Chp2

Glass + Silk

Glass(positive) + Silk (negative)

plastic + Paper

plastic (negative) + Paper (positive)

cloth and polythene

polythene becomes negatively charged and perspex/acetate becomes positive

like charges

repel

opposite charges

attract

Charging by contact

rod is negatively charged, so the ball will Aquire the same charge (negative) so they will both repel each other

Same result if we have a positive rod

Experimental observation: Glass and metal rod

The positive charge in the glass rod will be transferred onto the metal rod, which will be transferred onto the sphere (sphere becomes positive

—>The sphere will repel the metal rod because they are both positively charges

IMPORTANT (WRITTEN): If we touch the metallic rod by our hand, the charges will leak into our body and the ground, thus the metallic rod and the sphere will become less positive

—>Less repulsion, sphere and metallic rod come closer to each other

Experimental observation: Glass rod and plastic rod

the positive charge in the glass rod will not be transferred onto the plastic rod because plastic is an insulator

—>The metal-coated ball will not be repelled

sharing a charge over two conductors

decreases electric force

Electrical forces are proportional to the:

quantity of charge on the charged objects

Plastic

insulator

metals

conductor

electrolytes

conductor

wood

insulator

Earthing/grounding

To earth a charge objects means to bring it in contact with a much large conductor, which may or may not be the planet Earth

Induction

charging without touching

Induction and separation experiment:

1. Two uncharged metal sphere are touching each other

2. Positively charged rod is bought near one of the metal sphere without touching the sphere

3. without removing the glass rod, the two sphered are pulled apart

4. The glass rod is moved far away

5. The metal sphere that was nearer to the charged glass (positive) rod is found to have a negative charge, while the metal sphere that was farther away from the glass rod has a positive charge

Induction and Earthing

1. An uncharged metal sphere is put on a stand

2. A positively charged glass rod is brought near the sphere without touching it

3. Without removing the glass rod, the sphere is earthed (by touching it with a finger for example).

4. The glass rod is moved away

5. Metal sphere is left with negative charge

When oppositely charged spheres produced by induction and separation are brought in contact together

they lose their charge and become neutral

—>Charges that cancel each other (neutral) have charges that are equal in magnitude but opposite in direction

OR opposite charges

Electrostatic force between charged bodies _________ with an__________ in the distance between the charges

Electrostatic force between charged bodies decrease with an increase in the distance between the charges

Electrostatic force between charges is

1. Proportional to the charges

2. Inversely proportional to the square of the distance between them

3. K=9×10⁹

   Q=Charge

If d is doubled

force will decrease

F/4

Attraction of a neutral conductor by a charged object

A positive plastic rod is bought near to a neutral conducting sphere. The negative charges of the rod will the attracted to the positive charges in the sphere, while the negative charges inside the sphere will be repelled. Since the positive is near

—>the attractive force is greater than the repulsive force

greater distance

smaller force

smaller distance

larger force

charged and neutral bodies

the attractive forces are greater

Sample written

1. The sphere is attracted to the “positive” side then it will become positive

2. Then the sphere will repel the “positive” rod because they are both positive

3. Then it will move to the right and will get attracted to the negative side

4. But it will be repelled because the sphere will also obtain negative charges

5. The sphere will keep swinging until it loses it’s charges

positive charge is

equal in magnitude to the negative charge

Energy is

quantized

Q=Ne

Q=charge

N=number of electrons gained or lost

e=elementary charge 1.6 × 10^-19 coulombs

The net amount of electric charge produced in any process is

Zero

1mC

10^-3 coulombs

1 nano coloumb

10^-9 coulombs

1 micro coloumbs

10^-6

Electroscope case A

bringing a negatively charges polythene strip to negatively charge electroscope

—>Since like charge repel, the negative electrons will be repelled down towards the gold leaf. The leaf will diverge more

Electroscope case B

Bring a positively charge cellulose acetate strip near the negatively charged metal cap. The electrons on the metal cap will be attracted more to the metal plate. The positive charge will move down towards the golden leaf

—>The leaf will diverge less

Electroscope case C

Bring your hand near the metal cap. Your hand will have positive charges because all the electrons will be repelled away to Earth

—>Divergence of the leaf decreases

An increase in the divergence of the gold leaf shows that the object has:

the same charge as the electroscope

A Van de Graaff generator consists of:

rubber belt

2 rollers

2 brushes

motor

dome

concealed fingerprints can be revealed using

charged powder

The specimen to investigate is connected to the negative terminal and the powder is connected to positive terminal of a high voltage

electric field

vector quantity (magnitude and direction)

Farther the lines are apart

weaker field

-field line points away from the positive charge and towards a negative charge

two opposite charges in an electric field

represent lines as emanating from a positive charge and terminating on a negative charge

electric current

flow of electric charge wether negative or positive

current

flows from positive to negative

in metals

negative charges to positive charges

to move electrons from sphere A to B

1. energy must be supplied to pull the e- away against the force that attracts them to the nuclei in A

2. energy must be supplied to push the e- against the repulsive force by the electric of atom B

If we join sphere A and B by a conducting wire

1. each electron will be pushed away from B (excessive negative charge on B) towards A

2. Mobile electrons on A and B

3. Excess electrons on B repel each other into the wire and the electrons will pour from the wire into A

Energy used to push the electrons through the wire

-K.E increases

-internal energy

-light

-radiant energy

passage of electric current through a conducting wire

release of heat, emission of light, magnetic effect, chemical effect

p.d.

electrical energy converted to other form of energy per unit of charge

emf

amount of energy supplied by a generator to transfer a coloumb of charge from the positive terminal to the negative terminal of a battery

energy converted electrical energy per unit of charge

not a force

Q=IT

points A and B are said to have potential difference between them when charges:

move spontaneously from one to the other, electric P.E is changed to some other form of energy

how is P.d between two points measured?

in terms of how much energy is given out by each coloumb of charge that passes across the points

W=QV

P=IV

Storage battery is being converted to:

chemical P.E.

thermocouple

heat energy

electrical generator

mechanical energy

photoelectric cell

light energy

Power

work/time

Power=VI

kilowatt-hour

energy consumed in one hour by an electric device having a power of 1kW

low current devices

tenth of an ampere (fluorescent lamps and music players)

medium current devices

an ampere (filament lamps and telivisons)

high current device

ten amperes (central air conditioning and heating systems)

coloumb

small quantity of charge that may flow through an ordinary lamp every second

-not a reasonable amount of static electricity

electric generator

device that converts a certain available form of energy to electric energy

ammeter

-measure the electric current by its magnetic effect

-reads the current in the branch in which it is included, regardless of its own relative position in that branch

when reading an ammeter it is important to minimize the parallax effect by

making the needle cover its image in the mirror

voltmeter

measures the potential difference between the two points in a circuit and its terminals must be connected

-measures the potential difference between two points

any point connected to the Earth is though to be at zero potential

Kirchhoff’s laws

total current entering a junction in a circuit equals the total current leaving it

Kirchhoff’s second law

sum of the potential differences in the series equals the potential difference across the whole branch

potential difference between two points doesn’t depend on the path followed

Fixed resistor

variable resistor

rheostat

potential divider

signal lamp

filament lamp

diode/ recitifier

heater

thermistor

identical cells can be grouped in parallell in order to obtain larger but same e.m.f.

grouping in parallel

identical cells are grouped in series, so that larger e.m.f. values are obtained

grouping in series

Resistor

device that changes potential energy to heat energy

resistance

ratio of the potential difference across a conductor to the current through it

V=RI

R=V/I

Ohms law

R is always constant, no real material satisfies ohm’s law

R is not constant for any real circuit element

resistance of a uniform conductor is

directly proportional to its length and inversely proportional to its cross sectional area

poor conductor

high resistivity

good conductors (silver)

low resistivity

PVC is made of insulated copper

used for cables and cable conduits

coaxial

contains inner conductor and outer conductor separated by insulating layer

used for signal transmission in television

Electric tools such as pliers and screw drivers

handles are made of insulated rubber

linear resistance: Q/m, resistance per unit length

V=IR

W=Pt

resistance color coding

black beetles running over your garden bring very good weather

black brown red orange yellow green blue violet grey white