Electric Current Lecture

8/6/2019 Electric Current Lecture

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Introduction

1.Electrostatics stationary charges

What is the relationship betweenelectric charges and electricity?


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Electric Current

2. For an amount of current,

3. The current is measured in Ampere, A.

1 A = 1 C s-1

tQI !


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Potential Difference (p.d.), V

1.Current is anologous to water flowing in asystem of pipes. Water flows from a point ofhigher pressure to a point of lower pressure.T

hus, a pressure difference.2.Electric current flows from a point of higher

electric potential to a lower electric potential. Apotential difference, V is required for a currentflow.

3.P.d. between 2 points in a circuit is the energyrequired by a Coulomb of charge to move fromone point to other.

4. 1 Volt = 1 Joule per Coulomb


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Potential Difference (p.d.), V

Resistor

Higher

potential,V1

Lower

potential,V2

V = V1 V2

I


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Electric energy, E


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ElectricalPower, P


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Resistance


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Electromotive Force (e.m.f.)


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Electromotive Force (e.m.f.)


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For a current to flow continuously between two points

PQ in a circuit, a source of e.m.f. is required, such as abattery, to maintain the p.d. between the two points.

The source of e.m.f. is the energy source in an electriccircuit.

El tr tiv F r ( . .f.)

P Q

E

V


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Ohms Law

A steady current through a metallic

conductor is directly proportional to the

potential difference across the conductor

if the temperature and other physicalconditions remain constant

Current I w V if temperature is constant


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Ohmic conductor

Obeys Ohms Law:


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Non-Ohmic Conductor- are conductors not obeying Ohms Law


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Resistors in series

The same current Ipasses through all resistors.

The potential difference across each resistor is

The total potential difference,

Hence,


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Resistors in parallel

P.d. are the same for all resistors

Current Iis divided into I1, I2, I3.


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Hence,

IfR is the effective resistance,


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Example:


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Questions:

1. Two 1.5 V battery is connected toresistors 3 , 4 and 5 connected in series. Calculate (i)

the current in the circuit and (ii)the potential difference acrosseach resistor.

2. Three 1.5 V cells are connected in

parallel.This circuit is connected toa resistor of 5.0 . Calculate the

current flowing through theresistor.


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Electromagnetism


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Using electromagnets

Many objects around youcontain electromagnets.They are found in electricmotors and loudspeakers.Very large and powerfulelectromagnets are usedas lifting magnets in scrap

yards to pick up, thendrop, old cars and otherscrap iron and steel.

They are better thanmagnets because themagnetism can be turned

off and on.


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Electromagnet Corkscrew rule or

right hand rule


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Electromagnet

S N

Use right-hand rule todetermine the magnetic poles


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Flemings Left Hand Rule (motor)

Force(Thumb)

Field (First finger)

Current (seCond finger)


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Electromagnetic Induction

Faradays Law:

the induced e.m.f. is directly

proportional to the rate of change offlux linkage or rate of flux cutting

Lenzs law

the direction of the induced e.m.f.is such that it tends to oppose theflux change causing it.


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What is magnetic flux

Consists of many magnetic field linesin an area;

Also known as magnetic flux density,B;

Unit ofB is Tesla, T

A

B


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Electromagnetic induction

N N

1. Magnet moving intosolenoid;

2. Direction of current isaccording to Lenz Law.


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N S

1. Magnet moving out ofsolenoid;

2. Direction of current isaccording to Lenz Law.


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This is a rule to help you to remember their directions.(NOTNOT a physics principle!)

Direction of induced current

a Fleming's right-hand rule

For a wire cutting through a B-field...

motion or force F

magneticfield B

inducedcurrent I


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b Lenz's law

induced I opposesthe motion ofmagnet (beingattracted)

induced I opposes

the motion ofmagnet (beingrepelled)


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In both cases, magnet

moves against a force.Work is done duringthe motion & it istransferred as electrical

energy.

Induced I always flows to oppose themovement which started it.

b Lenz's law


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Example 1Current induced along a coil

A bar magnet passes through a coil:

(a) Indicate the direction of the induced I ineach case. Explain briefly.

(i) (ii) (iii)


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Example 1Current induced along a coil

(a) Indicate the direction of the induced I. Explain.

(i)

When magnets N-pole is movinginto coil,

induced I flows in such a direction asto produce a N-pole

to oppose the approaching of magnet.Lenzslaw

I

S N


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Induction

)( *!\ Ndt

d

Induced e.m.f. depends on:

1.No. of coils

2.Magnetic field strength

3.Velocity of magnetic field

4.Cross-sectional area of coil


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Self-inductance

A current-carrying coil has amagnetic flux, B;

When the current in the coilchanges, B in the coil changes;

This induces an e.m.f. in the coil;

Induced e.m.f. is due to the changein the current in the same coil.


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NsNp

Vp Vs

Ip

Is

Transformer

S

P

P

S

P

S

S

P

SSPP

S

P

S

P

N

N

I

I

II

VV

VIVI

poweroutputpowerInput

N

N

V

V

!

!

!

!

!


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Transmission of current

Powerstation

houses

Step-downtransformer

Step-uptransformer

cables

50 kV 100 kV

240 V

13 A

0.5 A

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Electric Current Lecture