Ch.8.5

Question 1

What is the use of the concept of resistance?

Answer 1

The resistance of a material measures the opposition of the material to the flow of electrical current.

Question 2

Why is there resistance?

Answer 2

This is because the atoms in a conductor can oppose the flow of free electrons.

Question 3

What is the unit of resistance (symbol) ?

Answer 3

ohm (Ω)

Question 4

Do different materials have the same resistance?

Answer 4

No.

Question 5

________________________ have _____________ resistance while ________________________ have
________________________ resistance.

Answer 5

Conductors, low, insulators, very high

Question 6

Some conductors, eg ________________, have lower _______________________ than some other conductors, eg ___________________ and ____________________.

Answer 6

copper, resistance, graphite, nichrome

Question 7

___________________________ have no resistance.

Answer 7

Superconductors

Question 8

The ___________________ the resistance of a circuit, the ___________________ the electric current. This is because __________________________________________________________________________________________________________________.

Answer 8

higher, smaller

a circuit of higher resistance has stronger opposition to the flow of electric circuit.

Question 9

What are the three factors affecting resistance?

Answer 9

Material of the wire, thickness of the wire, length of the wire.

Question 10

[thickness] A ___________________ wire has higher resistance.

Answer 10

thinner

Question 11

[length] A ___________________ wire has higher resistance.

Answer 11

longer

Question 12

[material] An ___________________ has higher resistance.

Answer 12

insulator

Question 13

What are overhead power lines usually made of? Why?

Answer 13

Thick wires. This is because they need to have low resistance to carry currents over long distances.

Question 14

What is a resistor?

Answer 14

It is a circuit component with a fixed value of resistance.

Question 15

What are resistors used for?

Answer 15

To control or limit the sizes of currents in the circuits of electrical appliances, eg a computer.

Question 16

The _____________________ the resistance of a resistor, the _____________________ the current.

Answer 16

larger, smaller

Question 17

What is a rheostat?

Answer 17

It is a circuit component whose resistance can be adjusted.

Question 18

What are the two types of rheostats commonly used in a school laboratory?

Answer 18

—sliding rheostat

—rotary-type rheostat

Question 19

What are the components of a sliding rheostat?

Answer 19

A coil of resistance wire, a metal bar, three terminals and a sliding contact.

Question 20

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?

Answer 20

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).

Question 21

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?

Answer 21

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.

Question 22

Consider the positions of the three terminals:

||@||

Answer 22

the electric current flows through a longer or shorter length of the resistance wire, the resistance of the rheostat changes

Question 23

Rheostats are used in many devices, eg:
—In ____________________________________________________________.
—__________________________________________________________________________________________

Answer 23

dimmer switches to adjust the current and thus the brightness of lamps
the volume controls of hi-fi systems

Question 24

A rheostat adjusts the _______________________, not the ____________________ of the electric cells.

Answer 24

current, voltage