Unit 3: Electricity

Question 1

All (conceptual) notes: https://drive.google.com/drive/u/0/folders/120q0zGzo2H3hzhk2vOOarJLNkT7IigKq

PLEASE STILL GO THROUGH THE SYLLABUS STATEMENTS, ESPECIALLY THE ONE WHICH TELLS YOU WHICH CIRCUIT SYMBOLS YOU HAVE TO KNOW. ALSO LOOK AT THE PAST PAPER QUESTIONS I PUT AT THE END BECAUSE THEY SUMM UP THE TRICKY CONCEPTS YOU NEED TO KNOW ABOUT STATIC ELECTRICITY.

ALSO READ CHAPTER 17,18,19 OF THE TEXTBOOK (PHYSICS SECTION) THERES A LOT IN THERE YOU NEED TO KNOW.

Question 2

State that there are positive and negative charges

Question 3

State that unlike charges ___(1)____ and that like charges ___(2)____

Answer 3

(1) attract
(2) repel

Question 4

Describe and interpret simple experiments to show the production and detection of electrostatic charges by friction

Answer 4

1. Rubbing a balloon against a jumper
2. Charged balloon running next to a tap of water

Question 5

State that charging a body involves the addition or removal of _______

Answer 5

electrons

Question 6

Distinguish between electrical conductors and insulators and give typical examples

Question 7

Describe an electric field

Answer 7

a region in which an electric charge experiences a force

Question 8

Demonstrate understanding of current, potential difference, e.m.f. and resistance

Answer 8

Current: A flow of charged particles

Potential difference: The amount of energy transferred by each unit of charge passing between two points.

e.m.f: The phenomena that allows charge to flow. it is the energy per unit charge.

Resistance: A measure of how much opposition there is to the flow of current. measured in ohms.

Question 9

Remember: IT IS ALWAYS AND ONLY THE _____(?)_______ CHARGES (ELECTRONS) THAT ARE TRANSFERRED. THE POSITIVE CHARGES ARE INSIDE THE NUCLEUS OF THE ATOMS AND DO NOT MOVE.

Answer 9

NEGATIVE

Question 10

State that current is related to the flow of charge

Answer 10

Remember that the flow of charge could also include the flow of cations/anions, it is only/mainly in metals where it is due to the flow of electrons

Question 11

State that current in metals is due to a flow of _________

Answer 11

electrons

Question 12

State that the potential difference (p.d.) across a circuit component is measured in volts

Question 13

Use and describe the use of an ammeter and a voltmeter, both analogue and digital

Question 14

State that the electromotive force (e.m.f) of an electrical source of energy is measured in volts

Question 15

Show understanding that a current is a rate of flow of charge and recall and use the equation
interlinking current, time, and charge

Answer 15

I = Q / t

Question 16

Define EMF

Answer 16

energy supplied by a source in driving charge around a complete circuit

Question 17

State that resistance = (what is the equation for resistance?) and understand qualitatively how changes in p.d. or resistance affect current

Answer 17

p.d. / current

Think of it like this:
current x resistance = potential difference
now just rearrange that equation

Current is directly proportional to electric potential difference and inversely proportional to resistance.

Question 18

Recall and use the equation R = V / I

Question 19

Sketch and explain the current-voltage characteristic of an ohmic resistor and a filament lamp

Question 20

Recall and use quantitatively the proportionality between resistance and length, and the inverse proportionality between resistance and cross- sectional area of a wire

Question 21

Draw and interpret circuit diagrams containing sources, switches, resistors (fixed and variable), lamps, ammeters, voltmeters and fuses (Symbols for other common circuit components will be provided in questions.)

Answer 21

fixed resistors = resistance doesn’t changed, variable resistor = resistance changes

Cell symbol - the logner line = positive side, shorter line = negative end

Question 22

Understand that the current at every point in a series circuit is the same

Question 23

Calculate the combined resistance of two or more resistors in series

Question 24

State that, for a parallel circuit, the current from the source is larger than the current in each branch

Answer 24

because total current = sum of the current in each branch.
Lets imagine this - a river splitting its course into two paths. Lets say that one of the paths has too many rocks, while the other path is a plain piece of land. Where does most of the water flow? Its the plain path isn’t it? It offers less resistance and hence favours the flow of water.

Yes. The same applies to current flow in circuits as well. Electric current is nothing but flow of electrons. And different paths in a circuit may have different resistances. So, currents in parallel branches may be different.

Question 25

State that the combined resistance of two resistors in parallel is less than that of either resistor by itself

Answer 25

Net resistance decreases cuz there are more pathways for the current to flow through

Question 26

State the advantages of connecting lamps in parallel in a circuit

Answer 26

Two bulbs in a simple parallel circuit each enjoy the full voltage of the battery. This is why the bulbs in the parallel circuit will be brighter than those in the series circuit. Another advantage to the parallel circuit is that if one loop is disconnected, then the other remains powered.

Question 27

Recall and use the fact that the sum of the p.d.s across the components in a series circuit is equal to the total p.d. across the supply

Question 28

Recall and use the fact that the current from the source is the sum of the currents in the separate branches of a parallel circuit

Question 29

Calculate the combined resistance of two resistors in parallel

Question 30

Draw and interpret circuit diagrams containing NTC thermistors and light-dependent resistors (LDRs) Describe the action of NTC thermistors and LDRs and show understanding of their use as
input transducers

Answer 30

An NTC thermistor is a temperature sensor that uses the resistance properties of ceramic/metal composites to measure the temperature.

LDRs (light-dependent resistors) are used to detect light levels, for example, in automatic security lights. Their resistance decreases as the light intensity increases: in the dark and at low light levels, the resistance of an LDR is high and little current can flow through it.

Question 31

Recall and use the equations P = IV and E = IVt

Question 32

Identify electrical hazards including:
– damaged insulation
– overheating of cables
– damp conditions

Answer 32

Overheating of cables:

Causes:
The more current you run through a conductor, the more resistance there is and resistance =heat.

Think of it like water. Voltage is the pressure in a hose, amperage is the volume of water running through the hose and watts is the amount of water that has gone through the hose. Changing one affects the other.

Say you are using 1 gallon a minute which is an ok flow for a garden hose. Now you want 2 gallons a minutes. You can either increase the pressure (volts) or increase the volume (amps) to get a certain amount of water (watts.) You are filling water ballons so too much pressure (volts) will ruin the ballon…it’s like trying to run a 12 volt light on 110 volt power. There’s just too much pressure and the light burns out (balloon bursts.)

So instead of increasing the pressure, you try to get a higher volume of water through the small hose. But the hose isn’t big enough to carry that much water and it resists the flow of higher amounts of water or the amperage is too much and the hose starts to bulge. In an electrical wire, this “bulge” of current flow causes it to heat up because that energy has to go somewhere, it just can’t all go through the wire so it is converted into heat. This is how an electric heater works…or, it’s how electrical fires start. This principle can be used for something good or it can cause bad things to happen like starting a fire.

Question 33

State that a fuse protects a circuit. Explain the use of fuses and choose appropriate fuse ratings

Question 34

Explain mains electricity

Answer 34

https://www.youtube.com/watch?v=fbu3o9wavHk

Question 35

Question 8 a)

A metal sphere, mounted on an insulating plastic stand, is positively charged. A smaller metal sphere, also mounted on an insulating plastic stand, is uncharged.
This smaller sphere is moved close to the positively charged sphere. Fig. 8.1 shows the two spheres.
***On Fig. 8.1, draw the distribution of charge on the smaller sphere.

Answer 35

• negative charge(s) on left (left = left side of the sphere) AND
  positive charge(s) on right (right = right side of the sphere)
• equal number of positive and negative charges AND number of
  each ≤ 7

Explanation:
- Remember CHARGE BY INDUCTION
- yeah, so the protons in the large sphere repel the protons on the small sphere and attract the electrons on the small sphere so the electrons get closer to the larger sphere (they move to the left side), and because of this the protons are left on the farther/right side

Question 36

Question 8 b)
(look at question 8 a) for context)

An earthed metal wire is touched against the smaller metal sphere.

***State and explain what happens to the charge on the smaller sphere.

Answer 36

• electrons / negative charges flow from Earth / on to sphere
  (NOT protons / positive charges / positive electrons move) -
  remember only electrons can move, but protons are stuck in
  the nucleus because of a force
• total charge negative OR (some) protons / positive charges
  cancelled

Explanation for the mark scheme:
- Its actually very simple: The electrons in the earth are attracted to the protons on the right side of the sphere so they move into the sphere. (and remember the electrons on the left side of the sphere are attracted to the protons in the big sphere so they are kind of ‘fixed in their position’)

Question 37

what are the two kinds of static electricity?

Answer 37

1. Charge by friction
2. Charge by induction

Question 38

A rod is rubbed against a cloth, the rod gets a positive charge. What material is the rod - plastic, or metal?

Answer 38

The rod has to be plastic since static electricity only occurs between insulators. As mentioned in the name STATIC, insulators are what allow the build up of charge as the electrons are static/stay in one place. Whereas in conductors the electrons can flow.