Chapter 9 Electricity

Question 1

Defining Electric Current

Answer 1

• is the flow of charge carriers and is measured in units of amperes (A) or amps
• Charge can be either positive or negative
• When two oppositely charged conductors are connected together (by a length of wire), charge will flow between the two conductors, causing a current

Question 2

In electrical wires, the current is a flow of

Answer 2

• electrons
• Electrons are negatively charged; they flow away from the negative terminal of a cell towards the positive terminal

Question 3

Conventional current is defined as the flow

Answer 3

• of positive charge from the positive terminal of a cell to the negative terminal
• This is the opposite to the direction of electron flow, as conventional current was described before electric current was really understood

Question 4

Current is measured using an

Answer 4

• ammeter
• Ammeters should always be connected in series with the part of the circuit you wish to measure the current through

Question 5

Quantisation of Charge

Answer 5

• The charge on charge carriers is quantised
• Charge comes in definite bits
• e.g. a single proton has a single positive charge, whereas a single electron has a single negative charge
• the quantity of charge can be quantised dependent on how many protons or electrons are present
  
  • positive and negative charge has a definite minimum magnitude and always comes in multiples of that magnitude
  • This means that if we say something has a given charge, the charge is always a multiple of the charge of an electron by convention
  • The charge of an electron is -1.60 × 10-19 C
  • The charge of a proton by comparison is 1.60 × 10-19 C (this is known as the elementary charge, denoted by e and measured in Coulombs (C) )

Question 6

Calculating Electric Charge

Answer 6

• Q = IT
• Q=charge(c)
• I=Current (A)
• t=Time(s)

Question 7

Current in a Current Carrying Conductor is due to?

Answer 7

• In a conductor, current is due to the movement of charge carriers
• These charge carriers can be negative or positive, however the current is always taken to be in the same direction
• In conductors, the charge carrier is usually free electrons

Question 8

The drift speed

Answer 8

• is the average speed the charge carriers are travelling through the conductor. You will find this value is quite slow. However, since the number density of charge carriers is so large, we still see current flow happen instantaneously
• The current can be expressed in terms of:
• number density (number of charge carriers per unit volume) n
• the cross-sectional area A
• the drift speed v
• the charge of the charge carriers q

Question 9

Current in a conductor equation

Answer 9

• I=Anvq
• I=current (A)
• A=cross sectional area(m^2)
• n=number density of charge carriers (m^-3)
• v= average drift speed on charge carriers (ms^-1)
• q=charge of each charge carrier (m^-3)

-The same equation is used whether the charge carriers are positive or negative

Question 10

Defining Potential Difference

Answer 10

• A cell makes one end of the circuit positive and the other negative. This sets up a potential difference (d) across the circuit
• The potential difference across a component in a circuit is defined as the energy transferred per unit charge flowing from one point to another
  • The energy transfer is from electrical energy into other forms
  • Potential difference is measured in volts (V). This is the same as a Joule per coulomb (J C-1)
  • The potential difference of a power supply connected in series is always shared between all the components in the circuit

Question 11

a voltmeter

Answer 11

• Potential difference or voltage is measured using a voltmeter
• A voltmeter is always set up in parallel to the component you are measuring the voltage for

Question 12

Calculating Potential Difference

Answer 12

• The potential difference is defined as the energy transferred per unit charge
• Another measure of energy transfer is work done
• Therefore, potential difference can also be defined as the work done per unit charge

V= W/Q

Question 13

Calculating Electrical Power

Answer 13

• Power P was defined as the rate of doing work
• Potential difference is the work done per unit charge
• Current is the rate of flow of charge

P=IV

Question 14

Power equation in terms of resistance

Answer 14

• -Using V = IR to rearrange for either V or I and substituting into the power equation means we also write power in terms of resistance R
• P= I^2R
• P=V^2/R

Question 15

Defining Resistance

Answer 15

• Resistance is defined as the opposition to current
  
  • For a given potential difference: The higher the resistance the lower the current
  • -Wires are often made from copper because copper has a low electrical resistance.
  • This is also known as a good conductor
  • The resistance R of a conductor is defined as the ratio of the potential difference V across to the current I in it

Question 16

Resistance of a component is the ratio of the potential difference and current

Answer 16

R= V/I

Question 17

resistance

Answer 17

• Resistance is measured in Ohms (Ω)
• An Ohm is defined as one volt per ampere
• The resistance controls the size of the current in a circuit
  
  • A higher resistance means a smaller current
  • A lower resistance means a larger current
    
    • All electrical components, including wires, have some value of resistance

Question 18

Ohm’s Law

Answer 18

• states that for a conductor at a constant temperature, the current through it is proportional to the potential difference across it
• Constant temperature implies constant resistance

Question 19

Ohm’s Law equation

Answer 19

V= IR

Question 20

An electrical component obeys Ohm’s law

Answer 20

if its graph of current against potential difference is a straight line through the origin

• —A resistor obeys Ohm’s law
• —A filament lamp does not obey Ohm’s law

-This applies to any metal wires, provided that the current isn’t large enough to increase their temperature

Question 21

Resistance in a Filament Lamp

Answer 21

• The I-V graph for a filament lamp shows the current increasing at a proportionally slower rate than the potential difference
• This is because:
  • As the current increases, the temperature of the filament in the lamp increases
  • Since the filament is a metal, the higher temperature causes an increase in resistance
  • Resistance opposes the current, causing the current to increase at a slower rate
  • Where the graph is a straight line, the resistance is constant
  • The resistance increases as the graph curves

Question 22

Resistance and temperature

Answer 22

• All solids are made up of vibrating atoms
  
  • The higher the temperature, the faster these atoms vibrate
  • Electric current is the flow of free electrons in a material
  • The electrons collide with the vibrating atoms which impedes their flow, hence the current decreases
  • So, if the current decreases, then the resistance will increase (V = IR)
  • Therefore, an increase in temperature causes an increase in resistance

Question 23

Resistivity

Answer 23

• All materials have some resistance to the flow of charge
• As free electrons move through a metal wire, they collide with ions which get in their way
• As a result, they transfer some, or all, of their kinetic energy on collision, which causes electrical heating

Question 24

The length and width of the wire affect its resistance

Answer 24

• The resistivity equation shows that:
• The longer the wire, the greater its resistance
• *-The thicker the wire, the smaller its resistance**

R = pL/A

• R=resistance(Ω)
• p=resistivity (ΩM)
• L= lenght (M)
• A= cross-sectional area (M^2)

Question 25

Resistivity

Answer 25

• is a property that describes the extent to which a material opposes the flow of electric current through it
• It is a property of the material, and is dependent on temperature
• Resistivity is measured in Ω m

Question 26

resistivity of materials means

Answer 26

• The higher the resistivity of a material, the higher its resistance
• This is why copper, with its relatively low resistivity at room temperature, is used for electrical wires — current flows through it very easily
• Insulators have such a high resistivity that virtually no current will flow through them

Question 27

Resistance in a Light-Dependent Resistor

Answer 27

• A light-dependent resistor (LDR) is a non-ohmic conductor and sensory resistor
• Its resistance automatically changes depending on the light energy falling onto it (illumination)
• As the light intensity increases, the resistance of an LDR decreases

Question 28

LDRs can be used as light sensors

Answer 28

• so, they are useful in circuits which automatically switch on lights when it gets dark, for example, street lighting and garden lights
• In the dark, its resistance is very large (millions of ohms)
• in bright light, its resistance is small (tens of ohms)

Question 29

Resistance in a Thermistor

Answer 29

• A thermistor is a non-ohmic conductor and sensory resistor
• Its resistance changes depending on its temperature
• As the temperature increases the resistance of a thermistor decreases

Question 30

As the potential difference across the LDR increases

Answer 30

• the light intensity increases causing its resistance to decrease
• Ohm’s law states that V = IR
• The resistance is equal to V/I or 1/R = I/V = gradient of the graph
• Since R decreases, the value of 1/R increases, so the gradient must increase
• Therefore, I increases with the p.d with an increasing gradient

Question 31

Thermistors

Answer 31

• are temperature sensors and are are used in circuits in ovens, fire alarms and digital thermometers
• As the thermistor gets hotter, its resistance decreases
• As the thermistor gets cooler, its resistance increases

Question 32

Charge on a drop

Answer 32

• F=Eq (electric field strength x charge)
• F=mg (mass x gravitational field strength)
• So q = mg/E