Reference: Nelson Physics VCE Units1 & 2 Chapters 17 & 18 Page 411 onwards

• The unit of charge is the coulomb (C)
• 1 coulomb = 6.25 x 10 ¹⁸ electronic charges (the charge on an electron)
• The charge on 1 electron = 1.60 x 10 ^-19 C.
• There are two types of charge. Positive and Negative.
• Objects can become charged when rubbed. This does not create charges, but separates the charges already there.
• Objects that allow charges to move along them are called conductors.
• Objects that do not allow charges to flow are called insulators.

The force between charges relies on two things:

1. The size of the charges.
2. The distance between them.

These relationships were discovered by Charles Coulomb and can written as

where k = coulomb’s constant = 8.99 ´ 10⁹ N m²C^-2

q₁, q₂ = the charges (in coulomb)

r = the distance of separation

Example

The electric force of attraction between two unlike charges is 4.0 ´ 10^-4 N when they are placed
10 cm apart. What is the force between the two charges when

1. The separation is 20 cm
2. The separation is 5.0 cm
3. One of the charges is doubled
4. One charge is doubled and the separation is 20 cm

Problem Set #1: Text Page 435 Questions 1 – 4, 6

Electric current is the movement of charges. It is the measure of the number of charges passing a point every second.

where Q = charge in coulomb

t = time interval in seconds

The units of current are Ampere (Amp).

The direction of current is the direction of movement of the positive charges. i.e. positive to negative. We know that it is really the electrons that move, but this definition stays for historical reasons.

When a charge passes through a battery it is given energy. When it passes around a circuit it loses this energy to the circuit elements (perhaps a light globe).

The term voltage is used to refer to the amount of energy that is given to a charge or is used up by a charge when moving around a circuit.

The voltage of a battery refers to how many joules are given to each coulomb of charge:

1 V = 1 J C^-1

Voltage is some times referred to as potential difference, the difference in potential energy of a charge at two different places in a circuit.

Prac #1: EXPT 2.9 Energy Transfer in a Circuit

The rate at which energy is supplied by a battery, or is used up in a circuit element, is called power

The energy supplied by a battery is voltage(V) ´ amount of charge(Q)

The time can be related to the current by

Power =

Power = V I Watt(W)

Problem Set #2: Text Page 462 Questions 1 – 4, 11 – 15

Prac #2: EXPT 2.10 Electric Currents in Circuits

I₂ + I₃ = I₁ In parallel the currents add up

V₁ = V₂ In parallel potential differences are the same

V1 + V3 = E.M.F. In series potential differences add up

Problem Set #3: Text Page 462 Questions 5 – 10, 16 – 25

Prac #3: EXPT 2.11 Ohm’s Law

In any Circuit, if a circuit element gives a straight line graph of I Vs V, as shown

It is said that the element obeys Ohm's Law

V µ I

V = I R

R is called the resistance and is measured in Ohms (W )

Problem Set #4: Text Page 464 Questions 26 – 30

Gardiner & McKittrick Set 56 Page 167 Questions 7 – 11

A resistor or a resistance is a circuit element that resists the flow of electricity (electrons). The higher the number the higher the resistance. The units of resistance are Ohms (W ).

Prac #4: EXPT 2.12 Series and Parallel Circuits

In a series circuit , the current leaves the battery, travels around the loop of the circuit passing through each circuit element. As the charges move through each circuit element they loose potential (energy) until they arrive back at the battery terminal with no energy left. In a series circuit the current is the same throughout the circuit.

We can say that the sum of the energies lost in the circuit = the energy supplied by the battery.

i.e. E = V₁ + V₂ + V₃ + ….......

from Ohm's law V = I R

we get I R_T = I R₁ + I R₂ + I R₃ + …........

since I is the same around the circuit

I R_T = I ( R₁ + R₂ + R₃ + ............ )

So R_T = R₁ + R₂ + R₃ + ...........

Example: Find the total resistance in the circuit below

In a parallel circuit the current splits up into two or more components while the voltage across each element in the parallel connection is constant.

Thus I_T = I₁ + I₂ + I₃ + ............

From V = I R

substituting we get

Since V is the same for all resistances

Example: Find the total resistance in the following circuit

Problem Set #5: Text Page 465 Questions 40 – 50

From section E.4 P = V I

but Ohm's law says V = I R

So we can get two alternate expressions for power

P = I² R

and

Example:

A light globe is rated as 75 W, it has a current of 500 mA flowing through it. What is the resistance of the light globe?

Many components used in circuits have voltage/current relationships which are quite different to those of Ohm's law. i.e. a graph of V vs I does not give a straight line.

Examples:

Solid State Diode

Neon Lamp

Light Dependant Resistor

Problem Set #6: Text Page 466 Questions 51, 52

Prac #4: EXPT 2.13 Internal Resistance of a Battery

Most power supplies have an internal resistance, this means that some of the energy provided by the power supply is used up inside the power supply, appearing as heat.

Thus in practice less energy is available for the external circuit than is indicated by measurement of the E.M.F.

Consider a battery of emf (e ) 1.5 V which can be measured when no current is flowing.

The voltmeter will read 1.5 V i.e. The emf (e ) = 1.5 V r = internal resistance of the battery

In the situation above the current is effectively zero, hence energy is not transferred by charges moving inside the source, and the volt meter reads the maximum potential difference for the terminals, this is the E.M.F.

If the battery was now connected in a circuit so that a current of 1 A flows through the battery the voltmeter reading would reduce to say 0.9 V, the 'lost' 0.6 V between the terminals is required to move the charge through the battery.

In this situation some energy is used to move the charge through the source and hence less energy is available for the external circuit.

Thus E.M.F. = V_terminals + V_int

Example:

A high resistance voltmeter reads 9.0 V when connected to a battery. When the battery is connected to an unknown resistance and supplies a current of 0.2 A, the voltmeter reads 8.4 V.

a) What is the E.M.F. of the battery?

b) What is the voltage across the internal resistance of the battery when it is supplying current?

c) What is the internal resistance of the battery?

d) What is the value of the unknown resistance?

Problem Set #7: Text Page 467 Questions 53 – 58

Close inspection of a car battery shows it to have six separate compartments. Each compartment is known as a cell with a collection of such cells forming a battery.

Consider these cells of emf e joined in series to form a battery of total emf 3e .

As a 1 C (Coulomb) charge of electricity passes through all three cells it therefore receives three times the energy it would have received from a single cell.

Consider the same three cells joined in parallel:

If each cell has an identical internal resistance then the current will divide so that I₁ = I₂ = I₃ and I = I₁ + I₂ + I₃. Each 1 C charge of electricity passes through only one cell and so the total emf is only e . However the energy drain on each of the identical cells in parallel is only one third of a single cell doing the same work.

1. If the emf of each cell below is 2 V calculate the total emf in the following arrangements:

The damaging effect of an electrical shock in the human body is the result of current passing through the body.

Recall from Ohm's Law:

Thus the current through the body depends on the applied voltage and the electrical resistance of the body. The resistance of a human body ranges from about 100 W , if the person is soaked with salt water, to about 50,000 W if the person's skin is very dry.

The following table describes the effects of different amounts of current on the human body.

If an active wire in a household supply is touched while a person is standing on the ground then a potential difference of 240 V exists between the person's hand and the ground. If the person's feet and the ground are wet there is a very low electrical resistance bond between the person and the ground which can easily produce fatal currents in the human body.

1. Assume the resistance of your body is 100,000 ohms. Determine the current passing through your body if you touched the terminals of a 12 volt car battery and describe the sensation that you would feel.

 

 

 

2. After emerging wet from a swimming pool, your skin being very wet, reduces your resistance to 1,000 ohms. Determine the current passing through your body if you now touched the terminal of a 12 volt car battery and describe the sensation that probably would be felt.

 

 

 

 

 

 

 

 

 

3. If, while wet, body resistance is 1,000 ohms, you touched a live wire of a 240 V appliance and you were standing on a concrete patio, determine the current that would pass through your body and the probable effect it would have on your body.

Electric shock overheats the tissues in the body and disrupts normal nerve functions. Thus such a shock can upset the nerve centre that controls breathing. In treating an electrocuted person, first turn off the electricity at the switch and pull out the plug or remove the conducting wires from the patient using a dry wooden stick or some other non-conducting material. if the person has no heart beat or is not breathing immediately commence emergency heart-lung resuscitation as summarised below:

1. Check for free air way and remove any foreign material from the mouth. Extend the neck and check breathing by placing your ear near the patient's mouth and by watching for chest movement.
2. If the patient is not breathing use mouth-to-mouth or mouth-to-nose ventilation. Give five quick full inflations (if ventilation is not effective extend the head back further to open the airway more fully). Maintain 12 inflations per minute until spontaneous breathing returns or the ambulance arrives.
3. After the first five ventilations described above, check for a pulse in the neck. If absent, continue ventilation and use external heart compression. Place the patient fiat on their back on a firm surface. Depress the middle of the lower half of the breast bone 4 - 5 cm, 60 times per minute (1 per second) while keeping fingers off the chest If effective the neck pulse will be felt with each compression and the skin will become pinker.

If there is only one person assisting the patient, then use:

4 cycles per minute

If there are two people assisting the patient, then use:

l2cycles per minute

If circuits suddenly carry more current than that for which they are designed then the distribution wires may become hot enough to ignite the surrounding substances. Recall that the heating effect is proportional to the square of the current. (E = V I t = I²Rt).

To avoid the danger of fire each circuit should have a safety device that will 'open' the circuit if excess current develops. This safety device is usually either a fuse (low melting point wire) or a circuit breaker (usually incorporating a bimetallic strip that bends away from a contact when it is heated).

Shocks can occur when the surface of an electrical appliance is at a different potential from the surfaces of other nearby devices. If a person touches surfaces at different potentials electrical current will pass through the person's body, often with fatal consequences. To overcome this problem an 'earth' wire, that is connected directly to the ground, is connected to the metal body of the appliance. If a wire breaks free or the insulation on the wire becomes frayed and an active wire touches the metal body of the appliance, then the current is 'short circuited' to the earth which would also mean the excess current flow would overheat and rupture the fuse or 'trip' the circuit breaker in the circuit

Below is a diagram of our 'earthed' appliance.

Switches are placed on the active side of the circuit. A switch on the neutral side would work but could be dangerous, e.g. a light switch in the neutral wore could mean that somebody changing a glove could touch a 'live' active contact even though the switch is turned off.

Often appliances do not have an earth 'pin' and such appliances are termed 'double insulated' appliances. In all such appliances the outer case is insulated (made from a non-conductor). Protection from frayed, ruptured or shorting wires is at two levels:

This is the insulation around the live wires.

This is insulation in the form of a plastic (non-conducting) case.

It is highly unlikely that both forms of insulation would fail simultaneously without the device being totally inoperative.

Earth leakage protection device continuously monitors the currents in Active and Neutral lines. These two currents will at all times be the same unless some of the current has leaked to earth. The device will switch off the power in a matter of milliseconds in order to prevent electrocution. ELP devices are mandatory in Victoria in all new buildings.

Problem Set #8: Jacaranda Physics 1 Page 420 Questions 2, 4 - 8, 10, 12, 14, 19

Problem Set #9: Text Page 468 Questions 59 – 78