Exp SPA - Chp 17 Current of Electricity

Current of Electricity

Learning Objectivesa) State that current is a rate of flow of charge and

that it is measured in amperesb) Distinguish between conventional current and

electron flowc) Recall and apply the relationship charge = current

× time to new situations or to solve related problems

Electric CurrentWhat is Electric Current? An electric current is formed by moving charges.

Definition:An electric current is a measure of the rate of flow of electric charge through a given cross-section of a conductor.

The SI unit of electric current is the ampere (A).

where I = current; Q = charge;

t = time taken.

QI t

Electric CurrentPractice ProblemIn one minute, 60 μC of charge passes through a point in a circuit. Calculate the current.

Solution

I = =

= 1 × 10−6 A = 1 μA

Q t60 × 10−6 C

60 s

Electric CurrentHow do we measure current? We make use of an ammeter to measure current. The ammeter should be connected in series to the

circuit. Current must flow into positive (+)

terminal and leave negative (–) terminal

Current in Current out

Electric CurrentConventional Current & Electron Flow Before the discovery of electrons,

scientists believed that electric current was caused by the movement of positive charges.

Although this idea was later proven wrong, the idea remains.

Conventional Current Flow Electron Flow

Movement of positive charges

From positive end of battery to negative end of battery

Movement of electrons (–)From negative end of battery topositive end of battery

Electric CircuitMain components of a circuit A typical electric circuit consists of four main

components.

A source of electromotive force that drives electric charges around

the circuit.

A load in which moving charges can do a useful job.

Conductors that connect the components together.

A method of opening or closing the circuit.

Dry cell

Wires

Switch

Bulb

Electric CircuitDrawing Circuit Diagrams Electric circuits can be represented by circuit diagrams. Can you identify what the symbols in the circuit diagram

below represent? ammeter

bulb

connecting wires

switch

cell

Electric CircuitDrawing Circuit Diagrams Some common components and their symbols are

listed in the tables below.Symbol Device

switch

cell

batteryd.c. power

supplya.c. power

supply

light bulb

Symbol Device

wires joined

wires crossed

fixed resistor

Variable resistor (rheostat)

fuse

coil of wire

Symbol Device

galvanometer

ammeter

voltmetertwo-way switchearth

connector

capacitor

Electric CircuitInterpreting Circuit Diagrams Open circuit

A circuit in which current is unable to flow through due to breaks in the circuit.

Electric CircuitInterpreting Circuit Diagrams Short circuit

An alternative path of lower resistance is present and hence, current flows through wire X instead of the bulb.

Learning Objectivesd) Define electromotive force (e.m.f.) as the work

done by a source in driving a unit charge around a complete circuit

e) calculate the total e.m.f. where several sources are arranged in series

f) State that the e.m.f. of a source and the potential difference (p.d.) across a circuit component is measured in volts

g) Define the p.d. across a component in a circuit as the work done to drive a unit charge through the component

Electromotive Force & Potential Difference

What is the role of the battery in the circuit?

Why do you need a battery to make the bulb light up?

We can compare flow of charges to flow of water.


Flow of water in a pipe Flow of charge in a wireThe water in a horizontal pipe will not flow unless there is a difference in pressure between X and Y.

The free electrons in a wire will not flow unless they are forced to do so.


Flow of water in a pipe Flow of charge in a wireA pressure difference across X and Y is required such that X is at a higher pressure than Y

A source of energy is needed to drive the charge round a circuit. It may be a cell, a battery or a generator

Water pump does work to drive the water around the pipe.

Battery does work to drive electrons around the circuit.


What is Electromotive Force?Definition:The electromotive force (e.m.f) of an electrical energy source is defined as the work done by the source in driving a unit charge around a complete circuit.

ε = e.m.f. of electrical energy source;W = work done (amount of non-electrical energy converted to electrical energy);Q = amount of charge.

The SI unit of e.m.f is the volt (V).

We QQ

We


How do we measure e.m.f.? We make use of a voltmeter to measure e.m.f. The positive and negative terminals of a voltmeter

should be connected to the positive and negative terminals respectively of the electrical source.


Group ActivityObjective Measure and observe the e.m.f. of different arrangements of

dry cells.

Instructions1. In groups, set up the circuits shown below.

2. Record the voltmeter readings.

+ −

+ −

+ −

+ −

+ −

+ −

+ −

Series Arrangement Parallel Arrangement


When cells are arranged in parallel, the resultant e.m.f. is equal to that of a single cell.

When cells are arranged in series, the resultant e.m.f. is the sum of all the e.m.f.s of the cells.


What is Potential Difference?Definition:The potential difference (p.d.) across a component in an electric circuit is the work done to drive a unit charge through the component.

V = p.d. across a component;W = work done (amount of non-electrical energy converted to electrical energy);Q = amount of charge.

The SI unit of potential difference is the volt (V).

WV Q


How do we measure p.d.? We make use of a voltmeter to measure p.d. The voltmeter should be connected in parallel with the

component.


Potential Differences around a simple circuit

The cells does work by pushing the electrons through the whole circuit. They gain electrical potential energy.

As they flow through each component, the electrical potential energy is converted to other forms of energy.

Total e.m.f. in a circuit is equal to the total p.d. across each component


Potential Differences around a simple circuitFor a single charge,


Electromotive Force vs. Potential Difference

Electromotive force Potential Difference

It is the work done to drive a unit charge through two points.

It is the work done by the source in driving a unit charge around a complete circuit.

Associated with two points in an electric circuit

Associated with an electrical energy source (e.g. a dry cell)


What will the voltmeter reading show? A The e.m.f. of the electrical source (the three cells in series) B The e.m.f. of the bulbC The potential difference across the bulb

Learning Objectivesh) State the definition that resistance = p.d. / currenti) Apply the relationship R = V / I to new situations or to

solve related problemsj) Describe an experiment to determine the resistance of

a metallic conductor using a voltmeter and an ammeter, and make the necessary calculations

k) recall and apply the formulae for the effective resistance of a number of resistors in series and in parallel to new situations or to solve related problems

Learning Objectivesl) state Ohm’s Lawm) describe the effect of temperature increase on the

resistance of a metallic conductorn) sketch and interpret the I/V characteristic graphs for a

metallic conductor at constant temperature, for a filament lamp and for a semiconductor diode

Resistance

QuestionPredict what will happen if a porous plate (an obstacle) is placed in the path of the water flow.

Earlier, we used the water pump analogy to help us understand e.m.f.

obstacle

Resistance

Resistance is a measure of how difficult it is for an electric current to pass through a material.

It is the property of the material that restricts the movement of free electrons in the material.

Resistor added

Rate of flow of electric charges

reduced

resistor

Current is reduced

Ammeter reading will be reduced

ResistanceWhat is Resistance?Definition:The resistance of a component is the ratio of the potential difference across the component to the current flowing through the component.

R = resistance of a component;V = p.d. across a component;I = current flowing through component.

The SI unit of resistance is the ohm (Ω).

VR I

ResistanceHow do I measure resistance?

A circuit is necessary to measure resistance.

The ammeter is connected in series with the bulb.

The voltmeter is connected in parallel with the bulb.

Use

+

−

−+

ResistanceIn the circuit shown,

All components have resistance.

Wires and ammeter assumed zero resistance current flows through without any loss of energy

Voltmeter considered infinite resistance no current flows through it

Light bulb Wires VoltmeterAmmeter

R = 0 Ω R = ∞ Ω

ResistanceWhat are resistors?

A resistor is a conductor in a circuit that is used to control the size of the current flowing in a circuit.

There are two types of resistors — fixed resistors and variable resistors (or rheostats).

some common fixed resistors a rheostat is a type of variable resistor

ResistancePractice Problem

A 3 V source is connected to a light bulb,as shown in the diagram.The ammeter registers a reading of 0.2 A.a) State the reading on the voltmeter.b) Determine the resistance of the light bulb.c) The circuit is switched on for five minutes.

i. Determine the amount of charge and; ii. energy dissipated in the bulb.

+

−

−+

ResistancePractice Question(a) Voltmeter reading = 3 V

(b) R = = 15 Ω

(c) (i) Q = It = 0.2 A × 5 × 60 s = 60 C

(ii) E = QV = 60 C × 3 V = 180 J

3 V0.2 A

Ohm’s LawOhm’s Law states that the current passing through a metallic conductor is directly proportional to the potential difference across it, provided that physical conditions remain constant.

where I = current; V = potential difference.

Ohm’s LawOhmic Conductors are conductors that obey Ohm’s Law.

The I−V graph of an ohmic conductor is a straight line that passes through the origin.

Ohm’s LawNon-Ohmic Conductors are conductors that do not obey Ohm’s Law.

They do not obey Ohm’s Law and their resistance R can vary.

Their I−V graphs are not straight lines, which means the ratio V/I is not a constant.

Ohm’s LawA filament lamp is the 1st non-ohmic conductor. As the current increases, the temperature of the

filament lamp increases. It results increase in resistance increases with increasing current decreases with increasing current

Ohm’s LawA semiconductor-diode is the 2nd non-ohmic conductor. It is a special device that allows current to flow in one

direction.

1) When potential difference is in the correct direction, current flows through it as though it is like a resistor.

2) When potential difference is in the reverse direction, almost no current flows through it.

Learning Objectiveso) Recall and apply the relationship of the proportionality

between resistance and the length and cross-sectional area of a wire to new situations or to solve related problems

ResistivityBesides temperature, the resistance R of a conductor depends on

1) its length l;

2) its cross-sectional area A;

3) the material it is made of (i.e. resistivity ρ).

l A

Resistivity The formula of resistance depending on its physical

properties is:

The formula of resistivity:

The SI unit of resistivity is the ohm metre (Ω m).

where ρ = resistivity of conductor;R = resistance of conductor;A = cross-sectional area of conductor; l = length of conductor.

Resistivity Different materials have

different resistivities.

Resistivity is a property of the material and it is independent of the dimensions of the material.

The lower the resistivity of a material, the better it is at conducting electricity.

ResistivityComparing a wire to a tunnel

We can compare a wire to a tunnel to remember the factors affecting resistance:

Cross-sectional area The wider a tunnel,

the greater the number of people can walk through at the same time

The lower the resistance

Length The shorter a tunnel

The shorter the time taken to completely walk through it

The lower the resistance

ResistivityTackling problems

Resistivity questions generally look like this:

1) I have a wire with resistance R1, length l1 and area A1.

2) If I use the same material but new length l2 and area A2, find its new resistance R2.

Since it is the same material, its resistivity r must be constant.

Therefore

Original wire’s

dimensions

New wire’s dimensions

ResistivityPractice Problem 1

A conductor has a length l of 4 cm and cross-sectional area A of 2 cm2. Given that its resistance R is 5 Ω, determine the resistance of each of the following. l = 4 cm

A = 2 cm2

l = 4 cm

A = 4 cm2

l = 8 cm

A = 2 cm2(a)

(b)

Resistivity

l = 8 cm

A = 4 cm2(c)

l = 8 cm

A = 1 cm2(d)

Read the solution for (a) and try the rest on your own first!

Resistivity

(a)

Original wire’s

dimensions

New wire’s dimensions

Attempt (b) to (d) using the same method before checking the answers!

Resistivity(c)

(b)

(b)


A length of resistance wire 50 cm long is connected in series with an ammeter and a 3 V battery. The ammeter reading is 0.15 A.

A

50 cm

3 V

0.15 A


(a) Determine the resistance of the wire.

(b) The length of the wire is doubled and its cross-sectional area halved. Determine the new resistance and hence ammeter reading.

(c) Given that the diameter of the 50 cm long wire is 5 mm, determine its resistivity.

Resistivity(a)

(b)

(c) Remember that wires are

usually circular!

Exp SPA - Chp 17 Current of Electricity

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Transcript of Exp SPA - Chp 17 Current of Electricity