Ch.8.3

Question 1

What is matter made up of?

Answer 1

Atoms.

Question 2

Describe the structure of atoms.

Answer 2

Each atom has a nucleus surrounded by tiny electrons.

Question 3

The nucleus of an atom carries a __________________ charge, while electrons carry a __________________ charge.

Answer 3

positive, negative

Question 4

What is the relationship between ions and atoms?

Answer 4

Ions are charged atoms — the neutral proton no. is not equal to the electron no.
Atoms are neutral — the neutral proton no. is equal to the electron no.

Question 5

What are free electrons?

Answer 5

The electrons that can escape from the atoms and move freely within metals.

Question 6

When a metal is connected to an electric cell to form a closed circuit, free electrons are driven to ______________________________________________________________________________________________________. This forms an _________________ _________________.

Answer 6

flow in a direction from the negative pole towards the positive pole of the electric cell
electric current

Question 7

Before the discovery of electrons, scientists believed that an electric current was ______________________________________________________________________________________________________. This is called the _________________ _________________.

Answer 7

a flow of positive charges from the positive pole of the electric cell to the negative pole
conventional current

Question 8

What are the directions of the flow of electrons in the concepts of electric currents and conventional currents respectively?

Answer 8

— ➞ +

+ ➞ —

Question 9

Do scientists still use the conventional current?

Answer 9

Yes.

Question 10

The size of an electric current is related to ________________________________________________________________________________________________________________.

Answer 10

the number of free electrons that flow through a point in the circuit every second

Question 11

The _________________ free electrons flowing through a circuit, the _________________ the electric current.

Answer 11

more, larger

Question 12

We can use an _________________ to measure the size of an electric current.

Answer 12

ammeter

Question 13

What are the units of an electric current?

Answer 13

ampere (A) or smaller, milliamperes (mA)

Question 14

1A=?mA

Answer 14

1000

Question 15

An ammeter has a _________________ terminal (_________) and a _________________ terminal (_________).

Answer 15

positive, red, negative, black

Question 16

How should be the ammeter connected?

Answer 16

It should be connected so that the electric current flows into the ammeter at its positive terminal and leaves from its negative terminal.

Question 17

In the experiment of using an ammeter to measure an electric current, why do we have to add a light bulb to the circuit?

Answer 17

This is because the current will be too large and may damage the ammeter without the bulb.

Question 18

The ________________ the current, the ________________ the brightness of a light bulb in a circuit.

Answer 18

larger, higher

Question 19

When an electric current flows through a conductor, ________________________________________________________________________________. This is the ________________ ________________ of ________________ ________________.
(hint: related to energy conversion)

Answer 19

some of the electrical energy changes to thermal or even light energy
heating effect, electric current

Question 20

The ________________ the electric current, the ________________ the heating effect.

Answer 20

larger, larger

Question 21

The heating effect of electric current is ________________.

Answer 21

useful

Question 22

Some ________________ ________________ such as electric kettles make use of the heating effect to work.

Answer 22

electrical appliances

Question 23

Explain the use of a fuse.

Answer 23

When the temperature of an electrical appliance is higher than its conductors’ melting points, the fuse breaks to open the circuit, and hence protects the current from being too hot.

Question 24

Besides heating effect, electric current also produces a ________________ ________________.

Answer 24

magnetic effect

Question 25

How can we test for the magnetic effect of an electric current?

Answer 25

When an electric current flows through a wire or a coil, the needle on a compass nearby changes direction.

Question 26

The ________________ the electric current, the ________________ the magnetic effect.

Answer 26

larger, larger

Question 27

When an electric current flows through a coil, the coil can ________________ iron paper clips like a ________________. The coil is called an ___________________________.

Answer 27

attract, magnet, electromagnet

Question 28

What are the pros of electromagnets?
—
—

Answer 28

the strength of the attraction can be controlled by the size of current
the magnetic effect can be switched on or off

Question 29

Under what circumstances can electromagnets produce a magnetic effect?

Answer 29

When an electric current flows through it.

Question 30

Electromagnets are commonly used in devices such as ________________ ________________ ________________, ________________ ________________ etc.

Answer 30

electric door locks, electric bells