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
How you should
be thinking
about electric
circuits:
Voltage difference (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)
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 a current round
a circuit.
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.
How you should
be thinking
about electric
circuits:
Resistance: friction that
impedes flow of current
through the circuit (like
rocks in the river)
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.
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 is known to affect the resistance to charge flow is 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.
How you should
be thinking
about electric
circuits:
Current: the actual
“substance” that is
flowing through the
wires of the circuit
(electrons!)
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:
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.
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.
simple circuits
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.
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;
cell
ammeter
battery
switch
voltmeter
lamp
Ohm’s Law
I= V
R
Georg Simon Ohm (1787-1854)
I
= Current (Amperes) (amps)
V
= Voltage (Volts)
R
= Resistance (ohms)
Ohm’s Law
The greater the battery
voltage (i.e., electric
potential difference), the
greater the current. And
the greater the
resistance, the less the
current. Charge flows at
the greatest rates when
the battery voltage is
increased and the
resistance is decreased.
I=V
R
I
= Current (Amps)
V
= Voltage (Volts)
R
= Resistance (ohms)
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?