CURRENT OF ELECTRICITY (PART 1)

Textbook Chp 17

Topics

Current Electromotive Force Potential Difference Resistance

Current

Symbol for current: I Units for current: Ampere (A) Definition: Rate of flow of electric

charge Formula: I = Q/t

Q is charge (in Coulombs, C) t is time (in seconds, s)

Current is a vector, it has both magnitude and direction

Direction of Current

If positive charges are moving, the direction of current is the same direction as the positive charges

If negative charges (e.g. electrons) are moving, the direction of the current is the opposite direction as the negative charges

Note: 99% of the time, it is electrons which are moving

Worked Example 1

2.0 C of positive charge moved from left to right in 1.0 s.

(a) what is the current? (b) what is the direction of the

current?

Worked Example 2

5.0 C of electrons moved downwards in 4.0 s.

(a) what is the current? (b) what is the direction of the

current?

Worked Example 3

A current of 2.0 A flowed for 0.3 s. How much charge did that current carry?

Worked Example 4

Simple Circuit Diagram:

direction of current

direction of electrons

Did You Know?

(not in syllabus) How much current does it take to kill

a person? Ans: 0.1 A, 0.0001 A if through the

heart How much current in a lightning bolt? Ans: 40 000 A (on average) Did you know that majority of people

survive a lightning strike?? (10-30% mortality rate)

Half-Time: Electricity Men? Mohan (India) http://

www.youtube.com/watch?v=PRf9MqqkMDM

Biba Struja (Serbia) http://www.youtube.com/watch?v=PpIXNZjAvpA

Voltage

In Primary School, you used the word “voltage” in electricity.

DO NOT EVER USE THIS WORD FOR O LEVELS

Actually this is not a wrong term, but O levels prefer you to differentiate between e.m.f. and p.d.

Electromotive Force (e.m.f) The electromotive force (e.m.f.) is a

measure of a source of electrical energy (usually a battery)

Symbol: ɛ (epsilon) Units: volts (V) Definition: the work done by the

source in driving a unit charge around a complete circuit

Electromotive Force (e.m.f) (not in syllabus) A battery with e.m.f. 1 volt will

supply 1 joule of energy to 1 coulomb of charge around a complete circuit

In equation form: ɛ = W/Q

E.M.F. in series

Recall from primary school when batteries are arranged in

series, the e.m.f. add up

Potential Difference (p.d.) Symbol: V Unit: volts (V) Definition: Work Done to drive a unit

charge through the component

E.M.F. vs P.D.

e.m.f. is a quantity describing sources of electrical energy (i.e. they supply electrical energy) batteries, electrical generators

p.d. is a quantity describing sinks of electrical energy (i.e. they use up electrical energy) resistors, bulbs, etc.

E.M.F. vs P.D.

How do I use a voltmeter ? When I attach a voltmeter across a

resistor, what am I measuring? When I attach a voltmeter across a

battery, what am I measuring?

Resistance

Symbol: R Units: Ohm (Ω) Definition: the ratio of the potential

difference across the component to the current flowing through it

Equation: R = V/I

Resistance Simple Circuit Diagram:

Resistance = (Voltmeter Reading )/(Ammeter Reading)

R = V/I

V

A

Resistors in Series

If there are two or more resistors in series, the total resistance is given by:

Rtotal = R1 + R2 + R3 + …..

Worked Example 5

What is the total resistance of this arrangement of resistors?

Rtotal = 1+2+3 = 6.00 Ω

1 Ω 2 Ω 3 Ω

Resistors in Parallel

When there are two or more resistors in parallel, the total resistance is given by:

1/Rtotal = 1/R1 + 1/R2 + 1/R3 + …..

Useful Hint!

Most questions only ask for you to calculate two resistors in parallel

It may be useful to memorize this equation:

Rtotal = R1R2/(R1 + R2) Note: this equation may only be used

for 2 parallel resistors. If 3 or more resistors, use back original formula

Worked Example 6

What is the total resistance of this arrangement of resistors?

Method 1: 1/Rtotal = ½ + ¼ = ¾

Rtotal = 4/3 = 1.33 Ω (3 sf)

Method 2: Rtotal = R1R2/(R1 + R2) = (2)(4)/(2+4) = 8/6 = 1.33 Ω

2 Ω

4 Ω

Important Concept

When a resistor is added in series, the total resistance always increases

When a resistor is added in parallel, the total resistance always decreases

Problem Solving Strategy

For more complex arrangement of resistors,

break it down into parts and determine subtotals of resistance before finally combining to find total resistance

Worked Example 7

What is the total resistance of this arrangement of resistors?

Step 1: find the subtotal of the parallel resistors first

Step 2: add this subtotal to the other resistor in series

Ans: 3.71 Ω (3sf)

2 Ω3 Ω

4 Ω

Worked Example 8

What is the total resistance of this arrangement of resistors?

Ans: 2.77 Ω

4 Ω

3 Ω

1 Ω

2 Ω

3 Ω2 Ω

Summary

I = Q/t Conventional Currvent vs Electron

Flow Electromotive Force Potential Difference Resistance

R = V/I Resistors in Series Resistors in Parallel

Quiz will only be done after Part 2 is completed

No Quiz!