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Circuit Symbols
A V
CURRENT
Current
• Definition: The number of electrons flowing through a point in a circuit in one second
• Unit: Current is measured in amperes (A)• Measuring Device: Current is measured by an
ammeter• Naming: Named after André-Marie Ampère, a
French physicist
The Coulomb• Because the number of electrons travelling
through a circuit is astronomically high, we need a different unit of measurement
• 6.241 x 1018 is one coulomb of charge• Therefore, 1 A is one coulomb per second• Charles-Augustin de Coulomb was a French
physicist
1A = 6 241 500 000 000 000 000 electrons passing in one second
6.241 x 1018
Does the addition of light bulbs in series
change the current?
A
A
A1 A
A
A
A1 A
Series CircuitAt=A1=A2=A3…
Does the addition of light bulbs in parallel
change the current?
A
1 A3 A
A A A
A
A
1 A3 A
A A A
AParallel CircuitAt=A1+A2+A3…
Some MathRemember: Current is the number of electrons that
pass a point in a circuit every second.
Quantity Symbol
Current I
Charge Q
Time t
Qt
I =Q
I t
1. How many amps are there in a circuit where 10C of charge pass by in 6 seconds?
2. How many coulombs of charge passed by a circuit if a 10A circuit was left on for 4 minutes?
3. How long does it take 18 coulombs of charge to pass through a 200mA circuit?
Current ratings of common loads
Electrical Device (Load) Electric Current (A)
Wristwatch 0.00013
Calculator 0.002
Electric clock 0.16
Light bulb (100W) 0.833
Television 4.1
Electric drill 4.5
Vacuum cleaner 6.5
Stove element 6.8
Oven element 11.4
Toaster 13.6
Water heater element 27.3
Car started motor (V-8) 500.0
Current Kills
Even a very small
amount of current is
deadly
Perception level: 0.001A
Muscles Tingle: 0.002A
Maximum safe level: 0.005A
Muscles convulse: 0.016ABreathing affected
Severe burns: 0.200Ano breathing
Current needed for 100W bulb: 0.833A
Heart stops: 0.050A