Electricity

Question 1

Current def

Answer 1

the rate of flow of positive charge carriers

Question 2

What happens when two oppositely charged conductors are connected together

Answer 2

Charge will flow between them, causing a current

Question 3

Conventional current def

Answer 3

the flow of positive charge from the positive terminal of a cell to the negative terminal

Question 4

How is current measured

Answer 4

Using an ammeter

Question 5

How should ammeters get connected with the part of the circuit you wish to measure the current through

Answer 5

In series

Question 6

Potential difference def

Answer 6

the work done per unit charge

Question 7

Resistance def

Answer 7

the opposition to current

Question 8

Why are wires often made of copper

Answer 8

because copper has a low electrical resistance and resistivity. This is also known as a good conductor as current flows through it easily.

Question 9

One ohm definition

Answer 9

one volt per ampere

Question 10

Describe how the resistance controls the size of a current in a circuit

Answer 10

• A higher resistance means a smaller current
• A lower resistance means a larger current

Question 11

What does Ohm’s law state

Answer 11

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

Question 12

Describe how the relations between potential difference across an electrical component (in this case, a fixed resistor) and the current can be investigated through a circuit

Answer 12

Question 13

How is Ohm’s law represented graphically

Answer 13

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

Question 14

Does a filament lamp obey Ohm’s law

Answer 14

No

Question 15

Draw the current-voltage graphs for a resistor, semiconductor diode and filament lamp

Answer 15

Question 16

Explain the current -voltage graph for a semiconductor diode

Answer 16

A diode is used in a circuit to allow current to flow only in a specific direction:
- When the current is in the direction of the arrowhead symbol, this is forward bias. This is shown by the sharp increase in potential difference and current on the right side of the graph
- When the diode is switched around, it does not conduct and is called reverse bias. This is shown by a zero reading of current or potential difference on the left side of the graph

Question 17

Explain the current-voltage graph for a filament lamp

Answer 17

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
- The filament lamp obeys Ohm’s Law for small voltages

Question 18

Explain why resistance exists

Answer 18

• 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
• Since current is the flow of charge, the ions resisting their flow causes resistance

Question 19

What does resistance depend on

Answer 19

the length of the wire, the cross-sectional area through which the current is passing and the resistivity of the material

Question 20

What does the resistivity equation show

Answer 20

• The longer the wire, the greater its resistance
• The thicker the wire, the smaller its resistance

Question 21

What is resistivity

Answer 21

• Resistivity 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

Question 22

Resistivity unit

Answer 22

Ohm metres

Question 23

Why does electric current decrease (and resistance increase) as temperature increases

Answer 23

The electrons collide with the vibrating atoms which impede their flow, hence the current decreases

Question 24

Describe the rules of Ohm’s law on temperature

Answer 24

• An increase in temperature causes an increase in resistance
• A decrease in temperature causes a decrease in resistance

Question 25

What happens to the resistance of a thermistor as temperature increases

Answer 25

the resistance of the thermistor decreases (and vice versa)

Question 26

Draw the temperature - resistance graph for a thermistor

Answer 26

Question 27

Superconductor def

Answer 27

A material with no resistance below a critical temperature

Question 28

Critical temperature def

Answer 28

The temperature at which a material becomes superconducting

Question 29

What is a common superconducting material and what is its critical temperature

Answer 29

Mercury - it has a critical temperature of 4.2K

Question 30

Draw a Resistivity against Temperature graph for a superconductor vs. a normal metal

Answer 30

Question 31

What is the transition temperature

Answer 31

The temperature threshold before the critical temperature

Question 32

What are superconductors useful for

Answer 32

for applications that require large electric currents.
For example:
- The production of strong magnetic fields
- The reduction of energy loss / dissipation in the transmission of electric power

Question 33

Define resistivity

Answer 33

Question 34

Answer 34

Question 35

Answer 35