Chapter 9: Energy, power and resistance

Physics AS Level - OCR Gateway A need to complete... ♡

The flow of current in circuit symbols

The flow of current in circuit symbols

In on the left

Out on the right

Potential difference

The same as voltage

A measure of the energy transferred by charge carriers

WORK IS DONE BY THE CHARGE CARRIERS

Definition of p.d

One volt is the p.d across a component when 1J of energy is

transferred per unit charge

1V = 1JC^-1

Equation of p.d

V = W / Q

V = p.d measured in volts

W = Energy transferred (by charge Q) / Work done

measured in Joules

Q = Charge (elementary charge on an electron - 1.6 × 10^-19 )

measured in coulombs

Electromotive force (emf)

EMF is when charged particles gain energy from a source

Greater the emf, the more energy per coulomb has

been transferred

WORK IS DONE ON THE CHARGE CARRIERS

Definition for emf

The measure of the electrical energy per unit

charge produced by the source (e.g. battery or cell)

Equation of emf

Difference between emf and p.d

Difference between emf, p.d, and work done

EMF - picking charge up

P.D - using it

Work done - dropping it off

Relationship between emf, terminal voltage and lost volts

EMF - theoretical max value (voltage) a cell can put out / make

TERMINAL VOLTAGE - reading on a voltmeter

LOST VOLTS - difference between the two

emf = V_T + V_L

V_T - terminal voltage

V_L - lost volts

Thermionic emission

The metal (a wire filament) is heated by passing an electric current through it. The filament (cathode), is in an evacuated glass tube with a metal plate (anode).

The electrons emitted from the filament are attracted to the anode by connecting a high voltage power supply between anode and the cathode.

There are no gas molecules in the tube to stop the electrons, so the electrons are accelerated to the anode where they pass through a small hole to form a narrow beam.

Electrons emitted from cathode by thermionic emission.

The speed of each electron

eV = ½ mv²

e = fundamental charge on an electron

The electron volt

The KE of an electron after it has been accelerated from rest through a p.d of 1V.

the energy gained in one eV= accelerating voltage

[1V - 1eV]

Converting between Joules and eV

J to eV

divide by 1.6 × 10^-19

eV to J

multiply by 1.6 × 10^-19

Relationship between current and resistance

↑ Current (I) when ↓ Resistance (R)

↓ Current (I) when ↑ Resistance (R)

^inversely proportional

Resistance

Resistance is how hard it is for current to flow

• high resistance means low current

1 Ω of resistance is when 1V flows per 1A

Ohm’s Law

For a conductor at a constant temperature, the current ∝ p.d across it

ohmic conductor / metal → the resistance doesn’t change as the voltage does

How to change the current

add a variable resistor anywhere

I-V graph for resistor

I-V characteristics for resistor

• V is directly proportional to I

• obeys Ohm’s law and is an ohmic conductor

• resistance is constant - straight line

• behaves the same way regardless of polarity

I-V graph for a filament light bulb

I-V characteristics for a filament light bulb

• V is not directly proportional to I

• doesn’t obey Ohm’s and isn’t an ohmic conductor

• resistance isn’t constant and can be determined by V/I at a point

• behaves the same way regardless of polarity

I-V graph for diode

I-V characteristics for diode

• V is not directly proportional to I

• doesn’t obey Ohm’s and isn’t an ohmic conductor

• resistance isn’t constant and can be determined by V/I at a point

• behaviour depends on the polarity