WELCOME TO PERIOD 21
CAUTION:
Keep your phones, tablets,
calculators, watches, etc. off of the table.
Homework Exercise #20 is due today.
Midterm 2 grade ranges: Average = 26.5
A: 30 – 33
B: 26 – 29
C: 19 – 25
D: 15 – 18
E: 0 – 14
PHYSICS 1103 – PERIOD 21
•How do electrical transformers change voltage
and current?
•How do transformers use induced current and
magnetism?
•How can transformers make electricity
transmission safer and more efficient?
•Put away your phones and electronics. The
large magnets will damage them.
Electrical transformers
Transformers trade high voltage for low current or low
voltage for high current.
A step-up transformer increases
voltage and decreases current.
A step-down transformer decreases
voltage and increases current.
Note: Transformers work only with
alternating (AC) current. They do not
work with the direct (DC) current
provided by batteries or capacitors.
Induced magnetism and current
1) A changing current in a coil of wire induces a changing
magnetic field around the coil.
2) This changing magnetic field can induce a changing
current (AC) in another nearby coil of wire.
Transformers
• A transformer consists of two coils of wire.
• The wire coils are wrapped around iron cores that
concentrate the magnetic fields.
Iprimary
Isecondary
Current
enters the
primary coil
Current leaves from
the secondary coil
An alternating current in the primary coil
induces a changing magnetic field around
the primary.
This changing magnetic field induces a
changing (AC) current in the secondary coil
of wire.
Primary and
secondary coils
of copper wire.
Transformers
• Transformers trade high voltage for low current or low
voltage for high current.
• The power into the primary equals the power out of the
secondary (except for small joule heating losses).
Power into the
primary coil =
Iprimary x Vprimary
Power out of the
secondary coil =
Isecondary x Vsecondary
Power = I V
Ip Vp = Is Vs
Step-up and step-down transformers
• The number of turns of wire in the primary (Np) and
secondary (Ns) coils determines the voltages.
• More turns of wire equals higher voltage.
Step down reduces voltage
Np = 5
Step up increases voltage
Ns = 3 Np = 3
Np
Ns

Vp
Vs
Ns = 5
Transmitting electricity
• Large amounts of electricity cannot be easily stored.
• Therefore, electricity must be generated as it is
needed and transmitted to consumers.
• What is the most efficient way to transmit electricity
and reduce waste due to joule heating?
Joule heating
Joule heating occurs when a wire is heated as current
flows through it.
Pjoule = I2 R
Pjoule
I
R
= power (in watts)
= current (in amps)
= resistance (in ohms)
High joule heating during the transmission of
electricity is dangerous and wastes power.
Reducing joule heating
How can joule heating be reduced? Should electricity
be transmitted at..
• High voltage or low voltage?
• Very large currents or smaller currents?
Which circuit will transmit more power (have brighter
bulbs)?
Circuit #1
Circuit #2
Step
Down
Step
Down
Transmitting electricity
U.S. power grid: 300,000 km of lines
operated by about 500 power companies
BEFORE THE NEXT CLASS…
Read textbook chapter 22
Complete Homework Exercise 21
Bring a blank Activity Sheet 22 to class.