Topic 5, electricity & magnetism Flashcards
Does current get used up in a circuit?
NO!! Energy (voltage) is used up, NOT current. Current=charge/time passing a point = same eg before and after light bulb
What drives/causes current?
Difference in electric potential
Voltage
Voltage is a measure of + unit
Energy carried by charges. J/C = volt
Can think of it as electric pressure (difference)
Current is a measure of + unit
Amount of charge per time unit passing through a point. C/s = ampère (A)
Flow of charge
What does it mean if a battery has the voltage 10V?
Each Coulomb of charge has 10 Joules of energy
What does it mean if the current is 5A?
5 coulombs of charge pass a point each second.
What happens to V at a long continuous piece of wire?
Explain resistor, loss
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
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)
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
What happens if you have resistors in parallel w 1 Ohm vs 1kOhm (1000 Ohms)? What appr is the Req?
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)
What happens if you have 3 resistors in parallel: 10 Ohm, 20 Ohm, 500 Ohm. What appr is the Requivalent?
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
What is the Requivalent in relation to the individual resistors in parallel?
ALWAYS LESS THAN THE SMALLEST RESISTOR
What is “the same” in a PARALLEL CIRCUIT?
The VOLTAGE (all resistors (in parallel)) receive the battery voltage (expl continuous piece of wire). The current divides (not nec equally).
What is “the same” in SERIES CIRCUIT?
CURRENT. The V across resistors differs but all resistors in series have the same current
What is a primary cell?
Disposable battery
What is a secondary cell?
Rechargeable battery
How does cell voltage of a cell change during its lifetime?
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
What is emf
Voltage of a source when no current flows. Voltage measured in an open circuit.
(No voltage drop from the internal resistance)
What happens to the magnitude of the internal resistance when cells are connected in series vs parallel (compared to single cell)
Series: r INCREASES (think adding series resistors)
Parallel: r DECREASES (think adding parallel resistors)
Older batteries have higher or lower r?
Higher!
How do you think for and draw electric field lines?
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!!
Since electric fire F prop to 1/r^2 what happens to the force if the distance is doubled? (PAPER 1 kinda Q)!!
F = 1/2^2 = 1/4 so 1/4
k=1/4piEpsylon0, what is epsylon0?
Permittivité of vacuum (constant) see FB
How do you think of you have several point charges and want to know where one would go // vector for resultant force?
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
Is electric field strength a vector quantity?
YES!!! The direction matters!!! Also think F is so E is too (E=F/q)
