- State that resistance = p.d./current and understand qualitatively how changes in p.d. or resistance affect current.
The electrical resistance is a measure of the difficulty to pass an electric current through a conductor.
Resistance is calculated by dividing the voltage between two points by the current flowing through the points.
R = V ÷ I
From this formula, we can conclude that an increase in voltage will increase resistance. Reducing voltage will reduce the resistance.
- Recall and use the equation R = V / I
If the voltage is 6 volts and the current is 2 amps, the resistance = 6 ÷ 2 = 3 ohms
This formulas is known as Ohm’s law and rearranged to find each of the values
I = V ÷ R
V = I * R
- Describe an experiment to determine resistance using a voltmeter and an ammeter.
Voltmeter measured voltage/p.d. and ammeter measures current, so you can use these devices to determine the resistance between two points in a circuit.
- Set up an ammeter somewhere in the series circuit; this will give you the amount of current flowing in the circuit.
- Next set up a voltmeter in parallel to the object, in this case a light bulb, to find the potential difference across it.
- Using the equation R = V/I , we can find the resistance.
- Relate (without calculation) the resistance of a wire to its length and to its diameter.
As the length of the conducting wire increases, the resistance of the current flowing increases. Resistance and length of a wire are directly proportional
The greater the diameter of the wire, the smaller the resistance. Resistance and cross-sectional area of a wire are inversely proportional.
- Recall and use quantitatively the proportionality between resistance and length, and the inverse proportionality between resistance and cross-sectional area of a wire.
We now know that resistance is directly proportional to length, denoted as R α length.
Resistance is also inversely proportional to cross-sectional area, denoted as
R α (1 ÷ cross-section area)
Combining these two we get:
R α length ÷ cross-section area OR R α l ÷ A
we can add a constant ρ (pronounced as rho, in greek alphabet) to rewrite it as:
R = ρ * (l ÷ A)
In order to find the constant (by what proportion resistance changes with respect to length and cross-section area), we can rearrange the formula to:
ρ = R * (A ÷ l)
Notes submitted by Lintha
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