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Elec mcq help pls

Htx_x346

Hi how do i do this question pls? Whats the fastest way of doing it?

(edited 1 year ago)

Reply 1

Driving_Mad

The current is the pd across the variable resistor / resistance of variable resistor. Then work out total resistance by doing emf / current.

This allows you to work out the internal resistance of the cell which you should label in your diagram.

From then on, it’s okay I would say

Reply 2

Htx_x346

Original post by Driving_Mad

Okay so r is 4
Do i need to work out the new current? Then do V=i/r again?

literally why is this question like 5 parts long lol

Reply 3

Driving_Mad

Original post by Htx_x346

Okay so r is 4
Do i need to work out the new current? Then do V=i/r again?

literally why is this question like 5 parts long lol

You could, or just use the potential divider formula, whichever you find easier

Reply 4

Htx_x346

Original post by Driving_Mad

You could, or just use the potential divider formula, whichever you find easier

Ok idk what im doing wrong

v=2(4/12) gives 0.67

Reply 5

Driving_Mad

Original post by Htx_x346

Ok idk what im doing wrong

v=2(4/12) gives 0.67

The resistance of the whole circuit is 16, you want the pd across the terminals (I.e circuit so emf - lost volts), so you use the load resistance, meaning you’d use 12/16 instead

Reply 6

Htx_x346

Original post by Driving_Mad

The resistance of the whole circuit is 16, you want the pd across the terminals (I.e circuit so emf - lost volts), so you use the load resistance, meaning you’d use 12/16 instead

Ohhhh wait
They want the voltage across the terminals of the cells. So 4/16?
sorry i dont get why you did 12/16

Reply 7

Driving_Mad

Original post by Htx_x346

Ohhhh wait
They want the voltage across the terminals of the cells. So 4/16?
sorry i dont get why you did 12/16

Terminals of the cell means the pd across the whole circuit, this doesn’t include the lost volts. If you did 4/16, you’d be finding the lost volts.

Terminal pd means the pd across the circuit that isn’t lost (emf - lost volts)

Reply 8

Skiwi

Original post by Htx_x346

Okay so r is 4
Do i need to work out the new current? Then do V=i/r again?

literally why is this question like 5 parts long lol

Just going to point out that you could skip the entire first step just by spotting that the voltage is split in a 1:1 ratio. Though calculating it by finding the current works just as well.

Original post by Driving_Mad

The resistance of the whole circuit is 16, you want the pd across the terminals (I.e circuit so emf - lost volts), so you use the load resistance, meaning you’d use 12/16 instead

If it makes it any easier to think about you could use 4/16 and just find the lost volts directly and then subtract that from 2V to find the potential difference in the rest of the circuit. What @Driving_Mad does just skips the additional step of subtracting the lost volts and finding the Pd in the actual circuit, same thing but using 12/16 is faster and leaves less room for error if you're in a rush

(edited 1 year ago)

Reply 9

Skiwi

Just realized i quoted the wrong post :h:

Original post by Htx_x346

Ohhhh wait
They want the voltage across the terminals of the cells. So 4/16?
sorry i dont get why you did 12/16

Reply 10

Htx_x346

Original post by Driving_Mad

Oh i didnt know that, i always assumed terminals of the cell meant p.d across the cell.
thanks!

Original post by Skiwi

Just going to point out that you could skip the entire first step just by spotting that the voltage is split in a 1:1 ratio. Though calculating it by finding the current works just as well.

If it makes it any easier to think about you could use 4/16 and just find the lost volts directly and then subtract that from 2V to find the potential difference in the rest of the circuit. What @Driving_Mad does just skips the additional step of subtracting the lost volts and finding the Pd in the actual circuit, same thing but using 12/16 is faster and leaves less room for error if you're in a rush