#### Transcript Slide 1

### AS Level Electricity - Circuits

### Taking Measurements

• The p.d. across a component in a circuit is measured in volts (V) using a voltmeter connected across (in parallel with) the component.

### Taking Measurements

• The current ( an ammeter

*I*

) flowing through a component in a circuit is measured in amperes (A) using connected in series with the component.

### Current

• A current will flow through an electrical component (or device) only if there is a voltage or potential difference (p.d.) across its ends. • The bigger the potential difference across a component, the bigger the current that flows through it.

### Model

• You can think of electrical potential as being the topography of the electrical environment.

• The flow of charged particles is affected by the steepness of the ‘slope’.

• The change in volts per metre is a measure of how steep the slope between two points is… the steeper the ‘potential gradient’ the faster the charge will flow.

### Current

• An electric current is a flow of charge (Q) measured in coulomb (C).

• The charges 'flowing' are usually electrons (in a wire) but can be ions (in a solution).

### Current

• It is the **'net' flow of charge **that makes the current. • Charges going in opposite directions cancel out each other's effect. • Double-charged ions will make double the current that single-charged ones would.

### Resistance

• Components resist a current flowing through them.

• The bigger their resistance, the smaller the current produced by a particular voltage, or the bigger the voltage needed to produce a particular current.

• Resistance (R) is measured in ohms ( W )

### Resistance

• When electrical charge flows through a resistor, electrical energy is transferred as heat according to the equation P=

*I V*

• This makes components get hotter as current goes through them.

• A change in temperature can change the resistance of the component. You need to appreciate this.

### Cells and Batteries

• An electric cell provides the potential difference for a battery powered circuit by changing chemical energy into electrical energy.

### Cells and Batteries

• If more than one electrical cell is connected together the term for the power source is ‘battery’ – a single cell is just called an electric cell.

### Cells and Batteries

• A cell’s potential difference between its terminals has a chemical source and that this can **‘run down’ **with use or incorrect storage providing less of an electrical gradient for the current (i.e. the voltage stamped on a battery might not be correct).

### Electrical Energy Transfer

• As an electric current flows through a circuit, energy is transferred from the battery or power supply to the components in the electrical circuit. • An electric current is a flow of charge. • Charge (Q), measured in coulomb (C) is a property of the electrons that move in the wire. Each electron has a very tiny charge of 1.6 X 10 -19 C

Equations you should already know from GCSE When electrical charge flows through a resistor, electrical energy is transferred as heat.

The rate of energy transfer (power) is given by: P =

*I*

V Where:

*I*

P = power (in watts, W) V = potential difference (in volts, V) = current (in ampere, A) 1 watt is the transfer of 1J of energy in 1s.

### Equations you should KNOW

The higher the voltage of a supply, the greater the amount of energy transferred for a given amount of charge which flows. E = VQ Where E = energy transferred (in joule, J) V = potential difference (in volt, V) Q = charge (coulomb, C)

### Equations you should KNOW:

Q =

*I*

t Where: Q = charge (coulomb, C)

*I *

= current (in ampere, A) t = time (in seconds, s)

### Equations you should KNOW

V =

*I*

R Where: V = potential difference (in volts, V)

*I*

= current (in ampere, A) R = resistance (in ohm, W )

### Equations you should KNOW

E = Pt Where: E = energy transferred (in joule, J) P = power (in watts, W) t = time (in seconds, s)

For all equations you should be able to: • recall the equation • manipulate it • know the symbols, values and units • use it in calculations • be able to use S.I. Prefixes with the units

### Symbols

connecting wire connection between two crossing wires two crossing wires that are not connected to each other switch (open) switch (closed) signal lamp filament lamp

### Symbols (cont)

cell battery power supply fuse resistor diode variable resistor thermistor

### Symbols (cont.)

ammeter voltmeter L.D.R. (light dependant resistor) You have to be able to draw these symbols and incorporate them into circuits.

They must be drawn carefully.

**Never **put a symbol in a ‘corner’.

**Never **leave a gap.

Use a sharp pencil to draw the circuits.

### Series Circuits

When components are connected in series: • their total resistance is the sum of their separate resistances R TOTAL = R 1 + R 2 + ..........R

N ; • the same current flows through each component; • the potential difference from the supply is shared between them.

### Parallel Circuits

When components are connected in parallel: • there is the same potential difference across each component; • the current through each component depends on its resistance; the greater the resistance • the total current through the whole circuit is the sum of the currents through the separate components - this follows from Kirchhoff's

### Characteristic Curves

• Current-voltage graphs are used to show how the current through a component varies with the voltage you put across it.

• They are called **characteristic curves **

**of the components.**

The current through an ohmic conductor (e.g. a wire) is proportional to the voltage across the resistor at constant temperature. This is known as **Ohm's Law**. The straight line shows proportionality – the fact it goes through the origin shows it is directly proportional – double the voltage and the current doubles!

The resistance of a filament lamp increases as the temperature of the filament increases. When the filament is very cool the graph is a straight line – it curves most as the temperature changes rapidly (when it goes through the red glow to white glow stage). When it is really hot it gets to a steady temperature and the line straightens out again.

The current through a diode effectively only flows in one direction only. It acts like a closed switch when connected in forward bias and an open switch when in reverse bias. When connected in forward bias its resistance is very low (provided it has a potential difference of more than 0.6 volts across it). The diode has a very high resistance in the reverse bias therefore only a tiny current flows. Zero p.d. gives zero current.

### You also need to KNOW

• The resistance of a light dependent resistor decreases as the light intensity increases. • The resistance of a thermistor decreases as the temperature increases. (There are some thermistors which behave in the opposite way to this but all of your questions will be set on this version).