5. Electricity

Question 1

What is electrical current? What is its units?

Answer 1

The rate of flow of charge. It is measure in Amperes

Question 2

What is potential difference?

Answer 2

The work done moving a unit charge between 2 points in a circuit
V = W/Q

Question 3

What is resistance?

Answer 3

How difficult it is for current to flow through a circuit

R= V/I

Question 4

What is meant by an ohmic conductor?

Answer 4

A conductor that obeys Ohm’s law, meaning the current is directly proportional to the potential difference providing physical conditions such as temperature stay constant

Question 5

How can you measure the current in a circuit?

Answer 5

By using an ammeter in series with the circuit

Question 6

How do you measure the potential difference across a component?

Answer 6

By connecting a voltmeter parallel to the component being measured

Question 7

What does the gradient of a current-potential difference graph represent?

Answer 7

1/R

Question 8

Why does current increasing on a filament lamp increase resistance?

Answer 8

As current flows through the lamp , electrical energy is converted to heat energy so the metal ions vibrate with increased amplitude. This impedes the movement of electrons through the lamp as they collide with the ions

Question 9

What is a diode?

Answer 9

An appliance that allows current to travel in one direction

Question 10

Unless stated in the question should you assume voltmeters to have 0 or infinite resistance?

Answer 10

You should assume they have infinite resistance. Current takes the path of least resistance so if the voltmeter is connected in parallel with a component all current should flow through the component and no the voltmeter

Question 11

Why should you assume that the ammeter has 0 resistance unless stated otherwise?

Answer 11

This assumption means there would be no potential difference across it so no energy lost : ; does not affect circuit

Question 12

What is a Light Dependent Resister (LDR)?

Answer 12

A semiconductor that is sensitive to light so as light increases resistance decreases

Question 13

How does a thermistor work?

Answer 13

Similar to LDR but as temperature increases resistance decreases

Question 14

What is resistivity?

Answer 14

p = RA/L
p = resistivity (ohm m) R = resistance (ohm)
A = cross sectional area (m^2) L = length(m)

Question 15

Describe an experiment to determine the resistivity of a metal

Answer 15

1) Measure the diameter with a micrometer of the wire and workout the cross sectional area
2) Set up the circuit with an ammeter in series and a voltmeter parallel to the wire
3) Vary the wire length by a set amount each time and record the voltage and current for each length
4) Use R = V/I to work out resistance
5) Plot a graph of resistance against wire length
6) Gradient = resistivity / cross sectional area
7) So resistivity = cross sectional area x gradient

Question 16

What is a superconductor?

Answer 16

A material with a resistance of zero at or below a critical temperature. The critical temperature is an inherit property of the material

Question 17

How do you find the total resistance in a series circuit?

Answer 17

Add the individual resistances of the components

Question 18

How does the current vary between each component in a series circuit?

Answer 18

The current through all of the components is the same so current does not vary

Question 19

Is the current in parallel components the same?

Answer 19

No , each branch of parallel components can carry different currents through them according to Kirchhoff’s first law

Question 20

What is Kirchhoff’s first law?

Answer 20

All of the current going into a junction is equal to the total current leaving the junction

Question 21

What is Kirchhoff’s second law?

Answer 21

For any path (loop) of the circuit, the sum of all the potential differences must be equal to the total emf of the circuit

Question 22

What happens when two cells connect negative to negative in series?

Answer 22

They cancel out

Question 23

What is power?

Answer 23

The rate of energy transfer

Question 24

Give an equation for power in terms of current and voltage

Answer 24

P = VxI

Question 25

What is the purpose of a potential divider?

Answer 25

1) To provide variable potential difference

2) To provide a constant specific potential difference

Question 26

What is emf?

Answer 26

Electromotive force: the electrical energy transferred by a power supply per unit charge

Question 27

What are two applications of superconductors?

Answer 27

1) Power cables which would reduce the energy lost through heat to zero through transmission
2) Strong magnetic fields that do not need a constant power source

Question 28

How do you calculate the total resistance of resistors in parallel?

Answer 28

1/R total= 1/R1 + 1/R2 + 1/R3+……+ 1/Rn