Calculating Thermal Stress on Steel Train Rails: Winter to Summer Transition

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jsalapide
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Steel train rails are laid in 10m long segments placed end-to-end. The rails are laid on a winter day when the temperature is -10 degrees Celsius.

a. if the rails are originally laid in contact, what is the thermal stress on them on a summer day when their temperature is 35 degrees Celsius?

b. To prevent thermal stress during summer, what minimum gap between rails must be left when they are originally laid?

my answer in (a) is -3.6x10^7

my answer in (b) is 1.8x10^-3

am i correct?
 
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Do you have any work to back those answers up?
 
sorry it was the wrong one..

i resolved it and:

a. the young's modulus for steel is 20x10^10 and its coefficient of linear expansion is 1.2x10^-5..
i used the formula (F/A)=-(coefficient of linear expansion)(young's modulus)(delta T)
F/A is the thermal stress..
my answer is -1.08x10^8..

b. i just used the formula delta L=(coefficient of linear expansion)(original length)(delta T)

my answer is 5.4x10^-3 m

is that correct?