Physics 05: Electric Currents - PowerPoint Presentation

Slide1

Physics 05: Electric Currents

BY HEI MAN KWOK 12N03S Slide2

5.1 electric potential difference, current and resistanceSlide3

Model of electric conduction in a metal – energy transfer

Charge carriers have kinetic energy

These collide with lattice ions

Increasing amplitude of vibrations

This is seen in an increase in temperature

Electrons loses it energy and transfers it to thermal energy

Speed of electron through a conductor = drift speedSlide4

Charge (Q)

Measured in Coulombs (C)

Charge of 1 electron =

1 Coulomb =

electrons

 Slide5

Electric Potential Difference

Energy taken per unit charge

1 volt =

wd

per unit charge, 1 joule per coulomb

 Slide6

Change in potential energy when a charge moves between two points at different potentials

Potential energy (J) = Voltage (V) x Charge (C)

Insert diagram



 Slide7

Electronvolt

(

eV

)

Amount of energy (work done) one electron would gain by moving through a potential difference of 1 volt

eV

=

Charge of a electron =

 Slide8

Electric Current (in a conductor)

The rate

flow of electric charge (from

positive

to negative potential)

Ampere = force per unit length between parallel current-carrying conductors why?

2x 10^7 N

 Slide9

Resistance

Ratio of Voltage to Current

How easily does current flow through

Resistor = a component with a known resistance

 Slide10

Resistivity

More current flows through a short fat conductor than a long thin one

 Slide11

Ohm’s Law

The ratio of potential difference to current are proportional at

constant temperature

V = IR

Eg

. Wires and fixed resistors Slide12

Non-Ohmic – Filament lamp

As current increases, temperature increases, atoms vibrate more, collisions between electrons and metal atoms are more frequent so resistance

increases, and graph flattens

The current is not directly proportional to the voltage – disobeys ohm’s law

Graph = symmetrical Slide13

Diode

One way value for electrons

No current when negative V

Positive = current flows easily as diode has low resistance above about 0.7V

LEDs also one way :D Slide14
Slide15

Component’s Potential Difference

Cannot be equal to the

p.d

. of the battery:

Some voltage will be lost to internal resistance and/ or the resistance in wires

Cannot equal to zero

Low voltage requires very high resistance, max. resistance of the variable resistor cannot be infinite, there will always be some resistance from the component Slide16

5.2 Electric circuits Slide17

Electromotive Force (emf

)

Work done per unit charge made available by the energy source (cell or battery)

Power

supplied by the cell per unit current from the

cellSlide18

Law of Conservation of Energy

Energy cannot be created or destroyed – the energy converted from chemical to electrical in the cell must be equal to the amount converted from electrical to heat in the resistor

 Slide19

Internal Resistance

The resistance of the cell is ^

As current flows through the internal resistance – some energy is converted from electrical to heat inside the cell (so the cell gets hot)

This means that there is less energy to be converted to heat in the resistor –

p.d

. across the resistor is therefore less than the

emf

of the cell Slide20

Finding Internal Resistance Experimentally

V = E –

Ir

By

recording values of current and terminal

pd

as the external resistance changes you can plot the graph and find the internal resistance and the emf of the cell.If there is more than one cell in series the internal resistances of the cells must be added.Slide21

Power Dissipated

P = Power; I = Current; V = Voltage; R = Resistance

 Slide22

Power Delivered

In a real battery – actual power delivered will be a bit less, since there will be some power dissipated in the internal resistance

 Slide23

V, I, R in Series

 Slide24

V, I, R in Parallel

 Slide25

Ideal Ammeter

Zero resistance

so it does not change the current in the circuit

Connected in series so the current will flow through the ammeter as it flows through a component Slide26

Ideal Voltmeter

Infinite resistance

so it does not take any current from the circuit

Connected in parallel to see the difference in potential energy between two points Slide27

Thermistor and LDR

Made of semi-conducting material

Heat

and light frees more charge

carriers: as the temperature/ light intensity increases, the resistance decreases

The current is not directly proportional to the voltage

LDR Thermistor Slide28

Strain Gauge

Think metal wire

If stretch – length increases and cross-sectional area decreases = increase in resistanceSlide29

Potential Divider