Electric Charge and Current

7/28/2019 Electric Charge and Current

1/24

Physics Department SMK Sultan Ismail Johor Bahru

________________________________________________________________________________________________________________

Electric Charge and Current

Electric Charge

1. There are only two kind of electric charge, namely the positive charge and the

negative charge.

2. Like charge repel each other.

3. Unlike charge attract each other.

4. The SI unit of electric charge is Coulomb (C).

Unit of Charge

The SI unit of electric charge is Coulomb (C)

1Coulomb (C) = 1 Ampere Second (As)

Example

Charge of 1 electron = -1.6 10-19 C

Charge of 1 proton = +1.6 10-19 C

Formula - Total Charge

Example 1

1.251019 electrons are added into an object. Find the nett charge of the object in the unit of

Coulomb. [Charge of 1 electron = -1.610-19]

Answer:

Number of electrons, n = 1.251019

Charge of 1 electron, e = -1.610-19

Total charge, Q = ?

Q = ne

Q = (1.251019)(-1.610-19) = -2C

http://www.one-school.net/Malaysia/UniversityandCollege/SPM/revisioncard/physics/heat/thermal.html#4

Prepared by: En Adnan Shamsudin
http://www.one-school.net/Malaysia/UniversityandCollege/SPM/revisioncard/physics/heat/thermal.html#4http://www.one-school.net/Malaysia/UniversityandCollege/SPM/revisioncard/physics/heat/thermal.html#4


2/24


________________________________________________________________________________________________________________

Current

An electric current I is a measure of the rate of flow of electric charge (Q) through a given

cross-section of a conductor.

Direction of Current

Conventionally, the direction of the electric current is taken to be the flow of positive charge.

The electron flow is in the opposite direction to that of the conventional current.

Unit of Current

The SI unit for current is the ampere (A).

Therefore, we can say that a current of one ampere is a flow of charge at the rate of one

coulomb per second. (Note: This is not a definition of ampere.)

Formula of Current

Example 2

If 30 C of electric charge flows past a point in a wire in 2 minutes, what is the current in the

wire?




3/24


________________________________________________________________________________________________________________

Answer:

Charge, Q = 30C

Time taken for the charge flow, t = 2 minutes = 120s

(Since the unit of current, Ampere (A) is also equal to Coulomb per second (Cs-1), the unit of

time must be changed to second)Current, I = ?

Electric Field

Electric Field

An electric field exists in a region of space where a small positive charge experiences an

electric force.

Line of force

1. The direction of the field is defined as the direction of the force on a small positive

charge.2. Lines of force are used to represent the direction of an electric field.

3. The lines of force are directed outwards for a positive charge and inwards for a

negative charge.




4/24


________________________________________________________________________________________________________________

Strength of Electric Field

The strength of the electric field is indicated by how close the field lines are to each other.

The closer the field lines, the stronger the electric field in that region.

Field Pattern of 2 Point Sources




5/24


________________________________________________________________________________________________________________

Ping Pong Ball in an Electric Filed

The ball will still remain stationary. This

is because the force exert on the ball by

the positive plate is equal to the force

exerted on it by the negative plate.




6/24


________________________________________________________________________________________________________________

If the ping pong ball is displaced to the

right to touch the positive plate, it will

then be charged with positive charge and

will be pushed towards the negative

plate.

When the ping pong ball touches the

negative plate, it will be charged with

negative charge and will be pushed

towards the positive plate. This process

repeats again and again, causes the ping

pong ball oscillates to and frocontinuously between the two plates.

Candle in an Electric Field

The heat of the candle flame

removes electrons from the air

molecules around it, and thereforeionised the molecule.

If the candle is placed in between 2

plates connected to a Extra High

Tension (E.H.T.) power supply, thepositive ions will be attracted to the

negative plate while the negative

ions will be attracted to the positive

plate.




7/24


________________________________________________________________________________________________________________

Potential Difference, Ohm's Law and

Resistance

Electrical Potential

The electric potential V at a point in an electric field is the work done to bring a unit ( 1

Coulomb) positive charge from infinity to the point.

Potential Difference (Voltage)

The potential difference (p.d.) between two points is defined as the energy converted from

electrical to other forms when one coulomb of positive charge passes between the two points.

Unit of Potential Difference

The SI unit of potential difference is the same as that for e.m.f., i.e. the volt. We define the

volt as follows:

The potential difference (p.d.) between two points in a conductor is 1 volt if 1 joule of energy

is converted from electrical to other forms when 1 coulomb of positive charge flows through

it.

Formula of Potential Difference

Example 1

How much energy had been transfer when 5 C charges moved across a potential difference of

10V?

Answer:




8/24


________________________________________________________________________________________________________________

The charge, Q = 5C

Potential difference, V = 10V

Energy, E = ?

Ohm's Law

The current flowing in the metallic conductor is directly proportional to the potential

difference applied across its ends, provided that the physical conditions ( such as

temperature ) are constant.

Formula:

Any other conductors, other than metallic conductors, which obey Ohms Law are described

as Ohmic conductors.

Example 2

What is the current through an 8 toaster when it is operating on 240V?

Answer:

(In this question, 2 physical quantities are given, they are the "240V" and "80". The

question doesn't tell what quantites they are. However we can recognise these quantities from

its unit. is the unit of resistance whereas V is the unit of potential difference.)

Resistance, R = 80Potential difference, V = 240V

Current, I = ?

V = IR

(240) = I(80)

I = 3A




9/24


________________________________________________________________________________________________________________

Resistance

The resistance R of a material is defined as the ratio V : I, where Vis the potential difference

across the material andIis the current flowing in it.

Unit:

The SI unit of resistance is the ohm (). One ohm is the resistance of a material through

which a current of one ampere flows when a potential difference of one volt is maintained.

Resistivity

The resistance R of a given conductor depends on the:

length l,

Longer wire - Higher Resistance

cross-sectional area A,

Thicker wire - Lower Resistance

temperature

Higher temperature - Higher Resistance

the type of material

copper has resistance lower than iron

Superconductor

Superconductors are materials where their electrical resistance is exactly zero at some

relatively low temperature.




10/24


________________________________________________________________________________________________________________

Application of Superconductor

1. Magnetic Resonance Imaging (MRI)

2. Magnetic-Levitation Train (MagLev)

3. Electric generators

Series Circuit and Parallel Circuit

Resistance, Current and Potential Difference in Series

Circuit

Effective Resistance: R = R1 + R2

Current: I1 = I2 = I3

Potential Difference V = V1 + V2




11/24


________________________________________________________________________________________________________________

Resistance, Current and Potential Difference in Parallel

Circuit

Effective Resistance: R = (R1 + R2 + R3)-1

Current: I = I1 + I2 + I3

Potential Difference V = V1 = V2 = V3

Example 1

What is the effective resistance of the connection shown in the picture below?

a. b.

Answer:

a. Effective resistance, R = 2 + 3 + 6 = 11

b. Effective resistance, R = (1/5 + 1/5)-1 = 2.5

Example 2

Find the resultant resistance of the arrangement below.




12/24


________________________________________________________________________________________________________________

a. b.

Answer:

a. Effective resistance = 3 + (1/2 + 1/2)-1 = 4

Current in Series Circuit

The current flow into a resistor = the current flow inside the resistor = the current flows out

from the resistor (I1 = I2 = I3)

In a series circuit, the current at any points of the circuit is the same.

Current in Paralle Circuit

The current flow into a parallel circuit is equal to the sum of the current in each branches of

the circuit.

(I = I1 + I2)

Example:




13/24


________________________________________________________________________________________________________________

If the resistance of the 2 resistors is the same, current will be divided equally to both of the

resistor.

Example 3

In each of the diagrams below, find the reading of the ammeter.

a. b.

Answer:

a. In a series circuit, the current at any points of the circuit is the same. Therefore, the reading

of the ammeter is also 0.5A.

b. Reading of the ammeter, I = 6A - 2A = 4A

Example 4

In the diagram above,

a. find the reading of the ammeter.




14/24


________________________________________________________________________________________________________________

b. find the current flows through each of the resistors.

Answer:

a.

Resistance of the (whole) circuit = 2 + 4 = 6Potential difference across the whole circuit, V = 12V

Current, I = ?

V = IR

(12) = I(6)

I = 2A

Reading of the ammeter = 2A

b.

Since in a series circuit, the current at any points of the circuit is the same. Therefore, thecurrent flows through each of the resistors is also 2A.

Potential Difference in Series Circuit

The sum of the potential difference across individual resistor in between 2 points in a series

circuit is equal to the potential difference across the two point.

V = V1 + V2

Example




15/24


________________________________________________________________________________________________________________

Potential Difference in Parallel Circuit

The potential difference across all the resistor in a parallel circuit is the same.

V = V1 = V2

Example

Find the reading of the given voltmeter(s) in the diagrams below

a. b

.

Asnwer:

a. Reading of the voltmeter, V2 = 12 - 7 = 5V

b. The potential difference across all the resistor in a parallel circuit is the same. Therefore,

the reading of the voltmeter V1 is also 5V

Example 6




16/24


________________________________________________________________________________________________________________

Find the potential difference across each of the resistors in the diagram above.

Answer:

The potential difference across the whole circuit = 12V, but the potential across the 2 resistor

R1 and R2 are unknown. In order to find the potential difference across the resostors, we

need to find current passing through the resistors.

V = 12V, R = 6, I = ?

V = IR

(12) = I(6)

I = 2A

For resistor R1,

R = 2, I = 2A, V = ?

V = IR

V = (2)(2) = 4V

For resistor R2,

R = 4, I = 2A, V = ?

V = IR

V = (2)(4) = 8V

The potential difference across the resistors R1 and R2 are 4V and 8V respectively.




17/24


________________________________________________________________________________________________________________

Potential Difference and E.M.F

If we assume that there is no internal resistance in the cell, the potential difference across the

cell is equal to the e.m.f. of the cell.

Potential Change in a Series Circuit

V1 = V2 = V5

V3 + V4 = V5

Potential Change in a Parallel Circuit




18/24


________________________________________________________________________________________________________________

V1 = V2 = V3 = V4

Example 7

Find the reading of the voltmeter in each of the circuit below.

a. c.

b

.

d

.

Answer:

a. Reading of the volmeter = e.m.f. = 3V




19/24


________________________________________________________________________________________________________________

b. Reading of the volmeter = e.m.f. = 3V

c. Reading of the volmeter = e.m.f. = 3V

d. Reading of the volmeter = e.m.f. = 3V

Card 10: Empty Card

Electromotive Force and Internal

Resistance

Electromotive Force

In a circuit, electromotive force is the energy per unit charge converted from the other formsof energy into electrical energy to move the charge across the whole circuit.

Unit:

The unit of e.m.f. is JC-1 or V (Volt)

Formula of Electromotive Force

Comparing E.M.F. and otential Difference

Electromotive Force Potential Difference

Similarities:

Have same unit (Volt)

Can be measured by Voltmeter




20/24


________________________________________________________________________________________________________________

Definition

The electromotive force (e.m.f.) is

defined as the energyper unit

charge that is converted from

chemical, mechanical, or otherforms of energy into electrical

energy in a battery or dynamo.

Definition

The potential difference (p.d.)

between two points is defined as

the energy converted from

electrical to other forms whenone coulomb of positive charge

passes between the two points.

Symbol:

Denote by the symbol, E.

Symbol:

Denote by the symbol, V

Internal resistanceThe internal resistance of a source (cell or generator) is the resistance against the moving

charge in the source.

Load Resistance

The load resistance in a circuit is the effective resistance against the moving charge outside

the source of electric.

Terminal Potential Difference Terminal potential difference or terminal voltage is the potential difference across the

two terminal (the positive terminal and the negative terminal) of an electric source

(cell or generator).

If the internal resistance of the cell is ignored, the terminal potential difference is

equal to the e.m.f.

Formulae of Electromotive Force and Internal Resistance




21/24


________________________________________________________________________________________________________________

Finding E.M.F and Internal Resistance - The Open Circuit

Method

In open circuit ( when the switch

is off), the voltmeter shows the

reading of the e.m.f.

In close circuit ( when the switchis on), the voltmeter shows the

reading of the potential difference

across the cell.

With the presence of internal resistance, the potential difference across the cell is always less

than the e.m.f.

Finding E.M.F and Internal Resistance - Linear Graph

Method




22/24


________________________________________________________________________________________________________________

Gradient od the grapf, m = - internal resistance

Y intercept of the graph, c = e.m.f.

Electrical Energy and Power

Electrical Energy

From the definition of potential difference, the electric work is given by the formula:

W = QV

(W = Work done; Q = Charge; V = Voltage)

Since the work done must be equal to the energy to do the work, therefore we can also say

that, the electrical energy ( E )is also given by the formula

Electrical Power

1. The electrical power, P is defined as the rates of energy that supply to the circuit ( orthe rates of work been done ) by sources of electric.




23/24


________________________________________________________________________________________________________________

2. The unit of electric power is the watt (W).

3. One watt of power equals the work done in one second by one volt of potential

difference in moving one coulomb of charge.

Formulae of Electrical Power

Resistance and Power

In a series circuit, the higher the resistance of a resistor, the higher the power of the resistor.

In a parallel circuit, the higher the resistance of a resistor, the lower the power of the resistor.

Sum of Power

The effective power in a series circuit is equal to the sum of the power of each resistor in the

circuit.

P = PR1 + PR2




24/24


________________________________________________________________________________________________________________

Sum of power in a Parallel Circuit

The effective power in a parallel circuit is also equal to the sum of the power of each resistorin the circuit.

P = PR1 + PR2

Calculating The Cost Of Electricity Consumption

The cost of electricity consumption is based on the number of kilowatt-hours (kWh) of

electrical energy used. The kilowatt-hours are sometimes known as the domestic units of

electricity.

Formula

Electric Charge and Current

Documents