CHAPTER 9:

ELECTRICITY

CIRCUIT SYMBOLS page 61, Syllabus Book

Current Electricity

• Current Electricity: Net flow of charges in a certain direction.
• All matter is made up of tiny particles called atoms, each consisting of a positively charged nucleus with negatively charged electrons moving around it.
• Charge is measured in units called coulombs (C). The charge on an electron is -1.6 x 10⁻¹⁹ C.

• Normally atoms have equal number of positive and negative charges, so that their overall charge is zero.

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• For some atoms, it is relatively easy to remove an electron, leaving an atom with an unbalanced number of positive charges. This is called positive ion. - metal

• Atoms in metals have one or more electrons which are not held tightly to the nucleus.
• These free (or conduction) electrons wander at random throughout the metal.

https://youtu.be/O4lkxkbeo3s?si=ztKDfKxhG-SDU78v

• But when a battery (or source) is connected across the ends of the metal, the free electrons drift towards the positive terminal of the battery (or source) producing an electric current.

https://youtube.com/shorts/YsBqFiptmvw?si=NTc1KYJozO5kuaL5

• Conventional current: Direction of current from +ve to –ve.
• When connected to wire (metal) it exerts an electrical force on the electrons.
• electron flow away from –ve towards +ve.

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Charge Carriers- positive or negative?

Electrolyte: a solution which conducts the current and contains both +ve & -ve ions. It allows current in batteries, electrolysis, body functions

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Charge carriers: any charged particles which contribute to an electric current, such as p-type and n-type of semiconductor below

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•The size of the electric current is given by the rate of flow of charge and is measured in units called amperes with symbol A.

•A current of 3 A means that 3C pass a point in the circuit every second. In 5 seconds, a total charge of 15C will have passed the point.

Q = It

I = Q/t

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Charge & Coulomb

•From the definition of electric current, I,(recall?) we obtain,

Q = It

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•Electric charge, Q flowing through a section of a circuit is the product of the electric current and the time that it flows.

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• Charge carrier is quantized (discrete) - all Q must be multiple of e (1.6 x 10^-19 C)

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• Quantized: A quantity is said to be quantized if it can only take specific, fixed values — not any value continuously.

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• Q = Ne , Q = total charge, N = number of charge carrier,

e = 1.6 x 10-19 C (elementary charge)

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ANS: B

ANS: B

From the diagram the following can be implied:

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• volume of the cylindrical section = LA
• no. of free electrons in the section = nLA
• quantity of mobile charge Q in the section Q = nLAq = nLAe

(e for electron)

For metal:

L

• The time t for all the electrons in the section to travel from one face to the another is the time for one electron (on the far right) to travel the whole length:

and

If current increases, the drift velocity must increase.

If the wire is thinner, the electron move more quickly for a given current.

Answer : C

Answer : B

Answer : B

• The three voltmeters are measuring three voltages or potential difference.
• The voltage across the power supply is equal to the sum of the voltages across the resistors.

• Voltmeter readings indicates the energy transferred to the component by each unit of charge.
• Voltmeter across power supply, measures e.m.f of the supply.
• Voltmeter across the resistors, measure the p.d (potential difference).

Voltage

Potential difference between two points, is the energy given up (equal to work done) by unit charge as it moves from a point to another point in a circuit

Brightness of identical bulbs/ lamps depends on potential difference, V across it

e.m.f is defined as the total work done when unit charge goes round a complete circuit (charge gain electrical energy from power supply or a battery)

Answer : A

Electric Resistance

Resistance: the ratio of the potential difference to the current

To determine the resistance, need to measure both V and I.

1. The ammeter is connected in series with conductor there is same current in both
2. Voltmeter is connected across (in parallel with) conductor

Ohm’s Law

Ohm’s law states that the current I through a given conductor is directly proportional to the potential difference V between its end points.

Ohm’s law allows us to define resistance R and to write the following forms of the law:

Electric Power

Electric power P is the rate at which electric energy is expended, or work per unit of time.

Recall: Work done = qV

Substitute q = It , then:

P = VI

Unit: Watts = amps x volts

Calculating Power

Using Ohm’s law, we can find electric power from any two of the following parameters: current I, voltage V, and resistance R.

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Power as energy transferred/ dissipated in unit time

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Ohm’s law: V = IR

The brightness of a lamp depends on power its dissipated

A lamp dissipates more power produces more light and appears brighter

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P=VI

A lamp becomes brighter when either the voltage across it or the current through it increases, because its power increases

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P=I2R

If a greater current, I, flows through an identical lamp ( R constant), it dissipates more power and therefore glows more brightly.

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P=V2/R

For identical lamps (R constant), a larger voltage across the lamp produces a larger power, so the lamp becomes brighter

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RELATIONSHIP BETWEEN POWER AND BRIGHTNESS- WHY A LAMP GLOWS?

A lamp glows because electrical energy is converted into light and heat.

The rate at which energy is converted per second is called power.

Power =energy per second =rate of energy

So higher power → more energy per second → brighter light.