Sam's parents took her to the World Park last week. He saw Big Ben, the Tower Bridge, Disneyland in America, the CN Tower, Niagara Falls and the Sydney Opera House. His father told her Big Ben and the Tower Bridge are in London. Her mother told her Niagara Falls is in Canada and the Sydney Opera House is in Australia. He learnt about many things of the famous places around the world.

两个分别带有电荷量﹣Q和+3Q的相同金属小球（均可视为点电荷），固定在相距为r的两处，它们间库仑力的大小为F．两小球相互接触后将其固定距离变为 $\frac{r}{2}$ ，则两球间库仑力的大小为（   ）

【猜想与实验设计】：小红猜测：黑色粉末是CuO．她设计了如下图所示的实验装置．

实验步骤取适量该粉末放入玻璃管中，打开K通入CO，排尽空气后加热．

【实验现象】