Electric Currents Chapter 34 What is an electric current? What makes current flow?
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Transcript of Electric Currents Chapter 34 What is an electric current? What makes current flow?
Why does the water in a river flow?
PE
Gravity is pulling it down, but a general rule of thumb is that an object has a tendency to move towards positions of lower energy.
Water will stop flowing when there is no PE difference
This is also when the pressures are equal on both sides
Water will stop flowing when there is no PE difference
In order to keep water flowing we would need to maintain a difference in height
An electrical current is made up of little charged particles (electrons).
What tells you the Potential Energy of an electron. (Hint J/C)
Electric Potential (Volts)
Electrons travel from high to low voltages
-
2.0 V
+- +- +- +
1.5 V1.0 V0.5 V0.0 V
As electrons flow the potential difference is reduced.
In order to keep current flowing we need an electron pump.
1.5 V
A battery keeps current flowing by maintaininga voltage difference (until it “dies”)
e-
electrons are produced when the zinc “corrodes” at the anode
electrons used up at the cathode
Chemical reactions in a battery pump electrons around a loop
The rate at which electrons “flow” is measured in current
I =Qt
Electrical Current: Amperes or Amps for short
the amount of charge that flows through a wire (C)
per time (s)
1 ampere =1 Coulomb
1 second
One light bulb draws 1/3 amp
Another draws 2/3 amp.
How many more electrons pass through the second as compared to the first in one second?
The bigger the electric potential difference, the faster the flow of electrons.
As Voltage Current
Voltage can be thought of as electrical pressure
Electrical RESISTANCE of something (like a wire) is how much it resists electrons flowing through it.
It is measure in OHMS
The Electrical Resistance of an object depends on 4 things
1.) THE MATERIAL
Copper wire Glass rod
Which will have the higher resistance?
The Electrical Resistance of an object depends on 4 things
2.) The Length
Which will have the higher resistance?
Short Long
The Electrical Resistance of an object depends on 4 things
3.) The thickness
Which will have the higher resistance?
THICK
THIN
The Electrical Resistance of an object depends on 4 things
4.) The Temperature
The hotter an object the more it resists current.
When some materials are cooled WAY DOWN, they actually lose all resistance and become SUPERCONDUCTORS
switch
resistor
battery
+-
valve
Tube
pumpwater flow e- flow
pressure drop acrossvoltage drop across
A 9-volt battery is place on your wet tongue which has a resistance of about 3000 ohms. How many amps run through your tongue?
Dry skin has a resistance of about 500,000 WET skin has a resistance
of about 3,000
If you touch a 120 V circuit
low currenthigh current
Why is the bird safely perched on a power line
Time
What happens to the current that flow through the wire?
At times the current is moving:left or right or not at all
Current 0
Are there any electrons in a copper wire which is not connected to a battery?
e-
In a neutral wire, an electron moves randomly but doesn’t really go anywhere
-+
Path of an electron in an electric field
drift speed
An electron has a total speed of about 1,500,000 m/sbut usually has a drift speed of about .0001 m/s
With a drift speed of about .0001 m/s
Why does it not take several minutes for the lights to turn on, when you hit the switch?
The wires are already full of electrons, when one enters one exits.
-+
The electron bumps into atoms in its path
This slows its drift speed, & is also why a wire gets hotwhen current passes through it.
The hotter the wire, the faster the random motion of the electron is.
The more frequently bumps into an atom, and loses drift speed.
(the greater the resistance)
Temp
Electron motion applet
-+
Path of an electron in an electric field
Which way would field lines point?
The electric company does not sell electrons
all they do is jiggle the electrons in your wires which requires ENERGY
When you get electric shock, its your own electrons that are shocking you!
9V battery9 V
0 V
No resistance = No energy loss
EnergyEnergyEnergyEnergyEnergy
the change in PE of the electronthe flowrate of electrons
P =
For electric power, it is very similar
ICurrentC/s
VVoltageJ/C
PowerJ/s(Watt, W)
9V battery
EnergyEnergyEnergy
I P
9V battery
EnergyEnergyEnergyEnergyEnergyEnergy
The voltage is the same so each electron gives the same amount of energy, but more electrons pass through each second
I P
9V battery
0 VEnergy
V P
EnergyEnergyEnergyEnergy
120 V battery
Energy
Even if the current is the same, the Power increases because each electron drops off more energy!
V PEnergyEnergyEnergyEnergyEnergy
When resistor are positioned in series the total resistance just adds up
4 4
Total or EQUIVALENT resistance
8
4 8 12
4 8 12
8 V
Because the current can only go in 1 path, the current through both resistors is the same
4 4
1.0 A
1.0 A 1.0 A1.0 A
24 V
What is the voltage drop across the first resistor?
4 8
What is the voltage between the resistors?What is the voltage drop across the 2nd resistor?
EnergyEnergy
EnergyEnergy
EnergyEnergy
EnergyEnergy
A lamp with a resistance of 360 and one that is 144 are connected in series to a 120 V circuit. Which burns brighter?
9 V
3 light bulbs each with a resistance of 2 , are connected to a 9V battery. What is the current and voltage at each point along the circuit
Which bulb is the brightest?
There are Twice the paths open to the water
The resistance is only 1/2 , so twice as much water will flow
Which will be easier to pump water through?
4 4
4
Resistors can also be added in Parallel
Which circuit has the higher Resistance to current flow?
4
8
What is the equivalent resistance
9 V
What is the total current out of the battery?
REQ=2.7
Is the current the same through each resistor?
I=3.3 A
8
24
9 V
What would happen the the Voltage, Current, and Resistance if another resistor was added in parallel?
4
9 V
9 V
How does the brightness change if another bulb is added in series?
What changed?
½ Resistance
Twice current
battery drains twice as fast
A light bulb with a thick filament and one with a thin filament are connected in parallel to a battery.Which will burn brighter?
9 V
Hot From Power company
Circuit Breakeror fuse (15 A)
to other circuit breakers
7 AFridge unused outlet
8 AmpMicrowave
Hot From Power company
Circuit Breakeror fuse (15 A)
to other circuit breakers
7 AmpFridge
8 AmpMicrowave
1 Amplight
Circuit Breakeror fuse (15 A)
to other circuit breakers
7 AmpFridge
8 AmpMicrowave
1 Amplight
Hot From Power company
4
4 4
4
4 4
What is the equivalent resistance of the circuit below
How much current would flow from a 100 V batttery?
12 V
20 80
40
I1
What is the current out of the battery?
Which of these resistors will have more current?
9V battery
Energy
9V battery
Energy
24 V
What is the voltage drop across the first resistor?
4 8
What is the voltage between the resistors?What is the voltage drop across the 2nd resistor?
EnergyEnergy