Transcript How you should be thinking about electric circuits

INTRODUCTION TO ELECTRIC CIRCUITS
All you need to be an inventor is a good imagination and a pile of junk.
-Thomas Edison
What we’ll cover today:
• 3 characteristics of a circuit
• How we draw a circuit & some symbols used.
• Ohm’s Law- The way to calculate 1 characteristic if you know the other 2.
Tomorrow: Circuit types (Series vs. Parallel)
How you should
be thinking
about electric
circuits:
Voltage difference,
voltage potential, or just
voltage: a force that
pushes the current
through the circuit
(in this picture, it would be
equivalent to the height difference
that forces the water to flow)
Symbol = V Units = volts (V)
A 12-V battery would be
analogous to a 12-meter
waterfall. A 6-V battery
would be a 6-meter
waterfall. A greater
voltage is a greater
“push” on the current.
Voltage is supplied by The CELL
The cell stores chemical energy and transforms it
to electrical energy when a circuit is connected.
When two or more cells are
connected together, we call this
a Battery.
The cell’s chemical energy is
used up pushing electrons
around a circuit.
How you should
be thinking
about electric
circuits:
Current: the actual
“substance” that is
flowing through the
wires of the circuit
(electrons!)
Symbol = I
Units = ampheres, amps (A)
What is a current?
An electric current is a flow of electrons through
wires and components.
+
-
In which direction does the current actually flow?
• From the Negative terminal to the Positive terminal of a
cell. (Think of where an electron would go if you
released it near the battery.)
Check your understanding
Use the diagram to complete the following statements:
Consider this:
Current = charge’s velocity
So…Amps = Coulombs per second
A = C/s
a. A current of one ampere is a flow of charge at the rate of
_______ coulomb per second.
b. When a charge of 8 C flows past any point along a circuit in 2
seconds, the current is ________ A.
c. If 30 C of charge flow past point A in 10 seconds, then the
current is _________ A.
d. If the current at point B is 2.0 A, then the current at point D is
______.
Check your understanding
Use the diagram to complete the following statements:
True or False:
The current at point E is considerably less than the current at point
A since charge is being used up in the light bulbs.
How you should
be thinking
about electric
circuits:
Resistance: friction that
impedes flow of current
through the circuit (like
rocks in the river)
Symbol = R
Units = ohms (Ω)
Resistance
• Resistance is the hindrance to the flow of charge.
For an electron, the journey around a circuit is not a direct route.
Rather, it is a zigzag path that results from countless collisions with
fixed atoms within the conducting material. The electrons encounter
resistance - a hindrance to their movement.
• What do you think this does to the current?
• Will this have an effect on the total voltage?
Factors that affect resistance• First, the total length of the wires will affect the amount of resistance.
The longer the wire, the more resistance that there will be. After all, if resistance occurs as the
result of collisions between charge carriers and the atoms of the wire, then there is likely to be
more collisions in a longer wire. More collisions mean more resistance.
• Second, the cross-sectional area of the wires will affect the amount of resistance.
Wider wires have a greater cross-sectional area. Water will flow through a wider pipe at a higher
rate than it will flow through a narrow pipe. In the same manner, the wider the wire, the less
resistance that there will be to the flow of electric charge.
• A third variable that affects the resistance is what the material that a wire is made of.
Not all materials are created equal in terms of their conductive ability. Some materials are better
conductors than others and offer less resistance to the flow of charge. Silver is one of the best
conductors but is never used in wires of household circuits due to its cost. Copper and aluminum
are among the least expensive materials with suitable conducting ability to permit their use in
wires of household circuits.
For example…
simple circuits
Here is a simple electric circuit. It has a cell, a
lamp and a switch.
cell
wires
switch
lamp
To make the circuit, these components are connected
together with metal connecting wires.
Try answering the following questions without
yesterday’s notes, and then check if you’re right.
Can you define the following terms?
1.Voltage
2.Current
3.Resistance
What 3 things can affect resistance?
simple circuits
Closed Circuit- When the switch is closed, the lamp
lights up. This is because there is a continuous path of
metal for the electric current to flow around.
Open Circuit- If there were any breaks in the
circuit, the current could not flow.
circuit diagram
Scientists usually draw electric circuits using symbols;
cell
lamp
switch
wires
More symbols:
In circuit diagrams components are represented by
the following symbols;
battery
switch
Ammeter
Voltmeter
lamp
(measures
current)
(measures
voltage)
cell
Ohm’s Law
I
= Current (Amperes, A) (amps)
V
= Voltage (Volts, V)
R
= Resistance (ohms, Ω)
Georg Simon Ohm (1787-1854)- As a school teacher, Ohm began his research
with the new electrochemical cell, invented by Italian scientist Alessandro Volta.
Ohm’s Law
Consider the formula to fill-in-the blank:
• The greater the battery voltage (i.e.,
electric potential difference), the
____________the current.
• And the greater the resistance, the
____________the current.
• Charge flows at the greatest rates
when the battery voltage is ______and
the resistance is __________.
I = Current (Amps)
V = Voltage (Volts)
R = Resistance (ohms)
Quick thought: Will Ohm’s Law work if there’s no resistance?
Practice Problems:
1. What is the current in a 10V circuit if the
resistance is 2Ω?
2. What voltage is required to move 2A through
5Ω?
3. What is the resistance of a circuit with 20V
and 2A?
Ready to tackle some problems solo?