5.1 - Electric potential - questions

1 - Define electric potential difference !

3 - What is the formula for electric potential difference ?

4 - What are the units in the formula for electric potential difference ?

5 - Define current !

6 - What is causing a current ?

7 - If a current of 10A flows through a wire during 10s. How much charge has passed through the conductor ?

8 - How much energy is lost when +5C of charge moves from the positive side to the negative side of a 9 Volt battery ?

9 - Define resistance !

10 - What is the unit for resistance and how is it defined ?

11 - What is Ohms law ?

12 - What is the formula and unit for resistivity and how can it be used to describe the resistance of a wire ?

13 - Describe resistance in the microscopic model.

14 - The resistivity of copper is 1.72•10-8Ωm . What is the resistance of a 1 m long copper wire with 2 mm diameter ?

15 - Draw diagrams that describes an Ohmic resistor and explain how to get the resistance from them ?

15.5 - Draw diagrams that describes the filament of a light bulb and explain how to get the resistance from them ?

5.2 - Electric circuits - questions

16 - Define electromotive force and explain what it is !

17 - What does the potential difference over a resistance tell us ?

18 - What is the formula for a battery with internal resistance ?

19 - What is the potential difference for a battery with emf = 9V and internal resistance = 1Ω if
1) the battery is not connected,
2) the battery is connected to R=8 Ω,
3) the battery is connected to R=899 Ω ?

20 - What is power and what is its unit ?

21 - What is the formula for the power delivered by a battery ?

22 - What is the formula for the power dissipated by a resistor ?

23 - What is the formulas for resistors coupled in series and parallel ?

24 - What is the resistance for this circuit:   

25 - How is a voltmeter connected and what is an ideal voltmeter ?

26 - How is a ampmeter connected and what is an ideal ampmeter ?

27 - How does a potential divider work ?

28 - What is this:   

29 - What is this:   

30 - What is this:   

31 - What is this:   

32 - Draw a V-I diagram for a wire in a light bulb and a thermistor

33 - What is the definition of an electron volt ?

34 - How large is the drift velocity of electrons in a conductor ?

35 - Which of these units are fundamental: Volt, Ampere, Ohm, Joule, Watt, Second, meter ?

36 - What is the charge of one electron or one proton ?

37 - What is the formula for acceleration of an electron or proton over a potential difference ?

5.1 - Electric potential


The unit for potential is Volts (V)
The unit for work is the same as for energy: Joule (J)
The unit for charge is Coulomb (C)

Look at the data booklet:  
DEFINITION: The current (I) is the flow of charge (Δq) per unit time (Δt) in a conductor.

1 Ampere of current = 1 Coulomb of charge passing through a wire in one second.

Look at the data booklet:  
I = 10A and Δt= 10s so Δq = I•Δt = 10•10 = 100 C

V = W/q so W = V•q = 9•5 = 45 Joule

Look at the data booklet:  
DEFINITION: Resistance is the potential difference over a resistor divided by the current in the resistor.

Resistance is a term that describes the forces that oppose the flow of electron current in a conductor.

Look at the data booklet:  
The unit for resistance is ohm (Ω). One ohm is the amount of electrical resistance that exists in an electrical circuit when one amp of current is flowing with one volt being applied to the circuit.

Look at the data booklet:  
The resistance of a conductor is constant if its temperature is constant and the current is then proportional to the potential difference.

Look at the data booklet:  
The unit for resistivity is Ωm.

The formula describes how the resistance depends on the length of a wire (L) and its cross section area (A) with the resistivity (ρ) being the constant of proportionality. A good conductor such as Copper has a low ρ value and a bad conductor (an insulator) such as plastic has a high ρ value.


When the electrons move through the metal they give energy to the metal atoms, causing the metal to heat up. Take two conductors with different resistance and pass the same current (number of electrons through them). The one with high resistance is taking up more energy from the electrons and heats up more.

The resistivity of copper is 1.72•10-8Ωm . What is the resistance of a 1 m long copper wire with 2 mm diameter ?

Diagrams for an Ohmic resistor.

Diagrams for a light bulb.

5.2 - Electric circuits

The potential difference over a resistance = the energy converted from electrical to heat per unit charge (V = W/q).

Power: P = W / t   →   Power = work done per unit time    Unit: Watt (W) or J/s

For a battery the power delivered is P = ε•I . The power is in this case the amount of chemical energy converted to electrical energy per unit time.

Look at the data booklet:  
The power is in this case the amount of electrical energy converted to heat per unit time.

Look at the data booklet:  

Look at the data booklet:  Variable Resistor  

A variable resistor consists of a resistive wire and a dial that touches it. When one turns the dial the resistance change.

Look at the data booklet:  Potentiometer  

A potentiometer is a variable resistor with 3 connections as in the picture. It is used as a potential divider i.e. it is used to change a potential by changing a resistor.

Look at the data booklet:  Light Dependent Resistor  

Conductors have low resistance and insulators have high resistance. A semiconductors is between a conductor and an insulator. The photoresistor is made of a semiconductor (Cadmium Sulphide) with very high resistance with only a few free electrons. When light falls on this material, photons from light is absorbed by these materials and energy is transferred to electrons which become free resulting in lower resistance.

Look at the data booklet:  Thermistor  

The resistance of a thermistor goes down when the temperature goes up. (This is opposite to a wire in a lamp, because its resistance goes up when it gets hot.)

An electron volt is the energy gained by an electron when it moves through a potential difference of 1 Volt.
Look at the data booklet:  :  
The formula for potential difference is: V=W/q which means that the energy corresponding to 1 eV is W = V•e = 1•1.6x10-19=1.6x10-19 J.

The actual velocity of electrons through a conductor is measured as an average speed called drift speed. Individual electrons do not continue through the conductor in straight line paths, but instead they move in a random zig-zag motion, changing directions as they collide with atoms in the conductor. Thus, the actual drift speed of these electrons through the conductor is very small in the direction of current. It is in fact only about 1 mm per second.

So how does an electrical device turn on near instantaneously? If you think of a copper wire as a pipe completely filled with water, then forcing a drop of water in one end will result in a drop at the other end being pushed out very quickly. This is analogous to initiating an electric field in a conductor.

Fundamental units: Ampere, Second, meter Not fundamental units: Volt, Ohm, Joule, Watt

Look at the data booklet: