AS Level Physics question on electricity. Help needed!

A 12.0 V battery is connected with its positiveterminal to A and negative terminal to B.(i) Calculate the current in the 8.0 Ω resistor.......answer.................................. A

The answer is 1.5A and to get it you do 12v/8 ohms to get 1.5A (from mark scheme). But I'm very confused as to why my method and answer from my method is wrong.

My method is this:

1/8+1/4= 3/8.

3/8^-1 = 8/3 or 2.67

12v/2.67ohms = 4.5A

2:1 ratio therefore 4.5/3 is 1.5

1.5*2 is 3A.

Sorry if this seems like a stupid question but I'm just VERY confused...m

Thanks!

Well as voltage is the same, between a parallel combination, it will be 12V across the 8 ohm (aswell as the diode & 4 ohm).

So therefore using I = V / R = 12 / 8 = 1.5A

As to why your method doesn't quite work, I'm not quite sure to be honest, that's the method I would've used if it was a more complex circuit (I wonder if the diode has something to do with it?), though the quick method I posted above does work.

A 12.0 V battery is connected with its positiveterminal to A and negative terminal to B.(i) Calculate the current in the 8.0 Ω resistor.......answer.................................. A

The answer is 1.5A and to get it you do 12v/8 ohms to get 1.5A (from mark scheme). But I'm very confused as to why my method and answer from my method is wrong.

My method is this:

1/8+1/4= 3/8.

3/8^-1 = 8/3 or 2.67

12v/2.67ohms = 4.5A

2:1 ratio therefore 4.5/3 is 1.5

1.5*2 is 3A.

Sorry if this seems like a stupid question but I'm just VERY confused...

Thanks!

The problem is with your ratio. 2:1 is the ratio of current through the 4Ω resistor to current through the 8Ω resistor, not the other way around.

In general, for two parallel resistors, I₂/I₁ = R₁/R₂. This can be derived from Ohm's law using the fact that the potential difference across each will be the same.

A 12.0 V battery is connected with its positiveterminal to A and negative terminal to B.(i) Calculate the current in the 8.0 Ω resistor.......answer.................................. A

The answer is 1.5A and to get it you do 12v/8 ohms to get 1.5A (from mark scheme). But I'm very confused as to why my method and answer from my method is wrong.

My method is this:

1/8+1/4= 3/8.

3/8^-1 = 8/3 or 2.67

12v/2.67ohms = 4.5A

2:1 ratio therefore 4.5/3 is 1.5

1.5*2 is 3A.

Sorry if this seems like a stupid question but I'm just VERY confused...

Thanks!