Download Current - 7th and 8th Grade Science

Electric
Current
What is electrical current?
• When electrical charges move, they are
known as electric current
• Electrical current is the flow of electrons
through a substance (a conductor).
• Usually, it is the flow of electrons through
wires
Conductors and Insulators:
• Conductors - materials through which
charges move easily
– metals (especially copper and aluminum)
• Insulators - materials through which
charges will not move easily
– glass, dry wood, most plastics, rubber, dry air
The two types of electrical current:
• Direct Current (DC) – electrons flow only in
one direction (battery powered devices)
• Alternating Current (AC) – electrons move
back and forth (plug-in outlet devices)
Circuits:
• A series of conductors
moving current around is
known as a circuit
• Circuits must have a
completely closed “loop”
for current to flow
• If there is a break in the
circuit, current cannot
travel
Batteries and DC
• In a DC circuit, electrons flow from negative to
positive.
Fan motor etc.
+++
+++
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AC Current
• In circuits with alternating current (AC), the
electrons move back and forth in the wires
• AC is produced by a generator which is
sometimes far from the point of use.
flow of
current
Properties of Electrical Current:
• Voltage (V) – a measurement of the amount
of force driving electrical current through a
circuit
• Voltage is measured in volts (V)
• Voltage is also known as electric potential
Properties of Electrical Current:
• Current (I) – a measurement of the amount of
electricity flowing through a circuit
• Current is measured in Amperes (A or Amps)
• more voltage  more current
Properties of Electrical Current:
• Resistance (R) – a measurement of how hard
it is for electrical current to flow through a
circuit
• Resistance is measured in the unit Ohms ()
• Anything in a circuit that slows current down,
such as an appliance, a bulb, or an insulator is
called a resistor
Hose Analogy:
• Compare water flowing through a hose
to current flowing through a wire:
– Voltage is the water pressure
– Current is how much water flows through the
hose
– Resistance is anything that slows the water
down (hose diameter, blockages, “kinks”)
voltage
resistance
current
Ohm’s Law:
I = V÷R
V = I×R
V = voltage (V)
I = current (Amps)
R = resistance ()
R = V÷I
Ohm’s Law Problem #1
1. A 9 volt battery is placed in a circuit
with 2 ohms of resistance. How much
current flows in the circuit?
Given
Unk Equation
V=9v I=?
I = V÷R
R=2Ω
Solution
I = V÷R
I=9÷2
I = 4.5 A
Ohm’s Law Problem #2
2. A circuit with a 12V battery produces a
current of 3.2 Amps. How much
resistance is in the circuit?
Given
Unk Equation
V = 12v R=?
R=V÷I
I = 3.2A
Solution
R = V÷I
R = 12 ÷ 3.2
R = 3.75 Ω
Ohm’s Law Problem #3
3. A circuit with 0.75 of resistance has a
current of 2.0 Amps running through it.
What amount of voltage is in the circuit?
Given
Unk Equation
R=0.75 Ω V=?
I=2.0 A
V=I×R
Solution
V = I×R
V = 0.75 × 2.0
V = 1.5 v
Problems to solve:
1. A 9.0V battery is in a circuit with a bulb
that provides 1.5 of resistance. What
amount of current flows in the circuit?
2. 4.0A of current flow through a circuit with
30.0 of resistance. What is the voltage
of the power source?
3. A battery provides 1.5V of potential to a
circuit with a motor. If the circuit draws
0.55A of current, what resistance does
the motor provide?