3.5.1 CURRENT ELECTRICITY

Question 1

What is electric current defined as?

Answer 1

The rate of flow of charge (Q/t)

Question 2

What is potential difference?

Answer 2

The work done per unit charge (J/Q) [V]

Question 3

What is the difference between current in a wire, and current in a salt solution?

Answer 3

The charge carriers - In a wire, electrons carry charge, in a salt solution, ions carry charge

Question 4

Describe properties of an insulator

Answer 4

Electrons are fixed to atoms, and even when a voltage is applied, no current can pass, as electrons cannot move.

(too much energy to move from valence to conduction band)

Question 5

Describe properties of a conductor

Answer 5

Most electrons are fixed to atoms but some are delocalised. When a voltage is applied, electrons will move from the negative to positive terminal

Question 6

Describe properties of a semiconductor

Answer 6

Number of charge carriers increases with temperature. (Energy transferred allows electrons to move from conduction to valence band, also opening “holes” of positive charge that are filled by other electrons, driving a current). This means that the resistance of a semiconductor decreases with increasing temperature, making it useful in thermistors and other NTC components`

Question 7

What is resistance?

Answer 7

A characteristic of a component, measuring the opposition to current flow. It also shows the drop in potential difference per ampere of current inside it

Question 8

Describe the relationship between power dissipated and resistance in series and parallel

Answer 8

Power dissipated is inversely proportional to resistance for a constant voltage as P = V^2/R
(in parallel)

Power dissipated is directly proportional to resistance for a constant current as P = I^2R
(in series)

Question 9

What is the equation relating resistance and resistivity?

Answer 9

ρ = RA/l
R = ρl/A

Where l = length of wire and A is the cross sectional area

Question 10

What is the purpose of voltage?

Answer 10

Voltage causes electrons to flow in a chosen direction (neg to pos), and without it, delocalised electrons would move randomly, which isn’t very useful

Question 11

Explain why the current in a filament lamp rises to a high value before falling to a steady value, and why it is more likely to fail when being switched on than other times

Answer 11

initial resistance is low so initial current is
high
* temperature of filament increases (or filament
heats up)
* resistance increases as temperature rises
* increase in resistance causes current to fall
* current is steady when energy supplied =
energy lost from filament (or when temperature
is constant)
* maximum heating is produced at start when
current is highest
* melting of filament causes it to fail (could be
mechanical failure caused by temperature rise)
* when switched on energy is supplied more
rapidly than it is lost so filament melts

Question 12

What is the approximate threshold voltage for a diode?

Answer 12

around 0.6/0.7 V

Question 13

What is a superconductor?

Answer 13

A component/substance with no electrical resistance

Question 14

What does the term “transition/ critical temperature” mean?

Answer 14

The temperature at which a substance becomes a superconductor

graph of R against T shows
high temperature graph with abrupt
discontinuous vertical line indicating that R has
become zero at a certain temperature (label transition)
R decreases steadily with T until transition temp

Question 15

Name 2 applications of superconductors

Answer 15

power cables, electromagnets, generators,
motors, transformers, MRI scanners, monorail
trains, particle accelerators, fusion reactors

Question 16

Explain why emf is different to the voltage in the external circuit

Answer 16

work is done inside the battery/there is resistance
inside the battery
so less energy is available for the external circuit/some voltage
is lost between the terminal/mention of lost volts

Question 17

What is emf?

Answer 17

Amount of chemical energy transferred to electrical energy per coulomb of charge

the (total) energy transferred/work done when
one unit/coulomb of charge
is moved around a circuit/provided by the supply

Or potential difference across the terminals when the current = 0

Question 18

Describe the heating effect of an electric component

Answer 18

• Heating effect due to the resistance
• Charge carriers repeatedly collide
  with positive ions of colliding
  material
• Net transfer of energy from charge
  carriers to positive ions
• After charge carrier loses Ek from
  collision, force due to pd across
  component accelerates it until it
  collides with another ion

Question 19

Does the power supplied to a component depend on the direction of current?

Answer 19

no

Question 20

Potential divider equation

Answer 20

V1 = Vt . (R1/R1+R2)
series components, Vt is total input voltage

Question 21

Describe how a simple potential divider circuit can be used as a heater

Answer 21

• Connect voltage source to resistor
  and thermistor in series
• Connect heating element to
  thermistor in parallel
• Cold temperatures, R of thermistor
  increases, so V increases
• V across heater also increases
• Heats

Question 22

Conservation of charge and energy in circuits

Answer 22

Charge - current entering a junction = current exiting a junction

Energy - sum of emf in a closed loop = sum of pds across each component,

(Work done on a charge carrier = potential drops across components)

Question 23

What are some uses of potential divider circuits?

Answer 23

As switches, with LDRs or thermistors

To turn on a fan

Calibrating voltmeters

Question 24

What type of resistor has a high resistance at low temperatures?

Answer 24

an NTC thermistor

Question 25

How do potentiometers work?

Answer 25

Potentiometers work by having a resistive element inside. Both end terminals are attached to it, and do not move. The wiper travels along the strip when the knob is turned. The closer the wiper is to the end terminal it is wired in conjunction with, the less the resistance, because the path of the current will be shorter

cgp book explains well

Question 26

What type of temperature coefficient does a metal have

Answer 26

A positive one, as resistance increases with temperature

(Positive ions in metal vibrate more when temp increases, electrons can’t pass as easily when a pd is applied)

Question 27

What are two methods to measure IV characteristics of a component

Answer 27

Use a potential divider to vary voltage from 0, or use a variable resistor to vary current from a minimum

(adv of potential divider is that it can vary voltage to 0, which can’t be done w variable resistor)

Question 28

IV graph for a NTC thermistor at constant and varying temperature

Answer 28

• For a constant temperature, IV
  graph is a straight line

For changing temperature, graph is one of inverse proportion

Question 29

Why are intrinsic semiconductors NTC components?

Answer 29

At higher temperatures, more charge carriers are freed (conduction electrons move and form holes).
R decreases non linearly as temp increases

+percentage change of resistance per kelvin is greater than metals, so often used in temperature sensing circuits

Question 30

What is a sensor circuit?

Answer 30

A circuit that produces an output pd as a result of a physical change e.g. temperature

Question 31

How does adding identical cells in parallel decrease the internal resistance of the circuit?

Answer 31

It decreases it by a factor of 1/n , where n is the number of cells
Less lost volts as IR = ε - Ir/n

Question 32

Define internal resistance

Answer 32

Loss of p.d per unit current in a source when current passes through the source

Question 33

Give an advantage and disadvantage of using cells in parallel

Answer 33

• If one of the cells fails, other cells
  can still provide emf
• More energy is stored in banks of
  cells, less power dissipated
  -Current is shared by cells, takes
  longer to convert energy stored in
  each cell
• However total emf cannot be
  changed

+increased “capacity” according to breakenridge

Question 34

Pros/cons of using superconductors in electrical transmission

Answer 34

• Zero resistance, Reduce power loss
• Difficult to maintain below critical
  temperature (+over long distances)

Question 35

Why does the gradient of an IV graph not give you the resistance

Answer 35

This is because the resistance is the voltage divided by the current at a particular point. It’s not the change in voltage divided by the change in current at that point, which is what the gradient represents

Question 36

Symbol for heater and motor

Answer 36

Heater is box w 3 stripes, motor is a circle w M then weird line

(google)

Question 37

When potential divider eq can’t be used

Answer 37

When load resistance (connected to vout) is negligible, otherwise can be incorporated as a resistor in parallel

Question 38

Maximum power delivered from a circuit w internal resistance

Answer 38

When r=R, curve looks like a bell w max at R=r

(don’t think i need to know this)

Question 39

Cells in parallel + lost volts

Answer 39

Each time an electron passes through the circuit, only passes through one of the cells, so no extra pd

lost volts = Ir/n

Question 40

Conservation of charge in circuits

Answer 40

Total charge passing through any cross-section of a conductor in a given time is always the same. This is because electric charge cannot be created or destroyed

current in = current out at a junction

Question 41

LDRs and photoelectric effect

Answer 41

We can say the principle behind the LDR is based on the Internal PhotoElectric effect. When the light is absorbed by the material the conductivity of the material increases i.e the electrons in the valency band are excited to the conduction band and so the resistance decreases

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