Voltage, Current, and Resistance

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Voltage, Current, and Resistance

• Mr. V. Perez
• Grade 6 Science

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Conservation of Charge

• According to the Law of Conservation of Charge,
  charge can be transferred from object to object,
  but it cannot be created or destroyed
• Whenever an object becomes charged, electric
  charges have moved from one place to another
• Remember the balloon?
• The electrons moved from the hair to the balloon
  creating a negative electric charge on the
  balloon AND a positive electric charge on the
  hair
• There are THREE ways that a net charge can build
  up on an object or move from one object to
  another
• Charging by Friction
• Charging by Induction
• Charging by Conduction

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Electromotive Force - Voltage

• Voltage is the measure of how much electric
  energy an electric circuit can gain from a
  battery
• The force or pushing power that moves the
  electrons is called electromotive force, or EMF
• EMF is measured in a unit called volts.
• Voltage is measured with an instrument called a
  voltmeter

• When the voltage increases
• Current flows faster
• Bulbs get brighter, buzzers get louder, and
  appliances get more power

Digital Voltmeter
Voltmeter
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The Current in a Simple Circuit

• A simple electric circuit contains
• A source of voltage, such as a battery or power
  plant
• A load (or device), such as a light bulb, that
  has resistance
• Wires (conductors) that connect the load to the
  battery terminals.
• Switches that can make a circuit open or closed
• When the wires are connected to the battery
  terminals, the battery produces an electric field
  in the wire and the current flows in the closed
  path.

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Current and Amps

• The amount of electric current depends on the
  number of electrons flowing through a wire
• The unit for measuring electric current is the
  ampere, or amp
• The ampere is a measure of the number of
  electrons flowing in a circuit in a given amount
  of time

• Ex One ampere is the amount of current passing
  a point each second
• Current is measured with a device called an
  ammeter

Ammeter
Digital Ammeter
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Resisting the Flow of Current

• Resistance is the tendency for a material to
  oppose the flow of electrons, changing electrical
  energy into thermal energy and light
• When electric current flows through a wire, the
  electrons are slowed down by the wire
• This resistance opposes the flow of electrons
  (because resistance is similar to friction)

• Resistance is measured by the unit ohms (?)
• Insulators have MORE resistance than conductors

A multimeter is an instrument used to measure
resistance, current, and voltage
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Resistance

• FOUR things affect the resistance of a wire
• Length The longer the wire, the more resistance
  it has. Why? The electrons have to be pushed
  further.
• Width The thinner a wire is, the more
  resistance it has.
• Material Wires made of poor conductors have
  more resistance than wires made of good
  conductors (where atom are closer together).
• Temperature As a wire gets hotter, its
  resistance increases

______ wire
Resistance Circuit Symbols
lamp
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The Current in a Simple Circuit

• Every closed circuit has THREE things the
  voltage (V), electric current (I), and resistance
  (R)
• Current, voltage, and resistance are different
  from circuit to circuit
• Different power sources have different amounts of
  voltage (Ex 9 volt radio battery lt 12 volt car
  battery)
• The resistance varies depending on the type of
  wire used (Long wire gt Resistance short wire,
  Thin wiregtResistance thick wire)
• The current is affected by the voltage AND the
  resistance

More Resistance Long Wire
More
Less Resistance Short Wire
Less
There is less resistance in the circuit on the
bottom. The bulb is glowing more brightly due to
the short wire.
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Ohms Law

• Ohms Law states that the current is equal to the
  voltage divided by the resistance
• Voltage (V) is measured in volts
• Current (I) is measured in amps
• Resistance (R) is measured in ohms
• Voltage Current x Resistance
• Current Voltage Resistance
• Resistance Voltage Current

X
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Ohms Law Word Problems

• Suppose a 12 volt battery is connected to a
  circuit with a resistance of 6 ohms. What is the
  current?

I V R I 12 volts 6 ohms I 2 amps
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Ohms Law Word Problems

• What is the voltage if the resistance is 3 ohms
  and the current is 4 amps?
• V I x R
• V 4 amps x 3 ohms
• V 12 volts

• In a circuit, the voltage is 18 volts, and the
  current is 3 amps. What is the resistance?
• R V I
• R 18 volts 3 amps
• R 6 ohms

X
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Resistance

• As the electrons flow through the filament in a
  light bulb, they bump into the metal atoms that
  make up the filament
• In these collisions, some of the electrical
  energy of the electrons is converted into thermal
  energy
• Eventually, the metal filament becomes hot enough
  to glow, producing radiant energy that can light
  up a dark room.