# Current Electricity

Section 3

Current Electricity

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## Circuit Symbols

Light Emitting                          Light Dependent                                 Thermister
Diode                                      Resistor

Fuse                                       Variable Resistor                                 Resistor

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## What Is Current?

Current is the rate of flow of charge
Think of it as water flowing through a pipe, the amount of water depends on the flow rate and the time. Its the same with current, though in an electrical circuit the charge is carried through the wires by electrons

Where Q = Charge in Coulombs (C)     I = Current in Amperes (A)     t = Time in Seconds (s)

Coulomb is a unit of charge, 1 C is defined as the amount of charge that passes in one second when the amperes is 1 A

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## What Is Potential Difference?

The P.D. is the work done per unit of charge
Energy needs to be transferred to a circuit to make the electric charge flow, this energy needs to come from a power source like a battery or a cell.
When a charge flows through the power source it is 'raised' through a potential and energy is transferred to the charge as electrical potential energy.

When this electrical energy is transferred work is done - this means that the power source does work to move the charge around the circuit.
The Potential Difference, P.D. or Voltage between two points is defined as the work done in moving a unit charge between the points

V=W/Q Where V = P.D. in Volts (V) W = Work Done in Joules in (J) Q = Charge in Coulombs (C)
W=VQ

The potential difference across a component is 1 V when you convert 1 J of energy moving 1 C of charge throught the component

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## Measuring Current and Voltage

Current
The current flowing through part of a current is measured using an ammeter
The ammeter is placed in series in the circuit

Remember that a convential current always goes +to-
P.D.
A voltmeter can be used to measure the P.D. but this must be done in parallel

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## Resistance

How much current you get  for a particular potential depends on the resistance.

The resistance is a measure of how difficult it is to get the current to flow through and is measured in ohms (Ω) and changing the resistance changes the current

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## Ohmic Conductors

Ohm's law is a rule to predict how the current would change when the voltage changed.

Provided that the temperature is constant then IV

Most metals are ohmic conductors (though most conductors don't follow Ohm's law)

The gradient of this graph is the resistance

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## I-V - Characteristics

I-V - Characteristics is another way of saying an I-V graph. The shallower the gradient the higher the resistance and a curve shows that the resistance is changing

Metallic Conductors  are ohmic at constant temperatures and the current through a metallic conductor is directly proportional to the voltage  so the characteristic graph shows a straight line.

Filament Lamps  show a curve that starts steep but gets shallower as the voltage rises
The filament is a thin metal wire so logically should follow Ohm's law but the current flowing through it raises the temperature which increases the resistance
The current raises the temperature because some of the electrical energy is transferred to heat energy and causes the metal to heat up and this causes the particles in the metal to vibrate a lot more  and these vibrations make it harder for the electrons to travel through as easily therefore increasing the resistance
The graph levels off because there is a limit of the amount of current that can travel through

Filament Lamps can blow because the cold temperature before turning it on means it has a lower resistance so the initial current is very high. However the filament heats up quickly so the rapid temperature change can also get it to blow up

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## Semiconductors - Thermistors

Semiconductors aren't as good as metals because they have fewer charge carriers, however if energy is suppled to a semiconductor e.g. by an increase in temperature, more charge carriers can be released meaning they can make good sensors for detecting changes in the environment examples are thermistors, diodes and LDRs
A thermistor is a component with a resistance which depends on the temperature . NTCs are Negative Temperatures Coefficient where the resistance decreases as the temperature goes up
Increasing the current through the thermistor increases the temperature and decreases the resistance
Warming the thermistor  gives the electrons more energy to move around on their own so there are more charge carriers available

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## Diodes

Diodes, including LEDS only let the current flow in one way, forward bias is the direction the current is allowed to go and most diodes require a voltage of 0.6V in the forward direction beofre they will conduct
This is called the threshhold voltage
In reverse bias the resistance of the diode is very high and the current that flows is very tiny

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## LDRs

An LDR is a light dependant resistor, the greater the intensity of light the lower the resistance

Light rather than heat provides the energy that releases the electrons and lowers the resistance

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## Resistivity

Resistivity  is the property of a material  so the resistance depends on:

• Length (l) measured in metres. The longer the wire, the more difficult it is to make a current flow through it so the resistivity is proprtional to the material
• Area (A) measured in square metres. The wider the wire the easier it will be for electrons to pass through
• Resistivity (p) measured in ohm metres. This is a measure of how much a material resists the flow of a current
• p = RA/L
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## Power and energy

Power is the rate of transfer of energy and is measured in watts and 1 watt is equivelant to 1 J per second

P = E / t P= Power in watts E= Energy in Joules and t= time in seconds
P=VI V= p.d. in volts I= Current in Amps
P=
V^2/R
P= I^2R

Working out the energy  means having to substitute P=E/t into the power equation

E = ItV E=(V^2/R)t  E=I^2 Rt

In a car engine the starter motor which provides the energy needs to have a very high power as it needs to prouce a lot of enery in a small amount of time, however the p.d. is usually very small as the current flowing through the motor needs to be very high P=IV

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## Conservation of charge and energy

Charge cannot be usedup in a circuit  and this means that whatever charge flows through a junction needs to come back out again, as current is the rate of flow of charge it means that the current must also be conserved

Kirchhoff's first law: The toral current enterring a junctuin = the total current leaving it

Energy  is also conserved and in circuits the energy is transferred round the circuit

Kirchoff's second law: The total emf around a series curcuit = the sum of the p.d. across each component  E = EIR

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