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ELECTRICAL CIRCUITS
The CELL
The cell stores chemical energy and transfers 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.
Where does the energy come from?
Remember… electric potential…
V = PE
q
In a circuit, a battery gives electrons a
push
Electric Current
Phet – Basic circuit (electron vs. conventional flow)
Analogy: A Ski hill
Battery: Chair Lift
Load/Resistor: the Downhill
Short circuit
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, current must flow through a
closed loop via conducting (metal) wires.
circuit diagram
For our simple circuit:
cell
lamp
switch
wires
circuit diagrams
In circuit diagrams components are represented by
the following symbols;
cell
ammeter
battery
voltmeter
switch
motor
lamp
buzzer
resistor
variable
resistor
Electric Current
Measure of the charge passing in a circuit in a
given time
OR
The Rate of Flow of charge in a circuit
I = ∆Q /∆t
I: Current (Ampere 1 A = 1 C/s)
Q: Amount of charge (C)
t: time (seconds)
Electric Current
A charge (Q) is just a certain number of electrons!
Q = Ne
Q: Charge (Coulombs)
N: Number of charges
e: elementary charge (1.6 x 10-19 C)
(and therefore:
I∆t = Ne )
Example
A 1.50 volt AA cell is rated at 0.750 Amp-hours.
When connected to a small electric motor 0.250 A
of current
a) How much charge is stored in the cell?
b) How many electrons are stored in the cell?
c) How long will the cell power the motor?
Ohm’s Law
Resistor/Load: converts electrical energy into other forms
of energy. Think of the downhill skiing (or water slide)
analogy.
The Voltage drop across a resistor is equal to the current
through it multiplied by it’s resistance
V = IR
V: Voltage (V)
I: Current (Ampere or C/s)
R: Resistance (Ohms Ω)
Example
A 25 Ω motor is connected to a 6.0 V battery.
a) Draw a circuit diagram
b) Find the current
Measuring Current and Voltage
Ammeter: -Measures current flow
-connected in SERIES
Voltmeter: -Measures electric potential gain/drop
across a battery (gain) or resistor/load (drop)
Electric Power
P = E/t = Energy transferred/time
And E=qV (Energy, not electric field!)
P = qV/t
P = IV
(and I = Q/t)
(1)
P = I(IR) = I2R
(2)
P = (V/R)V = V2/R
(3)
Example
Find the resistance of a 40 W car headlight
designed for a 12 V battery.
Paying for Energy (not Power)
Paying for Energy (not Power)
E = Pt
When calculating the COST of energy, you pay per
kW•hr that you use, because the unit of Joules is
too small.
Power: kW
Time: hr
Energy: kW•hr
Example
An electric heater draws 15.0 A on a 120 V line.
a) How much power does it use and b) how much
does it cost per month (30 days) if it is operated
3.0 hr per day and BC Hydro charges 10.5 cents
per kWh?
Series and Parallel Circuits
SERIES CIRCUITS
PARALLEL CIRCUITS
SERIES CIRCUITS
The current is the same in all parts of the circuit
Total resistance (Rtot) in a series DC circuit is the
sum of the individual resistances (Rtot = R1 + R2 +
R3 + …)
If one bulb ‘blows’ it breaks the whole circuit and all
the bulbs go out.
PARALLEL CIRCUITS
– The current has a choice of routes and is NOT the same in all parts
of a DC circuit
– The total current is the sum of the current in each branch of the
circuit. Itot = I1 + I2 + I3 + …
– The voltage drop across each resistor is the same (loop rule)
– Total resistance (Rtot) is found using this equation:
1/Rtot = 1/R1 + 1/R2 + 1/R3 + …..
– Adding resistors in parallel decreases total resistance
– If one bulb ‘blows’ there is still a complete circuit to the other bulb
so it stays alight.
measuring current
Electric current is measured in amps (A) using
an ammeter connected in series in the circuit.
A
Measuring Current
This is how we draw an ammeter in a circuit.
A
A
SERIES CIRCUIT
PARALLEL CIRCUIT
Measuring Current
SERIES CIRCUIT
• current is the same
at all points in the
circuit.
2A
2A
2A
PARALLEL CIRCUIT
• current is shared
between the
components
2A
2A
1A
1A
Measuring Voltage
The ‘electrical push’ which the cell gives to the current
is called the voltage. It is measured in volts (V) on a
voltmeter
V
Measuring Voltage
Different cells produce different voltages. The
bigger the voltage supplied by the cell, the bigger the
current.
Unlike an ammeter a voltmeter is connected across
the components (in parallel)
The term Potential Difference is used when talking
about voltage.
Measuring Voltage
This is how we draw a voltmeter in a circuit.
V
SERIES CIRCUIT
V
PARALLEL CIRCUIT
Measuring Voltage
V
V
V
V
Series Circuit
Voltage is shared between the components
3V
1.5V
1.5V
Parallel Circuit
Voltage is the same in all parts of the circuit.
3V
3V
3V
Measuring Current & Voltage
Copy the following circuits on the next two
slides.
Complete the missing current and voltage
readings.
a)
6V
4A
A
V
V
A
b)
4A
6V
A
V
A
V
A
Answers
a)
b)
4A
6V
6V
4A
6V
4A
4A
3V
2A
3V
4A
6V
2A
Given that I = 1 A in this circuit,
Find:
a)The resistance of resistor R
b)Voltage drop across R
Find:
a)Req for this circuit
b)The current flowing through the circuit
c)Current through R4