P4: Electric circuits

What does electrical charge need to flow through a closed circuit?

Circuit must have a source of PD

What happens when you close the switch of a circuit?

• Electrons flow out of the cell

• They move around the circuit

Charge

Group of electrons

Electric current

Flow of electrons

Size of electric current =

Rate of flow of electrical charge

In what direction does electric current flow?

Negative to positive end of cell

Are electrons positive or negative?

Negative

What are elctrons carrying?

Energy from cell

What do electrons do with the energy from the cell they are carrying?

Pass energy to components

What happens to the energy transferred to a lamp?

Electrical energy transferred to thermal + light energy

When electrons return to the positive end of the cell, are they carrying more or less energy than when they left the negative end?

Less

What direction is conventional current drawn in?

Positive to negative

What is electrical charge measured in?

Coulomb (C)

1 Amp current =

1 coulomb of charge flowing per s

Charge flow equation

charge flow = current × time

• Q = It

Unit of electric current

Ampere (Amp)

What is used to measure current in a circuit?

Ammeter

Is current used up in a circuit?

No-never

In a series circuit (single closed loop) current is…

Same value at any point

Series circuit

No branches, single closed loop

In a series circuit, current can only flow in…

1 path

Parallel circuit

Has branches

How does current flow in a parallel circuit?

• Current splits

• Some passes thru both branches

How does current behave in a parallel circuit?

Current in branches adds up to total current leaving the cell

Energy transfers involved in this?

1. Cell has chemical energy store

2. Transferred to electrical energy + carried out by electrons (current) passing out the cell

3. Electrons pass thru components, EE transferred to other forms of energy

4. If lamp, EE transferred to TE + LE

Potential difference (voltage)

Measure of WD / energy transferred to a component by each coulomb of charge that passes thru it

PD of 1 volt =

1 joule of energy transferred to each coulomb of charge moving thru the circuit

How is PD measured in a circuit?

Voltmeter

How is a voltmeter connected in a circuit?

In parallel to the component you want to measure the voltage of

How is a ammeter connected in a circuit?

In series

In series, why are both lamps dimmer than if there were just 1?

1. PD in each lamp is lesser

2. Total energy carried by current in shared betw 2 lamps

3. So lesser electrical energy transferred to light energy

How does voltage behave in a series circuit?

Shared

• Total PD across components = total PD across cell

Voltage (PD)

Amt of energy transferred to each coulomb of charge moving thru a circuit

What does a PD of 6V show us?

Current carrying 6J of energy per coulomb of charge

How does PD behave in a parallel circuit?

For components connected in parallel, the PD across each component is the same

Battery

2 or more cells connected tog

The cells in a battery must be…

Connected in the same direction

• Eg positive ends both pointing left

If each cell has a PD of 9V, what is the overall PD of the battery?

27V

PD of this battery

0V

• Cells pointing in diff directions → PD of each cells cancel out

PD of each battery if each cell has a PD of 9V

9V

• 2 × 9 = 18

• 18 - 9 9

What happens whenever charge flows in a circuit?

Work is done (ET)

Energy transferred equation

E = QV

What does this show in terms of energy?

All electrical energy carried by current is transferred by lamp to other forms of energy

What about components causes them to transfer energy?

Resistance

Electric current

Flow of electrons thru a conductor (eg metal wire)

What does resistance tell us?

PD needed to drive a current thru a component

• How much energy needed to push a coulomb of charge thru

Resistance

What happens as electrons move thru a conductor (metal wire)?

1. e^- collide w atoms in the metal

2. So electrical energy transferred to other forms of energy (eg thermal)

Which lamp has a lower resistance and why?

Lamp A = lower resistance

• Current being driven by PD of only 1V

• So only small amt of energy needed to drive the current thru the lamp

Which lamp has a higher resistance and why?

Lamb B = higher resistance

• Current being driven by PD of 10V

• So large amt of energy needed to drive current thru lamp B (more needed than A)

Resistance equation

R = V/I

Unit for resistance

Ohm

How does a variable resistor work?

1. Contains long piece of wire in a coil

2. Use slider to change length of wire current runs thru → hence ↑ or ↓ resistance

What is a variable resistor used for in this circuit?

To control PD across lamp

• Increase resistance → makes lamp dimmer

Ohm’s law

Current thru a resistor at a constant temp is directly proportional to PD across the resistor

What does current thru a component depend on?

• Resistance of component

• PD across component

Higher resistance means…

More energy transferred as the curent moves

When is resistance useful?

Resistor

Device that limits the flow of electrical current in a circuit

The greater the resistance of the component…

The smaller the current for a given PD across the component

Factors affecting resistance

Thickness, length, temperature of wire

How do thicker wires decrease resistance?

Moving electrons have more space to move

How do shorter wires decrease resistance?

Moving electrons have a shorter distance to travel

How do hotter wires increase resistance?

• Metal atoms in wire move faster

• So electrons collide more with them

When is a resistor used?

When want to add resistance into a circuit

The lamp is very bright (20V), how can you make the lamp dimmer?

1. Use cell with lower PD

2. Add extra resistance into circuit

How can you make this lamp dimmer by adding extra resistance into the circuit?

1. Add resistor in series with the lamp → PD shared betw lamp + resistor

2. So less electrical energy transferred to lamp → not as bright

Current thru a resistor is…

Directly proportional to PD

What does it mean if current thru a resistor is DP to PD?

Resistance is constant (R doesn’t change if current changes)

• Straight line

Ohmic conductor

• Components (resistors) that follow Ohm’s law

• Conductor for which PD + I are DP → R = constabt

• Fixed resistors, wires

A resistance will only stay constant if?

Temp is constant

Current thru an ohmic conductor (at a constant temp) is?

DP to PD across the resistor

What happens to current flowing thru a resistor if PD is increased?

Increases

Why does a PD of 0V give a current of 0 amps?

If PD is 0, electrons have no energy → can’t move thru resistor

How does this graph show that current thru a resistor is DP to PD?

Straight line passing thru origin

IV graph for an ohmic conductor

When are resistors used in circuits?

To control PD across other components

Circuit used to change PD across resistor + measure current flowing thru it

What does it mean if resistance is not constant

Resistance changes w the current through the component

Examples of non-ohmic conductors (resistance isn’t constant)

1. Filament lamp

2. Diodes

3. Thermistors

4. LDR

Is a filament lamp energy efficient?

No

Filament

Tightly coiled wire

How does a filament lamp work?

1. Wire gets v hot when electric current passed thru it

2. Causes it to glow + give out light

IV graph for a filament lamp

As PD increases, current no longer increases as much → R is increasing

Current thru a filament lamp is…

Not DP to PD

Why is current in a filament lamp not DP to PD (why isn’t resistance constant?)

1. Filament gets hot → causes R to increase

2. At high temp, atoms in filament vibrate more

3. So electrons in current collide more w atoms

4. So more energy needed to push current thru the filament

What happens to the resistance of a filament lamp as temp increases?

R increases

Diode

Non-ohmic conductor that allows current to flow in 1 direction only (forward)

What is special about the current thru a diode?

Flows in 1 direction only

Why does the current thru a diode flow in 1 direction only?

Diode has v high resistance in reverse direction, preventing current flow

What does the direction of the arrow in the symbol for a diode show?

Direction conventional current must flow in

• For the diode to allow it thru

What happens in relation to the diode if the cell is switched around?

• Direction of current switches

• So diode won’t allow current to pass thru

• Due to v high R in reverse direction

IV graph for a diode

Describe the IV graph for a diode

1. No current can flow in the reverse direction

2. In forward direction, current increases as PD increases

What are diodes useful for?

Controlling flow of current in circuits

Light emitting diode (LED)

• Gives off light when current flows thru

• Only allows current to flow in forward direction

Are LEDs an efficient soure of light?

Yes

What does a diode have in the reverse direction

V high resistance

Resistance of diode in forward + reverse direction

1. Forward: low

2. Reverse: high

How do resistors in series behave?

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