Physics Electricity

Cell: Provides the circuit with a source of potential difference.

Battery: 2 or more cells 

Switch: Turns the circuit on (closed) or off (open)

Fixed Resistor: A resistor limits the flow of current. A fixed resistor has a resistance it cannot change.

Variable resisitor: The resistance can be varied.

Thermistor: The resistance depends on its temperature. As its temperature increases its resistance decreases and vice versa. 

Light-Dependent-Resistor(LDR): The resistance of an LDR depends on the light intensity. As the light increases its resistance decreases and vice versa.

Diode: Allows current to flow in only one direction.

Light-Emmiting Diode(LED): Emits light when current passes through it. These are used for aviation lighting and displays (TVs,Road signs)

Ammeter : Used to measure the current in a circuit. Connected in series with other components.Voltmeter: Used to measure the potential difference of an electrical component. Connected in parallel with the relavent component.

For Circuit diagrams to work effectively they require:

An energy source- This is a source of potential difference so a current can flow. This can be a cell, battery or a power supply. 

A closed path or a complete circuit- Electrons need to flow in a complete loop for current to flow. A circuit can be open and closed using a switch. 

Electrical components- These could act as sensors that respond to the enviroment (LDR,thermistor) or measure a value(ammeter,voltmeter) or transfer electrical energy to other forms of energy (LED,lamp). These must be drawn with the correct circuit symbol. 

An ammeter is always connected in series

A voltmeter is always connected in parallel to the component the voltage is being measured.

The direction of current flow is from positive to negative terminal of the power supply.

For electrical charge to flow through a closed circuit it must include a source of potential difference (P.d).

Sources of p.d include: a cell, batteries and electrical generator.

A cell makes one end of the circuit positive and the other negative

This sets up a p.d across the circuit. This i sometimes known as voltage. 

Symbol for  p.d is V

The p.d across a component in a circuit is defined as: The energy transferred per unit charge flowing from one point to another

The energy transferred is always called the work done 

Potential Difference (V) = Work done (J) / Charge (Q)

P.D is measured in volts. This is the same as a Joule per coulomb

Due to p.d being measured in volts. This is the same as Joule per coulomb

If a bulb has a voltage of 3V every coulomb of charge passing through the bulb will lose 3J of energy.

p.d is measured using a voltmeter

Question: Calculate the work done in  moving a charge of 5C through a potential difference of 20V

Answer: 

Charge Q = 5C

Potential Difference V = 20V

V=W/Q

W=VQ

W=20 x 5 = 100J  

Electric Current is the flow if electrical charge

It is measured in units of amperes (A) or amps 

The symbol for current is I

The size of the electric current is the rate of flow of electrical charge

In other words how much charge passes through a point each second 

In metals, such as a copper wire the electrical charge that flows is electrons. 

Therefore the current in a circuit is a flow of electrons.

The unit of Charge is coulomb(C) 

This is defined as the quantity of charge that passes a fixed point per second when a current of 1A is flowing.

The coulomb is equal to the quantity of electricity conveyed in one second by a current of one ampere i.e 1C = 1A (second)

Charge flow, current and time are related by equation:

Q = It

Charge(C) = Current(A) x Time(s)

Question: Calculate the current through a lamp when a charge of 4C passes through it in 500s

Answer: 

Charge Q = 4C

Time s = 500s

Q= IT

I= 4/800 = 8 x 10^-3 A

Electrons are negatively charged 

Therefore they flow away from the negative terminal of a cell towards the positive terminal.

Conventional current is defined as the flow of positive charge from the positive terminal of a cell to the negative terminal 

This is opposite to the direction of electron flow, as the conventional current was described before current was really understood.

Current is measured using an ammeter ( which is connected in series with the part of the circuit you wish to measure the current through)

In a circuit that is a closed loop such as a series ciruit the current is the same value at any point

This is because the number of electrons per second that passes through one part of the circuit is the same number that passes through any other part.

This means that all components in a closed loop have the same current.

Resistance is defined as the opposition to current

The higher the resisitance of a circuit lower the current

This means that good conductors have a low resistance and insulators have a high resistance 

The symbol for resistance is R

It is measured in Ohms  Ω   The symbol is the Greek capital letter "Omega" 

An ohm is defined as one volt per ampere (1 V/A)

The resistance of a circuit can be increased by adding resistors ( or variable resisitors) to it.

Every electrical component has a resistance even wires 

In an exam question the resistance of the wires and batteries are assumed to be negligible (unimportant).

The current through a component depends on both the resistance of the component and the potential difference across the component

The greater the resistance the lower the current for a given potential difference across the component

The lower the resistance the greater the current for a given potential difference across the component

The current, resistance and potential difference of a component in a circuit are calculated using the equation:

V=IR

Potential difference = Current x Resistance

Question: Calculate the potential difference through a resistor of resistance 10 Ω  of there is a current of 0.3 A through it.

Resistance R = 10 Ω 

Current I = 0.3A 

V=IR

V = 0.3 x 10 = 3V

Equipment : 

Power supply (battery/cell) - Source of p.d to the circuit

Wires - To connect all the components in the circuits

Crocodile Clips: To connect different lengths of the resistance wire

Ammerter - To measure current through the circuit

Voltmeter - To measure the potential difference through the resistors

2 or more resistors-  To measure the resistance of something

This resistance wire -  To measure the resistance of something

Meter ruler- To measure the length of the resistance wire 

Resolution of measuring equipment: 

Meter ruler  = 1mm 

Ammeter = 0.01A

Voltmeter = 0.1V

The aim of this experiment is to investigate how the length of a wire at a constant temperature affects the resistance of electrical circuits

Independent Variable = Length of resistance wire( L) 

Dependent variable  = Resistance (R)

Control Variable: P.d of the power supply, Temperature of the wire

Resistors come in 2 types: Fixed and variable resistors

Fixed resistors have a resistance that remain constant

Variable resistors can change the resistance through the circuit. This therefore can vary the amount of current through the circuit

For fixed and variable resistors once the resistance is set it will stay at this value no matter how the current changes.

However the resistance of components such as lamps, diodes ( also LEDs), thermistors and light-dependent resistors changes the current through the component.