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Chapter 34 and 35 Motes: Electric Current and Circuits
Q: What is electricity?
 Electricity is the movement of electrons through something (“medium”)
 Electrons move for 2 reasons
o They are attracted to… PROTONS! (Thanks Paula Abdul!)
o They are “hit” by another electron and bounce out of the way.



Electricity is similar to water moving in a hose.
Water flows from higher pressure to lower pressure. The flow will cease
when the difference in pressure ceases.
Water continues to flow because a difference in pressure is maintained with the pump. The same is true
of electric current.
Q: What is a circuit?
 In a simple circuit, electrons flow from the
negative part of a battery through a wire
or foil to the side (or bottom) of a bulb,
through a filament inside the bulb, and
out the bottom (or side) and through the
other piece of wire or foil to the positive
part of the battery. The current then
passes through the interior of the battery to complete the circuit.
 A complete pathway for electrons to travel
o In a complete circuit, electrons must be able to travel back to their starting point!
 Has 3 main parts:
o Source of electrons (ie. battery)
o Medium for electrons to travel (ie. wire and other conductors)
o Something for electrons to do work on (ie. light bulb)
 Can have other parts
o Resistor- designed to slow down the flow of electrons.
 Made of material that isn’t quite considered to be a conductor, but isn’t an insulator
either….
 Different materials allow circuits to have different resistances.
o Switch- allows you to disconnect the circuit when you want electrons to stop flowing and
reconnect when you want electrons to resume flowing
 In a flashlight, when the switch is turned on to complete an electric circuit, the mobile conduction
electrons already in the wires and the filament begin to drift through the circuit.

Circuits are often drawn as a diagram using the symbols given below:
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Q: What are some “types” of circuits?
Complete vs. Incomplete Pathway
 Closed circuit
o Just means the circuit is complete and electricity CAN travel thru it.
 Open Circuit
o Means the circuit is incomplete and electricity CANNOT travel thru it.
o Usually, this occurs when you move a switch to the off position.
Organization of Pathway
 Series Circuit
o All the objects that work is done on (ie. light bulb) are in a straight path.
o In a series circuit, you can travel from one object to the next only ONE way. There are no
options!
 Parallel Circuit
o Objects that work is done on (ie. Light bulb) are NOT in straight path.
o In a parallel circuit, you have options as to how you can travel from one object to the next!
Electrical Currents and Calculations
Q: What is an electrical current?
 Electrons in motion
 Comparison to water hose: this is the moving water
Q: How is current measured?
 Current is measured in terms of the rate at which electrons travel
 In an equation, current is symbolized by I
 Units: amperes or amps (A).
 Tool Used to Measure: Ammeter
 Comparison to water hose: this is like if you sat in one spot and counted how many water molecules
passed you in a certain amount of time
 Two Main Types of Current
o Alternating (AC): electrons constantly change directions, like an outlet
o Direct (DC): electrons move only in one direction, like a battery
Q: What is voltage?
 Voltage is a measure of the amount of force that keeps the electrons flowing.
 In an equation, voltage has the symbol V
 Units: Volts (V).
o The voltage of a typical small battery is 1.5 V. The voltage of a typical car battery is 12 V.
 Tool Used to Measure: Voltmeter
 Comparison to water hose: this is like the water pressure.
Q: What is resistance?
 The amount of resistance present tells us how much the flow (current) is restricted by the material
(resistor)
o Resistance is caused by friction.
 In an equation, resistance is symbolized by r.
 Units: Ohms (Ω)
 Comparison to water hose: this is like squeezing the hose in one spot.
Q: How are all of these related?
 Ohms Law: The current thru a circuit (I) is directly related to the force keeping the electrons flowing (V)
and inversely related to the resistance (r) present in the circuit.
 Equation: I = V/r
 Units: I in amps, V in volts, r in ohms.
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PRACTICE PROBLEMS
1)
The headlights of a typical car are powered by a 12 V battery. What is the resistance of the headlights if
they draw 3.0 A of current when they are turned on?
Answer:
R=?
R
=
2)
=
12 V
3A
=
4Ω
A clothes dryer is equipped with an electric heater. The heater works by passing air across an electric
wire that is hot because of the current in it. The wire’s resistance is 10.0 Ω and the current in the wire
equals 24 A. What is the voltage across the heater wire?
Answer:
V=?
V
=
3)
V
I
IR
=
(10.0 Ω)(24 A)
=
240 V
An electric car is equipped with a 50 hp (horse power) motor. The voltage across the motor’s terminals
equals 500 V and the resistance in the motor’s circuit 7.5 Ω. How large is the current in the motor?
Answer:
I=?
I
=
V
R
=
500 V =
7.5 Ω
67 A
Q: What is electrical power?
A: Electrical power is the rate at which electrical work is done and is also called power rating.
 Electrical power is the energy associated with electrical charges, whether moving or at rest.
o The symbol for electrical power is P. The unit of measure for electric power is watts and the
symbol for watts is W
 Electric power can be calculated by multiplying the total current (I) in a circuit and the total voltage (V)
across a circuit.
 Formula – Electric power - P= current x voltage = IV
~Electric Power Problems~ Try these problems on separate sheet of paper
1. When a hair dryer is plugged into a 120 V outlet, it has a 9.1 A current in it. What is the hair dryers power
rating?
Answer: P = IV = 9.1A * 120V = 1092W
2. A color TV has a power rating of 320 W. How much current is in the TV when it is connected to a 120 V
power outlet?
Answer: I = P/V = 320W/120V = 2.7 A
3. The current in the heating element of an electric iron is 5.0 A. If the iron dissipates 590 W of power, what is
the voltage across it?
Asnwer: V = P/I = 590 W/5A = 120 V
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Q: What happens when a current has too much power?
 When the resistance of a circuit is too low, the wires carry too much current. When this happens, the
circuit is said to be overloaded.
o Circuits can be overloaded when too many appliances are plugged into a single power source.
 There are two ways to prevent overloading circuits:
o Fuse
 A fuse is an electrical device contains a metal strip that melts (thus opening the circuit)
when the current in a circuit becomes too great.
 Once the metal strip inside a fuse melts, it must be replaced.
o Circuit breaker
 A circuit breaker is a device that protects a circuit from current overloads by opening the
circuit when the current becomes too great.
 Circuit breaker act like switches and can be reset to close the circuit again.
Q: What is a short circuit?
 Caused by improper wiring.
 The current travels around the resistor, therefore overloading the circuit.
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