Electric Current
symbol: I
Current = flow of charges
unit: Amperes (A)
(The rate at which charge flows by
a given cross section)
1C
1A =
1s
I=
𝑄
𝑡
Electric Current
symbol: I
Current = flow of charges
unit: Amperes (A)
(The rate at which charge flows by
a given cross section)
1C
1A =
1s
I=
𝑄
𝑡
To have an electric current, you need two things:
• A closed circuit / path for the charges
• A power supply maintain the potential difference
Electric Current
symbol: I
Current = flow of charges
unit: Amperes (A)
(The rate at which charge flows by
a given cross section)
1C
1A =
1s
I=
𝑄
𝑡
To have an electric current, you need two things:
• A closed circuit / path for the charges
• A power supply maintain the potential difference
Remember: Current will flow from high potential to low
potential, but charges need to be pushed (by an energy
source) back from low potential to high potential.
Closed vs. Open Circuits
No, the
switch is
open, so
the path
is not
complete
Yes,
charge
will flow,
BUT the
light will
not light
No, the
circuit
needs to
go from +
to -
Maybe! If
one
battery
has
higher V
than the
other
Direction of Current
Current is defined as the direction positive charges
would flow
• From high potential to low potential
• From + side of battery to – side of battery
Fun Fact
Benjamin Franklin defined
current in this manner long
before we knew much
about charges.
Now, we know that positive
charges stay put and
negative charges flow. So,
electrons actually flow
opposite current.
Calculating Current
Current is the rate of charge flow per unit time
𝑄
I=
𝑡
So, if you have 6 C of charges passing through a
section of wire every 2 seconds, then your current is:
I = 6C / 2 s = 3 A
An important note about current:
Although potential decreases across
a circuit, current is the same
everywhere in a circuit!
Circuit Analogy – CFU
Circuit Analogy - CFU
E
F
B
C
A
Electrical resistance (symbol R)
• Electrons flow due to potential difference. If the potential
difference is removed, then
• As electrons move, they bump into other atoms, this slows
them down and impedes their motion.
atoms
(actually
positive ions)
free electron
.
Resistance (R) is a measure of the degree to which
an object impedes the flow of current.
Resistance is measured in Ohms ()
path
OHM’S LAW - Current, Voltage and Resistance
Current = the potential difference
applied across a circuit divided by
the total resistance of the circuit.
• R - resistance
• I – current
V
I=
R
V – potential difference across R
Examples
• If a 3 volt flashlight bulb has a resistance of 9 ohms, how
much current will it draw?
• I = V / R = 3 V / 9  = 1/3 Amps
• If a light bulb draws 2 A of current when connected to a
120 volt circuit, what is the resistance of the light bulb?
• R = V / I = 120 V / 2 A = 60 
Check for Understanding
Which of the following would cause the current in a circuit to
decrease the most?
1)
2)
3)
4)
Increased voltage and increased resistance
Increased voltage and decreased resistance
Decreased voltage and decreased resistance
Decreased voltage and increased resistance
Check for Understanding
If the resistance of a circuit were tripled, then the
current through the circuit would be ____.
1. one-third as much
2. three times as much
3. unchanged
4. ... nonsense! There would be no way to make
such a prediction.
Just for Fun …
Effects of electric current on the BODY- electric shock
Current (A)
Effect
0.001
can be felt
0.005
painful
0.010
involuntary muscle contractions (spasms)
0.015
0.070
loss of muscle control
if through the heart, serious disruption; probably
fatal if current lasts for more than 1 second
human body resistance varies:
100 ohms if soaked with salt water;
moist skin - 1000 ohms;
normal dry skin – 100 000 ohms,
extra dry skin – 500 000 ohms.
What would be the current in your body if you touch the
terminals of a 12-V battery with dry hands?
I = V/R = 12 V/100 000  = 0.000 12 A
quite harmless
But if your hands are moist and you touch 12 V battery,
how much current would you draw?
I = V/R = 12 V/1000  = 0.012 A
a dangerous amount of current.