3.5 Electricity

Question 1

What is electric current?

Answer 1

The rate of flow of charge

Question 2

What is potential difference?

Answer 2

The rate of energy transfer (work done) per unit charge when electrical energy is converted into another form of energy

Question 3

How is volt defined?

Answer 3

The unit of potential difference, work done per unit charge

Question 4

What is resistance?

Answer 4

The ratio of potential difference over current

Question 5

What is Ohm’s Law?

Answer 5

The current through an ohmic conductor is directly proportional to the potential difference across it, provided physical conditions (temperature) are constant

Question 6

What is an ohmic conductor?

Answer 6

A component that follows Ohm’s Law

Question 7

What is the threshold voltage of a semiconductor diode?

Answer 7

The smallest voltage needed to allow current to flow (forward bias), hence the shape of the I-V graph which has a sudden positive gradient from having no gradient

Question 8

In the direction of the reverse bias of a semiconductor diode, why can only a very small current flow?

Answer 8

The resistance of the diode is extremely high

Question 9

Describe a filament lamp.

Answer 9

It contains a length of metal wire (filament) which heats up as current increases, so the resistance of this component increases as current increases

Question 10

What is a thermistor?

Answer 10

A resistor in which an increase in temperature results in resistance decreasing

Question 11

What is an LDR?

Answer 11

Light Dependent Resistor - resistor in which an increase in intensity of light decreases resistance

Question 12

How much resistance should an ideal ammeter have?

Answer 12

Zero, so it doesn’t affect the measurement of current in a circuit at all

Question 13

How much resistance should an ideal voltmeter have?

Answer 13

Infinite, so no current can flow through it, so the measurement of potential difference across a component is exact

Question 14

What is resistivity?

Answer 14

• The measure of how easily a material conducts electricity, how much a material opposes the flow of electric current through it
• The product of resistance and cross-sectional area, divided by the length of the material
• The value of resistance through a material of length 1m and cross-sectional area 1m^2

Question 15

What is the relationship between temperature and resistance for a metal conductor?

Answer 15

An increase in temperature of a metal conductor increases its resistance

Question 16

Why does an increase in the temperature of a metal conductor increase its resistance?

Answer 16

The atoms in the metal gain kinetic energy and move more, causing more collisions between electrons carrying charge and the atoms, slowing the electrons down, decreasing the current and therefore the resistance

Question 17

What is the relationship between temperature and resistance for a thermistor?

Answer 17

As the temperature of a thermistor increases, its resistance decreases

Question 18

What is a superconductor?

Answer 18

A material that has zero resistivity below its critical temperature (different for each material)

Question 19

What are examples of applications of superconductors?

Answer 19

• Power cables - reduces energy loss through heat to zero
• Production of strong magnetic fields - used in maglev trains where there would be no friction between the train and rail, or in certain medical applications

Question 20

What is power?

Answer 20

The rate of energy transfer (work done)

Question 21

How is watt defined?

Answer 21

The unit of power, the energy transferred per unit time

Question 22

What is electrical power?

Answer 22

The rate of electrical energy transfer

Question 23

What is conventional current?

Answer 23

The flow of positive charge from positive to negative

Question 24

What is the rule for current in a series circuit?

Answer 24

The current is the same everywhere in the circuit

Question 25

What is the rule for potential difference in a series circuit?

Answer 25

The EMF is shared across all elements in the circuit, so the total sum of the potential differences across all elements is equal to the EMF

Question 26

What is Kirchhoff’s First Law?

Answer 26

The total current entering a junction is equal to the total current leaving the junction

Question 27

What is Kirchhoff’s Second Law?

Answer 27

The total potential difference across the EMF is equal to the sum of potential differences across components in any single loop

Question 28

What is Kirchhoff’s First Law representative of?

Answer 28

The conservation of charge

Question 29

What is a potential divider?

Answer 29

A circuit with several resistors in series connected to a power source used to supply constant or variable potential difference

Question 30

What are examples of components that could be used in a potential divider?

Answer 30

Variable resistors, thermistors, LDRs

Question 31

What is the terminal potential difference?

Answer 31

The potential difference across a cell

Question 32

What is EMF?

Answer 32

Electromotive force - the rate of energy transfer per unit charge when one form of energy is converted into electrical energy

Question 33

What does ‘r’ represent in the emf formula?

Answer 33

Internal resistance

Question 34

What is internal resistance?

Answer 34

Resistance caused by electrons colliding with atoms inside a battery, causing some energy to be lost before electrons leave the battery

Question 35

What is the pd across ‘r’ known as?

Answer 35

Lost volts because it is equal to the energy wasted by the cell per unit of charge