FLOW OF ELECTRIC CHARGE:

ELECTRICITY

WHAT WE NEED:A DEVICE THAT ALLOWS US TOHAVE A SUPPLY OF CHARGE WHENEVER WE NEED IT

AN ELECTROCHEMICAL CELL (OR MANY CELLS CALLED A BATTERY)

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Electric Current

• An electric current I is a measure of the rate of flow of electric charge Q through a given cross section of a conductor.

• Symbol of Electric Current = I

• SI Unit of Electric Current = ampere (A)

I = Q/t

where I = current in ampere (A)Q = amount of charges in coulombs (C) t = time in seconds (s)

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Conventional current flows from the positive to

the negative ends

Electric charges flow from the negative to the

positive ends

Conventional Current and Electron Flow

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Measuring current• An ammeter is an instrument used for

measuring electric current.

• Ammeters must be connected in series in a circuit

A ammeter symbol

Positive (negative) side of ammeter is connected to the positive (negative) terminal of the cell / battery.

Conventional Current and Electron Flow

The digital multimeter (DMM) is starting to replace the ammeter. has a wide range of

between a few hundred A to several A

can be used for direct current (D.C.) and alternating current (A.C.)

able to read voltage and resistance too

Measuring current

Conventional Current and Electron Flow

Since the circuit consists of only one loop, the same current flows through the circuit; does not matter where the ammeter is placed on the circuit

A4A4

resistor

+ -

A3A3

A2A2 A5A5

A6A6A1A1

cell

Measuring current

Conventional Current and Electron Flow

electric current is produced when there is a flow of charges

a source of energy (provided by a cell, group of cells or generator) is needed to enable charges to be pumped or forced around a circuit

electromotive force is the electric force that provides the pumping action for electric current to flow from the positive terminal to the negative terminal of the battery

I

+ -

lamp

cell

Electromotive Force (e.m.f)

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Electromotive Force (e.m.f)

Definition

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

– is the potential difference between the two terminals of the cell or battery. (From higher p.d. to lower p.d)

– A point of high potential is a region where there is a large number of positive charges whereas a point of low potential has lesser positive charges

(more negative charges)

Electromotive Force (e.m.f)

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Electromotive Force (e.m.f)

• Symbol of Electromotive Force = • SI Unit of Electromotive Force = volts (V) or joules per

coulomb (JC-1)

= W/Q

where = e.m.f. (V)W = Energy converted from non–electrical forms to electrical form (J) [work done]Q = amount of charge in coulombs (C)

Electromotive Force (e.m.f)

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Potential Difference (p.d.)

• The Potential Difference (p.d.) between two points in an electric circuit is defined as the amount of electrical energy converted to other forms of energy when one coulomb of positive charge passes between the two points

• Symbol of Potential Difference (p.d.) = V

• SI Unit of Potential Difference (p.d.) = volts (V)

V = W/QwhereV = Potential difference (V)W = Energy converted from electrical form to other forms (J)Q = amount of charge in coulombs (C)

Potential Difference

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Measuring p.d./e.m.f.• An voltmeter is an instrument used for measuring

potential difference or electromotive force. • As charges flow round a circuit, they lose their P.E.,

transforming P.E. into other forms of energy.• It is connected in parallel to the circuit.• The SI unit for p.d. / e.m.f. is volt (V)

Voltmeters will measure the potential difference across 2 points of the circuit, so we connect it in parallel with respect to those 2 points

V

voltmeter symbol

Potential Difference

VV

+

V3

1

VVV1

VVV2

-

2

Potential difference around a simple circuit

sum of all the e.m.f.’s of the cells must be equal to the sum of potential differences across all the components in the circuit

1 + 2 = V1 + V2 + V3 1 + 2 = V1 + V2 + V3

Potential Difference

In a circuit, the size of the current depends on the resistance in the circuit.

Any component of a circuit resisting the flow of electricity is called a resistor

The greater the resistance in a circuit, the lower the current.

different types of resistors

Resistance

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Definition:

• Resistance R of a component is the ratio of the potential difference V across it to the current I flowing through it.

• Symbol of Resistance = R

• SI Unit of Resistance = ohms ()

WhereR = resistance in ohms ()V = p.d. across the component in volts (V)I = current in ampere (A)

Resistance

VV

RI

Ohm’s Law

Ohm's law states that the current through a conductor between two points is directly proportional to the potential difference or voltage across the two points, and inversely proportional to the resistance between them.

where I is the current through the resistance in units of amperes, V is the potential difference measured across the resistance in units of volts, and R is the resistance of the conductor in units of ohms. More specifically, Ohm's law states that the R in this relation is constant, independent of the current.

If a cell is connected to a resistance, the current gets smaller as the resistance increases.

Resistance

All metals have finite resistance.

Materials Uses

Low resistance

copper, gold, silver, aluminium connecting wires, conductors or connectors

High resistance

tungsten used in light bulbs

nichrome (an alloy of nickel and chromium)

heaters, such as coils of electric kettles

carbon resistors for radio and television sets

Uses of high and low resistances materials.

Resistance

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Resistors• Is a conductor that has a known value of resistance

• Primary purpose is to control the size of the current flowing in the circuit.

• Two types: fixed resistors & variable resistors (or rheostats)

• Variable resistor (or rheostat) allows resistances to be changed easily

fixed resistor symbol variable resistor symbol

Resistance

are variable resistors used for controlling the size of the current in a circuit

are used as brightness controls for lights, volume controls on radio and television sets

Rheostats

Resistance

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Measuring Resistance

• To determine the resistance of a metallic conductor, we use the following circuit:

• We can find the current flowing through R from the ammeter reading.

• We can find the potential difference across R from the voltmeter reading

• R can be calculated from the equation:

R = V / I

Resistance

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Experiment to Determine Resistance of a resistor

battery rheostat

ammeter

voltmeter

R

1. Set-up the apparatus as shown in the diagram.

2. As a safety precaution, adjust the rheostat to the

maximum resistance so that a small current

flows in the circuit initially.

3. Record the ammeter reading (I) & voltmeter

reading (V).

4. Adjust the rheostat to allow a larger current to flow

in the circuit. Again record the values of I and V.

5. Repeat Step 4 for at least 5 sets of I and V

readings.

6. Plot the graph of V(V) against I (A). Determine

the gradient of the graph.

Note that:

Always connect:Voltmeter in Parallel Ammeter in Series

Resistance

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Experiment to Determine Resistance of a resistor

Result:

The gradient of the graph gives the resistance of the

load, RV / V

I / A0

Gradient = V / I

= resistance

Resistance

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Factors Affecting Resistance

1. Cross-sectional area of wire / thickness of wire

There are several factors that affect the resistance of an object such as a wire:

Resistance

thicker wire smaller resistance (R 1/A)

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2. Length of wire

Factors Affecting Resistance

Resistance

longer wire larger resistance (R l)

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Factors Affecting Resistance

Resistance

3. Type of material

Wires of the same length and thickness but made of different materials will have a different resistances. This is because they have different resistivities. (Units: Ωm)

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• These factors can be placed together to find resistance

R = l /A

Where R = resistance in ohms () = resistivity in ohm meter (m) l = length of wire (m) A = cross-sectional area in meter square (m2)

Resistance

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Example

• The diameter of the copper wire used in a circuit is 2.0 mm. If the resistively for copper is 1.7 x 10-8 m, what is the resistance for 50 cm of the wire?

Solution

L = 50 cm = 0.5 mdiameter = 2.0 mm = 0.002 mA = (d/2)2 = (0.002/2)2 = (0.001)2 m2

R = (1.7 x 10-8)(0.5) / (0.001)2 = 0.0027

Resistance

resistors in series

R1I R2 R3

V1 V2 V3

RtI

V

effective resistance

is equivalent

to

since resistors are in series, current I passing through each resistor is the same

Rseries = R1 + R2 + R3Rseries = R1 + R2 + R3

Resistance

RI

V

effective resistance

since resistors are in parallel, potential difference across each resistor is the same

R3

I

V

R2

R1

I3

I2

I1

is equivalent

to

Resistance

resistors in parallel

Temperature Dependence

Near room temperature, the electric resistance of a typical metal increases linearly with rising temperature, while the electrical resistance of a typical semiconductor decreases with rising temperature. The amount of that change in resistance can be calculated using the temperature coefficient of resistivity of the material using the following formula:

R = Ro[α(T-To)+1] -- Formula not in syllabus

where T is its temperature, To is a reference temperature (usually room temperature), R0 is the resistance at T0, and α is the percentage change in resistivity per unit temperature. The constant α depends only on the material being considered.

V

V

I

I

The uniform gradient shows uniform resistance

(a) Pure metalO

O(b) Copper sulphate solution

Ohmic Ohmic ConductorsConductors

Pure metal,carbon and copper

sulphate

Non-Ohmic Conductors

filament bulb

Constant resistance

Higher resistance due to higher temperature

V

O I

At low temperature, the tungsten wire obey Ohm’s Law but at higher temperature it is not obeyed the Law.

Non-Ohmic ConductorsSemiconductor diode

A diode allows an electric current to pass in one direction (called the diode's forward direction) while blocking current in the opposite direction (the reverse direction). Thus, the diode can be thought of as an electronic version of a valve.

Forward Voltage DropElectricity uses up a little energy pushing its way through the diode, rather like a person pushing through a door with a spring. This means that there is a small voltage across a conducting diode, it is called the forward voltage drop and is about 0.7V for all normal diodes which are made from silicon. The forward voltage drop of a diode is almost constant whatever the current passing through the diode so they have a very steep characteristic (current-voltage graph).

Reverse VoltageWhen a reverse voltage is applied a perfect diode does not conduct, but all real diodes leak a very tiny current of a few µA or less. This can be ignored in most circuits because it will be very much smaller than the current flowing in the forward direction. However, all diodes have a maximum reverse voltage (usually 50V or more) and if this is exceeded the diode will fail and pass a large current in the reverse direction, this is called breakdown.

Bridge Rectifiers

Rectifier diodes are used in power supplies to convert alternating current (AC) to direct current (DC), a process called rectification. There are several ways of connecting diodes to make a rectifier to convert AC to DC. The bridge rectifier is one of them and it is available in special packages containing the four diodes required.

References

http://www.cartft.com/image_db/1n4001.jpg

http://image.wistatutor.com/content/current-electricity/vacuum-diode-graph.gif

http://cyberchalky.files.wordpress.com/2010/03/web_ohms_law_triangle.gif

http://www.kpsec.freeuk.com/components/diode.htm