Chapter 8: Ohm’s law describes the relationship of current, voltage and resistance.
Current Electricity
In static electricity, the electricity produced by rubbing materials together did not move.
(Static= at rest). Once the electrons were transferred from one object to another, they
were at rest.
In current electricity, the electrons do not remain at rest, but move from one place to
another. Remember it is only the electrons in the outermost shell of the atom that can
move.
Current Electricity: The continuous
movement of charged particles through a
conductor (material that allows electricity to
pass through it easily)
Electric Potential Energy and Voltage
Electric potential energy – gained when unlike charges move apart
Example; a battery has electric potential energy because the electrons have stored energy
and the ability to do work after they leave the battery.
Electric Potential Difference–The amount of electric potential energy per coulomb of
charge.
- Commonly known as Voltage
- SI Unit of measurement is volts (V)
- Measured in a circuit using a voltmeter
Think of electric potential difference (voltage) as a hydroelectric dam. Energy
(water) is stored behind the dam in a reservoir. The higher the dam, the more
potential energy the water has.
Electric (Chemical) Cells and batteries
An Electrochemical cell produces a supply of electrical charge.
Electrochemical cell: a chemical device for producing electric current.
An Electrochemical cell requires two electrodes of different materials and an electrolyte.
Electrode: Part of an electric cell
that allows electrons to enter or
leave the cell; it is usually made of a
metal or of carbon.
Electrolyte: A solution that conducts
electricity (can be found in a cell).
Battery:
-Two or more cells joined together.
-These cells are additive. A 1.5V cell and another 1.5V cell creates a 3.0V battery.
Correcting the falsehoods
A 1.5V “battery” you buy in the store is actually only a “cell”. A battery is two or more of
these cells connected to produce an electrical current.What is the voltage of four 1.5V
cells and a 9V battery? 15V
The potential energy of a battery depends on two
factors
1. The voltage (potential difference) and 2. The
charge (coulombs)
(Even though two 6V batteries make up the same
voltage as a 12V car battery it cannot be used
because the charge is different.)
Coulomb: Unit of electric charge (C)
1 coulomb = the addition/subtraction of 6.25x1018 electrons = number of electrons that
pass into a 100W light bulb per second.
Current: the continuous flow of charge in a complete circuit.
- SI units of measurement Ampere (A)
- Measured in a circuit using an ammeter
Think of current as two rivers of equal size and volume, the river that flows faster past a
given point (like a bridge) has a higher current.
Electric Circuit: A complete pathway that allows electrons to flow
4 parts to a circuit
Load: An electric device
Source of electrical energy: example, A cell (or battery)
Conductor: The wire
Control/Switch: A device used to open or close an electric
current.
Switch=open—power off
Switch=closed– power on
Electric potential is provided at the source and is used (converted) by the circuit
elements.
Circuit diagrams and Circuit Symbols
Know the symbols for wire, cell, battery, bulb, voltmeter, ammeter, closed switch, open
switch, resistor.
Know the rules for drawing diagrams (4 bullets on page 262)
Energy from another source is needed to produce energy in the form of an electric
current. Some common energy sources are
batteries (chemical energy converted to electrical energy),
solar cells (light converted to electrical energy),
thermocouples (heat converted to electrical energy), and
generators (mechanical energy converted to electrical energy).
Resistance
A measure of how much a material resists the passing of electricity through it.
Imagine if you walked down an empty corridor, there would be little resistance. Now
imagine if the corridor was filled with wrestlers and you had to walk though it now there is
more resistance. This is the same for electricity moving though different materials there
is a different amount of resistance.
Factors affecting Resistance
1. The nature or type of wire.
Some materials offer more resistance than others. Copper is a good conductor,
electrons easily flow through little resistance. Tungsten is not a good conductor
and electrons transform lost energy to light and heat such as in the light bulb.
2. The length of wire.
Longer – more energy used up to get through. Shorter – takes less energy to get
through
Think about trying to get through a 10 meter hall full of people versus a hall 100
meters long full of people.
3. The diameter of the wire. Thicker- less resistance; thinner – greater resistance
Think about a wide hall full of 100 people and a thin hall full of 100 people.
4.
Temperature can be considered another factor to affect resistance
Warmer offers more resistance because the particles inside are moving faster and
more erratically.
Resistor:
A device made from a material that resists the
flow of electricity.
The greater the resistance of a material, the
greater the amount of energy electrons give up as
they pass through the resistor.
Eg: An ordinary light bulb. The filament that
glows to give us light is made from a very
resistant metal called
tungsten. As electrical current meets the
tungsten filament, it tries to “push through”. The
tungsten resists the flow, so it heats up. This
causes it to glow,
giving off light. Heating elements of stove, toasters,
kettles, are all resistors. Televisions and most other
appliances contain resistors.
Relationship of Resistance, current, and voltage - OHMS LAW
Resistance = voltage
Current
See page 273-274
Lab: Resistors and Ohm’s Law
What we did– connected a number of different voltage of batteries to a circuit containing
a resistor and measured the voltage drop across the resistor and the current in the
circuit.
Things we learned – current is measured by hooking the ammeter up directly in the
circuit. – voltage is measured by hooking the voltmeter on both sides of the resistor –
random errors are associated with any measuring the device, the voltmeter itself can
cause a voltage drop and a change in current to the circuit. –line of best fit graph for ohms
law.