I've shown here a cartoon of, we can say, any material.

Perhaps a copper wire.

When charged particles flow through that material,

they encounter an electrical resistance.

And the electrical resistance is determined by the material's

cross sectional area A.

It's length L and its resistivity rho.

Now specifically, resistance R is the product of rho and

L divided by the cross sectional area A.

The resistivity is an intrinsic property

that quantifies the material's opposition to charge flow.

And just for reference, the resistivity of metals which are excellent conductors or

have very low resistance.

The resistivity values range from 10 to the minus 8 to 10 to the minus 3 ohm

meters.

But if you consider rubber.

Rubbers which are not good conductors of electricity, they have high resistivity,

or high opposition to charge flow.

Their resistivity ranges from about 10 to the 5, to about 10 to the 15 ohm meters.

The variable that we'll use for resistance is R,

and the unit will be volt amps or ohms.