1.1. Electric currentElectric current

2.2. Electromotive forceElectromotive force

& Potential Difference& Potential Difference

3.3. ResistanceResistance

Part I

Chapter 14

At the end of the chapter, you should be able to:

state that current is a rate of flow of charge and that it is measured in amperes

recall the relationship charge = current x time

apply the relationship to new situations or to solve related problems

Current flow• Actual electrons flowing from -ve to +ve terminal.

Chapter 14Pg 241

Definition: Current is a rate of flow of charge.

Actual

• ConventionalCharges flowing from +ve to –ve terminal.

Conventional

Quantity SI Unit

Current (I)

Charge (Q)

Time (t)

Ampere (A)

Coulomb (C)

second (s)

Q_______I t

Definition: Current is a rate of flow of charge. The amount of charge passing thru a given pt in 1 sec.

Formula:

I = or Q = I tQ

t

Chapter 14Pg 241

Example 1: A current of 10 A flows through an electric heater for 10 minutes. What is the total charge circulated through the heater?

[Solution]t = 10 min x 60 = 600 sI = 10 A

Q = I t = 10 A x 600 s

= 6000 C

The total charge is 6000 C

Chapter 14

Q_______I t

Example 2: In an electrical circuit, a charge of 60C flows past a point in 10s. What is the current in the circuit?

Chapter 14

[Solution]t = 10 s ; Q = 60 C

Q = I t

I =

= 60 / 10 = 6 A

The current is 6 A

Q_______I t

Qt

Example 3: A lightning flash carries 25 C of charge and lasts for 0.01 s. What is the current?

[Solution]Q = 25 C ; t = 0.01 s

Q = I t 25 C = I x 0.01s 25 / 0.01 = I

I = 2500A

Current is 2500A

Chapter 14

Example 4: A current of 2 A is flowing through a conductor. How long does it take for 10 C of charge to pass any point?

[Solution]I = 2 A ; Q = 10 C

Q = I t10C = 2A x t

10 / 2 = t t = 5 s

Time taken is 5 s

Chapter 14

A

AA

+

+

Ammeter• measures the current

in a circuit

• connects in series

• measures in A or mA

• has very low resistance

There must be a closed path in order for current to flow.

Chapter 14Pg 241

More common symbols can be found on pg 243

Chapter 14Pg 243

Variable resistor

Bulb

Ammeter

Voltmeter

BatteryFixed

resistor

Switch

Chapter 14Pg 243

1.1. Electric currentElectric current

2.2. Electromotive force & Electromotive force & Potential DifferencePotential Difference

3.3. ResistanceResistance

Chapter 14Pg 245

Part II

At the end of the chapter, you should be able to:

define electromotive force (e.m.f.) as the work done by a source in driving a unit charge around a complete circuit

state that the potential difference (p.d.) across a circuit component is measured in volts

Chapter 14Pg 245

Chapter 14Pg 245

Definition: Electromotive force is defined as the total work done by a source in driving a unit charge around a complete circuit

1 Unit charge = 1 coulomb of charge

Chapter 14Pg 245

Sources of e.m.f are:

• Electrical cells (i.e. batteries)

• Thermocouples

• Generators

• etc

2V2J of energy

2J of energy

2 J of work is done when 1 C of charge moves round the circuit

Mr Coulombgoes back to the source forenergy

Note:2J of electrical energy2J of light and heat energy

2 J of energy is supplied by the cell in moving1 C of charge round

Hi I’m Mr Coulomb (1 C)

Chapter 14Pg 245

Direction of current travel

Cell-Source of energy-Produces e.m.f that pushes the charges round the circuit.

Work done/energy is used to light upthe bulb.

The pump pushes the water to flow

flow of water

Work done/energy is usedto move the mill

Chapter 14Pg 246

Definition: The p.d. between two points is the energy required to move 1 C of charge between them.

Potential Difference (p.d.) OROR Voltage (V)

SI Unit : V (volts)

The p.d. between 2 points is the energy required to move 1 C of charge between the two points.

energy E p.d. = --------------- , V = ------ or E = VQ charge Q

e.g. 2V = 2 J/C

E_______V Q

Formula:

2J of energy

2J of energy

V

Voltmeter

• measures the p.d. / voltage between 2 points

• connects in parallel across 2 points

• measures in V or mV

• has very high resistance

+

+

Chapter 14Pg 247

The diagram shows a battery with an electromotive force of 6 V in a circuit. How much energy is needed to drive 30C of charge round the circuit?

E = VQ = 6V x 30C = 180 J

or

6 V

[Solution]

An electrical quantity is defined by “the energy converted by a source in driving unit charge round a complete circuit”. What is this quantity called?

A. Current B. Electromotive force

C. Potential difference D. Power

B

When a current of 0.5 A flows for 10 minutes through an electrical heater, 2400 J of energy is transformed.

(a)Calculate the total charge moving through the heater.

(b) Calculate the potential difference across the heater.

Q = I t = 0.5A x (10 x 60)s = 300 C Total charge is 300 C

E = V Q2400J = V x 300C V = 2400 / 300

= 8 V The p.d. is 8 V

1.1. Electric currentElectric current

2.2. Electromotive force & Electromotive force & Potential DifferencePotential Difference

3.3. ResistanceResistance

Chapter 14Pg 247

Part III

Chapter 14Pg 247

The resistance is a measure of how difficult it is for an electric

current to pass through a substance.

Chapter 14Pg 247

Definition: The resistance of a conductor is defined as the ratio of the potential difference across the conductor to the current flowing in it.

Formula:

R = SI Unit : Ohms ()

VI where R = resistance

V = p.d / voltageI = current

or V = IR

The size of the current depends on the resistance in the circuit.

A A

A

2 5

10

With the same cell used (i.e. voltage is the same), as resistance, R increases, current, I ____________

20 V 20 V

20 V

decreases

I = 10 A I = 4 A

I = 2 A

• Resistance resists the flow of current

• Resistance is low in conductors and very high in insulators.

Flow of current

Resistance

Chapter 14Pg 248

V = I R 6 = I x 46 / 4 = I I = 1.5 A

Reading on the ammeter is 1.5 AR I

_______V

A 4 resistor is connected in series with an ammeter and a 6 V battery, as shown. What is the reading shown on the ammeter.

Chapter 14Pg 249

The resistance R (= V / I) of a metallic conductor is CONSTANT under

steady physical conditions

For Ohmic conductors (Conductors that obeys Ohm’s law)e.g. pure metal

For non-Ohmic conductore.g. filament lamp bulb

I /A

V/V

I /A

V/V

Metal AMetal B

I /A

V/V

Chapter 14Pg 248

• an electrical component designed to reduce the flow of current.

• converts electrical energy to heat energy.(e.g. resistors used in electric fire and filament bulb

convert electrical to heat and light energy)

• represented by the symbol

Rheostat• a variable resistor that controls the size of a current in a circuit represented by

Procedure:• Set up the apparatus as shown above.• Adjust the variable resistor to allow the smallest possible current to flow in the circuit• Note the corresponding ammeter reading (I)and the voltmeter reading (V)• Adjust the variable resistor in steps to increase current flow in the circuit and note the values of I and V for at least five sets of readings.• Plot a graph of V against I. The graph plotted must be a best straight line passing through the origin.• The gradient of the best straight line obtained gives the resistance of the resistor, R.

To determine the unknown resistance, R of a fixed resistorPg 253

Fixed resistor

A

V

Variable Resistor/Rheostat

The unknown resistance of the resistor is found by obtaining the gradient of the straight line graph.

I /A

V/V

Precaution : To prevent a rise in the temperature of the resistor, which may change its resistance,• open the circuit between readings• use small amount of current

Chapter 14Pg 253

C

C

B

B

Besides physical conditions (e.g. temperature), the resistance R of a given conductor also depends on:

• its length l

• its cross-sectional area A

• the type of material

Formula:

lR

A

where R = resistance

ρ = resistivity

l = length

A = cross-sectional area

Simulation from Crocodile Physics