CHAPTER 17 Current Electricity © 2013 Marshall Cavendish International (Singapore) Private Limited.

CHAPTER 17Current Electricity

© 2013 Marshall Cavendish International (Singapore) Private Limited

17.1 Electric Current

17.2 Electromotive Force and Potential Difference

17.3 Resistance

17.4 Resistivity

Chapter 17 Current Electricity

Learning Outcomes

At the end of this section, you should be able to:

• define the term current and state its SI unit;

• differentiate between conventional current and electron flow;

• apply the formula charge = current × time to solve problems;

• draw electric circuit diagrams.

17.1 Electric Current

Electric Current

What is Electric Current?

Video on Lightning & Thunderhttp://www.youtube.com/watch?v=Sp9bKDHRfsM

So how do Electrons Flow?

So how does Lightning work?

Why does the lightning hit the tree?

Why is the rod good?

Electric Shock!

What conducts Electricity?

How do you protect yourself?

Good Electrical ConductorsThings that have free electrons

• Metals (copper & zinc?• What’s the best conductor?• How about water?

Good InsulatorsThings that do not allow electrons to flow

• rubber

An electric current is formed by moving charges.

17.1 Electric Current

What is Electric Current?

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

Electric current is a measure of the rate of flow of electrons through a conductor.

where I = current; Q = charge;

t = time taken.

Electron Flow

• Electric current is actually caused by the flow of electrons from the negative terminal to the positive terminal.

17.1 Electric Current

• We make use of an ammeter to measure current.

• The ammeter should be connected in series to the circuit.

17.1 Electric Current

How do We Measure Electric Current?

• 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.

17.1 Electric Current

Main Components of a Circuit

Dry cell

Wires

Switch

Bulb

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

below represent?

17.1 Electric Current

Drawing Circuit Diagrams

ammeter

bulb

connecting wires

switch

cell

Some common components and their symbols are listed in the tables below.

17.1 Electric Current

Drawing Circuit Diagrams

Open circuit

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

17.1 Electric Current

Interpreting Circuit Diagrams

17.1 Electric Current

Interpreting Circuit Diagrams

Short circuit

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

17.1 Electric Current

17.2 Electromotive Force and Potential Difference

17.3 Resistance

17.4 Resistivity

Chapter 17 Current Electricity

Learning Outcomes

At the end of this section, you should be able to:

• define electromotive force (e.m.f.) and potential difference (p.d);

• state the SI unit of e.m.f. and p.d.;

• calculate the e.m.f. when a few sources are arranged in series.

17.2 Electromotive Force and Potential Difference

What is the role of the battery in the circuit?

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

Recall

We saw this circuit in Section 17.1.

17.2 Electromotive Force and Potential Difference

• A battery functions like a water pump.

• A water pump does work (providing energy) to drive the water around the pipe.

• Likewise, a battery does work to drive electrons around the circuit.

What is Electromotive Force?

17.2 Electromotive Force and Potential Difference

SI unit of e.m.f is the joule per coulomb (J C–1) or volt (V).

What is Electromotive Force?

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

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

17.2 Electromotive Force and Potential Difference

How do We Measure E.m.f.?

• We use 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.

17.2 Electromotive Force and Potential Difference

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.

Arrangement of Cells

Series arrangement

Parallel arrangement

17.2 Electromotive Force and Potential Difference

What is Potential Difference?

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

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

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

17.2 Electromotive Force and Potential Difference

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.

17.2 Electromotive Force and Potential Difference

E.m.f. versus P.d.

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)

Potential differenceElectromotive force

17.2 Electromotive Force and Potential Difference

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

Question

17.2 Electromotive Force and Potential Difference

17.1 Electric Current

17.2 Electromotive Force and Potential Difference

17.3 Resistance

17.4 Resistivity

Chapter 17 Current Electricity

Learning OutcomesAt the end of this section, you should be able to:

• define the term resistance;

• apply the formula resistance =

to solve problems;

• describe an experiment to determine resistance;

• state Ohm’s Law;

• understand and draw the I−V characteristic graphs for ohmic and non-ohmic conductors;

• describe the relationship between the resistance of a metallic conductor and its temperature.

17.3 Resistance

p.d.current

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

17.3 Resistance

Recall

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

obstacle

Resistor added

Rate of flow of electric charges

reduced

17.3 Resistance

What is Resistance?

resistor

Current is reduced

Ammeter reading will be reduced

• Resistance is the difficulty for an electric current to pass through a material.

• It restricts the movement of free electrons in the material.

17.3 Resistance

What is Resistance?

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

The resistance of a component is the ratio of the potential difference across the component to the current flowing through the component.

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

1. In groups, design a circuit that can be used to determine the resistance of a bulb.

2. Using the materials given, check if your design works.

17.3 Resistance

Activity (Group)

Instructions

ObjectiveDesign an electric circuit that can be used to measure the resistance of a component.

p.d.current

Resistance =

17.3 Resistance

Circuit for Measuring Resistance

Note that:• The ammeter is

connected in series with the bulb.

• The voltmeter is connected in parallel with the bulb.

17.3 Resistance

What 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).

Ohm’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.

17.3 Resistance

Ohm’s Law

17.3 Resistance

Ohmic and Non-ohmic Conductors

• Ohmic conductors are conductors that obey Ohm’s Law. • Non-ohmic conductors are conductors that do not obey

Ohm’s Law.

Ohmic conductors

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

Non-ohmic conductors

• 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.

17.3 Resistance

17.1 Electric Current

17.2 Electromotive Force and Potential Difference

17.3 Resistance

17.4 Resistivity

Chapter 17 Current Electricity

17.4 Resistivity

Learning Outcome

At the end of this section, you should be able to:

• apply the relationship of the proportionality of resistance to the length and cross-sectional area of a wire to solve problems.

Recall

Ohm’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.

Other than temperature, what physical conditions affect the resistance of a component?

17.4 Resistivity

1. its length l;

l A

conductor

Other than temperature, the resistance R of a conductor also depends on

3. the material it is made of (i.e. resistivity ρ).2. its cross-sectional area A;

17.4 Resistivity

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

Rewriting , we can obtain the formula for resistivity.

Resistivity

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

17.4 Resistivity

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.

17.4 Resistivity

Worked Example

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) 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.

17.4 Resistivity

(a) R = V ÷ I

= 3 ÷ 0.15

= 20 Ω

(b)

(c)

17.4 Resistivity

Solution

Electric current I

(SI unit: A)

related to

Resistance R

(SI unit: Ω)

Charge Q(SI unit:

C)

Electromotive force ε

(SI unit: V)

defined as rate of flow of

Chapter 17 Current Electricity

Potential difference V(SI unit: V)

related to

(continued on next slide)

where W = work done by

source to drive a unit

charge around the circuit

ε =

WQ where

W = work done to drive a unit charge through a component

V =

WQ

where t = time

I =Qt wher

e

where

where

Chapter 17 Current Electricity

Electric current I

(SI unit: A)

related to

Resistance R

(SI unit: Ω)

related to resistivity ρ

Obeys Ohm’s Law I α V

where l = lengthA = cross-sectional

area

ρ =

RAl

where

if constant

where V = potential differenceI = current

R =

VI

if not constant

Does not obey Ohm’s Law

Ohmic conductors

Non-ohmic conductors

(continued from previous slide)

URL

Acknowledgements(slides 1−47) plasma globe © Stuartkey | Dreamstime.com(slides 6, 15) battery © Vladwitty | Dreamstime.com(slides 6, 15) bulb © Monsieurpix | Dreamstime.com(slide 7) ammeter © Arsty | Dreamstime.com(slide 18) voltmeter © Arsty | Dreamstime.com(slide 22) circuit © Marshall Cavendish International (Singapore)

Private Limited(slide 33) fixed resistors © Sergpet | Dreamstime.com(slide 33) rheostat © Arsty | Dreamstime.com

The URLs are valid as at 15 October 2012.

Chapter 17 Current Electricity