Current electricity

MODULE 3: ELECTRICITY AND MAGNETISM

Electric Circuits

Power sourceTo supply

electrical power

ConductorTo conduct electricity

A device that uses energy

A switch that controls the flow of electricity is a good addition

Basic Circuit

Cell

Battery

Light Bulb

Resistor

Circuit Components

Switch

Ammeter Voltmeter

Circuit Components

Conventional Current

𝒆−Electrons are

supplied at the negative pole of

the battery

Flow from negative to

positive

Electrical current is the flow of electrons

Conventional Current

Historically it was thought electricity was the flow of +

charges

Flow from positive to negative

We still use the convention today

Potential Difference

Potential Difference

Energy gets supplied by the battery

Energy gets used up by circuit components

• Symbol: V• SI-Unit: Volt (V)• Formula :

Where V = Potential difference (V)W = Work done/energy

transferred (J)Q = Charge transferred (C)

Potential DifferenceThe electrical work done per unit charge

Potential DifferenceMeasured with:

Voltmeter

Potential Difference: EMF

Electromotor Force:The maximum amount of electrical energy that a battery can supply to the charge that flows

through it

Potential Difference: EMF

VMeasured across the poles of the battery when

switch is open (no current)

Potential Difference:Example 1

10 J energy is transferred when 5 Coulomb charge moves from point A to

point B. Calculate the potential difference between the two points.

Current

CurrentNumber of particle being pushed through the

circuit

The rate at which charge flows

• Symbol: I• SI-Unit: Ampère (A)• Formula :

Where I = Current (A)Δt = Time (s)Q = Charge transferred (C)

Current

1 Ampère is the current when 1 Coulomb charge moves past a point in 1 second

CurrentMeasured with:

Ammeter

Current: Example 2It takes 5 C charge 2 min to move from point A to point B. Calculate the current

in the wire.

Current: Example 3Calculate the current if 10 J energy is

converted to heat by a lightbulb in 50 s. The potential difference over the ends of

the lightbulb is 5 V.

Resistance

Magnified representation of a wire

+

+

+

+

++ + +

+ +

+

++

+ ++

+

++

++ +

+

+

+

+

+

• Delocalized electrons collides wit positive atomic rests

• Causes Resistance

Tendency to inhibit the flow of electrons

Resistance: Factors

Resistance increases if temperature increases

Resistance decreases if thickness increases

Resistance: Factors

Resistance increases if length increases

Different materials does not conduct electricity to the

same extent

ResistanceThe relationship of the potential difference (V) across a component to the current (I) through

that component

• Symbol: R• SI-Unit: Ohm (Ω)• Formula :

Where R = Resistance (Ω)V = Potential difference (V)I = Current (A)

ResistanceA conductor has a resistance of 1 Ω if a current

of 1 A flows through it while the potential difference across the ends of the conductor is

1 V

V

I R

Resistance: Example 4The ammeter reading is 2 A

and the Voltmeter

reading is 10 V.

a) Calculate the resistance R.b) What is the emf of the battery?

Series CircuitsResistors in series act as voltage dividers

while the current stays the same

𝑉 𝑇=𝑉 1+𝑉 2

Series CircuitsAdvantage:

Lowers electricity usage

Disadvantage:If one lightbulb fuses none will work

Series Circuits𝑅𝑇=𝑅1+𝑅2+𝑅3

𝑉 𝑇=𝑉 1+𝑉 2+𝑉 3

𝐼𝑇=𝐼 1=𝐼 2

V

I R

Series Circuits: Example 5

Each cell has an emf of 10 V. The resistors have the

following resistances:R1= 3 Ω; R2 = 2 Ω; R3 = 5 Ω

Calculate:a) The total

voltageb) Total

resistancec) Ammeter

Readingd) The readings

on each of the voltmeters

Parallel CircuitsResistors in parallel act as current dividers

while the voltage stays the same

Advantage:If one lightbulb fuses the other will still work

Disadvantage:Increases electricity usage

Parallel Circuits

Parallel Circuits1𝑅𝑇

=1𝑅1

+1𝑅2

+1𝑅3

𝑉 𝑇=𝑉 1=𝑉 2=𝑉 3

𝐼𝑇=𝐼 1+ 𝐼2+𝐼 3

V

I R

Parallel Circuits: Example 6

Each cell has an emf of 1,5 V. The ammeter

readings are as follows:A1= 1 A; A2 = 2 A

Calculate:a) The total voltageb) Total resistancec) The readings on

each of the voltmeters

d) Each of the resistances

Combination CircuitsResistor in series with parallel connection

1𝑅𝑃

=1𝑅2

+1𝑅3

𝑉 𝑇=𝑉 1+𝑉 𝑃

Calculate Rp

𝑉 𝑃=𝑉 2=𝑉 3

𝑅𝑇=𝑅1+𝑅𝑃

Calculate RT

Combination CircuitsResistor in parallel with series connection

1𝑅𝑇

=1𝑅𝑠

+1𝑅3

𝑉 𝑠=𝑉 1+𝑉 2

Calculate Rs

𝑉 𝑇=𝑉 𝑠=𝑉 3

𝑅𝑠=𝑅1+𝑅2

Calculate RT

Combination Circuits: Example 7

The potential difference across R1 is 10 V and the reading on A1 is 0,4 A. The resistances of R2 and R3 is 45 Ω and 157,5 Ω respectively.

a) What is the reading on V1 if switch S2 is open?

b) What is the potential difference over the battery if switch S2 is open?

Both switches are now closed.

c) Calculate the resistance of R1.

Combination Circuits: Example 7 Contd.

d) Calculate the total resistance.e) What is the reading of V1?f) What is the readings on A2 and A3?g) What is the current through R3?

The switch S2 is now open.

h) What is the new readings on A1 and V1?i) How did the voltage across resistor R1

change? Explain why

Combination Circuits: Example 7 Contd.

V1

R1 R2

R3

V2

V3

Vs

V1

R1

R2

R3

V2

V3

VT

I1

I2

I3

IT

A

V

R

Series Circuits

V1

R1 R2R3

V2 V3

VT

I1 I2

IT IT

A1

R1

R2

R3

A2

A3

S1

V1

S2

24 V