What current and potential difference actually are, Ohms law, the circuit symbols you must know, and the required practical on the resistance of a wire.
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1.What is electric current?
The rate of flow of electrical charge.
2.What is the unit of current?
The ampere, or amp, symbol A.
3.State the equation linking charge, current and time.
charge flow = current x time
4.What is the unit of charge?
The coulomb, C.
5.A current of 3 A flows for 20 s. Calculate the charge.
3 x 20 = 60 C.
6.What is potential difference?
The energy transferred per unit charge passing between two points.
7.What is the unit of potential difference?
The volt, V. One volt is one joule per coulomb.
8.What is resistance?
A measure of how difficult it is for charge to flow through a component.
9.What is the unit of resistance?
The ohm.
10.State the equation linking potential difference, current and resistance.
potential difference = current x resistance
11.A component has 12 V across it and a current of 4 A. Calculate its resistance.
12 divided by 4 = 3 ohms.
12.A 6 ohm resistor carries 2 A. Calculate the potential difference.
2 x 6 = 12 V.
13.What happens to current if resistance increases at constant potential difference?
The current decreases, because current is inversely proportional to resistance.
14.What is needed for a current to flow?
A complete closed circuit and a source of potential difference.
15.How is an ammeter connected?
In series with the component, because it measures the current through it.
16.How is a voltmeter connected?
In parallel with the component, because it measures the potential difference across it.
17.Why must an ammeter have a very low resistance?
So that adding it to the circuit does not significantly change the current it is measuring.
18.Why must a voltmeter have a very high resistance?
So that almost no current flows through it, and it does not affect the circuit.
19.What does a variable resistor do in a circuit?
It allows the resistance to be changed, which changes the current.
20.In the required practical on the resistance of a wire, what is measured?
The current and potential difference for different lengths of wire, so resistance can be calculated for each length.
21.What is the relationship between length and resistance?
Resistance is directly proportional to length, so doubling the length doubles the resistance.
22.Why should the circuit be switched off between readings?
The wire heats up when current flows, and a higher temperature increases its resistance and affects the results.
23.Why is a thin wire used?
It has a higher resistance, so the change in resistance with length is easier to measure.
24.What would a graph of resistance against length look like?
A straight line through the origin, showing direct proportion.
25.What is an electrical conductor?
A material that allows charge to flow through it easily, because it has free charge carriers.
26.What is an insulator?
A material with very few free charge carriers, so charge cannot flow through it easily.
27.Which particles carry the current in a metal?
Delocalised electrons.
28.Why does a longer wire have more resistance?
The electrons have further to travel and more collisions with the ions in the metal, which opposes their flow.
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