Transcript Current - mvsd.ca

Current
“You’ll get a charge out of this”
Current is measured in Amperes
(amps or A)
1 amp = rate of flow such that 1C of charge moves
past a point in 1 second.
q
I
t
or
q  It
q = charge (C)
I = current (A)
T = time (s)
If 1.6 C moves past a point in 1.2s, what is the current?
q  It
1.6  I (1.2)
I  1.3 A
Work Done
Recall W = qV
Since work is done to move e-, then
moving e- can do work.
Since q=It and W = qV
Then W=ItV
Or
W
 IV
t
Recall that Power =
So
P = IV
W
t
e.g. A 100 Watt bulb is connected to 120 V (household
voltage) . What current flows through the bulb?
P = IV
100 = I(120)
I = 0.83 A
Electrical work is measured in kilowatt hours.
1 kWhr = power consumed in 1 hour
e.g. How much does it cost to operate a 100 W bulb for 8
hours at 5¢ / kWhr?
100 W = 0.1 kWhr
P = 0.1 kW x 8 hours
P = 0.8 kWhr
Cost = 0.8 x 5 = 4¢
Current is measured with an ammeter. To properly measure
current, the ammeter must be placed in series. To measure very
small currents, a galvanometer is used.
An ammeter in a diagram is symbolized by a
A
Voltage in a circuit is often provided by a battery (dry or wet
cell). Batteries produce a force which moves e-. This force is
known as an electromotive force (EMF). If the battery is 12 V
then it produces an EMF of 12 volts.
Dry cell – chemicals in the form of a paste (AA, D,etc)
Wet cell – liquid chemicals – (car battery)
A battery is actually a collection of cells. A battery with 3 cells
should be drawn like this
can be shown as
EMF is measured with a voltmeter
V
These are placed in parallel with the circuit to be
measured.
EMF – Electromotive Force
EMF – the ability of certain devices to produce a potential
difference or electric current.
Lesser known sources of EMF are friction and heat. Useful
sources of EMF share the following characteristics:
- produce a fairly constant voltage
- cause a flow of electric current when connected to a circuit
- exhibit polarity (+ & - terminals)
Sources include
1. Electromagnetism – current is produced by a conducting loop
rotating in a magnetic field. (generator)
2. Photo-electric cell – electrons are released from the surface of
a photosensitive metal when illuminated by light (silicon solar
cell)
3. Thermoelectric – when different metals are placed in two
different temperatures a current is produced (thermocouple)
4. Piezoelectric – certain crystals will when subjected to
mechanical stress produce a current (e.g. crystal microprobes)
5. Chemical – REDOX reactions will produce a current
(batteries)
Note: in Physics the terminals of a battery are defined on their electric
charge rather than based on the REDOX reaction (pg. 419)
SO:
cathode (-)
anode (+)