Ch.8.5 Flashcards
What is the use of the concept of resistance?
The resistance of a material measures the opposition of the material to the flow of electrical current.
Why is there resistance?
This is because the atoms in a conductor can oppose the flow of free electrons.
What is the unit of resistance (symbol) ?
ohm (Ω)
Do different materials have the same resistance?
No.
________________________ have _____________ resistance while ________________________ have
________________________ resistance.
Conductors, low, insulators, very high
Some conductors, eg ________________, have lower _______________________ than some other conductors, eg ___________________ and ____________________.
copper, resistance, graphite, nichrome
___________________________ have no resistance.
Superconductors
The ___________________ the resistance of a circuit, the ___________________ the electric current. This is because __________________________________________________________________________________________________________________.
higher, smaller
a circuit of higher resistance has stronger opposition to the flow of electric circuit.
What are the three factors affecting resistance?
Material of the wire, thickness of the wire, length of the wire.
[thickness] A ___________________ wire has higher resistance.
thinner
[length] A ___________________ wire has higher resistance.
longer
[material] An ___________________ has higher resistance.
insulator
What are overhead power lines usually made of? Why?
Thick wires. This is because they need to have low resistance to carry currents over long distances.
What is a resistor?
It is a circuit component with a fixed value of resistance.
What are resistors used for?
To control or limit the sizes of currents in the circuits of electrical appliances, eg a computer.
The _____________________ the resistance of a resistor, the _____________________ the current.
larger, smaller
What is a rheostat?
It is a circuit component whose resistance can be adjusted.
What are the two types of rheostats commonly used in a school laboratory?
—sliding rheostat
—rotary-type rheostat
What are the components of a sliding rheostat?
A coil of resistance wire, a metal bar, three terminals and a sliding contact.
Consider the positions of the three terminals:
———[]—————Y
X————————Z
Terminals X and Y are used.
When the sliding contact is moved to the right, what will happen?
When the sliding contact is moved to the right, the current flows through a longer length of the resistance wire. Therefore, the resistance of the rheostat increases. As a result, the current becomes smaller (and the bulb becomes dimmer).
Consider the positions of the three terminals:
———[]—————Y
X————————Z
Terminals X and Z are used.
When the sliding contact is moved to the right, what will happen?
The current of the circuit and brightness of the bulb will remain unchanged. It is because the current will flow through the entire length of the resistance wire, no matter where the sliding contact is.
Consider the positions of the three terminals:
||@||
the electric current flows through a longer or shorter length of the resistance wire, the resistance of the rheostat changes
Rheostats are used in many devices, eg:
—In ____________________________________________________________.
—__________________________________________________________________________________________
dimmer switches to adjust the current and thus the brightness of lamps
the volume controls of hi-fi systems
A rheostat adjusts the _______________________, not the ____________________ of the electric cells.
current, voltage
