Current & Charge

New vocabulary .

• Nucleus • Electron (-)• Proton ( +)• Neutron (0)• Ion• Current• Semiconductor• Conductor • Insulator

Diagnostic test

Q1 : Draw the structure of an atom :

Q2 : Where do you use the electric energy ?

The electric energy change in to …………….. In the …………….

Q3 : How does the electric energy move from the power station ?

Flow through cables .

Q4 : Draw the direction of ( electron and conventional current ) in the circuit ?

A semiconductor :is a material that has electrical conductivity between those of a conductor and an insulator; it can vary over that wide range either permanently or dynamically.

Semiconductors are essential in electronic technology. Semiconductor devices, electronic components made of semiconductor materials, are essential in modern consumer electronics, including computers, mobile phones, and digital audio players. Silicon is used to create most semiconductors commercially.

Semi-conductor devices consist of a diodes, transistors, SCR’s, triacs, and LED’s

Electric current Current is measured in amperes (A) using an ammeter .

1A=6X1018 electrons in each second .

The charge on 1 electron =1.6x10-19 C

Current ( I ) amps =

Charge ( Q ) coulomb / time ( t ) second

Measuring Current

If 1 Coulomb of charge (6.25 x 1018 electrons) passes a point each second, the current is 1 Ampere.

So, 1 Ampere = 1 Coulomb

Q1 : current of 2A passes through a lamp for 3 minutes.A- how much charge passes through the lamp.B- how many electrons pass through the lamp.

Q2 : current of 15A passes through a radio for 4 minutes.A- how much charge passes through the radioB- how many electrons pass through the radio.

Q3 :If there is a current of 10 amperes in a circuit for 10 minutes, what quantity of electric charge flows in through the circuit?

Q4:. How much current must there be in a circuit if 100 coulombs flow past a point in the circuit in 4 seconds?

Q5 :. How much time is required for 10 coulombs of charge to flow past a point if the rate of flow (current) is 2 amperes?

Conduction in liquid and gases :

Electrons in a circuit do not move quickly - they actually “drift” at about 1 mm/s.It is the electric field that moves quickly - at about the speed of light - through the circuit and carries the energy.

Current and drift velocity

I = n A v e

I = currentN = number of electron A = part of a wire of cross-sectional V = drift velocityE = number of coulombs per second

Q1 : copper contain 1x1029 free electron per m3. What is the drift velocity of electrons In a copper wire of cross – sectional area 0.55 mm2 carrying a current of 0.35A?

1m2= 1000mmx1000mm=1x106 mm2

1mm2= 1/1000m x 1/1000m=1x10-6 m2

Pop Quiz

Q1• One example of :

• A- Semiconductor • B- Insulator• C- Conductor

Q2:

• What is the name of this device :

Q3 :• What is the unit of :• A- Current • B- Charge

Q4 :

• A solution of sugar in pure water will not conduct . Why not ?

Q5 :

• When can air be a conductor of electricity ?

Potential Difference

Charges can “lose” potential energy by moving from a location at high potential (voltage) to a location at low potential. Charges will continue to move as long as the potential difference (voltage) is maintained.

Potential difference or ( p.d.)Connecting a cell or battery in a complete circuit causes a potential difference across each component. Potential differences are measured in volts (V). The number of volts tells you the amount of energy given up as the current flows through a component. All of the energy put in by the cell is used up as the current flows through the component

p.d.(Volt) = W( joule ) /Q(coulomb )

p.d.=Potential differenceW = energy transferredQ = charge

Example

Q1 : Calculate the p.d. across a wire if the energy transferred is “A- 600 j when a charge of 50C passes through it .B- 450 j When there is steady current of 0.5 A for 20 second .

Q2 : A lamp has a p.d. of 12 v across it. Calculate how much electrical Energy is transferred when :A- a charge of 400 C passes through it .B- a current of 2.5A passes through it for 30 second .

How do you measure p.d.?You use a voltmeter which is placed across the lamp as shown

W = I t p.d

Q1 : a 230v kettle transfers 6.9x105 j of energy in 5 minutes . What is the current In the kettle ?

Electrical Resistance

Most materials offer some resistance to the flow of electric charges through them. This is called electrical resistance.

Voltage Current Resistance

Definition

Symbol

Unit

Device

Complete the table

Resistance

Resistance of a conductor depends on:

Material - Gold is best

Length - longer conductors have more resistance.

Cross section - thick wires have less resistance than thin wires

Temperature - higher temperature means more resistance for most conductors

Ohms’ Law

In symbols:V = IR

                               VI R

                

Complete the table Voltage

)volt (Current

)A(Resistance

)ohm (

110 50

30 2

44 20

15 60