Define// Electricity

Question 1

Define electric current

Answer 1

Electric current is defined as the rate of flow of charge. Measured in amperes (A).

Question 2

Define potential difference

Answer 2

The potential difference is defined as the work done per unit charge. Measured in volts (V).

Question 3

Conventional current travel direction

Answer 3

By definition, conventional current always goes from positive to negative.

Question 4

Define resistance

Answer 4

R=V/I, it’s the opposition to the current. It is inversely proportional to current.

Question 5

State the rules for current in series and parallel circuits

Answer 5

Current is the same in all components in series.
Current separates at junctions in parallel, the current entering must be equal to the current coming out.

Question 6

State the rules for pd in series and parallel circuits

Answer 6

Pd is the same in parallel in all branches of the circuit.
It is divided in series.

Question 7

Explain what is meant by an ideal ammeter

Answer 7

An ammeter whose resistance is zero.

Question 8

Explain what is meant by an an ideal voltmeter

Answer 8

A voltmeter with infinite resistance (hence has little to no current)

Question 9

State the rules for resistance in series

Answer 9

R tot= R1+R2+R3

Question 10

State the rules for resistance in parallel circuits

Answer 10

1/Rtot= 1/R1+ 1/R2 + 1/R3

Question 11

State the rules for cells in series

Answer 11

V tot= V1+V2+V3
the current remains the same throughout the circuit
Rtot=R1+R2+R3

Question 12

State the rules for cells identical cells in parallel

Answer 12

their voltages remain the same as one of a single cell
I tot= I1+ I2 +I3
the resistance of the circuit is reduced, as the total resistance is less than that of a single cell.

Question 13

Explain how the rules for series and parallel circuits show how both charge and energy are conserved in series circuits

Answer 13

In a series circuit, the same current flows through each component, so the total charge flowing through the circuit is conserved. The voltage of the circuit is also conserved, as it is the sum of the voltages of each component. Energy conservation is demonstrated by the fact that the total power dissipated in the circuit is equal to the sum of the power dissipated in each component.

Question 14

Explain how the rules for series and parallel circuits show how both charge and energy are conserved in parallel circuits

Answer 14

In a parallel circuit, the total current flowing through the circuit is conserved, as it is the sum of the current flowing through each branch. The voltage across each branch is also conserved, as it is the same as the voltage of the source. Energy conservation is demonstrated by the fact that the total power supplied by the source is equal to the sum of the power consumed by each branch.

Question 15

State Ohm’s law

Answer 15

For a conductor at constant temperature, the current through it is proportional to the potential difference across it.

Question 16

Define an ohmic conductor.

Answer 16

A conductor which obeys ohm’s law.

Question 17

Ohmic conductor I-V graph

Answer 17

• I proportional to V
• demonstrated by straight line through the origin

Question 18

Semiconductor diode I-V graph

Answer 18

A diode only allows current to flow in one specific direction.
- when I is in the direction of the arrow, this is forward bias. It is characterised in the graph by the sharp increase in V on the +ve quadrant
- when diode is switched around, it does not conduct and it is called reverse bias. This is shown by the zero reading of current or pd on -ve quadrant.

Question 19

Filament lamp I-V graph

Answer 19

The I-V graph for a filament lamp shows the current increasing at a proportionally slower rate than the potential difference.
This is due to:
-as I increases temperature increases
-this causes a higher resistance as it is metal
-resistance opposes current causing current to increase at slower rate

It obeys ohms law for small voltages.

Question 20

Define resistivity

Answer 20

Resistivity is a measure of a material’s ability to resist the flow of electric current through it. It is defined as the resistance of a unit length of a material with a unit cross-sectional area, and is denoted by the Greek letter rho (ρ)

Question 21

Describe and explain the effect of temperature on the resistance of metal conductors

Answer 21

Metals have a lattice of ions which will vibrate as the temperature increases, this means that they are more likely to collide with the charges as they flow through the wire hence higher resistance.

Question 22

Define a superconductor

Answer 22

A superconductor is a material with no resistance below a critical temperature.

Question 23

Define the critical temperature

Answer 23

The temperature at which a material becomes superconducting (around 10K).

Question 24

Describe and explain the applications of superconductors

Answer 24

Superconductors are useful for applications that require high amounts of electricity, hence, useful for production of large electric fields and reduction if energy loss in transmission of electric power.

Applications can be:
-MRI scanners
-Transformers and generators
-Maglev trains (require strong electromagnets)
-electromagnets

Question 25

Describe and explain the effect of temperature on a negative temperature coefficient thermistor

Question 26

Define emf

Answer 26

The amount of chemical energy converted to electrical energy per column charge (C) when passing through a power supply.

(EMF = pd. when no I flowing)

Question 27

Define terminal potential difference

Answer 27

It is the potential difference across the terminals of a cell. Equal to emf when no internal resistance.

Question 28

Define lost volt

Answer 28

The work done per unit charge (coulomb) to overcome the internal resistance