Topic 5, electricity & magnetism

Question 1

Does current get used up in a circuit?

Answer 1

NO!! Energy (voltage) is used up, NOT current. Current=charge/time passing a point = same eg before and after light bulb

Question 2

What drives/causes current?

Answer 2

Difference in electric potential
Voltage

Question 3

Voltage is a measure of + unit

Answer 3

Energy carried by charges. J/C = volt
Can think of it as electric pressure (difference)

Question 4

Current is a measure of + unit

Answer 4

Amount of charge per time unit passing through a point. C/s = ampère (A)
Flow of charge

Question 5

What does it mean if a battery has the voltage 10V?

Answer 5

Each Coulomb of charge has 10 Joules of energy

Question 6

What does it mean if the current is 5A?

Answer 6

5 coulombs of charge pass a point each second.

Question 7

What happens to V at a long continuous piece of wire?

Explain resistor, loss

Answer 7

NOTHING! (Assuming that the R=0) V is constant.

The voltage changes due to resistance. Loss of E, E is dissipated, used up. The resistor would get hot and heat produced. HOWEVER, the current doesn’t change across resistor, current same. Current not used up, the E is used up

Question 8

What happens if you have 2 EQUIVALENT resistors in PARALLEL? Say 60 Ohms
What happens if you have 3 EQUIVALENT resistors in parallel? (Still 60 Ohms each)

Answer 8

2 equivalent pathways makes it 2x as easy for the current to flow (think analogy two roads between two towns twice as much traffic flow). Thus Req=1/2 the R and thus Req=60/2=30 Ohms.

If 3, 3x easier for current to flow and Req=R/3 = 20 Ohms

OBS THIS IS ONLY FOR EQUIVALENT RESISTORS (eg quick P1 sol)
Otherwise use formula

Question 9

What happens if you have resistors in parallel w 1 Ohm vs 1kOhm (1000 Ohms)? What appr is the Req?

Answer 9

The Req will be approximately 1 Ohm, but slightly less. Appr 1 Ohm since almost all current passes through that one, but slightly less as some passes through the high R one (additional path (not only the 1 ohm R) means easier for current and less R)

Question 10

What happens if you have 3 resistors in parallel: 10 Ohm, 20 Ohm, 500 Ohm. What appr is the Requivalent?

Answer 10

Requivalent will be LESS THAN 10 Ohms. Why? Because the other two are as extra roads added, hence making the Req lower than 10 since easier for current to flow

Question 11

What is the Requivalent in relation to the individual resistors in parallel?

Answer 11

ALWAYS LESS THAN THE SMALLEST RESISTOR

Question 12

What is “the same” in a PARALLEL CIRCUIT?

Answer 12

The VOLTAGE (all resistors (in parallel)) receive the battery voltage (expl continuous piece of wire). The current divides (not nec equally).

Question 13

What is “the same” in SERIES CIRCUIT?

Answer 13

CURRENT. The V across resistors differs but all resistors in series have the same current

Question 14

What is a primary cell?

Answer 14

Disposable battery

Question 15

What is a secondary cell?

Answer 15

Rechargeable battery

Question 16

How does cell voltage of a cell change during its lifetime?

Answer 16

Starts at higher V initially (shortly)
Then little lower, relatively constant (but slight decr) most it’s lifetime
Then decreases abruptly when begins to die out

Question 17

What is emf

Answer 17

Voltage of a source when no current flows. Voltage measured in an open circuit.
(No voltage drop from the internal resistance)

Question 18

What happens to the magnitude of the internal resistance when cells are connected in series vs parallel (compared to single cell)

Answer 18

Series: r INCREASES (think adding series resistors)

Parallel: r DECREASES (think adding parallel resistors)

Question 19

Older batteries have higher or lower r?

Answer 19

Higher!

Question 20

How do you think for and draw electric field lines?

Answer 20

Think: WHERE WOULD A POSITIVE TEST CHARGE GO?
If the charge pos they will repel if neg they will attract. If several then probs BEND!!

Question 21

Since electric fire F prop to 1/r^2 what happens to the force if the distance is doubled? (PAPER 1 kinda Q)!!

Answer 21

F = 1/2^2 = 1/4 so 1/4

Question 22

k=1/4piEpsylon0, what is epsylon0?

Answer 22

Permittivité of vacuum (constant) see FB

Question 23

How do you think of you have several point charges and want to know where one would go // vector for resultant force?

Answer 23

COVER SOME POINTS: DIVIDE THE QUESTION: first look at two points; what is the vector for Fres of those? Then for the other point; Fres. Then add the vectors

Question 24

Is electric field strength a vector quantity?

Answer 24

YES!!! The direction matters!!! Also think F is so E is too (E=F/q)

Question 25

Can you use the word “voltage” in exams?

Answer 25

NO!!! DONT!!! Say potential difference instead!!!!! Never ever say voltage!!

Question 26

Does path taken matter for the potential difference (don’t think circuit just general)

Answer 26

No!! It’s just the total work done/charge.

Question 27

What is an electronvolt, eV??

Answer 27

Charge of 1 ELECTRON * 1 VOLT. Measure of ENERGY. CHARGE * POTENTIAL DIFFERENCE

A key equation would be eV=1/2mv^2

e = the elementary charge of an electron. Charge is quantized. 1eV=1.6*10^-19 J

Question 28

How would you solve a Q: find KE of an electron that is accelerated across a potential difference of 1.0V?

Answer 28

eV=energy
1e1V = (1.610^-19C)(V) = 1.6*10^-19 J

Question 29

Define electric field

Answer 29

Thé le troc field in a point P is Force divided by Charge in a small positive test charge.

Question 30

What do field lines tell you? 2 things

Answer 30

Show direction of pos charge
DENSITY of the field lines shows HOW STRONG THE FIELD IS; MAGNITUDE

Question 31

What is the angle and v and B in the formula F=qvBsintheta ? And which 2 are vectors?

Answer 31

v=SPEED
Angle=ANGLE BETWEEN B AND v (in degrees)
B=MAGNETIC FIELD STRENGTH
Vectors: F and B

Question 32

When do you use the formula F=qvBsintheta (1) VS F=BILsintheta (2)

Answer 32

1: for MOVING SHARGE IN A MAGNETIC FIELD
2: for WIRE IN MAGNETIC FIELD

Question 33

What is the angle in the formula F=BILsintheta?

Answer 33

ANGLE BETWEEN B AND I (in degrees)

Question 34

What way does a magnetic field go from a magnet? (South to north or north to south?)

Answer 34

FROM NORTH TO SOUTH

Question 35

Say you have a magnetic field, how do you know which direction a compass would point at a point?

Answer 35

EXTEND THE MAGNETIC FIED LINES SO THEY PASS THROGH THE POINT. THE COMPASS WILL POINT IN THE SANE DIRECTION AS THE FIELD LINES. (Think fied lines can be though of / defined as the direction in which a compass would point)

Question 36

Where does a compass point?

Answer 36

IN THE SAME DIRECTION AS THE FIELD LINES.
Can say THUMB in RHR2 is NORTH and the compass will point away from there (as the field lines)
To find direction compass, draw field lines and EXTEND THEM to just reach/pass through the point.

Question 37

What happens to the path of a neutron going through a magnetic field directed downwards? And which hand rule do you use

Answer 37

NOTHING!!! Trick question!! A neutron has NO charge and is not deflected in a magnetic field. It is undeflected and continues its path straight through!!

Question 38

When do you use the LEFT PALM HAND RULE?

Answer 38

For NEGATIVE CHARGES ONLY!!! “ELEFTrons”.

Question 39

When do you use the RIGHT PALM HAND RULE?

Answer 39

For POSITIVE or CURRENT.

Question 40

What is the current/velocity and B and Force in the “PALM HAND” RULE?

Answer 40

have thumb at 90 deg
shoot lightning out of fingers, B
thumb is speed/current

Question 41

Which hand/palm rule do you use for electrons?

Answer 41

LEFT HAND RULE!! eLEFTrons

Question 42

Which hand/palm rule do you use for positive charges or current?

Answer 42

RIGHT HAND RULE!