Current,Voltage&Resistance

8/12/2019 Current,Voltage&Resistance

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ELECTRIC CURRENT, VOLTAGE

AND RESISTANCE



CURRENT

• Electric current is the rate of flow of electrical

charges.

• It is the quantity of electrical charges thatflow through a point in one second.



CURRENT

• The quantity of electric current is measuredusing an ammeter.

• An ammeter has to be connected in series in

an electric circuit.

ammeter



• Current is measured in amps (amperes), A.

• 1A is quite large, so mA (milliamperes) and µA

(microamperes) are often used.

• 1000mA = 1A, 1000µA = 1mA, 1000000µA = 1A.

Circuit symbol for amperes

The symbol I is used for current in equations.



Remember !!! • The ammeter must be connected in

series with the component.(Remember, in a series circuit, electricaldevices are placed one after the other in acontinuous line in the circuit between thepositive and negative poles of the battery.)



NEGATIVE TERMINALPOSITIVE TERMINAL



The positive terminal of the ammetermust be connected to the positive terminal of the electrical supply.

The negative terminal of the ammetermust be connected to the negative terminal of the electrical supply.



VOLTAGE

What is Voltage?

Voltage is the force

that pushes the electronsin a conductor toflow in one direction

It is possible to have voltage withoutcurrent,

but current cannot flow without voltage.



•

Voltage also is a measure of the energycarried by the charge.

Strictly: voltage is the "energy per unit charge".

•The proper name for voltage is potential differenceor p.d. for short, but this term is rarelyused in electronics.



• Voltage is measured with a voltmeter,

connected in parallel.

• Voltage is measured in volts, V.

• The symbol V is used for voltage in

equations.



Voltage, V

Connecting a voltmeter in parallel



• Voltage is supplied by the battery

(or power supply).

• Voltage is used up in components, but not inwires.

• We say voltage across a component.

• Voltmeters have a very high resistance.



The switch is closed making

a complete circuit socurrent can flow,the bulb is light on.

The switch is open so thecircuit is broken

and current cannot flow,The bulb does not light on.



Without the cell there is no sourceof voltage so current cannot flow.

The bulb does not light on.



Resistor

• A resistor is an electrical component thatopposes the flow of electricity.

• A resistor is usually used to control theflow of electric current in a circuit.



Two types of Resistor

1. Fixed resistor – has fixed resistance. Itsupplies a constant resistance.

http://en.wikipedia.org/wiki/Image:Old_Radio_Resistors.jpg

http://en.wikipedia.org/wiki/Image:3_Resistors.jpg



2. Variable resistor or rheostat - supplies

resistance that can be changed.

• A good electrical conductor has low

resistance.

• A poor electrical conductor has high

resistance.



The resistance in a wire depends on thefollowing factors:

1. The length of the wire: the longer the wire, themore resistance it has.

2. The thickness of the wire: the thicker the wire,the less resistance it has.

3. The type of metal: different types of metals

have different resistances.



Conductors, Semiconductors andInsulators

The resistance of an object depends on itsshape and the material from which it is

made.

For a given material, objects with a

smaller cross-section or longer lengthwill have a greater resistance.



Materials can be divided into three

groups:

1. Conductors which have low resistance.

• Examples: metals (aluminium, copper, silver

etc.) and carbon.

• Metals are used to make connecting wires,switch contacts and lamp filaments.



2. Semiconductors which have moderateresistance.• Examples: germanium, silicon.

Semiconductors are used to make diodes,

LEDs, transistors and integrated circuits(chips).



3. Insulators which have highresistance. • Examples: most plastics such as polythene

and PVC (polyvinyl chloride), paper, glass.

• PVC is used as an outer covering for wires to

prevent them making contact.



Connecting a resistor to the electrical component.

the resistor is connected series to the ammeter but parallel

with the voltmeter.



Summary

Quantity Symbol S.I Unit Symbol

Current I Ampere A

Voltage V Volt V

Resistance R Ohm Ω



Quantity Diagram of circuit

Current

ammeter mustbe connected in

series with the

Component.

Voltage

Voltmeter mustbe connected in

parallel with the

Component.

Resistorthe resistor is

connected series

to the ammeter but

parallel with the

voltmeter.



http://www.st-andrews.ac.uk/~www_pa/Scots_Guide/info/comp/passive/resistor/e12/e12.htm

Current,Voltage&Resistance

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