Download Electric Current and Circuits

Electric Current and Circuits
Ch. 18
Electric Current
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A net flow of charge
Variable = I
Unit = Ampere (A)
I = Δq/Δt
Conventional current is the direction a positive
charge would flow
Potential Difference
• Just like a ball will not fall if there is not a
difference in gravitational potential, an
electron would not move (ie no current
generated) if there is not a difference in
electric potential
• To have a current, you need a potential
difference.
EMF
• Potential difference maintained by an ideal
battery
• EMF is measured in volts (V)
• Measure of the work done by the battery per
unit of charge
• W = Ԑq
Current, Water, and Batteries
• Water runs down an incline passing through a
current
water
wheel. When the water
current is at the
resistor
bottom, a person
current back up to
battery carries the water
the top.
Batteries and Voltage
• A 9V battery keeps a positive terminal that is
9V higher in potential difference than the
negative terminal.
• The battery does 9 J of work for every C it
pumps through. The battery does work by
converting stored chemical energy into
electric energy.
More about Batteries
• Batteries come in different EMFs (voltages)
(1.5V, 6V, 9V, etc) and different sizes (AAA, AA,
C, D…)
• The common batteries all have 1.5V. This
means a larger batter can last longer or supply
charge faster than a smaller one.
Types of Currents
• Direct Current
– The current in any branch always moves in the
same direction
• Alternating Current
– The currents periodically reverse directions.
Electrons and Current
• Since current was defined (by Albert Einstein)
to be the direction a positive charge would
flow…
• Electrons move in the direction opposite the
current.
Resistance
• The current (I) that flows through a conductor
is proportional to the potential difference (ΔV)
that supplies it. (Ohm’s Law)
• Some materials allow current to flow more
freely than others. A measure of how well the
current flows is called resistance.
• R = ΔV/I
• Or more commonly… V = IR
• Resistance is measured in ohms (Ω)
Resistance of Materials
• R = ρL/A
• Long wires provide more resistance than short
wires
• Skinny wires provide more resistance than fat
wires
• When in doubt, think of a water hose.
Superconductors
• Materials with a resistivity approaching zero
when cooled to a very low temperature (close
to absolute zero)
• Resistance also increases when the
temperature increases.
Resistors
• In a circuit, resistors are materials that cause a
drop in voltage
• Typically the resistance is known
Kirchhoff’s Rules
• At a junction, the current entering the
junction is equal to the current leaving a
junction.
• The net voltage drop around a circuit is zero.
All the potential created by the battery must
be used up by the resistors.
Series Circuits
• The same current flows through each resistor
Series Circuits
• The total resistance in a series circuit is a sum
of all the individual resistors connected in
series
• RT = R1 + R2 + R3 + …
• The total resistance is larger than any of the
individual resistances
Series Circuits
• Things that are connected in series have the
same current, but different voltages (unless
they have the same resistance)
Series Circuits
• For a Resistor
V = IR
• For a capacitor
V = Q/C
• For multiple capacitors in series the total
capacitance is
• 1/C = 1/C1 + 1/C2 + 1/C3 + …
Parallel Circuits
• Resistors are wired so that the potential
difference across them is the same.
Parallel Circuits
• Things that are connected in parallel have the
same voltages, but different currents (unless
they have the same resistance).
• Benefits to parallel circuits…
– When one light bulb goes out, the current still has
a path to travel through so the other light bulbs
stay lit.
Parallel Circuits
• 1/RT = 1/R1 + 1/R2 + 1/R3 + …
• The total resistance for a parallel circuit is
smaller than any of the individual resistors.
• Capacitors in a parallel circuit:
C = C1 + C2 + C3 + …
Drawing Circuits
• Things you must have…
– Battery – long side is the positive terminal and
short side is negative terminal. The current leaves
the positive end.
– Wire
– Resistor – Drawn as zig zag lines, not light bulbs.
Each resistor must be labeled.
– Switch – to open or close the circuit (not always
necessary)
Solving Circuit Problems
• Simplify the resistors
• Assign variables to the current in each branch (I1,
I2, I3…) and choose a direction for each. Draw the
circuit with the current flow indicated by arrows.
• Apply the Junction Rule
• Apply the loop rule
– If your loop goes against the current in a resistor, V is
+. If your loop goes with the current, V is –
– If your loop goes from – to + terminal in a battery, the
voltage is +. From + to – is a negative voltage.
Electric Power
• P = IV
• P = I2R
• P = V2/R