Few Level 3 Questions

[m4tty]

Hi,

Just got a few questions that im trying to get head round and would appreciate some help please.

1. What current will flow in a 20Ω resistor when it is connected to a 50V supply ?

a) 0.4A

B) 1.6A

c) 2.5A

ohms law 50/20 = 2.5A

2. A current of 500mA flows in a resistance of 12Ω . What power is dissipated by the resistor ?

a) 3W

B) 6W

c) 18W

ohms law again 0.5A x 12 = 6W

3. Three 12Ω resistors are connected in parallel. What will the resistance of this combination be?

a) 3Ω

B) 4Ω

c) 48Ω

1/12 + 1/12 + 1/12 = 3/12 transposed to 12/3 = 4

4. A power of 240W appears in a load for a total time of 120 seconds. What energy is used ?

a) 2J

B) 2kJ

c) 28.8kJ

Not sure about this one can someone help with the formula needed

5. A charge of 1mC appears in a capacitor of 2uF. What voltage appears across the plates of the capacitor ?

a) 50V

B) 200V

c) 500V

Not sure on this one either

6. Capacitors of 1uF and 2uF are connected in series. What is the capacitance of the series combination ?

a) 0.33uF

B) 0.67uF

c) 3uF

1/1 + 1/2 = 3/2 which transpose to 2/3 which is 0.67uF

Any help would be greatly appreciated

Cheers

Matt

 

[Apache]

Hi,Just got a few questions that im trying to get head round and would appreciate some help please.

1. What current will flow in a 20Ω resistor when it is connected to a 50V supply ?

a) 0.4A

B) 1.6A

c) 2.5A

ohms law 50/20 = 2.5A YES

2. A current of 500mA flows in a resistance of 12Ω . What power is dissipated by the resistor ?

a) 3W

B) 6W

c) 18W

ohms law again 0.5A x 12 = 6W retract my answer Blushing P does = volts * amps but we need to transpose the formula to get resistance Blushing

4. A power of 240W appears in a load for a total time of 120 seconds. What energy is used ?

a) 2J

B) 2kJ

c) 28.8kJ

Not sure about this one can someone help with the formula needed

1 watt is 1 joule per second. so 240W/120s = 2 joules

My red.

**Bump**

I'm not an electrician!

 

[jimleen]

Hi Chaps,

I'd have thunk that number 2 is a, 3W, from P=I2R.

In number 4, P=E/t so 240=E/120, or

E=240 x 120 = 28.8kJ

I could be wrong of course.

Regards

 

[Apache]

Hello and welcome

Hi Chaps,I'd have thunk that number 2 is a, 3W, from P=I2R.

Ohm's Law states V=IR

To work out power in watts we multiply voltage by current

P = VI

We are given

current of 500mA

resistance of 12Ω

and need to find power.

so P = VI, we have the current but no volts.

to find volts we use Ohm's law V=IR

so P = (IR)*I

so P = (0.5*12)*0.5

P = 3 watts.

you are quite correct. I have edited my post, not to make myself look cleverer than I am, but for clarity!

Thanks for pointing that out.

In number 4, P=E/t so 240=E/120, or E=240 x 120 = 28.8kJ

Power in watts = energy (in joules) multiplied by time

1 watt is 1 joule per second.

So to transpose

P=Et

E=P/t

E=240/120

E=2 watts

I stand by this one!

 

[Apache]

3. Three 12Ω resistors are connected in parallel. What will the resistance of this combination be?

a) 3Ω

B) 4Ω

c) 48Ω

1/12 + 1/12 + 1/12 = 3/12 transposed to 12/3 = 4 Yes, if you remember that resisters in parallel the total resistance reduces then as they are all the same the resistance of 3 12ohms resisters will be a third of that ie 4ohms

6. Capacitors of 1uF and 2uF are connected in series. What is the capacitance of the series combination ?

a) 0.33uF

B) 0.67uF

c) 3uF

1/1 + 1/2 = 3/2 which transpose to 2/3 which is 0.67uFApplaud Smiley Yes, it's the same method as resistors in series

My red

 

[Apache]

5. A charge of 1mC appears in a capacitor of 2uF. What voltage appears across the plates of the capacitor ?

a) 50V

B) 200V

c) 500V

Not sure on this one either

Now if C = q/V ,

where:

* C is the capacitance in farads.

* q is the accumulated charge in coulombs.

* V is the voltage difference between the capacitor plates.

to transpose,

V = C/q

V = 0.001 / 0.000002

V = 500V

Remember mili 10-3, micro 10-6

I love doing your homework. I really should go to electrical college



I made it red for continuity

 

[Apache]

Does that all make sense? I have tried to include all working on the ones you haven't attempted (or that we both got wrong )

 

[m4tty]

Thanks for helping here mate. Its not homework just questions ive found off the net. Ive got all my Level 3 exams soon so trying to get prepared. Honest its not homework lol

 

[Apache]

Thanks for helping here mate. Its not homework just questions ive found off the net. Ive got all my Level 3 exams soon so trying to get prepared. Honest its not homework lol

I'm fine with it either way