Download Electric Current - Spring Branch ISD

Electric Circuits & Magnetism
Electric Current
In 1800, Alessandro Volta invented the electric battery, which produced the first steady flow of
electric charge, and transformed civilization.
Electric current is the rate at which charges move (flow of charge)
Electric Current=
charge passing through a given area
time interval
I = ∆Q / ∆t
1C
s
Current in common conductors is due to the motion of electrons because protons are stuck in
the nucleus and electrons are easily transferred from atom to atom.
The direction of conventional current is the direction that positive charges would flow. Therefore,
the direction we say current flows is opposite to the direction that electrons actually move. This
is because back in the day they didn’t know protons didn’t move.
Electrons are slow! They don’t flow rapidly like we think they would (since lights turn on
instantly).
When you flip a switch...
• an electric field is established which causes charges to move.
• the electric field travels through the wire at close to the speed of light
• the electric field causes a force that sets the electrons into motion, creating current
• the electrons are moving randomly and colliding with the atoms in the conductor as they
flow slowly along the conductor against the electric field.
I = current - measured in Amperes (A) (also called Amps) 1A=
Sources and Types of Current
For current to be maintained there needs to be a ∆V.
Positive charges (and conventional current) move from higher ∆V to lower ∆V
Batteries and generators maintain ∆V by converting other forms of energy to electrical energy.
Batteries
• Batteries convert chemical energy into electrical energy
• supply energy to circuits so charges can move
• A battery dies because it is out of energy, not because it runs out of charge
Generators
• convert mechanical energy into electrical energy
• source of potential difference across 2 holes of a wall socket
• supplies energy to run appliances
• average ∆V=120V
Types of Current
1. Direct Current (DC)
• charges move in one direction
• batteries are always DC because the terminals are fixed positive and negative
• generators can be DC
2. Alternating Current (AC)
• the motion of the charges continuously changes direction
• terminals of potential difference source are constantly changing sign
• charges basically vibrate back and forth (moving approx. 10 atoms in each
direction)
• occurs very quickly (60 times per second) so we don’t notice the flickering in lights
• supplied to homes and buildings
• generators can be AC
Resistance
To produce electric current in a circuit there needs to be a ∆V
George Simon Ohm established that I is proportional to ∆V, but voltage is not the only factor
that affects current.
Materials that make up wires affect current also because some allow charges to flow more
easily than others.
Resistance is the opposition to motion of charge through a conductor
R =∆ V/ I
R = resistance (measured in Ohms - Ω)
We rewrite this as Ohm’s Law
V=IR
Resistance is independent of V because resistance is constant over a wide range of ∆V.
Resistance depends on
Factor
Length
Cross Sectional Area
Material
Temperature
Lesser R
shorter
Larger
silver, copper
lower
Greater R
longer
Smaller
aluminum
higher
A resistor is an electrical element that
• provides a specified resistance
• is used when you need to reduce current, but cannot reduce ΔV
• is used in circuit boards
Our skin’s resistance
• when wet, resistance decreases - that’s why you don’t unplug an electrical device with
wet hands
•
•
when wet with salt water, resistance decreases more because the ions in salt water
conduct electricity easily
perspiration decreases resistance - some lie detectors use this because perspiration
increases when nervous.
Electric Power
Electric energy is useful because it’s easily transformed into other forms of energy.
• motors transform into mechanical work
• heaters, stoves, hair dryers transform into thermal energy
• light bulb transforms into visible light (10%) and thermal energy (90%)
recall: Power = rate at which work is done (energy is converted)
Electric work = rate at which electrical energy is converted (rate of energy transfer)
P=IΔV
P = power (Watts)
Power dissipated by a resistor
P=I2R
V2
P= R
Light bulbs are labeled by power ratings (wattage)
Electric companies charge for energy, not power. They measure energy in kilowatt hours (kWh)
How does electrical energy get to a house without losing most of the energy in the long wires
(long = more R)
Wires are made to have as little resistance as possible
V, lose only small amounts of energy
Power plants transport elect
V of up to 765000V.
V is reduced to about 4000V using a transformer.
When it gets to the home, another transformer reduces it to the 120V needed at the wall outlets.
Electric circuit – path charges can be conducted through
- Usually consists of 1. source of potential difference and electrical energy (i.e. a battery)
and 2. load – any element that dissipates energy (i.e. light bulb)
- Wires have such a low resistance they are not considered part of the load but are very
important in the circuit!
To work with electric circuits, need to be able to speak the language.
Often draw circuit diagrams which are easy to read because use a standard set of symbols
Ex. Of simple circuit diagram: battery connected to a light
bulb
Symbols we will use:
Wire
Resistor
Light bulb
Plug
Battery
Open switch
Closed switch
Capacitor
Closed vs. Open Circuits:
Closed circuits – no breaks in circuit therefore current can flow
- has a closed path for electrons to follow
- switch must be closed
Open circuits – has break in circuit therefore no current
- path is not closed, so electrons can’t continue to flow
- switch is open
Short Circuits:
Occur when circuit contains little resistance to flow of charges – i.e. no bulb or other resistor
present
Ex. 1 wire connected to one terminal of battery contacts another wire with little resistance that is
connected to the other terminal of the battery
Current usually increase and overheats wires which leads to trouble (potential fire)
Can you light a light bulb using only wires and no battery?
Nope because where is no potential difference so no charge can flow and therefore no current
The battery is needed to provide the potential difference and electrical energy (is a source of
emf)
Any device that increases potential energy of charges circulating ina circuit is a source of emf
emf – energy per unit charge supplied by source of electric current
- used to stand for electromotive force, but that name is misleading because it’s not a
force but rather a potential difference
- measured in volts
- voltage on battery is the emf of the battery
Terminal voltage – the potential difference across the battery’s terminals
- slightly less than battery’s emf because within the battery there is internal resistance to
the current due to moving charges colliding with atoms inside the battery while moving
charge moves from one terminal to the other
- we will use terminal voltage for a battery not its emf when looking at potential difference
Two types of simple circuits – series and parallel
Series Circuits:
In a circuit with 1 light bulb and battery – potential difference across bulb = terminal voltage of
battery and can find current using Ohm’s Law (V = IR)
If a second light bulb is added, the charges that pass through the first bulb
must pass through the second bulb – this is an example of objects
connected in series
Light bulb filaments are resistors so we can draw the circuit diagram of the above circuit as:
According to conservation of charge, charges can’t build up or disappear at a point, so the
amount of charge entering one bulb in a certain time interval equals the same amount leaving in
same Δt
There is only one pat for charge to flow, so amount of charges in second bulb = amount in first
bulb
Since current is the amount of charge – the current is the same for both light bulbs
This is true for any resistors connected in series (current same in each resistor connected in
series)
Total current in a series depends on the number of resistors and how much resistance for each
one
To find total current, need equivalent reistance (total resistance in a circuit) - Req
Req = R1 + R2 + R3 + …
(sum of all resistance in circuit)
Once have Req can find current using Ohm’s Law
V = IR  Itotal = V/Req
Because the current is the same in each light bulb can use ΔV = IR to calculate potential
difference across each resistor
In series circuit if a single bulb goes our, no mores lights because circuit is broken (open)
(if a light bulb burns out, its filament is broken which causes a break in circuit so current doesn’t
flow and the string of lights won’t work )
If this happens, why arrange resistors in series?
1. To regulate current
a. Christmas (holiday) lights – each additional bulb decreases current because
adds resistance
2. Use several small resistors to add up to a single greater resistance (especially if needed
resistance is unavailable)
3. Sometimes it is important to have a circuit with no current if any part fails
a. Some burglar alarm systems
Parallel Circuits:
Wiring arrangement that provides alternative pathways for movement of charge
Ex:
More than one path for
charge to follow
Resistors in parallel have same potential
difference across them but current can vary
When charge leaves + terminal of battery and reaches a branch point (A on digram), some
charge will go through top bulb and some charge goes through bottom bulb.
If one bulb has less resistance, more charge will move through that bulb because less
opposition to flow of charges
Since charge is conserved, sum of currents in each resistor equals total current that leaves from
battery
Itotal = I1 + I2 + I3 + …
But ΔV = IR so ΔV/Req = I1/R1 + I2/R2 + I3/R3 + … since ΔV is same across each resistor can
write Req as 1/Req = 1/R1 + 1/R2 + 1/R3 + …
(how to calculate equivalent resistance of resistors in parallel)
What happens in Christmas lights that are wired in parallel when one burns out?
-
there is no current in that branch of the circuit but each parallel branch offers an
alternative pathway and ΔV supplied to other branched and current in the branches
remains the same allowing the other bulbs to remain lit (and to get brighter?)
Household circuits designed in parallel
- allows us to turn on and off lights and appliances without affecting others
- allows appliance manufactures to standardize their design since ΔV is same at all points
in parallel circuit
- Can choose appropriate resistor so current flowing through is just right
Overview:
Series:
Circuit diagram:
Parallel:
Current:
I = I1 = I2 = I 3
Same for each resistor
I = I1 + I2 + I3
sum of currents
Potential
Difference
ΔV = ΔV1 + ΔV2 + ΔV3
ΔV = ΔV1 = ΔV2 = ΔV3
Sum of potential differences
same for each resistor
Req = R1 + R2 + R3
1/Req = 1/R1 + 1/R2 + 1/R3
sum of individual resistors
reciprocal sum of resistances
Equivalent
resistance
Question – are car headlights wired in series or parallel? How do you know?
In parallel so if one burns out, the other remains lit
Question – how would brightness of bulbs differ if they were wired in series instead?
Brightness would diminish because as you add resistors, the voltage drops and bulbs aren’t
able to reach their full ability therefore they don’t shine as brightly (Ohm’s Law)
Usually encounter circuits with both types of wiring so you get the best of both worlds
Household circuit – circuit breaker connected in series to numerous outlets that are wired to one
another in parallel
All three plugged into same outlet
-
parallel wiring allows one appliance to be off and others on
ensures identical ΔV across each appliance which allows standardization of appliances
to prevent excess current, must place fuse or circuit breaker in series with the outlet
Fuse: a metallic strip that melts if current is too high & must be replaced (can be a hassle)
More often  Circuit Breaker: triggers a switch when current reaches a certain value and switch
must be reset after overload is removed
Fuse or circuit breaker must be in series with the outlet so that excessive current doesn’t reach
any appliance
- use appropriate fuse/circuit breaker for circuit
- current in fuse/circuit breaker is same as total current in circuit (since in series)
To determine the equivalent resistance for complex circuit, simplify into groups of series and
parallel resistors