Charge, Current, and Potential Difference

At the end of this section

• You should understand the idea of current as a flow of charge. 
• You are expected to recall and use the relationships I = DQ/Dt and V = W/Q. 
• You should understand the power in a circuit, the idea of resistance, and the heating effect of a current. 
• You are expected to recall and use the formula R = V/I.

1. What is meant by a circuit?

2. What is a current?

3. Which way does current flow?

1. Write down the equation for power and learn it!

2. Make sure that you know all the units for electricity.

Ohm’s Law & Voltage Current Characteristics

·        You should understand Ohm’s Law as a special case where I µ V.

·        You should understand the voltage current graphs for an ohmic conductor, a semiconductor diode, and a filament bulb.

1. Draw a circuit diagram that would enable you to measure electrical quantities for a resistor.

2. Describe how you would do the experiment.

3. How would you modify the experiment to record the data on a computer?

4. You vary the voltage.  Why do we put current on the horizontal axis and voltage on the vertical axis?

5. The graph shown only looks at positive values.  Sketch the graph with negative values included.

1. What voltage does the diode start to conduct at?

2. What happens to the current at this voltage?

3. Compare the shape of the curve with that of the LED.  What similarities are there, and what differences are there?

4. In both these cases we have looked at the forward biased diode.  Sketch a graph for the diode both in forward and reverse bias?

5. Look at the graph for the Zener diode.  What reverse bias (negative) voltage does it break down at?

Resistivity

·    You will learn the concept of resistivity of materials. 

• You are expected to recall and use the equation r = AR/l. 

• You should be able to describe the qualitative effects of temperature on the resistance of metallic conductors and thermistors. 
• You will need to be aware of applications of resistivity.

1. What are the three external factors that affect resistance in a wire?

2. What is the internal factor that affects resistance in a wire

3. Explain why a light bulb has a higher resistance when it is hot than when it is cold.

4. Write down and learn the formula for resistivity.

5. What are the units for resistivity?

1. Draw the symbol for a thermistor

2. What is meant by negative temperature coefficient in a thermistor?  There are positive temperature coefficient thermistors, but you don’t need to know about them at this stage.

3. Sketch a graph to show how the resistance falls as the temperature goes up.

4. Why does the graph have a logarithmic scale?  Ignore the formula given!

5. Write down two uses for a thermistor.

Series and Parallel Circuits

At the end of this section you should be able to

·        understand the nature of series and parallel circuits

_·          derive the equations R_tot = R₁ + R₂ + R₃ + … + R_x

1   =    1  +   1 +   1

R_tot      R₁     R₂   R₃

·        be able to work out the single resistor equivalents.

1. How does the current change in a series circuit?

2. How does the voltage change in a series circuit?

3. How does the current change in a parallel circuit?

4. How does the voltage change in a parallel circuit?

5. Go to Checkpoint F.  How did you get on?  Did you have any problems?

1. What is an alternative name for a voltage divider?

2. Draw a circuit diagram to show a potential divider.

3. Write down the equation for a potential divider.

4. Work through the example given.

5. How would you change a light sensor to a dark sensor?

6. Do the questions in Checkpoint A.  Do you manage them?  Did you have any difficulty?

Kirchhoff’s Laws

In this section you should understand

·        The idea of conservation of charge and energy in simple D.C. circuits. 

·        You should also be aware of the relationships between currents, voltages, and resistances in series and parallel circuits.

1. In simple terms, what does Kirchhoff I state?

2. What does the animation show?

3. In simple terms, what does Kirchhoff II state?

4. Extension: Look at how Kirchhoff I and II are used in more complex circuits.

Electromotive Force and Internal Resistance

At the end of this section, you will need to understand that

·        sources of potential difference have an internal resistance.

·        You will tackle problems involving internal resistance. 

1. Explain why a battery gets hot in use.

2. How would you explain internal resistance to a student who is struggling with the concept?

3. Write down the formula and explain what each term means.

4. Work through the examples.

5. Explain how the relationship is consistent with the Principle of Conservation of Energy.

6. Describe an experiment that would enable you to get a graph like this:

    7.  Explain how you determine EMF the internal resistance of a cell using this graph.

Alternating Currents

At the end of this section you should be able to understand

·        Sinusoidally varying voltage and current. 

·        Root mean square and peak-to-peak values for sinusoidal wave forms.

1. Why is the first diagram wrong?  Copy the diagram and correct it.

2. Write down the features of alternating current.

3. Why could it be difficult to do calculations with AC?

4. What is the average voltage over a period?  Explain your answer.

5. How does the power in an AC circuit vary with time?  How does the frequency of the power compare with the frequency of the voltage?

6. What is the average power?

7. What the meant by rms value?  Draw a diagram to help you illustrate your answer.

8. Write down the relationship between peak voltage (current) and rms voltage and current.

1. Which controls do you use to change the trace?

2. Read through the pages.  Describe in a few lines how to set up and read the CRO.

3. Look through the pages on Signals.

4. Then try out the Signals Checkpoint Questions A and D.

Bulk Properties of Solids

At the end of this section you will need to be able to:

·        Explain density.

·        Recall and use the formula r = m/V. 

·        Explain Hooke’s Law. 

1. What do forces do to materials?

2. What does Hooke’s Law say?

3. Write this down as a formula.

4. What are the units for k?

5. What happens when the wire is loaded beyond the elastic limit?

6. Illustrate this as a sketch graph.

7. Show how you get the relationship F = ½ ke.

8. Draw the sketch graphs for copper, glass, and rubber.

9. Explain the shapes of these graphs.

1. Define stress and strain, writing down the formula needed for each.  Define the terms in SI units.  (Note they use s, “sigma”, a Greek letter ‘s’, for the code for stress, and e, “epsilon”, a Greek letter ‘e’, for strain.)

2. Why has strain got no units?

3. Define Young’s Modulus, and write down the relationship.

4. Rewrite this formula in terms of the Force, length, area, and extension.

5. What is the unit for Young’s Modulus?

6. What does Young’s Modulus tell us?

Remember:

• Do not blindly put the numbers in.
• Convert mm to m by multiplying by 1 ´ 10^-3.
• Convert mm² to m² by multiplying by 1 ´ 10^-6.
• Always quote the units.
• Marks thrown away like this can cost 6 to 8 marks, 12 to 16 percent.  That can be the difference of two to three grades!