13.7: Resistance in Circuits
pg. 564
Key Concepts:
1. Resistance is a measure of the opposition to the flow of electrons.
Electrical Resistance (R): is the ability of a material to oppose the flow of
electric current; measured in ohms (Ω).
- Resistance is the opposition to the movement of electrons through a
substance, such as; a conducting wire, or a load.
- The symbol for resistance is R.
- The unit of measure for resistance is an Ohm (Ω).
- A ball rolling a long a flat smooth surface receives very little resistance.
Roll the same ball a long the lawn, with grass and uneven surface the ball
slows down and stops over a short distance, this is similar to resistance in an
electric circuit.
- A conductor shows very little resistance to the movement of electrons,
while an insulator shows a greater resistance to the movement of electrons.
Factors That Affect Resistance
- The greater the resistance the slower the current in an electric circuit.
- The greater the resistance the warmer the material will get, as electric
energy is transformed into thermal energy.
-There are many factors that influence resistance; type of material, width or
diameter of material, length, and temperature.
Type of Material
- Metals are excellent conductors; they allow electrons to move freely,
unlike insulators.
- Metals such as copper, aluminum, gold, and silver express a low electrical
resistance.
Cross-Sectional Area
- The diameter of a cross section of substance such as a wire will indicate its
ability to transmit electrons. The thicker the wire the more easy it is to
conduct electricity, the thinner the more resistance.
Figure 1: The wire at the left has a smaller diameter cross-section than the
wire at the right. The thinner wire will have more internal resistance than the
thicker wire.
Length
- As the length of the conducting wire increases so does the resistance.
Temperature
- Temperature causes the electrons to become more active. Therefore the
more current can pass from atom to atom of the conducting wire.
Measuring Resistance
Ohmmeter: a device used to measure resistance.
- Current and voltage measurements are used to determine resistance.
- Resistance is measured using an ohmmeter.
- An ohmmeter must be connected in parallel with a load.
Figure 2: a) the circuit diagram symbol for an ohmmeter, b) An ohmmeter
measures the electrical resistance across a load. It is placed in parallel with
the load.
Resistors in Circuits
Resistor: a device that reduces the flow of electric current.
- A resistor is an electrical device that reduces current in a circuit.
- There are many different types of resistors.
Evidence of Learning …. Students can
- correctly identify and describe the various factors that affect resistance.
Check Your Learning
Questions 1 – 7, page 566
Summary:
- Electrical resistance is the opposition to the flow of electrons.
- The internal resistance of a wire increases by decreasing its cross-sectional
area, lengthening the wire, and increasing its temperature.
- All materials have some internal resistance. Materials that have less
resistance are usually used as conductors.
- Resistance causes electrical devices to warm up when a circuit is
functioning.
- Resistance is measured in units of ohms (Ω) using a meter called an
ohmmeter.
- Resistors are electrical devices that affect the electric current in a circuit.
- Ohmmeters are connected in parallel with a load when measuring
resistance. The circuit must be tuned off to measure the resistance.