ELECTRIC CURRENT, VOLTAGE AND RESISTANCE
8/12/2019 Current,Voltage&Resistance
http://slidepdf.com/reader/full/currentvoltageresistance 1/30
ELECTRIC CURRENT, VOLTAGE
AND RESISTANCE
8/12/2019 Current,Voltage&Resistance
http://slidepdf.com/reader/full/currentvoltageresistance 2/30
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.
8/12/2019 Current,Voltage&Resistance
http://slidepdf.com/reader/full/currentvoltageresistance 3/30
CURRENT
• The quantity of electric current is measuredusing an ammeter.
• An ammeter has to be connected in series in
an electric circuit.
ammeter
8/12/2019 Current,Voltage&Resistance
http://slidepdf.com/reader/full/currentvoltageresistance 4/30
• 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.
8/12/2019 Current,Voltage&Resistance
http://slidepdf.com/reader/full/currentvoltageresistance 5/30
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.)
8/12/2019 Current,Voltage&Resistance
http://slidepdf.com/reader/full/currentvoltageresistance 6/30
NEGATIVE TERMINALPOSITIVE TERMINAL
8/12/2019 Current,Voltage&Resistance
http://slidepdf.com/reader/full/currentvoltageresistance 7/30
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.
8/12/2019 Current,Voltage&Resistance
http://slidepdf.com/reader/full/currentvoltageresistance 8/30
8/12/2019 Current,Voltage&Resistance
http://slidepdf.com/reader/full/currentvoltageresistance 9/30
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.
8/12/2019 Current,Voltage&Resistance
http://slidepdf.com/reader/full/currentvoltageresistance 10/30
•
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.
8/12/2019 Current,Voltage&Resistance
http://slidepdf.com/reader/full/currentvoltageresistance 11/30
• Voltage is measured with a voltmeter,
connected in parallel.
• Voltage is measured in volts, V.
• The symbol V is used for voltage in
equations.
8/12/2019 Current,Voltage&Resistance
http://slidepdf.com/reader/full/currentvoltageresistance 12/30
Voltage, V
Connecting a voltmeter in parallel
8/12/2019 Current,Voltage&Resistance
http://slidepdf.com/reader/full/currentvoltageresistance 13/30
• 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.
8/12/2019 Current,Voltage&Resistance
http://slidepdf.com/reader/full/currentvoltageresistance 14/30
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.
8/12/2019 Current,Voltage&Resistance
http://slidepdf.com/reader/full/currentvoltageresistance 15/30
Without the cell there is no sourceof voltage so current cannot flow.
The bulb does not light on.
8/12/2019 Current,Voltage&Resistance
http://slidepdf.com/reader/full/currentvoltageresistance 16/30
8/12/2019 Current,Voltage&Resistance
http://slidepdf.com/reader/full/currentvoltageresistance 17/30
8/12/2019 Current,Voltage&Resistance
http://slidepdf.com/reader/full/currentvoltageresistance 18/30
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.
8/12/2019 Current,Voltage&Resistance
http://slidepdf.com/reader/full/currentvoltageresistance 19/30
Two types of Resistor
1. Fixed resistor – has fixed resistance. Itsupplies a constant resistance.
8/12/2019 Current,Voltage&Resistance
http://slidepdf.com/reader/full/currentvoltageresistance 20/30
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.
8/12/2019 Current,Voltage&Resistance
http://slidepdf.com/reader/full/currentvoltageresistance 21/30
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.
8/12/2019 Current,Voltage&Resistance
http://slidepdf.com/reader/full/currentvoltageresistance 22/30
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.
8/12/2019 Current,Voltage&Resistance
http://slidepdf.com/reader/full/currentvoltageresistance 23/30
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.
8/12/2019 Current,Voltage&Resistance
http://slidepdf.com/reader/full/currentvoltageresistance 24/30
2. Semiconductors which have moderateresistance.• Examples: germanium, silicon.
Semiconductors are used to make diodes,
LEDs, transistors and integrated circuits(chips).
8/12/2019 Current,Voltage&Resistance
http://slidepdf.com/reader/full/currentvoltageresistance 25/30
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.
8/12/2019 Current,Voltage&Resistance
http://slidepdf.com/reader/full/currentvoltageresistance 26/30
Connecting a resistor to the electrical component.
the resistor is connected series to the ammeter but parallel
with the voltmeter.
8/12/2019 Current,Voltage&Resistance
http://slidepdf.com/reader/full/currentvoltageresistance 27/30
Summary
Quantity Symbol S.I Unit Symbol
Current I Ampere A
Voltage V Volt V
Resistance R Ohm Ω
8/12/2019 Current,Voltage&Resistance
http://slidepdf.com/reader/full/currentvoltageresistance 28/30
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.
8/12/2019 Current,Voltage&Resistance
http://slidepdf.com/reader/full/currentvoltageresistance 29/30
8/12/2019 Current,Voltage&Resistance
http://slidepdf.com/reader/full/currentvoltageresistance 30/30