**BMAT Physics Notes: Electricity**

**BMAT Physics Notes: Electricity**

__Electrostatics__

Atoms are normally electrically

*neutral.*

The total negative charge is

*equal*to the total positive charge.

However, it is very easy to move electrons from an atom.

For example if an insulating rod is rubbed with a suitable cloth it is possible for electrons to move from surface atoms as shown.

This is called ‘charging by friction’.

The direction of the electron flow depends on the materials used.

If the cloth

**electrons then it becomes**

*loses***charged; the rod**

*positively***electrons and becomes**

*gains***charged.**

*negatively*

If the electrons flow in the

A build up of charge on a material can be dangerous. A spark may occur which could ignite any combustible (e.g. petrol vapour) material nearby.

For this reason materials are usually connected to the ground by a conductor so that the charge can flow to (or from) the ground. This prevents a build up of charge.

This is called

Experiment shows that objects that have the

[NOTE:

*opposite*direction the materials gain the**charge (as shown in the second diagram).***opposite**Conductors*cannot be charged because the charge will flow through the material (to earth).A build up of charge on a material can be dangerous. A spark may occur which could ignite any combustible (e.g. petrol vapour) material nearby.

For this reason materials are usually connected to the ground by a conductor so that the charge can flow to (or from) the ground. This prevents a build up of charge.

This is called

*‘earthing’.*Experiment shows that objects that have the

**charge***same***and objects that have***repel***charges***opposite***we say***attract,**‘like charges repel, unlike charges attract’.*[NOTE:

**Never**answer electrostatic questions in terms of ‘moving protons’ or ‘moving positive charges’. Although some books talk about a ‘build up of positive charges’ this is**due to moving positive charges or protons. Protons, which are positively charged, are held strongly in the nucleus and cannot be moved easily. A material ALWAYS becomes positive charged because it***never***electrons].***loses*

__Electrical Symbols__

__Electric CURRENT (I)__When a battery or cell is connected to a conductor (e.g. a metal) an

*electric current*will flow. This is due to a net movement of

*negative charges*. The ‘charge’ is carried by

**that move through the metal.**

*electrons*[

**REMEMBER:**Although protons are positively charged they DO NOT move through any material. This is because they are held by very strong forces in the nucleus of an atom].

*Definition: The CURRENT is the rate of flow of charge.*Hence

*Current =*

__Quantity of charge__*time*

*or,*

**I = Q/t or Q = I t**where Q = charge in

**I = current in Amps (Amperes)**

*coulomb (C)*

*(A)*t = in

*seconds (s)*[From this it follows that

**1 C = 1 A.s ]**

[You will

**NOT**be asked to recall the definitions BUT you will be expected to

**use**the equations in calculations].

__Examples 1:__1. How much

**flows past a point when:**

*charge*(a) 3A flows for 4s __________________________________

(b) 20 mA flows for 1 minute _________________________

(c) 500 mA flows for 10 minutes ________________________

2. Find the

**that flows if**

*current*(a) 10 C of charge pass a point in 5 s _____________________

(b) 1000 C pass a point in 4 minutes _____________________

(c) 12kC of charge pass a point in 10 minutes ______________

3. Find the

**taken when**

*time*(a) A current of 5A causes a charge of 20 kC to pass __________________

(b) A current of 10mA causes a charge of 20 C to pass __________________

(c) A current of 25 µA causes a charge of 20 mC to pass __________________

__d.c./a.c.__When a battery is connected in a circuit the current flows

**(from the**

*one way**positive*terminal around the circuit to the

*negative*terminal). This is called

**direct current (d.c.)**

If a dynamo is connected to a circuit then the current keeps

**This is called**

*changing direction.***alternating current (a.c.)**

__Voltage__Whenever current flows through a component the electrons

**(usually in the form of**

*lose energy**heat).*

The amount of energy transferred when

*one*coulomb of charge passes through a component is called the

*voltage*across the component. This leads to an important equation:

*Voltage =*

__energy__or V =__E__**charge**

*Q*

- You need to be able to use this equation to find V, E or Q;
- Also note that V must be in VOLTS; E in JOULES and Q in COULOMBS.
- Voltage is often called
*potential difference (p.d.)*

****1. (a) A charge of 20 C passes through a load and does 50 J of work. Calculate the p.d. across the load.**

__Voltage and Energy Calculations 2__

(b) 100J of heat are produced when 5 C of charge pass through a resistor. Calculate the voltage across the resistor.

_________________________________________

2. A current of 2A flows through a lamp for 10 minutes.

If 12 kJ of work is done calculate

(a) the amount of charge that passed _______________________

(b) the p.d. across the lamp _______________________

3. A current of 100mA flows through a lamp for 5 minutes. If the p.d. across the lamp is 240 V calculate:

(a) The quantity of charge that passes _____________________

(b) The work done by the charge _________________________

__Resistance__**The resistance of a material is the ratio of p.d. to current.**

*Def:***Resistance = voltage /current**or R =

__V__(In texts it is usually written: V = I R

I

[You will need to be able to use this equation to find V, I or R.]

__Unit of Resistance__When

**is measure in VOLTS and**

*V***in AMPS then resistance is in**

*I***OHMS (**

**Ω**

**)**

Good conductors (e.g. metals) have a

Poor conductors (insulators) have a

**electrical resistance.***low*Poor conductors (insulators) have a

**electrical resistance (such as rubber, plastic).***high*

__Examples 3__1. Calculate the p.d. when

(a) A current of 6A flows through a resistor of 2.5 Ω, ______________________________

(b) A current of 20mA flows through a resistor of 1.8kΩ, ____________________________

(c) A current of 50mA flows through a resistor of 0.5MΩ, ____________________________

2. Calculate the

**when**

*current*(a) A p.d. of 10V is applied across a resistor of 2.5Ω, ______________________________

(b) A p.d. of 6V is applied across a resistor of 2 kΩ, _____________________________

(c) A p.d of 12V is applied across a resistor of 0.1MΩ, _____________________________

3. Calculate the

**of a component if**

*resistance*(a) A p.d. of 12V produces a current of 0.5A, __________________________________

(b) A p.d. of 6V produces a current of 3 mA, __________________________________

(c) A p.d of 230V produces a current of 10µA _________________________________

ThermistorsA is a component whose thermistor resistance depends on its temperature.Its resistance could increase or decrease with increasing temperature. We only deal with resistance that with decreases .increasing temperature[Such a material is said to have a negative temperature coefficient of resistance (NTC.)]In the COLD its resistance is HIGH and when HOT its resistance is LOWThe graph shows how the resistance of a thermistor varies with temperature. The resistance can vary from a few hundred ohms when cold to a few ohms when hot. |

Light-Dependent-Resistor (LDR)A light dependent resistor is a component whose resistance depends on the falling on it.light intensity In the DARK its resistance is HIGH and in the LIGHT its resistance is LOWThe resistance can vary from a few million ohms (MΩ) to a few hundred. The diagram shows the variation of resistance with light intensity for a typical LDR. |

The DiodeA diode is a component that only allows current to flow through it. It allows current to flow in the direction of the ‘triangle.’one way It is used to protect devices from incorrect power connections. The diagram shows the I-V characteristics for a typical diode. It starts to conduct when the p.d. across it is about 0.5V and from thereafter the current rises quite steeply. This means that the resistance of the diode must be decreasing.Note that below about 0.5V the current is ZERO. This means that the resistance of the diode below 0.5V is _________________ |

Filament LampInside a lamp the filament is a coil of metallic wire. When COLD the filament has a LOW resistance. When the lamp is switched on the temperature of the filament increases. This increase in temperature causes the resistance of the filament to INCREASE. This means that a graph of I against V is non-linear |

So, the general result:

**In a SERIES circuit the CURRENT is the SAME at ALL points.**

where RT is the TOTAL resistance (sometimes called EQUIVALENT resistance).

[NOTE that the value of RT is ALWAYS

Calculate the total resistance when two resistors of 3Ω and 6Ω are connected in (i) series, (ii) parallel.

[NOTE that the value of RT is ALWAYS

**LESS**than either value of each resistor]

*Example 5*Calculate the total resistance when two resistors of 3Ω and 6Ω are connected in (i) series, (ii) parallel.

Short circuitIn the circuit shown current will flow through both resistors so the ammeter A and voltmeter V will both show readings. If the wire is placed across resistor then ALL the current will flow through the wire and NONE through the resistors. BOTH the ammeter and voltmeter will read ZERO.either This is sometimes called a ‘short circuit’. |

__Power & Energy__**POWER**is defined as the amount of ENERGY transferred PER SECOND.

This leads to a

*general*power equation:

For

Since V = I R we can substitute for V and rewrite the equation:

**circuits it can be shown that: Power = voltage x current or***electrical***P = V I**Since V = I R we can substitute for V and rewrite the equation:

[Unit: J] [ The time

You need to be able to use the equations in calculations.

**ENERGY**can be calculated by rearranging the ‘power’ equation: Energy = power x time or**E = V I t**[Unit: J] [ The time

**t**MUST be in**seconds**].You need to be able to use the equations in calculations.

__Example 7__A lamp is rated at ‘100W, 230V’. Calculate

(a) The

**flowing,**

*current*(b) The

**of the lamp,**

*resistance*(c) The

**transferred if the lamp is switched on for 1 hour.**

*energy*

__Example Answers__

**Examples 1**- (a) 12C (b) 20 1.2C (c) 0.3C
- (a) 2A (b) 4.17A (c) 20A
- (a) 4000s (b) 2000s (c) 800s

*Examples 2*- (a) 2.5V (b) 20V
- (a) 1200 C (b) 10V
- (a) 30 C (b) 7200J

*Examples 3*- (a) 15V (b) 36V (c) 25V

- (a) 4A (b) 3mA (c) 0.12 mA
- (a) 24W (b) 2000W (c) 23MW

*infinite*

**Example 4:***5.5A outwards*

**Example 5:****9W., 2W**

*Example 6*:

*Example 7*When the switch is closed the parallel resistor is ‘short circuited’. NO current flows through it. ALL the current flows through the switch. The voltmeter reads ZERO. The TOTAL circuit resistance DECREASES so the current INCREASES. So the answer is (B).

0.43A (b) 529W (c) 3.6 x 105 J