P2 Resistance and V = IR (page 180) Flashcards
What is the formula linkng Potential Difference and Current?
Potential difference (V) = Current (A) X Resistance (Ω) - look at symbol diagram on page 180.
A 4.0 Ω resistor in a circuit has a potential difference of 6.0 V across it. What is the current through the resistor?
Rearrange V= IR to give I = V ÷ R, then substitute in the values you have.
I = 6.0 ÷ 4.0 = 1.5
(Use the formulatriangle to rearrange. Just cover up the thing you’re trying to find, and what’s left visible is the formula you’re after) - page 180.
You can investigate the factors affecting Resistance, how?
The resistance of a circuit can depend on a number of factors, like whether components are in series or parallel (see p. 185) or the length of wire used in the circuit. You can investigate the effect of wire length using the circuit on page 180.
What is The Ammeter?
It measures the current (in amps) flowing through the test wire.
How should the Ammeter always be placed when whatever you’re investigating?
It must always be placed in series with whatever you’re investigating (see page 183-184 for more on series and parallel circuits).
What is The Volmeter?
it measures the potential difference (or pd) across the test wire (in volts).
How must the Volmeter always be placed when around whatever you’re investigating?
It must always be placed in parallel around whatever you’re investigating (see page 184). and NOT around any other bit of the circuit, e.g. the battery.
How do you investigate the effect of wire length using the circuit on page 180?
1) Attach a crocodile clip to the wire level with 0cm on the ruler (A thin wire will give you the best results. Make sure it’s as straight as possible so your length measurements are accurate).
2) Attach the second crocodile clip to the wire, e.g. 10cm away from the first clip. write down the length of the wire between the clips.
3) Close the switch, then record the current through the wire and the pd across it.
4) Open the switch, then move the second crocodile clip e.g. another 10cm, along the wire. Close the switch again, then record the new length, the current and pd.
5) Repeat this mor a number of different lengths of the test wire. (the wire may heat up during the experiment which will affect its resistance (pg 181). Leave the switch open for a bit between readings to let the circuit cool down.
6) Use your measurements of current and pd to calculate the resistance for each length of wire, using R = V ÷I (from V = IR).
7) Plot a graph of resistance against wire length and draw a line of best fit.
8) Your graph should be a straight line through the orgin, meaning resistance is directly proportional to length - the longer the wire, the greater the resistance.
9) If your graph doesn’t go through the origin, it could be because the first clip isn’t attached exactly at 0 cm, so all of your length readings are a bit out. This is a systematic error (see page 5).
You can also investigate the effect of diameter or material on the resistance of a wire.
An appliance is connected to a 230 V source. Calculate the resistance of the appliance if a current of 5.0 A is flowing through it (3 marks)
V = IR so R = V ÷ (1 mark)
= 230 ÷ 5.0 (1 mark)
= 46 Ω (1 mark)
