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Unit 3 Day 1: Voltage, Current,
Resistance & Ohm’s Law
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Batteries
Electric Current
Conventional Current
Resistance
Resistors
Energy Dissipated in a Resistor
Batteries
• Discovered by Luigi Galvani
(1780) And improved upon by
Alessandro Volta
• A battery produces electricity by transforming
chemical energy into electrical energy, using
various metal electrodes immersed
in an electrolytic solution
Batteries
• Exterior to the battery, the electrodes are connected to
terminals
• 2 or more batteries connected together in series ( + to –
terminals) adds the voltage
• Batteries are the source of potential difference ΔV =VA-B
& measured in SI units of volts (V)
Electric Current
• Electric current is the flow of electric charge in
an electric circuit
• An electric circuit consists of a source of ΔV
(battery), connecting wires, and a load (lamp)
Electric Current
• When a circuit forms a closed loop, conventional
current flows out of the (+) terminal of the
battery, into the load, and back into the (-)
terminal of the battery
Electric Current
Q
I
t
• Where ΔQ is the amount of charge that passes
through a conductor at any location, during
some interval of time.
• SI: Amperes (A)
1A = 1 Coulomb / sec
• Charge carriers are the electrons which flow out
of the (-) terminal of the battery and into the (+)
terminal
Electric Current
• Conventional current flows in the opposite
direction of electron flow !
Electric Current
+
E-Field
-
-
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Current flows because the potential difference produced by the battery sets
up an electric field parallel to the wires. Free electrons in the wire are
attracted to the (+) end of the electric field
• There will be a continuous flow of electrons as long as ΔV is
available
Resistance
• In 1850 Georg Simon Ohm determined that in a
circuit, I  V .
• The current is the circuit is limited or impeded
by the amount of voltage applied to the circuit.
• This impediment to current flow is due to
electrons colliding with metal atoms in the wire.
This impediment to current flow is called
resistance.
Resistance
V
V
R
or I 
or V  I  R
I
R
where R is the resistance, SI: Ohms (Ω)
1 Ω = 1 volt / ampere
• This is called Ohm’s Law
Resistors
• All electronic devices which represent a load in
an electronic circuit can be modeled as a
resistor
• A resistor is a device which contains a defined
amount of resistance and is used to control
current in an electronic circuit
Resistors
Load Resistances
I
+
+
VL
-
• In an electronic circuit, current flows into the load
resistor establishing a potential difference across
it in the polarity shown
Energy Dissipated into a Load Resistor
+
-
E-Field
I
e
A
B
• The energy dissipated into a load resistor is the energy
delivered by the battery per unit time:
U e  U A  U B  qVA  VB   q  V
U   U e   qi  V  Q  V

U  Q  V