How Do Braking Force, Circuit Components, and Safety Devices Work in Physics?

[reree17]

Please help -- review questions

Hi there. I have an exam TOMORROW. I have prepared a lot but I just really need to understand the following in order to do the best I can:

b) Calculate the average braking force acting on a car of 800 kg to bring it to rest if it was traveling at 20 m/s – I used the equation F= M X A (Mass x Acceleration). So speed is measured in m/s right? And acceleration is measured in m/s squared? So would I do 800 x 20 squared = 320,000 N or just leave it at 20 m/s?

c) http://postimage.org/image/1spa51pic/ Sorry the picture is a bit blurry, in case you can’t read it it says ‘The circuit below is used to investigate how the resistance of a lamp changes. Explain how component X allows a set of results to be obtained. – I wrote that Component X is a variable resistor and therefore varies the current. The higher the current the lower the resistance and the lower the current the higher the resistance – Is this the right answer? How can it be improved?

d) http://postimage.org/image/1mxtsq344/ Explain how the graph shows that the resistance of the lamp increases as the voltage increases – Following Ohm’s Law, The higher the current, the higher the voltage right? But the higher the current, the lower the resistance? And the higher the voltage the lower the resistance? So why does the resistance increase as the voltage increases?

e) Explain how the miniature circuit breaker protects against fire – I know that the electromagnets in the mcb become strong enough to separate a pair of contacts, but how exactly does separating a pair of contacts protect against a fire?

f) Explain how the residual current device protects the user – I know that the rcd detects a difference in the currents in the live and neutral wires but how does this protect the user? Does it break the circuit or trip the circuit? And what exactly does tripping a circuit mean?

Thanks so so so so much for answering in advance!

 

[berkeman]

b) You need to factor in the time to stop. A fast stop has a higher average deceleration.

c) and d) the pictures are not showing

e) What causes an electrical fire? How would opening the circuit breaker affect this?

f) Check wikipedia.org for info about Ground Fault Circuit Interrupters (GFCIs).

 

[reree17]

b) You need to factor in the time to stop. A fast stop has a higher average deceleration.

c) and d) the pictures are not showing

e) What causes an electrical fire? How would opening the circuit breaker affect this?

f) Check wikipedia.org for info about Ground Fault Circuit Interrupters (GFCIs).

b) ok thanks!
c + d have been changed to links so they can be easily accessed now
e) does overheating of the wires cause an electrical fire? and the circuit breaker...uhm...it breaks the circuit?...? :S
f) ok will do thanks!

 

[berkeman]

b) ok thanks!
c + d have been changed to links so they can be easily accessed now
e) does overheating of the wires cause an electrical fire? and the circuit breaker...uhm...it breaks the circuit?...? :S
f) ok will do thanks!

e) Yes, overheating from *overcurrent* causes electrical fires.

d) Remember that as the lamp filament heats up, its resistance goes up some. So the current versus voltage relationship for a lamp filament is non-linear. Kind of like the plot shows...

 

[reree17]

e) Yes, overheating from *overcurrent* causes electrical fires.

d) Remember that as the lamp filament heats up, its resistance goes up some. So the current versus voltage relationship for a lamp filament is non-linear. Kind of like the plot shows...

d) So as there is a bigger current, the filament lamp heats up. In turn, the increase in temperature results in the increase in resistance? So how does voltage come into this relation? Does the voltage increase as the current increases, or vice versa? :S

 

[berkeman]

d) So as there is a bigger current, the filament lamp heats up. In turn, the increase in temperature results in the increase in resistance? So how does voltage come into this relation? Does the voltage increase as the current increases, or vice versa? :S

With an incandescent light bulb, you put a voltage across it, and the current that flows is determined by the resistance of the filament. The resistance of the cold filament is lower than the resistance of a hot filament (the filament material is said to have a positive temperature coefficient of resistance).

 

[reree17]

With an incandescent light bulb, you put a voltage across it, and the current that flows is determined by the resistance of the filament. The resistance of the cold filament is lower than the resistance of a hot filament (the filament material is said to have a positive temperature coefficient of resistance).

Makes sense now, thanks so much! What about question c?