GRADE 8 PHYSICS MR. HUSSAM SAMIR ELECTRICITY

GRADE 8 PHYSICS MR. HUSSAM SAMIR

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ELECTRICITY

ELECTRIC CHARGE

PROPERTIES OF ELECTRIC CHARGE

Like charges repel. Unlike charges attract

We can make static electricity by induction

INDUCTION the process of charging a conductor by bringing it near another charged object and grounding the conductor


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An atom consists of protons, neutrons, and electrons. Some electrons can be moved away from their atoms.

Conductors cannot be charged by static electricity.

Lightning is a movement of electrons.

Dynamic electricity:-

Is the flow of electric charges through a conductor.

Potential difference is the work done to transfer a unit of charge between two points.

ELECTRIC CURRENT the rate at which electric charges pass through a given area

P.D =

J/COULOMB

OR VOLT


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The electromotive force (emf):-Is the total work done to transfer unit charge throughout the circuit,

outside and inside the source, it has units of volt.

The resistance (R):-It is the opposition of the conductor to the flow of electric current, it is measured in

ohm (Ω), it depends on:-

a. Length of the conductor. R α L b. Cross section area. R α 1 / A c. Type of material. d. Temperature.

Ohm’s law: - “The current intensity in a conductor is directly proportional to the potential difference

across its terminals at a constant temperature”

V = I R

Series Connection Parallel Connection

I is the same in all resistors.

The p.d (V) is distributed on all resistors

V = V1 + V2 + V3

I R = I R1 + I R2 + I R3

= I (R1 + R2 + R3)

Then Req. = R1 + R2 + R3

P.d (V) across all resistors is the same.

The total current is distributed among resistors

I = I1 + I2 + I3

321 R

V

R

V

R

V

R

V

Then

3R

1

2R

1

1R

1

eq.R

1


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The equivalent resistance is greater than the

biggest one.

Aim of this connection is to have large

resistance from a group of small resistances.

For a group of n equal resistors each of

resistance = r connected in series then

R = n x r

The equivalent resistance is smaller than the

smallest one.

Aim of this connection is to have a small

resistance from a group of large resistors.

For a group of n equal resistors each of

resistance = r connected in parallel then

R = n

r

For two resistors R1 and R2 in parallel

Req. = 21

21

RR

RR


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MAGNETISM

Any magnet has two poles North and South Pole.

Like poles repel, and unlike poles attract.

MAGNETIC DOMAIN a region composed of a group of atoms whose magnetic fields are aligned in the same direction

Some materials can be made into permanent

magnets as steel

Some materials can be made into temporarily

magnets as iron.

MAGNETIC FIELD a region in which a magnetic force can be detected

We can make a magnet by:- 1. Stroking.

2. Induction.

3. Electricity and called electromagnet.(DC current)

We can demagnetize a magnet by:- 1. Heating

2. Dropping

3. Hammering

4. Withdraw a magnet slowly from a solenoid carrying AC (alternating current)


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Magnetism from Electricity MAGNETIC FIELD OF A CURRENT-CARRYING WIRE

A long, straight, current-carrying wire has a cylindrical magnetic field

The right-hand rule can be used to determine the direction of the magnetic field

LOGIC GATES

1. NOT GATE

2. And gate

Input Output

1 0

0 1

A B Output

0 0 0

0 1 0

1 0 0

1 1 1


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3. OR GATE:

LED (Light emitting diode)

It emits light when electron passes through it.

DIODE

Change from AC (alternating current) to DC (direct current)

Allow the current to pass in one direction

LIGHT DEPENDENT RESISTOR (LDR):-

It is a resistor depending on the intensity of light.

As the intensity of light increases its resistance decreases, vice versa.

Buzzer Resistor Variable

resistor

Switch Battery Ammeter Voltmeter

A B Output

0 0 0

0 1 1

1 0 1

1 1 1

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GRADE 8 PHYSICS MR. HUSSAM SAMIR ELECTRICITY