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11.1 Electric Current
Circuit – continuous conducting path between terminals of a battery (or other source of EMF)
Electric Current – flow of charge (electrons)
I – current (amperes)
Q – charge (coulomb)
T – timet
QI
11.1 Electric Current
Ampere (for Andre’ Ampere)
Usually called an amp
Open Circuit – break in the
circuit, no current flow
sCA 11
11.1 Electric Current
Short Circuit – when the load is bypassed
Current increase
Ground – allows for a
continuous path for charge
flow
11.1 Electric Current
For historical reasons, current is defined as being in the direction that positive charge flows
11.3 Resistance and Ohm’s Law
George Simon Ohm
The actual values depend on the resistance of the conductor
Called Ohm’s LawR – resistance measured in Ohms ()
I V
IR V
11.3 Resistance and Ohm’s Law
Resistor – anything that uses
electric energy
Resistor – device used to control
current
The symbol for a resistor is
11.3 Resistance and Ohm’s Law
The resistance value of a resistor is indicated by the colored bands on the resistor
11.3 Resistance and Ohm’s Law
Misconceptions
1. Cells (batteries) do not put out a constant current. They maintain a constant potential difference.
2. Current passes through a wire and depends on the resistance of the wire. Voltage is across the ends of the wire.
3. Current is not a vector, it is always parallel to the conductor. The direction is from + to -.
11.3 Resistance and Ohm’s Law
Misconceptions
4. Current or charge do not increase or decrease. The amount of charge in one end of the wire comes out of the other end.
11.5 Superconductors
An element or compound that conducts electricity without resistance
Become insulators above a critical temperature
Uses
MagLev Trains
11.6 Electrical Energy and Power
The rate of energy flow for an electric circuit
That is more commonly written as
Combining with Ohm’s Law it can also be written
Vt
q
t
qV
t
WP
IVP
RIP 2R
VP
2
11.6 Electrical Energy and Power
The power company charges by the kilowatt-hour (kWh)
Just a cool picture
JsWkWh 000,600,3)3600)(1000(1
11.6 Electrical Energy and Power
Household circuits – wires will heat up as current increases
In a 20A household circuit
In a 15A household circuit
Circuits are typically designed to run at 80% of the rated power output
Different circuits have different gauge wires (diameter)
WVAIVP 2400)120)(20(
WVAIVP 1800)120)(15(
11.7 Sources of EMF
EMF – electromotive force – the potential difference between the terminals of a source when no current flows to an external circuit ()
11.7 Sources of EMF
A battery will have an internal resistance (r)
So there is a potential drop due to the current that travels through the cell
So the actual potential across the terminals of a cell will be
This is called the terminal
voltage
IrVc
IrEV
11.7 Sources of EMF
When resistors are place in a single pathway
They are said to be in
series
A schematic would look
like this
11.7 Sources of EMF
The current in a series circuit is the same throughout the circuit
The potential across the source of EMF is equal to the sum of the potential drops across the resistors
nT IIII ....21
nT VVVV ....21
11.7 Sources of EMF
Since potential can be defined as
We can rewrite the equation for potential as
IRV
nT VVVV ....21 nneqT RIRIRIRI ....2211 neq RRRR ....21
11.9 Resistors in Parallel
When resistors are place
in a multiple pathways
They are said to be in parallel
A schematic would look like this
11.9 Resistors in Parallel
The potential difference in a parallel circuit is the same throughout the circuit
The current through the source of EMF is equal to the sum of the current through the resistors
nT VVVV ....21
nT IIII ....21
11.9 Resistors in Parallel
Since current can be defined as
We can rewrite the equation for potential as R
VI
nT IIII ....21 n
n
eq
T
R
V
R
V
R
V
R
V....
2
2
1
1 neq RRRR
1....
111
21