Logging wrapper function review

Very new to .NET programming.
I am writing simple Logging wrapper function in class that can be called from many apps. This will create new log file everyday and log exceptions etc to that Log file.

I am using this code below: Please review especially this line:
using (StreamWriter sw = new StreamWriter(File.Open(file, System.IO.FileMode.Append)))

is it ok to open the file to append when writing to it everytime or will it create overhead?

public class LogUtlity
{
public static string logFile;
public static string logFilePath;
public static string file;

public LogUtlity()
{
logFilePath = ConfigurationManager.AppSettings["filePath"];
logFile = "log" + DateTime.Now.ToString("yyyyMMdd") + ".txt";

//Create new file everyday
if (!(File.Exists(logFilePath)))
{
file = Path.Combine(logFilePath, logFile);
}

}

public void WriteLog(string logMessage)
{
using (StreamWriter sw = new StreamWriter(File.Open(file, System.IO.FileMode.Append)))
{
sw.WriteLine(String.Format("{0:G}: {1}.", DateTime.Now, logMessage));
}
}