P2.3 Current Electricity

An atom has equal numbers of protons and electrons therefore it has zero charge/it is neutral. 

A charged atom is called an ion. An atom becomes an ion by gaining/losing electrons. If an atom gains an electron, it becomes a negatively charged ion.
If an electron, it becomes a positively charged ion.

Charging by friction

When two insulating materials are rubbed together, the friction between the materials can cause electrons to be transferred from one material to the other. The material which gains electrons will become negatively charged. The material which loses electrons will become positively charged.

• Like charges repel
• Opposite charges attract

Can bend water.

An object which can be places in an electrical current is called a component. It is important to know the symbol and function/job of the following components.

Cell Transforms chemical energy into electrical energy

Bulb Glows when the circuit is complete

Ammeter Measures the current, which is the flow off electrons through a component

Voltmeter Measures the voltage across a component

Resistor Slows down the flow of electrons in a circuit (reduces current)

Closed Switch Current can flow around the circuit

Open Switch Circuit is broken - no current can flow

Battery Two or more cells in series

Variable Resistor A resistor where the resistance can be changed

Fuse Fuse wire melts if too much current passes through it

Thermistor A resistor where the resistance depends on the temperature (resistance low if temperature is high)

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Light Dependant Resistor A resistor where the resistance depends on the light intensity (resistance low when light intensity high)

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Diode Only let the current flow in one direction

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Light Emitting Diode(LED) Only lets current flow in one direction. Doesn't light if connected the wrong way around

An electric current is the rate of flow of charge i.e. the amount of charge(electrons) which flows past a point each second.

I = Q / t

I = Current(A)
Q = Charge(C)
t = time(s)

Measuring electric current

Electric current is measured using an Ammeter. Ammeters are connected in the series so they measure the charge which passes through each second.

For electrons to flow there must be a source of p.d. in the circuit.

The p.d is a measure of the energy given to each unit of charge as it flows through the source of p.d.

V = W / Q

V = Potential Difference (V)
W = Work Done/energy transferred(J)
Q = Charge (C)

Ohm’s Law

Ohm’s Law states that the current through a conductor between two points in directly proportional to the potential difference across the two points. 

When electrons pass through components in a circuit, they have to push their way through vibrating positive ions. The ions resist the electrons passing through. The higher the resistance in a circuit, the lower the current.

The more the positive ions vibrate, the more resistance there is to electron flow (current).

If the temperature of the ions increased, the more the ions vibrate and the more resistance which means lower the current

For electrons to flow past a point each second (more current), there must be more energy for each electron (charge)(Potential difference)

If there is a directly proportional relationship between the current and the p.d there must always be the same resistance to the electron flow.

Increasing Resistance
If the resistance increases, it takes more energy to maintain a constant current.
Decreasing Resistance
If the resistance decreases, it takes less energy to maintain a constant current.

If the resistance is constant, if we double the p.d, we double the current.

If we double the p.d, and this causes and increase in resistance, the current will increase but not by double.

If we double the current, but less than double the p.d, the resistance must be decreased.

If the resistance is extremely high, the current will be extremely low/zero.

A thermistor is a resistor where the resistance depends on the temperature of the thermistor.

When the temperature is increased, the resistance of the thermistor decreases.

Used in thermostats to regulate heating. Type of resistor so follows Ohm's Law.

An LDR is a resistor where the resistance depends on the light intensity of the LDR.

When the light intensity is increased, the resistance of the LDR decreases.

Used in appliances designed to only come on in the day or at night.

Type of resistor so follows Ohm's Law.

Series Circuit

1. The current in a series circuit is the same at all points in the circuit.

A1 = A2 = A3

Current is the rate of flow of charge (electrons). As these all flow in the same direction at the same speed, the same number of electrons passes through each component every second. So the same current passes through each component in a series circuit.

2. The p.d (voltage) in a series circuit is shared between the components in the circuit.

V1 = V2 + V3  p.d of the supply(V1), the energy per unit charge put in to the circuit.

The more resistance a component has, the more p.d. which is required for the electrons to pass through the components.

3. The total resistance is a series circuit is equal to the sum of the individual resistors.

Rt = R1 + R2 + R3

4. The total p.d in a series circuit is equal to the sum of the p.d of each cell.

Vt = V1 + V2 + V3

If the cell is facing the wrong way, minus the p.d

1. The current in a parallel circuit splits between the different branches

A1 = A4                              A2 + A3 = A1/A4

Current is the rate of flow of charge(electrons). As the circuit splits the electrons can go two different ways therefore the current splits between branches. If the branches have the same resistance the current is equal. If one branch has a higher resistance than another, the one with higher resistance would have a lower current.

2. The p.d in a parallel circuit is the same across each branch, it is equal to the p.d of the supply.

V1 = V2 = V3