Download Electricity Definitions and Usage

AP Physics
End Slide
Current (I)
• The rate at which charge is flowing
through a circuit
• Units = Coulombs/sec = Amperes (A)
• Since each electron has a
specifically set amount of charge,
you can think of current as the
number of electrons that pass a
single point (but not how fast an
electron is moving)
End Slide
Current (cont.)
• Each electron does not move
from beginning to end at the
rate of electricity; the speed of
the electricity is actually a
measure of how fast the energy
is moving
• Example with Newton’s Cradle
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Junction Rule
• Kirchhoff’s 1st Law: Junction Rule
– Current is conserved in a circuit
𝑰𝟐
𝑰𝟏
𝑰𝟐
𝑰𝟏
𝑰𝟑
𝑰𝟏 = 𝑰𝟐
𝑰𝟏 = 𝑰𝟐 + 𝑰𝟑
End Slide
Voltage (V)
• The average amount of Potential
Energy per unit of charge in a
circuit
• Unit = Joules/Coulomb = Volts (V)
• Also called “potential difference”
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Voltage (cont.)
• Again, since each electron has a specifically
set amount of charge, you can think of
voltage as the amount of energy per electron
• This energy is caused by the negative end of
the battery pushing the charge out and the
positive end pulling it in
• Since the voltage difference between the
positive and negative ends of a 9-V battery is
9 Volts, then the electrons will use 9 Volts to
get from the negative to positive ends
End Slide
Resistance (R)
• The amount in which flow is
hindered
• Units = Ohms (W)
End Slide
Flow of Electricity
• Superconductors – allow electricity
to pass through very easily.
• Conductors – allow most electricity
to pass through easily.
• Semiconductors – allow only some
electricity to pass through.
• Insulators – allow little or no
electricity to pass through.
End Slide
Ohm’s Law
• Current is the balance between
Voltage forcing the flow and
Resistance hindering the flow
• What will Voltage do to Current?
• What will Resistance do to
Current?
V
I= V
R
more commonly
written as V = IR
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Ohm’s Law
• Suppose a 9.0 V battery was
lighting a bulb with a resistance of
3.0 W. What is the current?
I = V = 9.0 V = 3.0 A
R 3.0 W
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Ohm’s Law
• Suppose a 3.5 V power source
produced a current of 0.50 A
through a resistor. What is the
resistance?
R = V = 3.5 V = 7.0 W
I 0.50 A
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Ohm’s Law
• Suppose a 2.5 W resistor has a
current of 1.5 A through it. How
much voltage is the resistor using?
V = IR = (2.5 W) (1.5 A) = 3.75 V
End Slide
Loop Rule
• Kirchhoff’s 2nd Law: Loop Rule
– Energy is conserved in a circuit
Voltage
Supply
𝑹𝟏
𝑹𝟐
Voltage (V)
𝑹𝟑
𝑹𝟏
Voltage
Supply
𝑹𝟐
𝑹𝟑
Measuring Current &
Voltage
End Slide
• Current is measured in Series with an Ammeter
• Voltage is measured in Parallel with a Voltmeter
A
𝑹𝟏
𝑽
𝑹𝟑
𝑹𝟐
V
End Slide
Power
Power – the rate at which energy is
used
P = IV 
C/
sec
* J/C = J/sec
With a light bulb, this relates to how
bright it shines (a.k.a. Wattage).
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Power
• What is the current through a 60Watt light bulb if your house
receives 120 Volts?
P = IV
60 W = I * (120 V)
I = 60 W/120 V = 0.50 A
End Slide
Power
• Why is a light bulb with more resistance
brighter when in series with a bulb of
less resistance?
P = IV
P = I2 R
V = IR
Since current is the same through a series
circuit, higher resistance means more
power; thus, brighter.
End Slide
Equations for Circuits
Series
𝑽𝑻𝒐𝒕 = 𝑽𝟏 + 𝑽𝟐 + 𝑽𝟑 + ⋯
𝑰𝑻𝒐𝒕 = 𝑰𝟏 = 𝑰𝟐 = 𝑰𝟑 = ⋯
𝑹𝑻𝒐𝒕 = 𝑹𝟏 + 𝑹𝟐 + 𝑹𝟑 + ⋯
Parallel
𝑽𝑻𝒐𝒕 = 𝑽𝟏 = 𝑽𝟐 = 𝑽𝟑 = ⋯
𝑰𝑻𝒐𝒕 = 𝑰𝟏 + 𝑰𝟐 + 𝑰𝟑 + ⋯
𝟏
𝟏
𝟏
𝟏
=
+
+
+⋯
𝑹𝑻𝒐𝒕 𝑹𝟏 𝑹𝟐 𝑹𝟑