Chapter 5 - Electricity and Magnetism

Question 1

What is conventional current?

Answer 1

It’s a current that flows from positive to negative.

Question 2

What are conduction electrons?

Answer 2

They are negative electrons that flow the opposite way than conventional current does in a circuit.

Question 3

Why does the metal wires heat up in a circuit?

Answer 3

As the conduction electrons move the interaction between them and the lattice ions means work is done, so when the current flows the metal heats up.

Question 4

What is drift velocity?

Answer 4

The speed of the electrons due to the current.

Question 5

What’s the equation to calculate the electrical conduction?

Answer 5

I = nAvq
n = number of density of the charge-carries (the number per unit volume that are available to move) 
q = the charge on each carrier 
v = average speed

Question 6

What is current and its units?

Answer 6

Rate of flow of electrical charge
1A = 1Cs^-1
1As = 1C

Question 7

What is an ohmic material?

Answer 7

If current and potential difference are proportional

Question 8

What’s Ohm’s law?

Answer 8

The current flowing through a piece of metal is proportional to the potential difference across is providing that temperature remains constant.

Question 9

What’s a resistor?

Answer 9

A device with constant resistance. (Aka an ohmic device)

Question 10

What’s resistance?

Answer 10

The mathematical ratio between potential difference and current:

Potential difference/ current
1 ohm = 1VA ^ -1

Question 11

What would you need for a current to flow if something had a high resistance?

Answer 11

A large potential difference

Question 12

What are the three equations for power?

Answer 12

p = V x I 
p = I^2 x R 
p = V^2 / R

Question 13

What’s the first conversation law?

Answer 13

The conversation of charge (it’s never created or destroyed only transformed)
The sum of I = 0 (junction)
Current flowing in a junction is the same as the current flowing out.

Question 14

What’s the second conservation law?

Answer 14

The conservation of energy.
Sum of voltage = zero (loop)
Around any loop total energy per unit charge must sum zero.

Question 15

What happens if the chosen loop direction is from the negative side of the battery to its positive side?

Answer 15

It’s an increase in pd and the value is positive when calculating the sum

Question 16

What happens of ten direction around the loop is in the same direction as the current flowing through the component?

Answer 16

This is a potential drop and the value is negative when calculating the sum.

Question 17

What’s the relationship between pd, energy and charge?

Answer 17

V = E/Q

Question 18

Series circuits

Answer 18

Current is the same everywhere due to it being one loop so charge is conserved. pd is shared among the components.

Question 19

Features of an ammeter

Answer 19

Connected in series

A perfect ammeter would have zero resistance

Question 20

Features of a voltmeter

Answer 20

Placed in parallel with the component

A perfect voltmeter would have infinite resistance

Question 21

Parallel circuits

Answer 21

Each component has the same pd across it

Total current is the addition of currents in each branch

Question 22

What does a potential divider do?

Answer 22

It’s 2 resistors that divide up the potential difference of the battery.

Question 23

What’s an potentiometer?

Answer 23

A variable potential divider that is often the best way to produce a variable power supply.

Question 24

What is a light-dependant resistor?

Answer 24

A device whose resistance depends on the amount of light shining on the surface.

Question 25

What effect does a change of light shining on the LDR cause?

Answer 25

An increase of light causes a decrease in resistance

When light is shining on the LDR: decrease in pd across it, and a increase in pd across the fixed resistor.

Question 26

What’s a thermistor?

Answer 26

It’s a resistor whose value of resistance depends on its temperature.
An increase in temperature causes a decrease in resistance, as more current can flow through.
As it heats up the lamp gets brighter.

Question 27

What does the output of pd of a sensor circuit depend on?

Answer 27

On an external force

Question 28

How does a change of temperature effect the of pd the fixed resistor?

Answer 28

As temperature of the thermistor increases, the pd of the fixed resistor will increase.

Question 29

What is resistivity?

Answer 29

The resistivity, p, of a material is defined in terms of its resistance R, its length l, and it’s cross-sectional area A.
Unit: Ohm metres

Question 30

What happens to the resistivity if ‘l’ increases.

Answer 30

Increasing l is like putting another resistor in series. Doubling l is like putting an identical resistor in series

Doubling l means doubling R.
A graph of l vs R is a straight line through the origin

Question 31

What happens to the resistivity if you increase A?

Answer 31

Doubling A means halving R.

So the graph of R vs 1/A will be a straight line going through the origin.

Question 32

What’s the rule for applying Kirchoff’s laws?

Answer 32

With chosen direction around the loop in the direction shown, E and IR are both positive in the kirchoff equation.

Sum of emf = sum of IR

If chosen the direction opposite to that shown, values are negative.

Question 33

Equation for Emf, internal resistance.

Answer 33

V = IR = E - Ir 
E = V + Ir = IR + Ir

Question 34

Equation for potential dividers

Answer 34

V(1):V(2) = R(1):R(2)

V(out) = (R out /R total) x V total

Question 35

What’s the relationship between voltage and work done?

Answer 35

Voltage = work done / charge

Question 36

What’s the ratio between resistance and potential difference?

Answer 36

High resistance = large pd across it in order to get a current to flow.

Question 37

What material in particular obeys ohms law?

Answer 37

Metals

Question 38

How does connecting a filament lamp in series with thermistor protect components from an initial surge current?

Answer 38

Thermistor has a high resistance when it’s cold, so there isn’t an overflow in current

Question 39

What’s the short-cut for 2 resistors in parallel?

Answer 39

Divide on the the resistors resistance by 2