Transcript Electricity 2.2

Week 2
Electricity and circuits
For centuries
people have used
the energy of
flowing water to
help them do work.
What are the
limitations of
using water for
energy?
Would it work to put a water wheel in this pond?
Dams allow better control over the flow of water
There are some
similarities
between using
water for useful
work and using
electricity for
useful work.
Electric current is the flow of electrons
The amount of flow or current (I)
is measured in amperes (A or amps)
Current must flow through
something
going
somewhere.
Current is
measured
using an
ammeter.
Voltage (E) is the Electromotive force
that makes the electrons move.
Voltage is measured in volts (V) and the
. measurement is made by comparing one
point to another reference point.
It helps to understand electricity if
we compare it to flow of water.
Energy can be stored in water by
pumping it up to a higher elevation.
Gravity is the force
exerted on the
water trying to pull
it back down to
its original level.
The higher the water
is held above its original
level the more potential
energy that is stored in
the water.
Voltage is the
difference
between the 2
points.
Voltage is
sometimes
referred to as the
potential
difference.
The battery serves as the pump.
The push comes from the negative end (pole).
A voltmeter is
used to
measure
the voltage in
a circuit.
All materials exhibit some resistance
to the flow of its electrons.
This property is called resistance (R)
and is measured in ohms (W)
Materials with little resistance are conductors (metals)
Materials that prevent the flow of electrons
are insulators (glass, plastics, ceramics, nonmetals)
What is the name for the flow
of electrons in an electric circuit?
Current
In what unit is electrical
current measured?
Amperes
What is the basic unit of resistance?
The Ohm
What do we call materials that
offer little resistance to the
flow of electrons?
Conductors
What instrument is used to
measure the flow of current
in an electrical circuit?
Ammeter
What term is used to describe
opposition to current flow in
ordinary conductors such
as wires?
Resistance
Give an example of a material
that is a good electrical insulator.
Glass/ plastic/ non-metals
What instrument is used to measure
Electromotive Force (EMF) between
two points such as the poles of a battery?
Voltmeter
George Ohm discovered the relationship between
voltage, current and resistance in a circuit.
Ohm’s Law
The more resistance in a material, then the
less current will flow in response to the
voltage.
This diagram can help you
Remember Ohm’s Law.
V
I
R
What formula is used to
calculate current in a circuit?
Current (I) = Voltage (E) / Resistance (R)
What is the resistance of a circuit
when a current of 2 amperes
flows through a resistor
connected to 50 volts?
25 ohms
What is the current flow in a
circuit with an applied voltage
of 110 volts and a resistance
of 75 ohms?
1.5 amps
What is the voltage across the
resistor if a current of 1 amp
flows through a 5 ohm resistor?
5 volts
The rate at which electrical
energy is used is called power.
Power = current x voltage
The unit for power is the watt.
What is the formula used to calculate electrical
power in a DC circuit?
Power (P) = voltage(E) multiplied by current(I)
How much power is being used in a
circuit when the applied voltage is 13.8
volts DC and the current is 10
amperes?
138 watts
How much power is being used
in a circuit when the applied
voltage is 12 volts DC and
the current is 2.5 amperes?
30 watts
How many amperes are flowing in a
circuit when the applied voltage is
12 volts DC and the load is 120
watts?
10 amperes
Sometimes it is necessary to
convert from small units to larger
units or larger units to smaller units.
This is when an
understanding
of the metric
prefixes comes
in handy.
12 milliamps = ___ amps
24,000 watts = ___ kilowatts
70 millivolts = ___ centivolts
15 volts =
___ millivolts
___ microvolts
___ kilovolts
___ megavolts
Which of the following is equivalent
to 500 milliwatts?
A. 0.02 watts
B. 0.5 watts
C. 5 watts
D. 50 watts
How many volts are equal to
one kilovolt?
One thousand volts
How many milliamperes
is 1.5 amperes?
1500 millamperes
How many volts are equal to
one microvolt?
1 millionth of a volt
If an ammeter calibrated in amperes is
used to measure a 3000-milliampere
current, what reading would it show?
A. 0.003 amperes
B. 0.3 amperes
C. 3 amperes
D. 3,000,000 amperes