwise current of 15.0 A. – Find the force on each side of the loop and

the torque acting on it.