# Homework Help: Diameter of a wire with length, resistance and resistivity

1. Jan 1, 2010

### Elka

1. The problem statement, all variables and given/known data
This question is about the heated front windscreen of a car
The heater consists of resistance wires which are embedded in the glass

A) The power of the heater needs to be 180W for a satisfactory demisting
A car battery of Negligable internal resistance supplies 12V to operate on the heated screen

Calculate the current required to deliver a power of 180W
180/12=15A

Show that the resistance of the heater when operating is about 1ohm
12/15=0.8 ohms, rounded up=1ohm

B) the heater consists of 200 wires inside the glass
These wires are connected in parallel to the 12V supply

Each of the 200 wires has a length of 0.70m and a resistance R(subscript)p of 160 ohms
The material of the wire has resistivity p=6.0x10^-7ohmic metres

Calculate the DIAMETER of the wire

please people, I don't want the answer, can someone just help me out and give me the equation I need to use? I'm terrible at remembering equations I really need the equation, thanks x

2. Jan 1, 2010

### tiny-tim

Welcome to PF!

Hi Elka! Welcome to PF!

Resistance = resistivity times length over cross-section area.

3. Jan 2, 2010

### Elka

Thank you Tiny Tim! Thats great! You've helped me pick up 4 marks definately and maybe 5! (past papers homeworks) Thank you so much, you're a lifesaver :D