Transcript Electricity

Electricity
Part 2: Electric Current
Electric current is the amount of
charge moving past a point
Definition of current
I = current
Q = amount of charge that passes point.
t = time for charge to pass by.
Q
I
t
Units of Current
 Q  Coulomb 
I


 t  sec 
1 Ampere (Amp) = 1 Coulomb/second
Electric currents only flow in
wires.
1. True
2. False
50%
1
50%
2
Examples of Currents
Solar wind interacting with
the earths magnetic field
1) Coronal Mass Ejection
2) Aurora from space
3) Aurora from ground
CME animation
Aurora from space
Aurora effects
Examples of Currents
Lightning https://www.youtube.com/watch?v=1Pg6JQgjKnI
3 electron beams in a color TV
Tokamak Fusion Experiments
JET discharge
We usually think of currents in wires
Opening switch
Simple circuit
When a charged
particle passes
through the battery, it
gains energy.
When the particle
passes through the
light bulb it gives up
the energy as heat.
Battery doesn’t supply
charges
Ohm’s Law
V=IR
V= Voltage of the
Battery.
I=current in circuit.
R=Resistance in the
bulb/resistor.
(Depends on
materials and
geometry.)
Units of Resistance
R=V/I (volts/amps)
By definition, 1Ohm = 1 volt/amp,
or
1=1V/A.
Log Ride Analogy
Water circuit analogy
Example problem
How many amps of current would flow in
a light bulb that has a resistance of 60 if
it is connected to a 12 V battery.
I V/R
12V
I
 0.2 A
60
Power in a circuit
When Charge Q
passes through the
battery it gains an
amount of energy
E=(Q)V
(This is the amount of
work the battery does
on the charge.)
If the charge takes an amount of time t to
pass through the battery, the battery supplies
a power of (does work at a rate of)
E Q
P

V
t
t
But
Q
 I
t
Thus
P  IV
The power supplied by the battery must be
dissipated in the resistor.
We also know the V=IR.
Power dissipated in resistor
P  IV  I ( IR)
P I R
2
Which type of light bulb has a
larger resistance, a 100W bulb or a
60W bulb?
25%
25%
25%
2
3
25%
1. The 100 W bulb
2. The 60 W bulb
3. They will have the
same resistance
4. None of the above
1
4
Example Calculation
What is the resistance
and how much
current flows through
a 100 W bulb?
Note: The wattage on a
bulb is its power
output and assumes
that you will use it in
the US where the
voltage in 110 V.
I  P /V
I  100W

0
.
91
A
110V
RV/I
R  110V

121
0.91 A
Redo the calculations for a 60 W bulb
I  P /V
I  60W 110V  0.55 A
RV/I
R  110V 0.55 A  200