When calculating a confidence interval for the population mean with a known population standard deviation ,

When calculating a confidence interval for the population mean µ with a known population standard deviation σ, describe the effects of the following two changes on the confidence interval:
(1) Doubling the sample size,
(2) Quadrupling (multiplying by 4) the sample size. Give two reasons why this relationship does not hold true if you are calculating a confidence interval for the population mean µ with an unknown population standard deviation.