Download I = V

Current Electricity
Explore how a circuit works
• Follow the instructions on the handout to
investigate how an electrical circuit works.
• You will find 2 ways to light the bulb and 2
2 ways that do not light the bulb.
• You will sketch the ways that work and the
ways that do not work.
• You will use symbols to represent the
parts of the circuit.
Objectives
• Explain how electrical energy is
supplied to devices in a circuit.
• Use electrical symbols to draw
simple circuit diagrams.
• Distinguish between open and
closed circuits.
Which of these circuits will light?
B
N
F
C
N
G
N
D
Y
Y
H
Y
E
I
Y
N
J
N
A
N
The source of the electrons moving in the circuit is NOT the battery
or the wall outlet! The free electrons are contained within the wire
itself.
An individual electron does not actually travel all the way around a
circuit.
One electron bumps into the next that bumps into the next that
bumps into the next …..
It is the ENERGY that gets transferred all the way around the circuit.
You buy energy, not electrons from the electric company!
Flow of Charge
• Charge flows when there is
a potential difference
(voltage).
• Voltage pushes current like the
force of water in a water tower
due to its height pushes the
water to flow from high to low.
• Current flows through
wire/conductors like water
flows through pipes.
• Resistance to flow of
electrons depends on the
width and length of wire like
water is restricted by the
size of pipe
Current
CURRENT: a flow of charged
particles (electrons) through
a conductor
Current, I, is measured in
amperes, A, or “amps”.
charge
I
time
Andre Ampere
Example:
What is the current, I, if 8 C of charge
passes through a wire in 2 seconds?
I=q/t
I = 4 amps
The Damage Caused by Electric
Shock
1 mA = 0.001 A
Mild shock can be felt
5 mA = 0.005 A
Shock is painful
15 mA = 0.015 A
Muscle control is lost
100 mA = 0.1 A
Death can occur
60W light bulb - 0.5 A
Starter motor – 210 A
Clothes dryer – 18 A
Iron- 3 A
Voltage, V
SI unit is Volt, V
• Voltage is like electric “pressure” provided
by batteries (dry cells, wet cells) and
generators.
• Batteries provide energy from chemical
reactions.
• Generators provide energy from the
conversion of mechanical energy to
electrical energy using rotating
electromagnets.
Electrical Resistance, R
SI unit is Ohm, 
• Resistance in a conductor restricts the flow of
charge in a conductor. It depends on the
materials in the wire and the size/shape of the
wire.
• Different conductors have different conductivity.
• Thick wires have less resistance than thin
wires.
• Longer wires have more resistance than short
wires.
Finding Current:What is the current
through a 30 Ω resistance that has
a potential difference of 120 V?
•
•
•
•
Given:
V = 120 V
I=?
R = 30 Ω
• I = V/ R
= 120
=4A
V/ 30 Ω
Finding Voltage: How much voltage
is required to make 2 amperes flow
through a resistance of 8 ohms?
• Given:
•V=?
•I = 2 A
• R = 8Ω
V
I
R
V = I·R
= 2A·8Ω
= 16 V
Finding Resistance: Calculate the
resistance of the filament in a light
bulb that carries 0.4 A when 3.0 V
is impressed across it.
• Given:
• V = 3.0 V I
• I = 0.4 A
•R = ?
V
R = V/I
R = 3.0V ÷ 0.4A
= 7.5Ω
Many circuit elements do not
obey Ohm’s Law. Resistors
that get hot, like light bulbs
and heating elements, do not
keep a constant resistance.
Resistance generally increases as
objects become hotter.
Click here and here to run
simulations of Ohm’s Law.