Electric Circuits: Core Concepts

Question 1

What is the elementary charge?

Answer 1

The smallest unit charge comes in

Question 2

What is the formula connecting number of electrons, charge & the charge of one electron?

Answer 2

Q = n q

Question 3

Define current

Answer 3

The rate of flow of charge (the amount of charge passing a point in the circuit each second)

Question 4

What is the formula connecting time, current & charge?

Answer 4

Q = I t

Question 5

Should ammeters be placed in series or parallel & why?

Answer 5

Ammeters should be put in series because they measure the amount of charge flowing though a point in the circuit each second therefore meds to be in series

Question 6

What’s the ideal resistance of an ammeter?

and why

Answer 6

Negligible resistance (as close to zero as possible)if it had resistance it would affect the current & therefore decrease the number of electrons passing through

Question 7

What’s the relationship between conventional current & the flow of electrons?

Answer 7

They go in opposite directions (electrons go from negative to positive, conventional current goes from positive to negative)

Question 8

Describe the movement of electrons through a wire

Answer 8

They move randomly in all directions but drift in one direction

Question 9

What is mean drift velocity?

Answer 9

The average velocity of electrons (is much less than the electron’s actual speed)

Question 10

What’s the equation for mean drift velocity?

Answer 10

I = nAve

Question 11

Why are metals good conductors?

Answer 11

They have lots of free electrons per metre cubed

Question 12

What is an electrolyte?

Answer 12

A substance containing ions that conducts electricity

Question 13

Why do you get a spark when applying very high voltage to a gas?

Answer 13

Electrons get ripped out of atoms giving you ions along a path

Question 14

Define potential difference

Answer 14

The electric potential energy transferred per coulomb of charge which flows between 2 points in a circuit

Question 15

What’s the formula connecting work done, potential difference & charge?

Answer 15

V = W/Q

Question 16

Should a voltmeter be placed in series or parallel & why?

Answer 16

Parallel because it compares the energy of change carriers before & after the component, so it must be placed across the component in parallel

Question 17

What’s the ideal resistance of a voltmeter & why?

Answer 17

Infinitely large (many thousands of ohms) - this ensures that almost no current flows through the voltmeter instead it all flows through the component therefore giving an accurate measure of the energy of charge carriers before & after

Question 18

Define resistance

Answer 18

Whenever a current flows through a component, it experiences resistance. The greater the resistance, the more energy is transferred as a given current flows across the component

Question 19

What’s the formula connecting resistance, potential difference & current?

Answer 19

V = IR

Question 20

State ohm’s law

Answer 20

For a fixed resistor or metallic conductor @ constant temperature, the current through the resistor is directly proportional to the potential difference across it.

Question 21

Define power dissipated

Answer 21

The rate at which energy is transferred to other forms in an electrical circuit

Question 22

What’s the formula for power in terms of potential difference & current?

Answer 22

P = IV

Question 23

What’s the formula connecting potential difference, time, current and energy transferred?

Answer 23

W = VIt

Question 24

What’s 1kWh in J?

Answer 24

3.6 million J

Question 25

For an ohmic device, what is the relationship between the gradient of an IV graph & the resistance?

Answer 25

R = 1/gradient

Question 26

Explain why conductors increase in temperature as current through them increases

Answer 26

* As the current increases so does the rate of flow of charge (electrons) through the filament * As there are more electrons per second flowing through it, more collisions occur between the electrons and the positive metal ions per second. * When the electrons collide with the metal ions, the electrons transfer electrical energy to the ions, causing the ions to vibrate more (kinetic energy) and increase in temperature * This results in the metal ions colliding with still more electrons

Question 27

Explain what happens to the resistance of a conductor as the current through it increases

Answer 27

• Assuming potential difference is kept constant. • As current flowing in a conductor increases, the temperature of the conductor also increases and the conductor gets hotter. • When the temperature of the conductor increases, the positive metal ions in the conductor have more internal energy and vibrate with more internal energy about their mean positions. • The frequency of collisions between the free electrons and metals ions increases. Consequently the free electrons transfer more energy as they travel through the conductor and resistance increases

Question 28

Explain what happens to the resistance of a conductor as the current through it increases

Answer 28

• Assuming potential difference is kept constant. • As current flowing in a conductor increases, the temperature of the conductor also increases and the conductor gets hotter. • When the temperature of the conductor increases, the positive metal ions in the conductor have more internal energy and vibrate with more internal energy about their mean positions. • The frequency of collisions between the free electrons and metals ions increases. Consequently the free electrons transfer more energy as they travel through the conductor and resistance increases

Question 29

Explain the behaviour of the diode in terms of how its resistance changes with different potential difference values

Answer 29

The PD across the diode (or LED) is not directly proportional to the current through it. The diode (or LED) does not obey Ohm’s Law and can be described as a non-ohmic component When the diode is reverse-biased, its resistance is infinitely high and it does not conduct. When the diode is forward biased, as the PD increases, the resistance gradually starts to drop, The PD at which the diode starts to increase is called the threshold voltage Above the threshold voltage, the resistance drops rapidly for small increases in PD. Above this point the resistance is very low